Progress 10/01/10 to 09/30/15
Outputs
Target Audience:Scientists and lay persons interested in ecology and evolution. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?Schoener continues to interact with his former graduate students and postdoctoral: Jonathan Losos, Jason Kolbe, Dave Spiller, Jonah Piovia Scott, Mariana Eloy de Amorin. The first fhree are co-authors on the first paper described above; Spiller and Piovia-Scott are co-authors of the second paper, and the paper with Amorin being prepared is co-authored by Piovia-Scott. Field work is continuing with all of these people save Amorin. How have the results been disseminated to communities of interest?Publications as above. Also presentations to the Gordon Conference on predator-prey, a presentation at a conference in Toulouse (France) and a presentation at the 100th anniversary meeting of the American Society of Ichthyologists and Herpetologists in New Orleans. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
We prepared a paper for an international symposium (see below) that summarized our research so far. The abstract of of the current draft of that paper is as follows: Despite the call of Fussmann et al. in 2008 for more complete studies of eco-evolutionary dynamics and the subsequent publication of a number of relevant papers, we still have only a handful of multi-generational experiments in natural systems of eco-evolutionary dynamics. To perform such a study, in 2008 we introduced the lizard Leiocephalus carinatus, a predator (and possible food competitor) of the lizard Anolis sagrei, from natural populations on the adjacent mainland to seven islands having A. sagrei, with seven unmanipulated islands having A. sagrei as controls. Almost immediately into the study, L. carinatus, which is larger and more terrestrial than A. sagrei, caused a major habitat shift in the latter away from the ground and toward higher and thinner perches. Focal behavioral surveys showed that on introduction islands, A. sagrei produced less conspicuous visual displays. The expected pattern for density of A. sagrei is that it would decrease markedly at first via predation from the larger lizard, but then it would increase as the habitat shift selected for individuals better able to live in higher vegetation. Data through 2015 show this pattern, but the time-by-treatment interaction is not yet significant. A previous within-generation selection study and comparative data suggest that short legs should evolve as the lizards adapt to better maneuver on the thin perches of higher vegetation. However, no indication of the expected morphological change in limb length was present through 2015. Previous studies showed A. sagrei producing many effects on lower levels of the food web, some quite large. In this study through 2012, we found significant differences only in spiders (web and ground). We expected the more arboreal arthropods eventually to decline with the increasingly arboreal adaptation of A. sagrei, and plant damage from arthropod herbivory to decrease disproportionately in the higher vegetation, but neither happened. Because the morphological changes are not yet in the predicted direction, it is unsurprising that these food-web expectations are unfulfilled. A possible complication is that the study site was hit by two major hurricanes in the last five years, decreasing population sizes of both lizards. These natural disturbances caused great irregularities in the treatment, so we are continuing the experiment to give our hypotheses a thorough test. An advantage of the hurricanes is that they eliminated lizards from some islands, providing the opportunity to monitor natural recolonization. Annual surveys of the 46 islands that lost lizards showed that recolonization is rather slow. We also monitored morphology of 31 island populations for up to 19 years. Mean limb length oscillated across the 9 islands for which we sampled across the 19-year period, both increasing and decreasing substantially, yet the net effect over that period is almost no change. The time series suggests that in years following hurricanes, limb length tends to increase, followed by a decline. To relate all three of the ecological (species abundances), evolutionary (predator-trait distributions) and disturbance (hurricanes) dynamics, we are using individual-based, stochastic models; these predict the short-term dynamics of evolutionary resilience and the long-term effects of disturbances on community structure and species-trait distributions. In addition, we published a paper summarizing our studies (including the AES projects) on the relation of interaction strength to island area. The abstract of that paper is as follows: We ask empirically how the strength of the three major interactions - predation, competition, and mutualism - changes with increasing island area. We review and expand current theory bearing on these relationships. Data evaluating this question come from several Bahamian archipelagos and involve both experiments and observations; the latter can be especially long term, in one case collected over a 17-year period. We analyze the effect size of the following interactions across a range of island areas: 1) predation by lizards on spiders, 2) competition between two lizard species, 3) competition between two spider species, 4) ant-plant mutualism, and 5) plant-pollinator mutualism. Effect sizes for predation and competition mostly show a hump-shaped relationship to island area. Effect sizes for ant-plant mutualism were reduced on large islands when compared with smaller islands. Germination rate showed a steady increase with island area which we infer to be caused by an increase in pollinator limitation on smaller islands.We argue that the decreasing portion of the effect-size-to-area relation with larger island size has rather similar etiologies for predation, competition and to a somewhat lesser extent ant-plant mutualism, involving species diversity, top predators, plant defenses, and (for predation and competition) spatial heterogeneity with respect to refuges or resource use. The decreasing portion of the effect-size relation with smaller island size is argued to result from harsh abiotic conditions, marine subsidies, and stochastic events occurring primarily on very small islands. The monotonically increasing relation for plant-pollinator mutualism is suggested to be the result of diversity enhancing, rather than weakening, interaction strength.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2015
Citation:
2015. Schoener, T.W., D. A. Spiller and J. Piovia-Scott. Variation in ecological interaction strength with island area: theory and data from the Bahamian archipelago. Global Ecology and Biogeography 25:891-899.
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Progress 10/01/13 to 09/30/14
Outputs
Target Audience:Scientists and lay persons interested in ecology and evolution. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?Schoener continues to interact with his previous recent graduate students; Jonah Piovia-Scott, Chris Searcy and Amber Wright have all gotten ladder positions at major research universities. In addition, the Schoener lab hosted a visiting graduate student, Marianna Eloy de Amorin, from the University of Brasilia, Brazil -- a manuscript on Brazilian geckos has resulted from this interaction that is conceptually related to the core research and will be published in a major journal. How have the results been disseminated to communities of interest?Publications, as above. Schoener has agreed to speak to a Gordon Conference next January as a keynote speaker. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
The first paper above reports some of the main behavioral results of the experiment described in the core proposal Communication often is a major component of social interactions. Signaling individuals are faced with the challenge of capturing the attention of intended receivers while limiting eavesdropping by potential predators. We conducted an experiment in nature to evaluate the hypothesis that prey species can modulate the physical properties of movement-based displays in response to the presence of predators. We found that male brown anoles dramatically decreased the amplitude of their head-bob displays in the presence of a predatory lizard, resulting in less conspicuous signals. Although less conspicuous signals may be safer for the signaling individuals, they also reduce the distance from which potential mates or competitors can detect them, which might affect the territory size and reproductive success of signaling males. We will have another ms on behavior shortly. The second paper listed above resulted from one of the four symposia (during the 100th anniversary conference of the British Ecological Society in London) on eco-evolutionary dynamics, a subject which the investigator pioneered (and gave a plenary address about during the anniversary conference). As can be seen from the core proposal, eco-evolutionary dynamics form a major conceptual foundation for the current research. Our field work as described in the core proposal was carried out successfully.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2014
Citation:
Steinberg, D.S., J. B. Losos, T. W. Schoener, D. A. Spiller, J. J.Kolbe, and M. Leal.. Predation-associated modulation of movement-based signals by a Bahamian lizard. Proceedings National Academy of Sciences 111: 9187-9192.
- Type:
Other
Status:
Published
Year Published:
2014
Citation:
Schoener, T. W., J. Moya-Larano and J. Rowntree. Eco-evolutionary dynamics PREFACE. Advances in Ecological Research 50: XIII-XX
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Progress 01/01/13 to 09/30/13
Outputs
Target Audience: Scientists and lay persons interested in ecology and evolution. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided? Projects reported above have been part of the Ph.D. theses of two graduate students of Schoener, Jonah Piovia-Scott and Amber Wright. Piovia-Scott was also a postdoctoral student of Schoener and Louie Yang (Entomology, UCD). Piovia-Scott obtained a professorship at UC Riverside, and Wright obtained a professorship at U. Hawaii. Because professorships are very difficult to come by, this shows a major contribution of the project to the professional development of these two persons. How have the results been disseminated to communities of interest? Results were published in scientific journals. The small-island experiment, published in Science, attracted some lay publicity in the form of news articles, and Piovia-Scott gave a radio interview on the project. What do you plan to do during the next reporting period to accomplish the goals? We will proceed as given in the main proposal.
Impacts
What was accomplished under these goals?
A previous documentation of research for our project reported an experiment (published in Ecology) simulating the effects of seasonal seaweed-deposition events, caused by hurricanes and other storms, on species inhabiting certain Bahamian islands. We added seaweed to six shoreline plots and removed seaweed from six other plots for three months; all plots were repeatedly monitored for 12 months after the initial manipulation. Lizard density (A. sagrei) responded rapidly, and the overall average was 63% higher in subsidized than in removal plots. Stable isotope analysis revealed a shift in lizard-diet composition towards more marine-based prey in subsidized plots. Leaf damage was 70% higher in subsidized than in removal plots after 8 months, but subsequent damage was about the same in the two treatments. Foliage-growth rate was 70% higher in subsidized plots after 12 months. Results of a complementary study on the relationship between natural variation in marine subsidies and island food-web components were consistent with the experimental results We suggested two causal pathways for the effects of marine subsidies on terrestrial plants: 1) the "fertilization effect" in which seaweed adds nutrients to plants, increasing their growth rate but also susceptibility to herbivores, and 2) the "predator-diet-shift effect" in which lizards shift from eating local prey (including terrestrial herbivores) to eating mostly marine detritivores. A paper in Oecologia during the current period follows up on these results, substantial extending the time period of observation for the control plots and doing path analysis. We found statistical support for the fertilization effect (seaweed increased foliar nitrogen content, leading to greater herbivory) and a lizard numerical response effect (seaweed increased lizard densities, leading to reduced herbivory), but not for the lizard diet-shift effect (seaweed increased the proportion of marine-derived prey in lizard diets, but lizard diet was not strongly associated with herbivory). Greater seaweed abundance was associated with greater herbivory, and the fertilization effect was larger than the combined lizard effects. Thus, the bottom-up, plant-mediated effect of fertilization on herbivory overshadowed the top-down effects of lizard predators. These results, from unmanipulated shoreline plots with persistent differences in chronic seaweed deposition, differed from those of the previous experimental study of the short-term effects of a pulse of seaweed deposition: while the increase in herbivory in response to chronic seaweed deposition was due to the fertilization effect, the short-term increase in herbivory in response to a pulse of seaweed deposition was due to the lizard diet-shift effect. This contrast highlights the importance of the temporal pattern of resource inputs in determining the mechanism of community response to resource subsidies. In a recent Science paper, we used a whole-island field experiment to probe how major seasonal seaweed deposition - mimicking what occurs in an active storm year - influences the interactive effects of multiple predators on herbivory in a terrestrial food web. Twelve small islands, six with lizards and six without lizards, were used in the experiment. Seaweed was added or removed from islands in a crossed design - each combination of seaweed and lizard presence/absence was represented by three islands. Ant exclusions were established on branches of four (three in one case) Conocarpus erectus plants on each island; ants were excluded with a sticky resin which lizards were able to bypass by crossing a narrow gap between wire mesh cones. The experiment showed that ants and lizards had synergistic effects on leaf damage in the absence of seaweed subsidies: the combined effect of these two predators on herbivory was more than three times greater than expected based on the sum of their individual effects. In contrast, in the presence of seaweed subsidies, this synergy was absent. We suggest that the synergy between lizards and ants derives from the fact that these two predators are active at different times of day. The only lizard species present, A. sagrei, is diurnal. In contrast, the dominant ant Camponotus tortuganus, is nocturnal. This temporal partitioning of foraging activity may create a dilemma for herbivores - they can avoid A. sagrei by feeding at night, and they can avoid C. tortuganus by feeding during the day, but they cannot simultaneously avoid both types of predators. When seaweed subsidies were added to the system, the synergy between ants and lizards disappeared. We suggest that the seaweed caused both ants and lizards to spend more time on the ground foraging for detritivores associated with seaweed deposits, thereby reducing their combined impact on herbivory. A third publication (in Oikos) resulting from the subsidy work shows how the periodic reproduction of the lizard A. sagrei in the northern Bahamas intersects with the seasonal variation in seaweed subsidies to affect individual growth and ultimately reproductive rate. We found that experimentally subsidized lizards grew approximately 30% faster than control lizards; however, no differences were detected in survival or body condition. Because breeding is strongly seasonal in this species, accelerated growth roughly doubles the reproductive output of females in their first breeding season by allowing them to reach sexual maturity earlier. In short, these results show how changes in an individual trait can translate resource inputs into reproductive output, and highlight the importance of seasonal timing in governing individual- and population-level responses to pulsed resource subsidies. The general significance of this research is that it contributes to knowledge about the ecological consequences of rapid environmental change. Seaweed deposition is likely to become more common as overfishing and eutrophication facilitate shifts towards algae-dominated marine ecosystems, and as climate change causes more frequent and intense Atlantic storms. The research potentially aids efforts to predict the community-wide impacts of climate change with a particular focus on responses to extreme events.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2013
Citation:
Piovia-Scott, J., D, A. Spiller, G. Takimoto, L. H. Yang, A. N. Wright, and T. W. Schoener. The effect of chronic seaweed subsidies on herbivory: plant-mediated fertilization pathway overshadows lizard-mediated predator pathways. Oecologia 172: 1125-1132.
- Type:
Journal Articles
Status:
Published
Year Published:
2013
Citation:
Wright, A.N., J. Piovia-Scott, D. A. Spiller, G. Takimoto, L. H. Yang, and T.W. Schoener. Pulses of marine subsidies amplify reproductive potential of lizards by increasing individual growth rate. Oikos 122: 1496 1504.
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Progress 01/01/12 to 12/31/12
Outputs
OUTPUTS: Although abiotic and biotic factors can interact to shape the spatial niche of a species, studies that explore the interactive effects of both at a local scale are rare. We demonstrate that one of the main axes (perch height) characterizing the spatial niche of a common lizard, Anolis sagrei, varies according to the interactive effects of weather and the activity of a larger predatory lizard, Leiocephalus carinatus. Results were completely consistent: no matter how favorable the weather conditions for using the ground (mainly characterized by temperature, humidity, wind speed, rain), A. sagrei did not do so if the predator was present. Hence, great behavioral plasticity enabled A. sagrei to adjust its use of space very quickly. In short, exploring the dynamics shaping niche use of a species under a predator-prey scenario, we uncovered a clear interactive effect of weather conditions and predator activity on one of the main axes characterizing A. sagrei's niche: perch height. To the best of our knowledge, our results constitute the first field demonstration for anoles (and possibly for other animals as well) of how environmental conditions and the presence of a predator interact to produce short-term changes in utilization along a major niche axis. This study was published in the journal Ecology in December 2012 as an "express" report (this means the paper was given special priority for rapid publication). PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Two decades ago, Dunson and Travis emphasized the need to include abiotic factors as possible controlling variables in regulating ecological communities, stating that a full understanding of community structure can only be realized by studying both abiotic and biotic factors and their interactions. Today, while abiotic factors are routinely considered to affect organisms' distributions at a global scale, their role in shaping community assemblages at a local scale remains less explored. At this local scale, emphasis is given to biotic factors such as competition, predation, or mutualism; this is despite the fact that abiotic factors as well as biotic factors have contributed to the very definitions of the ecological niche. Niche use can change over space and time, and while a fair amount of attention is now being paid to how niches may change over decades or more less attention has been given to very short time periods. Yet often unpredictable abiotic factors, particularly weather, may affect niche use over such time scales, necessitating behavioral and physiological adjustments. We demonstrated that an important spatial niche dimension of A. sagrei, perch height, is influenced in the very short term by predator activity as well as by abiotic weather factors: rainfall/humidity, temperature, and wind speed. Our study also illustrates that it can be important to incorporate variation of the abiotic environment to understand species-interaction dynamics as they affect the niche: under predator-free conditions, lizards vary perch height in predictable ways with respect to weather conditions. Yet, no matter which abiotic factor or combination thereof was explored, A. sagrei's perch height was dependent on the activity of its predator. Our work showed that behaviorally driven changes in spatial niche use are finely adjusted to the small abiotic changes that themselves can be viewed as stochastic spatiotemporal dynamics: perch height of A. sagrei seems to result from relatively continuous behavioral decisions assessing the risk of predation. While its spatial niche use can be viewed as reflecting a compromise between avoiding predators and adjusting to short-term microclimatic variation, the compromise is heavily shifted toward the former: no matter how favorable weather conditions were for doing so, A. sagrei did not go to the ground in the presence of the predator. In short, A. sagrei shows great behavioral plasticity that allows it to "decide" its use of space relatively instantaneously. More generally, because the environment can vary in time and can fluctuate asynchronously at different spatial locations, abiotic factors interact with biotic factors in a context-dependent manner to govern species distributions. These processes can occur at a variety of scales, and our work illustrates them at the finest scale. We suggest that abiotic factors, and weather conditions in particular, be given more attention and integrated with biotic factors for a fuller understanding of species interactions in communities and the dynamics of the ecological niche.
Publications
- Schoener, T. W. and D. A. Spiller. 2012. Kinds of trait-mediated indirect effects in ecological communities: A synthesis. In Ohgushi, T., O. Schmitz and R.D. Holt, eds. Ecology and Evolution of Trait-Mediated Indirect Interactions: Linking Evolution, Community, and Ecosystems. Cambridge University Press, Cambridge UK.
- Lopez-Darias, M., Schoener, T. W., Spiller, D. A. and J. B. Losos. 2012. Predators determine how weather affects the spatial niche of lizard prey: exploring niche dynamics at a fine-scale. Ecology 93: 2512-2518.
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Progress 01/01/11 to 12/31/11
Outputs
OUTPUTS: The extent to which random processes such as founder events contribute to evolutionary divergence is a longstanding controversy in evolutionary biology. To determine the respective contribution of founder effects and natural selection, we conducted an experiment in which brown anole (Anolis sagrei) lizard populations were established on seven small islands from male-female pairs randomly drawn from the same large-island source. These founding events generated significant among-island genetic and morphological differences that persisted throughout the course of the experiment despite all populations adapting in the predicted direction-shorter hindlimbs-in response to the narrower vegetation on the small islands. Thus using a replicated experiment in nature, we showed that both founder effects and natural selection jointly determine trait values in these populations. This study was published by Science magazine, both on-line in Science Express, and then later in hard copy (Science). The lizard subject of the study was featured on the cover of the issue in which it appeared. It was featured in numerous newspaper articles, including detailed articles in the Washington Post and the San Francisco Chronicle (the latter by the renowned science writer David Perlman). PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Island populations are renowned for their extent of divergence from each other and from mainland source populations. Mayr argued that these differences often are triggered by random sampling when island populations are founded by a few colonizing individuals. The resultant founder effects-changes in the genetic and phenotypic composition of a population due to founding by a small number of individuals-have been proposed as an important cause of evolutionary divergence and even speciation for the last half-century. However, an alternative explanation is that island environments differ from each other and the source locality, and that these ecological differences result in divergent natural selection. The evolutionary significance of founder effects also has been questioned because their imprint may be short-lived if populations perish due to lack of genetic variation or to demographic stochasticity, or if natural selection overwhelms their effects. Data from nature are lacking because founder events are rarely observed, and thus their effects must be inferred post hoc. We capitalized on the extensive knowledge of how Anolis lizards adapt to their environment, combined with the opportunity to use small islands, recently cleared of lizards by a hurricane, as experimental units in a natural setting. Our focus was on limb length, and the extent to which lizard populations would adapt to the novel environments on these small and scrubby islands. Extensive comparative and experimental research supports an adaptive explanation for the positive relationship between hindlimb length and perch diameter. We introduced lizards to replicate islands to which we predicted they were not well adapted due to differences in structural habitat from their source, thereby simulating founding events and altering the selective regime for limb length. Specifically, founding propagules were established on small experimental islands, all of which are sparsely vegetated, covered primarily with short, narrow diametered vegetation compared to the more forested habitat of the source area on a nearby larger island. Our prediction is that if natural selection is the dominant force, then we would expect all populations to evolve shorter hindlimbs as they adapt to using narrower substrates. Conversely, if founder effects are dominant, then we would expect no general trend in limb-length evolution, with some populations increasing in limb length and others decreasing with respect to the source population, and limb variation being unrelated to vegetation differences among islands. Our results indicate that founder effects persist, even in the face of substantial adaptive differentiation. In support of the selectionist school, we show that differentiation from the source population is mostly the result of adaptation to the new environmental milieu on the experimental islands. However, the imprint of the founder effect remains apparent, even as this adaptive divergence has occurred; indeed, variation among experimental founder islands at the present time is better explained by initial phenotypes than by current environmental conditions.
Publications
- Kolbe, J. J., Leal, M., Schoener, T. W., Spiller, D. A. and J. B. Losos. 2011. Founder effects persist despite adaptive differentiation: A field experiment with lizards. Science 335: 1086-1089.
- 2 Estes, J. A., Schoener, T. W. 2011. Trophic downgrading of Planet Earth. Science 333: 3001-3006.
- Schoener, T. W. 2011. The newest synthesis: Understanding the interplay of evolutionary and ecological dynamics. Science 330: 426-429.
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Progress 01/01/10 to 12/31/10
Outputs
OUTPUTS: The effect of environmental change on ecosystems is mediated by species interactions. Environmental change may remove or add species and shift life-history events, altering which species interact at a given time. However, environmental change may also reconfigure multispecies interactions when both species composition and phenology remain intact. In a Caribbean island system, a major manifestation of environmental change is seaweed deposition, which has been linked to eutrophication, overfishing, and hurricanes. We showed in a whole-island field experiment that without seaweed two predators - lizards and ants - had a substantially greater-than-additive effect on herbivory. When seaweed was added to mimic deposition by hurricanes, no interactive predator effect occurred. Thus environmental change can substantially restructure food-web interactions, complicating efforts to predict anthropogenic changes in ecosystem processes. This study was published very prominently in Science, and a variety of news articles resulted as well as a radio interview. PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Global environmental change is expected to have a profound impact on the structure and function of ecological communities by changing interactions between their component species. Range shifts, extinctions and species introductions change community composition, deleting some interactions and adding others. In addition, changes in the seasonal timing of migration and other life-history events can produce phenological mismatches, which can affect communities even when species composition is unchanged. The absence of alterations in species composition and phenology, however, does not necessarily mean that ecosystems will remain unaltered: environmental change can also influence ecosystem processes by reconfiguring interactions in communities whose species lists remain intact. An important aspect of global environmental change is the mobilization and transport of resources between ecosystems. Seaweed deposition in particular is likely to become a more common feature in shoreline ecosystems as anthropogenic effects (such as overfishing and eutrophication) facilitate a shift towards algae-dominated marine ecosystems. Furthermore, intense storms, which are associated with the deposition of large amounts of seaweed, have increased in frequency, a trend expected to continue with increasing global warming. Such pulsed inputs of external resources can increase prey availability, thereby 'subsidizing' in situ predators and altering predator effects on lower trophic levels. Predicting the effects of environmental change on ecosystems is an important challenge. There is increasing recognition that species interactions strongly influence how environmental change affects ecosystem processes, complicating efforts to make reliable forecasts (K. B. Suttle, M. A. Thomsen, M. E. Power, Species interactions reverse grassland responses to changing climate. Science 315, 640-642 (2007).S. E. Gilman, M. C. Urban, J. Tewksbury, G. W. Gilchrist, R. D. Holt, A framework for community interactions under climate change. Trends Ecol Evol 25, 325-331 (2010)). Our results show that large seaweed-deposition events affect the structure and function of an ecological community by reconfiguring the effects of multiple predators on lower trophic levels. This suggests that predictions based on single-species responses or pair-wise interactions may not adequately represent community responses to environmental perturbations. Experiments such as the one we report here, conducted at a spatial scale large enough to capture community-wide dynamics, are particularly relevant for conservation and management decisions in the face of ever-increasing anthropogenic disturbances.
Publications
- Schoener, T.W. and D. A. Spiller. 2010 Trophic cascades. Islands. pp. 179-202 in J. Terborgh and J. Estes (Eds.). Trophic Cascades. Island Press.
- Piovia-Scott, J., D. A. Spiller and T. W. Schoener. 2011 Effects of experimental seaweed deposition on lizard and ant predation in an island food web. Science 331: 461-463.
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Progress 01/01/09 to 12/31/09
Outputs
OUTPUTS: The effect of resource subsidies on recipient food webs has received much recent attention. We investigated the effects of significant seasonal seaweed-deposition events, caused by hurricanes and other storms, on species inhabiting subtropical islands. The seaweed represents a pulsed resource subsidy that is consumed by amphipods and flies, which are eaten by lizards and predatory arthropods, which in turn consume terrestrial herbivores. Additionally, seaweed decomposes directly into the soil under plants. We added seaweed to six shoreline plots and removed seaweed from six other plots for three months; all plots were repeatedly monitored for 12 months after the initial manipulation. Lizard density (Anolis sagrei) responded rapidly and the overall average was 63% higher in subsidized than in removal plots. Stable isotope analysis revealed a shift in lizard-diet composition towards more marine-based prey in subsidized plots. Leaf damage was 70% higher in subsidized than in removal plots after 8 months, but subsequent damage was about the same in the two treatments. Foliage-growth rate was 70% higher in subsidized plots after 12 months. Results of a complementary study on the relationship between natural variation in marine subsidies and island food-web components were consistent with the experimental results. We suggest two causal pathways for the effects of marine subsidies on terrestrial plants: 1) the "fertilization effect" in which seaweed adds nutrients to plants, increasing their growth rate, and 2) the "predator-diet-shift effect" in which lizards shift from eating local prey (including terrestrial herbivores) to eating mostly marine detritivores. PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Understanding how climate change modifies species interactions is needed to predict the community-wide impacts of global warming. Disturbances associated with climate change may be responsible for the recent increase in widespread outbreaks of herbivorous insects (Logan 2003). Stireman et al. (2005) provide evidence that climatic variability disrupts top-down control by parasitoids on host populations and predict that herbivore outbreaks will increase as climate becomes more variable due to global warming. Tropical storm activity has increased markedly during the past decade in the Atlantic Basin, and the trend is predicted to continue during the next decade. We have documented the impact of major hurricanes on island food webs in two regions of the Bahamas (Spiller and Schoener 2007). We found in both regions that herbivory increased significantly following major hurricanes and suggested that this phenomenon was caused by two factors: 1) enhanced susceptibility of damaged plants to herbivory (Spiller and Agrawal 2003, Agrawal and Spiller 2004), and 2) reduced predator abundance (Spiller and Schoener 2007). This research focuses on a third factor, resource pulses of marine subsidies associated with hurricanes. Results so far show that a pulse of marine subsidies increases herbivory at least in the short term. We hypothesize that herbivory may decline in the long term, but if global warming increases hurricane activity in the future (Emanuel 2005, but see Landsea 2005), increased herbivory may become more widespread because the time interval between hurricanes would be too short for the long-term effect. However, the fertilization and other possible positive effects of increased subsidies may enhance plant growth and consumer populations. This research contributes toward our understanding of the significance of resource flows between different habitats (Polis et al. 2004) and the role of detritus in food-web dynamics (Dyer and Letourneau 2003, Moore et al. 2004). These issues may be applicable to biological control in agroecosystems (Settle et al. 1996, McNabb et al. 2001, Halaj and Wise 2002, Birkhofer et al. 2007). Specifically, enhancement of detritivore prey might be an effective method for increasing densities of generalist predators and subsequently providing better biological control of herbivorous pest species. Our results on the fertilization and other possible effects of seaweed on plant growth are also applicable to agroecosystems. Seaweed can provide many benefits to terrestrial plants, and has a long history of use in agriculture (Chapman and Chapman 1980). Previous studies have found that addition of seaweed to terrestrial soil increases nutrient levels, particularly N and K and enhances water retention, leading to higher crop yields (Haslam and Hopkins 1996, Lopez-Mosquera and Pazos 1997). In addition, mats of wrack washed onshore may reduce soil water evaporation, thereby reducing soil salinity and enhancing plant growth (Pennings and Richards 1998). Our research provides further information on this important use of seaweed in agriculture.
Publications
- Schoener, T.W. 2009 The niche. pp 3-13 in S. Levin (Ed.) Princeton Guide to Ecology. Princeton University Press, Princeton, NJ.
- Spiller, D.A. and T.W. Schoener. 2009 Species-area. pp 857-861 in R. Gillespie and D. Clague (Eds.), Encyclopedia of Islands. U. of California Press.
- Schoener, T.W. 2009 Evolutionary ecology. In M. Hutchins (Ed.). Evolution addendum to Grzimek's Animal Life Encyclopedia. Gale, a part of Cengage Learning, Farmington Hills, MI.
- Spiller, D.A., J. Piovia-Scott, A.N. Wright, L.H. Yang, G. Takimoto, T.W. Schoener and T. Iwata. 2010 Marine subsidies have multiple effects on coastal food webs. Ecology.
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Progress 01/01/08 to 12/31/08
Outputs
OUTPUTS: It is well known that for an isolated population, the probability of extinction is positively related to population-size variation: more variation is associated with more extinction. What then is the relation of extinction to population-size variation for a population embedded in a metapopulation and subjected to repeated extinction and recolonization In this case, the extinction risk can be measured by the extinction rate, the frequency at which local extinction occurs. Using several population-dynamics models with immigration, we found in general a negative correlation between extinction and variation. More precisely, with increasing length of the time series, an initially negative regression coefficient first becomes more negative, then becomes less negative and eventually attains positive values before decreasing again to zero. This pattern holds under substantial variation in values of parameters representing species and environmental properties. It is also rather robust to census-interval length and the fraction of missed individuals but fails to hold for high thresholds (population-size values below which extinction is deemed to occur) when quasi-extinction rather than true extinction is represented. The few departures from the initial negative correlation correspond to populations at risk: low growth rate or frequent catastrophes. The general theoretical result of a negative relation between temporal population-size variability and extinction rate agrees with the trend we found earlier for extensive time series of web spiders on c100 islands in the central Bahamas and supplements our earlier theoretical work (Schoener et al. 2003) on this topic PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The above results shed some light on differences between conclusions of studies finding positive vs negative slopes of extinction rate on temporal variability in population size. In terms of number of years, all studies are rather short-term, ranging from 6 years for a mammal study (Lima et al. 1996), through 9 years for a spider study (Schoener and Spiller 1992) and 13 years for a Panamanian bird study (Karr 1982), to 7-45 years for various British bird studies (Pimm et al. 1988).. While in the predicted direction, it seems unlikely that the slight differences in total time period between the bird studies and the others could have such an effect. More likely, perhaps, are differences in immigration rate between the spiders, the mammals and the birds: the slope attains maximally more negative values the lower the immigration rate. Ultimately and unfortunately, we may be faced with a conclusion similar to that for many other conservation issues: no simple consistent pattern will work for all species; rather most answers will be idiosyncratic. Our models are meant to represent a system whose populations have a reasonably high rate of extinction, i.e., reach zero from time to time. Thus we represent a metapopulation system whose subpopulations wink in and out, empty sites at which extinction occurred being eventually regenerated by immigration. This indeed is the kind of system described by many investigators. However much of the discussion on extinction, especially recently, is about quasi-extinction, in which a population that drops to a certain low percentage of its value is declared "quasi-extinct." Our initial attempts to simulate a kind of quasi-extinction by using a threshold population size below which a population is "quasi-extinct" did not always give results equivalent to true extinction. The major difference between these approaches deserves substantially more exploration, and it will be interesting to see how these and other comparisons between extinction schemes produce similarities and differences. Conservation managers will have to chose between the various approaches according to which is the most appropriate match for their system.
Publications
- Legendre, S., T.W. Schoener, J. Clobert and D. A. Spiller. 2008. How is extinction rate related to population-size variability over time A family of models for species with repeated extinction and immigration. American Naturalist 172: 282-298.
- Schoener, T.W. 2009. The MacArthur-Wilson Equilibrium Model: What it said and how it was tested. In J. B. Losos and R. E. Ricklefs (Eds.). The Theory of Island Biogeography at 40: Impacts and Prospects. Princeton University Press, Princeton NJ. In Press.
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Progress 01/01/07 to 12/31/07
Outputs
I investigated how temporal variation in rainfall influences the impact of lizards on spiders inhabiting small islands in Abaco, Bahamas. Repeated-measures ANOVA showed that annual variation in spider density (time) and in the lizard effect on spider density (lizard x time) were both significant. Correlation coefficients between the lizard effect (In ratio of no-lizard to lizard spider densities) and number of rainfall days were generally negative, and strengthened with length of the time period during which rainfall was measured prior to annual spider censuses. Spider density was also negatively correlated with rainfall days and strengthened with length of the prior time period. Longer time intervals included the hurricane season, suggesting that the strong negative correlations were linked to high rainfall years during which tropical storms impacted the region and reduced spider and lizard densities. Split-plot ANOVA showed that rainfall during the hurricane season
had a significant effect on the lizard effect and on spider density. Results in this study are opposite to those found in our previous 10-year study (1981-1990) conducted in the Exuma Cays, a moderately xeric region of the Bahamas, where the relation between rainfall and the lizard effect on spider density was positive. Combined data from the Exuma and Abaco studies produce a unimodal relation between trophic interaction strength and rainfall; we suggest that the negative effect of storms associated with rainfall was paramount in the present study, whereas the positive bottom-up effect of rainfall prevailed in our previous study. We conclude that climatic variability has a major impact on the trophic interaction and suggest that a substantial change in precipitation in either direction may weaken the interaction significantly.
Impacts
The results of our two 10-year studies show that climatic variability can have a major impact on a trophic interaction. Previously, we showed that herbivory increased markedly in Abaco following hurricane disturbance and suggested that this was caused by a reduction of many predatory species, including lizards, spiders and parasitoids. Hence, hurricane disturbance associated with extremely high rainfall may devastate predators in multiple trophic levels simultaneously and thereby decrease the overall trophic interaction strength of predators on herbivores. Because such major disturbance is continuing as part of the recent upsurge in Atlantic hurricane activity, possibly caused by global warming, we predict that top-down effects of predators on herbivores will continue to weaken and herbivory will become more severe.
Publications
- Spiller, D.A. and T.W. Schoener. 2008. Climatic control of trophic interaction strength: the effect of lizards on spiders. Oecologia 154:763-771.
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Progress 01/01/06 to 12/31/06
Outputs
Major abiotic disturbance can be an important factor influencing food-web dynamics, particularly in areas impacted by the recent increase in hurricane activity. We present a unique set of data on key food-web processes occurring on 10 small islands for three relatively calm years and then four subsequent years during which two hurricanes passed directly over the study site. Herbivory, as measured by leaf damage, was 3.2 times higher in the year after the first hurricane (2000) than in the previous year and was 1.7 times higher in the year after the second hurricane (2002) than in 2001. The effect of a top predator (the lizard, ANOLIS SAGREI) on herbivory strengthened continuously after the first hurricane and overall was 2.4 times stronger during the disturbance period than before. Overall abundance of lizards was 30 percent lower during the disturbance period than before, and abundances of web spiders and hymenopteran parasitoids were 66 percent and 59 percent lower,
respectively. We suggest that increased herbivory observed on all islands was caused, at least in part, by the overall reduction in predation by both lizards and arthropods, whereas magnification of the lizard effect on herbivory was caused by reduced compensatory predation by arthropods.
Impacts
A current prediction is that herbivore outbreaks will increase as climate becomes more variable due to global warming. Both the present study on the effects of Hurricanes Floyd and Michelle in 1999 and 2001 and a previously study conducted approximately 400 km farther south in the Bahamas (Exuma) on the effect of Hurricane Lili in 1996 showed that herbivory increased markedly following each hurricane. Such major disturbance is continuing as part of the recent upsurge in Atlantic hurricane activity: Hurricanes Frances and Jeanne impacted a large portion of the Bahamas in 2004. Were hurricane activity intensified further by global warming, increased herbivory, as reported here, may become widespread in the affected regions.
Publications
- Spiller, D.A. and T.W. Schoener. 2007. Alteration of island food-web dynamics following major disturbance by hurricanes. Ecology IN PRESS.
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Progress 01/01/05 to 12/31/05
Outputs
We monitored orb spiders on 41 islands in the northern Bahamas for 4 years before and then 4 years after the catastrophic Hurricane Floyd passed directly over the site, completely inundating the study islands with its storm surge. The respective recoveries of major community properties after this annihilation were far from synchronous. Before the hurricane the species-area relation was generally strong and the slope showed no temporal trend. After the hurricane the slope increased from near zero (7 months later) to a value in 3 years about equal to its pre-hurricane state. The lizard effect, measured by the difference in spider abundance or species richness between islands with and without the lizard ANOLIS SAGREI, was generally very strong before the hurricane; 7 months after the hurricane, the lizard effect on abundance was weak and the effect on richness had vanished. In subsequent years, the lizard effect on abundance became strong again, but the effect on species
richness remained weak. The strength of the lizard effect on both abundance and richness over the 8 years of the study was strongly positively related to the density of lizards measured on a subset of the study islands. During the year following the hurricane, species richness rebounded to the last pre-hurricane value, but abundance attained only about half that value; these results were remarkably similar to those found in an earlier study on spider populations impacted by Hurricane Lili in a different region of the Bahamas. Nonetheless, in the next three years species richness failed to increase further, part of its long-term decline at the study site.
Impacts
Severe hurricanes (categories 4 and 5) have been increasing in frequency in the hurricane belt worldwide, according to the most recent (late 2005) compilations. Recent papers have differed as to whether this increase results from warmer seawater temperatures that relate to anthropogenic global warming. Information on the ecological affects of hurricanes, especially if they diminish major components of food-webs, is becoming ever more valuable. Our work is pioneering empirical and conceptual understanding of the affects of these major natural disasters.
Publications
- Schoener, T.W. and D.A. Spiller. 2006. Nonsynchronous recovery of community characteristics in island spiders following a catastrophic hurricane. Proceedings of the National Academy of Sciences. In Press.
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Progress 01/01/04 to 12/31/04
Outputs
The lizard species ANOLIS SAGREI is a major predator in the subtropical island systems we study, affecting many lower-level elements of their food web. How this species is affected by natural disturbances (e.g. hurricanes) is therefore key to understanding the long-term properties of such food webs. Two recent hurricanes passed directly over the northern Bahamas two years apart, allowing a comparison of their effects on lizard populations inhabiting exactly the same islands. The hurricanes differed in two ways: one struck during the reproductive season and was relatively severe; the other struck after most reproduction had taken place and was milder. The late-season hurricane produced a significant relation between population reduction and lowness of the island that lasted at least through two seasons; the earlier hurricane produced no such relationship. The late-season hurricane wiped out populations of lizards on two islands (two of the three lowest) that the
earlier hurricane failed to exterminate even though it was stronger. We related these effects to the fact that the study lizards regenerated from the earlier hurricane only via the egg stage, whereas eggs were unavailable when the later storm struck and regeneration was via hatched lizards. We discriminate and illustrate four kinds of hurricanes, cross-classified by two contrasts: earlier vs. later and stronger vs. weaker. A later, stronger hurricane completely exterminated lizard populations at a second Bahamian site, whereas an earlier, weaker hurricane had no detectable effect at a third Bahamian site. Our results suggest that, in addition to severity, the timing of a hurricane as it coincides with reproductive scheduling or other phenological aspects may determine the magnitude of its effect on a variety of organisms.
Impacts
Hurricanes have recently been increasing in frequency in the West Indies, and may be doing so in other parts of the world (e.g. Australia, Japan, Southeast Asia). Although, to my knowledge, definitive evidence is still lacking, this increase could result from warmer seawater temperatures, in turn related to global warming that is generally agreed to be largely anthropogenic. Thus any information on the ecological affects of hurricanes, especially if they diminish major components of food-webs, is becoming increasingly valuable. Our work is pioneering a conceptual understanding of the varied affects of these major natural disasters.
Publications
- Schoener, T.W., D.A. Spiller and J.B Losos. 2004. Variable ecological effects of hurricanes: The importance of seasonal timing for survival of lizards on Bahamian islands. Proceedings of the National Academy of Sciences 101:177-181.
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Progress 01/01/03 to 12/31/03
Outputs
We analyzed extinction patterns for two species of orb spiders monitored annually on 77 islands for a continuous 20-year period. One species, ARGIOPE ARGENTATA, has a boom-and-bust pattern, with large populations sometimes crashing quickly to extinction and a much weaker relation of extinction likelihood to population size than does the other species, METEPEIRA DATONA. The models incorporate variation in life-history traits, as well as a population ceiling, demographic stochasticity and environmental stochasticity. Differences between the species in life-history traits estimated with measurements from the field are sufficient to explain differences in extinction patterns. In addition, incorporation of all three of a population ceiling, demographic stochasticity and environmental stochasticity is necessary to fit the observed extinction curves. As predicted from life-history patterns, long-term population-growth rates (and hence predicted extinction probabilities) are
relatively very sensitive to values of juvenile survivorship. Models are also sensitive to variation in the population ceiling and environmental noise, which tend to act in a complementary manner. A simple model with no age structure was able to describe the data on large initial population sizes for METEPEIRA. Where the simple model did not fit, age structure, small populations sizes (METEPEIRA) or length of the developmental period (ARGIOPE) seems to be responsible. Apparently, by increasing or lengthening the effect of stochastically produced perturbations through the transient phase, the life cycle acts as a destabilizing force, whose importance will depend on its exact interactions with the various sources of stochasticity.
Impacts
The study of extinction is crucial for successful species preservation. Our analysis and models indicate that inclusion of life-history structure, a population ceiling, demographic and environmental stochasticity may be appropriate to model similarly elaborate data sets for other organisms and systems.
Publications
- Schoener, T.W., J. Clobert, S. Legendre and D. Spiller. 2003. Life-history models of extinction: A test with island spiders. American Naturalist 162:558-573.
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Progress 01/01/02 to 12/31/02
Outputs
We tested the hypothesis that lizards intensify interactions among spider species. The most abundant lizard and web-spider species (ANOLIS SAGREI and METEPEIRA DATONA, respectively), were manipulated within field enclosures over a 30-mo period. The design was a 2X2 factorial: A. SAGREI present and removed, crossed with M. DATONA present and removed. A. SAGREI reduced abundance of EUSTALA CAZIERI, the second most abundant web spider, and tended to reduce abundance of all rare spider species combined. M. DATONA had no significant effect on abundance of E. CAZIERI or of all rare species combined. In each statistical analysis of abundance, the A. SAGREI X M. DATONA interaction term was not significant, indicating that lizards did not intensify (or reduce) interactions among spider species. Furthermore, data on spider food supply, body length, and web height showed no evidence of two proposed mechanisms in which interactions among spider species are stronger with lizards
than without. Thus, this experiment supports the hypothesis that direct predation is the major mechanism by which lizards reduce the abundance and species richness of spiders. The same experiment was also used to compare the impact of the commonest species of lizard and orb spider on lower portions of the food web, specifically on other spider species, aerial arthropods and leaf damage. Impact of the lizard was far greater overall, probably because of its larger size, greater flexibility in prey capture and relatively high density. The significance of removal experiments for the issue of whether species are "expendable" vs "important" was discussed in a contribution to a collection of conservation-oriented papers about to be published. In this paper, we propose the concept of the "assertion of lesser effect" used to rank species in this regard.
Impacts
Our experiments determine how species affect food-web elements from predators to plants. Specific information on leaf damage is useful in assessing the agricultural impact of species presence. Extinction of species by humans is becoming common, and conservationists are concerned as to which species may be most important to save (i.e., least "expendable")-our experiments give a way to assess this.
Publications
- Losos, J.B., T.W. Schoener, K.I. Warheit and D. Green. 2001. Experimental studies of adaptive differentiation in Bahamanian ANOLIS lizards. Genetica 112-113:399-415.
- Losos, J.B., M.A. Butler and T.W. Schoener. 2002. The relationship between habitat use and sexual dimorphism in size and shape of Caribbean ANOLIS lizards. In S. Fox, K. McCoy, and T. Baird (Eds.), Territoriality, Dominance and Sexual Selection. Johns Hopkins Press, Baltimore.
- Schoener, T.W., D.A. Spiller and J.B. Losos. 2002. Predation on a common ANOLIS lizard: Can the food-web effects of a devastating predator be reversed? Ecological Monographs 72:353-408.
- Schoener, T.W., D.A. Spiller and J.B. Losos. 2001. Natural restoration of the species-area relation for a lizard after a hurricane. Science 294:1525-1528.
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Progress 01/01/01 to 12/31/01
Outputs
The experiment described in last year's report was abruptly terminated by Hurricane Floyd, a huge Category IV, with maximum sustained winds of approximately 250 kph. We summarize the immediate and some longer-term effects of this catastrophic hurricane. 1) The hurricane led to the extinction of A. SAGREI populations on 4 of 6 islands on which the large predator had been introduced, whereas none of the control islands had a population that became extinct. Prior to the hurricane, the predator had reduced prey populations to values about half those on control islands. Two months after the hurricane, we found only recently hatched individuals; apparently lizards survived the inundating storm surge only as eggs. On predator-introduction islands, those hatchling populations were a smaller fraction of pre-hurricane populations than on controls. Egg survival allowed rapid recovery of prey populations to pre-hurricane levels on all control islands but on only a third of
predator-introduction islands; the other two-thirds lost their prey populations. Thus climatic disturbance compounded by predation brought prey populations to extinction. This study, which is one of the few illustrating an interaction between catastrophic disturbance and predation, is published in NATURE. 2) Web spiders were drastically diminished everywhere approximately 2 months later, being reduced by 94-98 percent (individuals) or 50-75 percent (species) from their previous value. By 2 years, spider numbers were much increased, but some major changes in species composition had occurred. Immediate effects of the hurricane on large and small arthropods were completely different. At the 2-month census, large arthropods were down; those on the ground were reduced 35-84 percent and those in the air were reduced by 72-90 percent. Conversely, small arthropods were up: ground 29-54 percent, aerial 35-68 percent. Thus we have found a striking ability among small arthropods to survive
and/or reconstitute populations quickly while large ones lag behind. Inasmuch as some of these small arthropods are eaten by spiders, the data also show a more rapid `recovery' of spider prey than of spiders after a major physical disturbance. 3) We measured the size of each C. ERECTUS shrub used in the leaf-damage study, as well as the size of all C. ERECTUS on the 8 islands for which the same measure was taken in 1998 and 1988. The data, not yet numerically compiled, appear to show little, if any, percent diminution on average; apparently the force of Floyd's storm surge was much less at our site than that of Hurricane Lili's (see previous reports) in exposed areas.
Impacts
This study shows how food-web levels respond differently to a catastrophic natural disaster. It also shows that extinction of a population from such a catastrophe is much more likely when that population has predators.
Publications
- Schoener, T.W., Spiller, D.A. and Losos, J.B. 2001. Predators increase the risk of catastrophic extinction of prey populations. Nature 412:183-186.
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Progress 01/01/00 to 12/31/00
Outputs
We determined the effect of the larger iguanid lizard LEIOCEPHALUS CARINATUS on lower elements of a largely buttonwood-based food web, including the smaller lizard A. SAGREI. The design had 3 treatments: 1) A. SAGREI present naturally and unmanipulated; 2) A. SAGREI present naturally and L. CARINATUS introduced; 3) A. SAGREI absent and unmanipulated. This experiment ran for 2.5 yrs until Hurricane Floyd, a huge Category IV with maximum sustained winds at approximately 250 kph, passed directly over our study site. Results up to the hurricane are: 1) The prediction that relative A. SAGREI density would decline happened immediately at the first census after introduction (3 mo). 2) We hypothesized that A. SAGREI would shift its habitat away from the large trunk, rock and ground surfaces preferred by L. CARINATUS to higher thinner perches. An immediate shift occurred, so that A. SAGREI were already higher at the next census. Over the entire course of the experiment,
percent on the ground went from initially 58 to 2 percent on introduction islands, while only dropping from 62 to 60 percent in the controls. Even without including ground observations, perch diameters became smaller in experimental than controls by 1 yr, as the A. SAGREI frequented thinner and thinner perches, which the bulkier L. CARINATUS avoided. 3) Because A. SAGREI have a huge negative effect on web spiders, we hypothesized that L. CARINATUS would indirectly increase web spiders. This occurred after 7 mo for species or 19 mo for density. 4) We hypothesized that small arthropods (at least on the ground) would become proportionally more abundant after L. CARINATUS introduction, because the latter eats larger arthropods. However, using the same size categories as above neither large nor small ground arthropods showed any change in 2 yrs. In contrast, large aerial arthropods, which would on average be much less accessible to L. CARINATUS, increased on introduction islands after 7
mo. The decline in A. SAGREI would be a possible explanation (as on unmanipulated islands large aerial arthropods are more abundant without than with A. SAGREI), although a reversal in abundance of large arthropods (albeit not significant) occurred at the 2-yr census. Small aerial arthropods were different: In our previous study we found that small aerial arthropods are this site are actually less abundant on no-lizard islands than on islands having A. SAGREI naturally; this was also true at the beginning of the present study for both ground and aerial arthropods. A positive effect of A. SAGREI on small arthropods suggests a 2-link (or 4-link) indirect effect via intermediate predators (e.g. spiders). No effect of the L. CARINATUS introduction on small aerial arthropods was found in 2 yrs. 5) Plant damage was hypothesized to increase with introduction of L. CARINATUS because of A. SAGREI's positive effect on plants and the former's tendency to occur away from foliage. On unmanipulated
islands without A. SAGREI, damage was greater for scars, mines and lines, but not holes, than on such islands with A. SAGREI. After 2 yrs, leaf damage had not yet responded to manipulation.
Impacts
This study shows how introduction of top predators will impact a variety of community and ecosystem characteristics, including reducing invertebrate predators. The study shows that direct and short-term indirect effects occur relatively quickly.
Publications
- Schoener, T.W. and Spiller, D.A. 2000. Effects of removing a vertebrate vs invertebrate predator on a food web, and is either species "expendable?" In S.A. Levin and P. Kareiva (Eds.), Expendable Species? Princeton University Press, Princeton, N.J. In press.
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Progress 01/01/99 to 12/31/99
Outputs
A system of neighboring small islands with and without the lizard ANOLIS SAGREI was used to stage a 7-year experimental study of the effects of an invading species. Lizard propagules were introduced to four islands, randomly selected from eight that lacked lizards naturally. Four other islands where lizards occurred naturally constituted a third treatment. Effects tested were possible lizard indirect effects on the commonest shrub (via its arthropod herbivores), two sizes of aerial arthropods, and hymenopteran parasitoids. Lizards reduced leaf damage; moreover, during the middle years of the experiment, damage was less on introduction islands than on islands having lizards naturally. Lizards increased the number of small aerial arthropods but had no effect on large aerial arthropods; the full effect on small arthropods was only evident near the end of the experiment. Lizard introduction increased the number of hymenopteran parasitoids at the end of the experiment;
however only islands having lizards naturally had significantly more parasitoids than no-lizard islands summed over the long term. In contrast to the indirect effects, the mostly direct effect of lizards on spiders was very strong; introduction transformed spider density to that on natural lizard islands relatively rapidly and monotonically. In addition to demonstrating how an introduced vertebrate predator can affect food webs even down to the producer level, this study illustrates the greater strength and regularity of direct as opposed to indirect effects.
Impacts
This study shows how introduction of lizards will impact a variety of community and ecosystem characteristics, in particular reducing damage to vegetation due to herbivorous arthropods. The study suggests that this introduction will have an immediate (first years) larger benefit than the longer term one, as introduction initially causes lack of damage to "overshoot" natural lizard islands.
Publications
- Schoener, T.W. and Spiller, D.A. 1999. Indirect effects in an experimentally staged invasion by a major predator. American Naturalist 153:347-358.
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Progress 01/01/98 to 12/01/98
Outputs
For the first paper listed below, we investigated how a major hurricane affected natural populations on small islands. Lizard and spider populations were censused immediately before and after Hurricane Lili on islands differentially affected by the storm surge. The results support three general propositions. First, the larger organisms, lizards, are more resistant to the immediate impact of moderate disturbance, whereas the more prolific spiders recover faster. Second, extinction risk is related to population size when disturbance is moderate but not when it its catastrophic. Third, following catastrophic disturbance, recovery rate among different types of organisms is related to dispersal ability. The absence of the poorer dispersers, lizards, from many suitable islands is probably the result of long-lasting effects of catastrophes. In the second paper listed below, we investigated how the magnitude of a predator's direct and indirect effects changed with island
size. An overwhelming tendency exists for lizards to affect spider density, species richness and composite diversity more, the smaller the island; dominance shows little difference. Herbivory is also affected on average more on small islands, but the variation in effect magnitude with island area is less. Aerial arthropods are also affected more on average on small islands, but unlike the other variable the direction of the effect can be negative or positive, and the effect is often very weak. Thus the mainly direct effects of lizards vary more in magnitude than do the mainly indirect effects of lizards.
Impacts
(N/A)
Publications
- SPILLER, D.A., LOSOS, J.B. and SCHOENER, T.W. 1998. Impact of a catastrophic hurricane on island populations. Science 281:695-697.
- SCHOENER, T.W. and SPILLER, D.A. In Press. Variation in the magnitude of a predator's effect from small to large islands. IN J.A. Alcover (Ed.), ECOLOGIA DE LES ILLES. Graphiques Miramar.
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Progress 01/01/97 to 12/01/97
Outputs
In the paper listed below, we investigated how foliage damage on natural islands is related to the presence of a predator (on herbivorous arthropods) in the moderately long term. Sea grape (COCCOLOBA UVIFERA) leaf damage was measured on 11 islands with diurnal lizards (ANOLIS SAGREI) and on 7 islands without diurnal lizards from 1986 to 1993. Two types of damage were common: scars (necrotic areas) and holes (entirely missing areas). A static measure in 1986 showed that both scar and hole damage tended to be higher on islands without lizards than on islands with lizards, but only the difference in holes was significant. A dynamic measure that followed tagged leaves each year from 1986 to 1993 showed that overall damage was higher on islands without lizards than on islands with lizards. Island area and isolation were not related to leaf damage. Total leaf damage varied substantially among years, and the temporal pattern of scars differed significantly from that of
holes. Mortality of tagged leaves varied substantially among years, but it was nearly identical on islands with and without lizards. Overall, this study showed that sea grape on entirely natural islands was affected positively by lizards and to about the same magnitude as in previous experimental studies on a large island.
Impacts
(N/A)
Publications
- SPILLER, D.A. and SCHOENER, T.W. 1997. Folivory on islands with and without insectivorous lizards: an eight-year study. Oikos 78:15-22.
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Progress 01/01/96 to 12/30/96
Outputs
In the paper listed below, we investigated how predator introduction affects oneaspect of prey diversity (number of species or species richness), and prey abundance. We ran a seven-year experiment on an entirely natural system of small islands, using the commonest local lizard as the predator and web spiders as prey. Lizard introduction caused rapid and devastating effects on spider diversity and abundance: within two years, islands onto which lizards had been introduced became almost identical to islands with natural lizard populations. The proportion of species becoming extinct was 12.6 times higher on `lizard-introduction' islands than on islands without lizards. Locally common and rare species were both reduced by the introduction of lizards, but nearly all of the latter became permanently extinct.
Impacts
(N/A)
Publications
- SCHOENER, T.W. and SPILLER, D.A. 1996. Devastation of prey diversity by experimentally introduced predators in the field. Nature 381:691-694.
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Progress 01/01/95 to 12/30/95
Outputs
The objective of the paper listed below was to ascertain whether the impact of lizards on spiders varies temporally, and if so, whether this variability is related to rainfall. We compared annual censuses of orb-spider populations on 24 islands with diurnal lizards present, and on 20 islands with diurnal lizards absent, to rainfall over a 10-year period. A strong positive correlation (Pearson r=0.877) was found between mean spider density on no-lizard islands and the number of days of rain that occurred 2 months prior to spider censuses; correlation coefficients declined with longer and shorter time periods prior to censuses. Correlation coefficients between mean spider density on lizard islands and rainfall showed a similar pattern but were generally lower than those for no-lizard islands. The strength of the impact of lizards on spiders, measured as the ratio of mean spider density on no-lizard islands, varied considerably and was positively correlated with rainfall;
the correlation was highest with number of rain days 6 months prior to spider censuses (r=0.741). Repeated-measures analysis of variance on the time series of spider densities showed that the lizard factor (present versus absent) varied significantly among years. Split-plot ANOVA with rainfall as a covariate indicated that spider density and the impact of lizards on spider density were both significantly correlated with rainfall.
Impacts
(N/A)
Publications
- SPILLER, D.A. and SCHOENER, T.W. 1995. Long-term variation in the effect of lizards on spider density is linked to rainfall. Oecologia 103:133-139.
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Progress 01/01/94 to 12/30/94
Outputs
In the paper listed below, we compared two experiments which identically measured the effect of lizards on lower food-web elements. Annual rainfall during Experiment 1 was about average, whereas during Experiment 2 number of days of rain in two of these years were the lowest recorded in the past 20 years. The magnitude of the lizard effect, measured by the ratio of mean value in lizard-removals to mean value in controls, was greater in Experiment 1 than in Experiment 2 for web spiders and for aerial arthropods, but this did not happen for leaf damage. The experiment-lizard interaction, which tested whether the lizard effect differed between experiments, was significant for aerial arthropods, nearly significant for web spiders, and not significant for leaf damage. This analysis indicates that lizard effects on web spiders and on aerial arthropods were both stronger in Experiment 1, which is consistent with the fact that lizards reduced food consumption by web spiders in
Experiment 1 but not in Experiment 2. We proposed that the strength of the lizard effect is modulated by rainfall, and offered three hypotheses on the causal mechanisms. 1. In dry years (Experiment 2), lizards may respond to desiccation by spending more time in unexposed microhabitats and less time foraging. 2. Lizards may respond numerically to higher food consumption in wet years. 3.
Impacts
(N/A)
Publications
- SPILLER, D.A. and SCHOENER, T.W. 1995. Food-web dynamics on some small subtropical islands: Effects of top and intermediate predators. IN G.A. Polis and C. Winemiller (Eds.), FOOD WEBS: INTEGRATION OF PATTERN AND DYNAMICS. Chapman Hall.
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Progress 01/01/93 to 12/30/93
Outputs
This ongoing experiment, provisionally analyzed in 1993, simulates the colonization of islands by top predators as it affects various groups lower in the food web. The design has large and small islands and 3 treatments: 1) lizards present naturally; 2) lizards introduced onto islands having lizards absent naturally; 3) lizards absent naturally. On small islands, colonization was erratic: no introductions failed on any large island, nor did any lose its lizards naturally. Spider density before manipulation was about 7X higher in Treatments 2 or 3 than in Treatment 1. It took about 2 yrs for spider density on formerly no-lizard islands to converge completely to that on natural lizard islands. This dramatic effect of lizard introduction did not occur on small islands. Large islands tended to have more large arthropods than small islands, both before manipulation and often during it. On large islands, large arthropods tended to be more abundant on no-lizard islands;
introduction islands began closer to this group and tended to converge to islands having lizards naturally. In contrast, islands having lizards naturally had more small arthropods than the other treatments. Types of leaf damage in the dominant plant (CONOCARPUS ERECTUS) varied in response. During the last 2 yrs analyzed, holes and scars (but not lines) were more prevalent on no-lizard islands than on other large islands. Surprisingly, hole and scar damage was lower on introduction islands than on natural lizard islands.
Impacts
(N/A)
Publications
- SPILLER, D.A. and SCHOENER, T.W. 1994 Effects of top and intermediate predators in a terrestrial food web. Ecology 75:182-196.
- SCHOENER, T.W. 1994 Temporal variability in lizard numbers: What is the appropriate kind of study population. Am Nat "In Press".
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Progress 01/01/92 to 12/30/92
Outputs
To determine the effects of lizards and web spiders on species in lower trophic levels, we manipulated their abundances within large field enclosures on Staniel Cay, Bahamas. The experimental design measured the separate effects of lizards and spiders and compensatory predation (does one predator increase its effect in the absence of another.). In treatments where web spiders were unaltered, mean number of web-spider individuals was 1.4 times higher in enclosures with lizards removed than in those with lizards present at natural densities. Total biomass of aerial arthropods caught in sticky traps was 1.4 times higher in treatments with web spiders removed than in treatments with web spiders unaltered. Lizards had no significant effect on aerial arthropods. Total amount of herbivore damage on sea grape leaves was 3.3 times higher in treatments with lizards removed than in treatments with lizards unaltered. Web spiders had no significant effect on leaf damage.
Compensatory predation was weak. Our results suggest that the interaction between top predators (lizards) and herbivores is strong, whereas the interaction between intermediate predators (web spiders) and herbivores is weak. Consequently, the net effect of top predators on producers is positive.
Impacts
(N/A)
Publications
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Progress 01/01/91 to 12/30/91
Outputs
Field work for 1991 was accomplished in the Spring (April-June), and during the period covered by this report the data was being analyzed and was reported in the termination report for AES project CA-D*-AZO-4810-H, which ended on Septemer 30, 1991. Preparations were begun for the 1992 field season.
Impacts
(N/A)
Publications
- NO PUBLICATIONS REPORTED THIS PERIOD.
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