Progress 10/01/04 to 09/30/09
Outputs
Studies were continued on the theory of competition between plant species, with a particular view to developments in practical terms methods needed to test theory. The focus is on detecting the operation of mechanisms that promote coexistence of competing species. These issues have practical applications in several different areas. If we are to successfully manage natural environments and halt the decline of biological diversity, we need a better understanding of how diversity is maintained in nature. Biodiversity has also been linked to the healthy functioning of both natural and agricultural ecosystems. Finally, the study of plant and animal competition has important application to the study of invasions, which cause serious economic and environmental problems. In this area, we ask, What are the conditions that allow natural communities to prevent invaders from establishing or growing to large densities? The theory goes under the headings of variable environment
community theory, and scale transition theory, and includes several different species coexistence mechanisms: the storage effect, nonlinear competitive variance and fitness-density covariance. If focuses on how differences in the environment in space and time allow species to coexist. This theory leads to quantitative measures of the coexistence promoting effect these mechanisms. The primary developments of this theory have come from this and related projects. During the project, new mathematical models exploring this theory were developed. These lead to more robust understanding of the application of the theory. Second, new statistical methods were developed to test the theory in nature. These methods are applied to several different kinds of experiments in nature. In the first kind, fluctuations in plant growth over time are studied with some species reduced in abundance, and others not deliberately manipulated. The methods compare fluctuations in growth between these different
conditions, and use the comparison to assess the operation of coexistence mechanisms. The second kind of experiment assesses also variation in space or time in the physical conditions that allow plant growth. Instead of basing the statistical methods on variance, these tests are based on covariance with physical conditions. The theory predicts how coexistence mechanisms can be detected and measured by changes in covariances as the abundances of the organisms are altered experimentally. Statistical methods for carrying out these tests were developed, and promise a new era in the testing of diversity maintenance mechanisms in ecology. These methods were applied to data collected by the PI, his students and collaborators on vernal pool annual plants, desert annual plants, and competition between California salamanders and newts. Much greater scientific rigor can be brought to this subject than has been possible previously. As a consequence, understanding of the maintenance of
biodiversity will be greatly facilitated, including understanding of how biodiversity can be lost through climate and land use change, and through the invasion of alien organisms.
Impacts
This study develops methods for testing hypotheses about interactions between plant species. It provides the means to gain a better understanding of the maintenance of biological diversity, and a better understanding of the ability of invading species to have large environmental impacts. It will facilitate management of natural and agricultural systems.
Publications
- Chesson, P., M. Donahue, B. Melbourne and A. Sears. 2005. Scale transition theory for understanding mechanisms in metacommunities. IN M. Holyoak, M.A. Leibold, R.D. Holt, eds, METACOMMUNITIES: SPATIAL DYNAMICS AND ECOLOGICAL COMMUNITIES, pp 279-306.
- Melbourne, B., A. Sears, M. Donahue and P. Chesson. 2005. Applying scale transition theory to metacommunities in the field. IN M. Holyoak, M.A. Leibold, and R.D. Holt, eds, METACOMMUNITIES: SPATIAL DYNAMICS AND ECOLOGICAL COMMUNITIES, pp 307-330.
- Snyder, R.E., E.T. Borer and P. Chesson. 2005. Examining the relative importance of spatial and nonspatial coexistence mechanisms. American Naturalist 166:E75-E94.
- Facelli, J.M., P. Chesson and N. Barnes. 2005. Differences in seed biology of annual plants in arid lands: a key ingredient of the storage effect. Ecology 86:2998-3006.
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Progress 10/01/04 to 12/31/05
Outputs
Studies were continued on the theory of competition between plant species, with a particular view to developments in practical terms methods needed to test theory. The focus is on detecting the operation of mechanisms that promote coexistence of competing species. These issues have practical applications in several different areas. If we are to successfully manage natural environments and halt the decline of biological diversity, we need a better understanding of how diversity is maintained in nature. Biodiversity has also been linked to the healthy functioning of both natural and agricultural ecosystems. Finally, the study of plant and animal competition has important application to the study of invasions, which cause serious economic and environmental problems. In this area, we ask, What are the conditions that allow natural communities to prevent invaders from establishing or growing to large densities? The theory goes under the headings of variable environment
community theory, and scale transition theory, and includes several different species coexistence mechanisms: the storage effect, nonlinear competitive variance and fitness-density covariance. If focuses on how differences in the environment in space and time allow species to coexist. This theory leads to quantitative measures of the coexistence promoting effect these mechanisms. The primary developments of this theory have come from this and related projects. During the project, new mathematical models exploring this theory were developed. These lead to more robust understanding of the application of the theory. Second, new statistical methods were developed to test the theory in nature. These methods are applied to several different kinds of experiments in nature. In the first kind, fluctuations in plant growth over time are studied with some species reduced in abundance, and others not deliberately manipulated. The methods compare fluctuations in growth between these different
conditions, and use the comparison to assess the operation of coexistence mechanisms. The second kind of experiment assesses also variation in space or time in the physical conditions that allow plant growth. Instead of basing the statistical methods on variance, these tests are based on covariance with physical conditions. The theory predicts how coexistence mechanisms can be detected and measured by changes in covariances as the abundances of the organisms are altered experimentally. Statistical methods for carrying out these tests were developed, and promise a new era in the testing of diversity maintenance mechanisms in ecology. These methods were applied to data collected by the PI, his students and collaborators on vernal pool annual plants, desert annual plants, and competition between California salamanders and newts. Much greater scientific rigor can be brought to this subject than has been possible previously. As a consequence, understanding of the maintenance of
biodiversity will be greatly facilitated, including understanding of how biodiversity can be lost through climate and land use change, and through the invasion of alien organisms.
Impacts
This study develops methods for testing hypotheses about interactions between plant species. It provides the means to gain a better understanding of the maintenance of biological diversity, and a better understanding of the ability of invading species to have large environmental impacts. It will facilitate management of natural and agricultural systems.
Publications
- Chesson, P., M. Donahue, B. Melbourne and A. Sears. 2005. Scale transition theory for understanding mechanisms in metacommunities. IN M. Holyoak, M.A. Leibold, R.D. Holt, eds, METACOMMUNITIES: SPATIAL DYNAMICS AND ECOLOGICAL COMMUNITIES, pp 279-306.
- Melbourne, B., A. Sears, M. Donahue and P. Chesson. 2005. Applying scale transition theory to metacommunities in the field. IN M. Holyoak, M.A. Leibold, and R.D. Holt, eds, METACOMMUNITIES: SPATIAL DYNAMICS AND ECOLOGICAL COMMUNITIES, pp 307-330.
- Snyder, R.E., E.T. Borer and P. Chesson. 2005. Examining the relative importance of spatial and nonspatial coexistence mechanisms. American Naturalist 166:E75-E94.
- Facelli, J.M., P. Chesson and N. Barnes. 2005. Differences in seed biology of annual plants in arid lands: a key ingredient of the storage effect. Ecology 86:2998-3006.
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Progress 01/01/04 to 12/31/04
Outputs
Studies were continued on the theory of competition between plant species, with a particular view to developments in practical terms methods needed to test theory. The focus is on detecting the operation of mechanisms that promote coexistence of plant species. These issues have practical applications in several different areas. If we are to successfully manage natural environments and halt the decline of biological diversity, we need a better understanding of how diversity is maintained in nature. Biodiversity has also been linked to the healthy functioning of both natural and agricultural ecosystems. Finally, the study of plant competition has important application to the study of invasions, which cause serious economic and environmental problems. In this area, we ask, What are the conditions that allow natural communities to prevent invaders from establishing or growing to large densities? The theory goes under the heading of the storage effect. If focuses on how
differences in the environment in space and time allow species to coexist. Storage effect theory leads to a quantitative measure of the coexistence promoting effect of the ways in which different species are in tune with varying characteristics of their physical environment. The primary developments of this theory have come from this and related projects. Over the past year, new mathematical models exploring this theory were developed. These lead to more robust understanding of the application of the theory. Second, new statistical methods were developed to test the theory in nature. These methods are applied to several different kinds of experiments in nature. In the first kind, fluctuations in plant growth over time are studied with some species reduced in abundance, and others not deliberately manipulated. The methods compare fluctuations in growth between these different conditions, and use the comparison to assess the operation of coexistence mechanisms. The second kind of
experiment assesses also variation in space or time in the physical conditions that allow plant growth. Instead of basing the statistical methods on variance, these tests are based on covariance with physical conditions. The theory predicts how coexistence mechanisms can be detected and measured by changes in covariances as the abundances of the organisms are altered experimentally. Statistical methods for carrying out these tests were developed, and promise a new era in the testing of diversity maintenance mechanisms in ecology. Much greater scientific rigor can be brought to this subject than has been possible previously. As a consequence, understanding of the maintenance of biodiversity will be greatly facilitated, including understanding of how biodiversity can be lost through climate and landuse change, and through the invasion of alien organisms.
Impacts
This study develops methods for testing hypotheses about interactions between plant species. It provides the means to gain a better understanding of the maintenance of biological diversity, and a better understanding of the ability of invading species to have large environmental impacts. It will facilitate management of natural and agricultural systems.
Publications
- Dewi, S. and P. Chesson. 2004. Age structured population growth rates in constant and variable environments: a near equilibrium approach. Theoretical Population Biology 65: 75 88.
- Chesson, P., R.L.E. Gebauer, S. Schwinning, N. Huntly, K. Wiegand, S.K.M. Ernest, A. Sher, A. Novoplansky and J.F. Weltzin. 2004. Resource pulses, species interactions and diversity maintenance in arid and semi-arid environments. Oecologia 141: 236 253.
- Snyder, R.E. and P. Chesson. 2005. How the spatial scales of dispersal, competition, and environmental heterogeneity interact to affect coexistence. The American Naturalist. In Press.
- Davies, K.F., P. Chesson, S. Harrison, B.D. Inouye, B.A. Melbourne and K.J. Rice. 2005. Spatial heterogeneity explains the scale dependence of the native-exotic diversity relationship. Ecology. In Press.
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Progress 01/01/03 to 12/31/03
Outputs
Studies were continued on the theory of competition between plant species, and the statistical methods needed to test this theory and to detect the habitat conditions that facilitate species coexistence. These problems have practical applications in several different areas. If we are to successfully manage natural environments and halt the decline of biological diversity, we need a better understanding of how diversity is maintained in nature. Biodiversity has also been linked to the healthy functioning of both natural and agricultural ecosystems. Finally, the study of plant competition has important application to the study of invasions, which cause serious economic and environmental problems. In this area, we ask, What are the conditions that allow natural communities to prevent invaders from establishing or growing to large densities? Understanding competition between plant species is hampered by the spatially localized nature of the interactions between individual
plants. Equations for population dynamics become very complicated by this feature of reality, but it is increasingly believed by plant ecologists that this feature is also essential for a proper understanding of plant species interactions. In addition, the physical environmental conditions encountered by a plant population vary on all spatial scales in nature, including the scales comparable to the area occupied by the root system of a plant individual. Plant ecology has been hampered by an inability to study these realistic features in theoretical models. My work over the past year, in collaboration with others in my lab, most notably postdoctoral fellow Robin Snyder, has provided new models of plant competition incorporating these critical features of nature. We have studied these models analytically to produce robust predictions about the nature of mechanisms of maintenance of plant species diversity, and the conditions for invasions to occur. This work has included the first
models to provide an analytical treatment of spatially local competition in a variable environment by means of competition kernels, which are functions representing the nature of the influence of one individual plant on another in terms of their distance apart. This work has revealed three mechanisms of species coexistence that arise in a spatially variable environment. In our models, mechanisms are expressed in statistical terms measurable in the field. Using these results, we have developed experimental designs and statistical procedures that facilitate empirical investigation. My graduate student, Anna Sears, is using these statistical methods on her own data and applying them also to data sets that have been given to her by the authors of various published studies of plant competition. This work is providing the first evidence of the workings of diversity maintenance mechanisms based on local interactions between plant species.
Impacts
This study develops theory for plant species interactions, and methods for testing this theory. It provides the means to gain a better understanding of the maintenance of biological diversity, and a better understanding of the ability of invading species to have large environmental impacts. It will facilitate management of natural and agricultural systems.
Publications
- Snyder, R.E. and P. Chesson. 2003. Local dispersal can facilitate coexistence in the presence of permanent spatial heterogeneity. Ecology Letters 6:301-309.
- Chesson, P., B. Melbourne, M. Donahue, A. Sears. 2003. Scale transition theory for understanding mechanisms in metacommunities. IN M. Holyoak, M.A. Leibold and R.D. Holt (Eds.), METACOMMUNITIES: SPATIAL DYNAMICS AND ECOLOGICAL COMMUNITIES.
- Melbourne, B., A. Sears, M. Donahue and P. Chesson. 2003. Applying scale transition theory to metacommunities in the field. IN M. Holyoak, M.A. Leibold and R.D. Holt (Eds.), METACOMMUNITIES: SPATIAL DYNAMICS AND ECOLOGICAL COMMUNITIES.
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Progress 01/01/02 to 12/31/02
Outputs
Studies were continued on an introduced winter annual plant (ERODIUM CICUTARIUM) and its impact on the native community at a the field site in the Chihuahuan Desert, USA. These studies continue to monitor the increase of the invader and its apparent displacement of the native community of winter annual plants. Field experiments have shown that this species competes strongly with native species at the desert site and may explain their displacement. This last year, a new study of the effects of E. CICUTARIUM was begun at a coastal California site near Bodega Bay. In conjunction with these field studies, new statistical methods were developed to determine how the effects of plant competition are modified by the physical environment. These methods have now been extended to cover a range of common designs for field experiments so that they can be used broadly by researchers studying competition between plant species. In particular, these methods have shown that E.
CICUTARIUM would not be found to limit itself intraspecifically if standard methods had been used. Our methods showed that proper accounting for spatial environmental variability does reveal intraspecific competition, and therefore that this species is beginning to limit its own density at our Chihuahuan Desert site. The understanding of ERODIUM CICUTARIUM's environmental requirements, and its competition with other species, should have broad application, as this species is an invader in many different habitats, especially in California. To gain a better understanding of invasive plants, research was continued on a theory of invasion resistance, and invasion impact. This work involves both theoretical models of invasions, and applications of existing theory in community ecology to the context of invasions of alien species. Given the importance of environmental variation as revealed by field studies of invasion, development of plant competition theory for variable environments was
continued. Features lacking in previous work were (a) an understanding the role of variation in rainfall patterns within a year, and (b) an understanding of the interaction between spatial environmental variation and local seed dispersal. In both cases strong effects were found. Different reaction times of species to rainfall events promotes their coexistence. Also, consideration of the local nature of seed dispersal showed that large scale spatial variation in the favorability of the environment for a species is particularly important in coexistence of competing plant species. This effect of the scale of variation was not appreciated previously because the effects of localized dispersal had not been considered. Both of these findings improve understanding of the situations in a which an alien invader may successfully enter a community, and they also improve understanding of the situations where the invader is unable to eliminate native species, but instead coexists with them.
Impacts
An invasive weed is having strong negative effects on native species in Arizona and California. Specific environmental conditions may limit the weed's impact. Statistical methods were developed to help better estimate the effects of different environmental conditions on the impact of an invader on native species. Theoretical models of plant competition were developed for predicting such impacts.
Publications
- Shea, K. and P. Chesson. 2002. Community ecology theory as a framework for biological invasions. Trends in Ecology and Evolution 17:170-176.
- Chesson, P. and C. Neuhauser. 2002. Intraspecific aggregation and species coexistence. Trends in Ecology and Evolution 17:210-211.
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Progress 01/01/01 to 12/31/01
Outputs
Studies were continued on an introduced winter annual plant (ERODIUM CICUTARIUM) and its impact on the native community at the field site in the Chihuahuan Desert, USA. These studies continue to monitor the increase of the invader and its apparent displacement of the native community of winter annual plants. New field experiments have shown that this species competes strongly with native species at the site and may explain their displacement. In conjunction with these field studies, new statistical methods were developed to determine how the effects of plant competition are modified by the physical environment. A field survey determined that invasion of ERODIUM CICUTARIUM is not complete, and in sandy soils of the San Simon Valley, Arizona, the native community of desert annual plants still exists in the absence of ERODIUM CICUTARIUM. In those localities, the native community retains the diversity that it had prior to the irruption of ERODIUM CICUTARIUM in other parts
of the valley. The understanding of ERODIUM CICUTARIUM's environmental requirements, and its competition with other species, should have broad application, as this species is an invader in many different habitats, especially in California. To gain a better understanding of invasive plants, research was begun on a theory of invasion resistance, and invasion impact. This study involves both theoretical models of invasions, and applications of existing theory in community ecology to the context of invasions of alien species. This work has led to the concept of niche opportunity, which defines conditions that promote invasions in terms of resources, natural enemies, the physical environment, interactions between these factors, and the manner in which they vary in time and space. Niche opportunities vary naturally between communities, but may be greatly increased by disruption of communities, especially if the original community members are less adapted to the new conditions. Applying
recent community theory, it was possible to clarify the prediction that low niche opportunities (invasion resistance) result from high species diversity. Conflicting empirical patterns of invasion resistance are potentially explained by covarying external factors. This work provides a predictive framework for invasion ecology.
Impacts
This study has shown that an invasive weed is having strong negative effects on native species in parts of the Chihuahuan Desert, USA, but shows also that some specific environmental conditions may limit its impact. Theoretical work shows how community ecology theory can lead to a better predictive understanding of alien species and their impact on existing biological communities.
Publications
- Hood, G.M., Chesson, P. and Pech, R.P. 2000. Biological control using sterilising viruses: Host suppression and competition between viruses in non-spatial models. Journal of Applied Ecology 37:914-925.
- Chesson, P. 2001. Metapopulations, p. 161-176. IN Simon A. Levin (Ed.), ENCYCLOPEDIA OF BIODIVERSITY, Vol 4. Academic Press.
- Chesson, P., Pacala, S. and Neuhauser, C. 2002. Environmental niches and ecosystem functioning, p. 213-245. IN Ann Kinzig, Stephen Pacala and David Tilman (Eds.), FUNCTIONAL CONSEQUENCES OF BIODIVERSITY. Princeton University Press.
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Progress 01/01/00 to 12/31/00
Outputs
Laboratory studies were continued on the germination requirements of ERODIUM CICUTARIUM, an introduced winter annual plant, and the native community that it is invading at the field site in the Chihuahuan Desert, USA. The study has continued to monitor the increase in this species and its apparent displacement of the native community of winter annual plants. New field experiments have begun to test competition between this species and the native community. A similar increase in E. CICUTARIUM has been observed in California. Hence, an understanding of E. CICUTARIUM's environmental requirements, and its competition with other species, should have broad application. Our hypothesis is that ERODIUM CICUTARIUM is being favored by the changed weather patterns of the last decade, which may be predictive of the direction the system will take under global climate change. Experimental and observation results obtained so far are consistent with this hypothesis. To gain
theoretical understanding of species coexistence and invasions of alien species, theoretical models of community dynamics were studied. These models examine the role of spatial environmental variation in mediating species coexistence, and also, they examine the role of patchiness in distributions that results from limited dispersal. Because plant competition occurs on a local spatial scale, such small scale patchiness can have profound implications for interactions between plant species. The models reveal a potentially very important role for spatial environmental variation, but show that there are important interactions between nature of the environmental variation (spatial, temporal or spatio-temporal; affecting germination, affecting survival or affecting growth), competition and plant life histories. For example, spatio-temporal environmental variation affecting survival is not predicted to have strong effects on species coexistence, but spatial variation affecting survival,
without a strong temporal component, is predicted to have a strong coexistence promoting effect when seeds are dispersed locally in space. Further work was done also on models of ecosystem functioning when species coexist by having different temporal niches, which, in the particular case investigated, means different, though overlapping, times of the year when the different species are growing most strongly. Such environmental niches are a component feature of the Mediterranean vegetation of California. It was found that species diversity has a very strong effect on the performance of ecosystem services. Net primary productivity, for example, increases with the diversity of species. The variance of the performance of these ecosystem services tends to decrease with increasing species diversity indicating more consistent performance with more species.
Impacts
The unusual weather of the last decade may be responsible for the increase of an invasive weed, and its displacement of native plants in the Chihuahuan Desert, USA, indicating potential effects of global change. Theoretical models indicate the importance of physical environmental diversity for the maintenance of biodiversity and the importance of biodiversity for the provision of ecosystem services, such as plant production.
Publications
- Chesson, P. 2000. Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics 31:343-66.
- Chesson, P. 2000. General theory of competitive coexistence in spatially varying environments. Theoretical Population Biology 58:1-27.
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Progress 10/01/99 to 12/31/99
Outputs
Laboratory studies were begun on the germination requirements of ERODIUM CICUTARIUM, an invasive winter annual plant found in many parts of the world but especially important in California and Arizona. At the Arizona field site, the study has continued to monitor the increase in this species and its apparent displacement of the native community of winter annual plants. Samples from the soil seed bank were taken at the end of Spring 1999 and demonstrate that this species accounts for more seeds than all other species combined. Germination studies show that ERODIUM CICUTARIUM from the field site has broader germination requirements than the native species, and is more likely than the native species to germinate under warmer than average fall conditions. Further studies on ERODIUM CICUTARIUM and the native community are planned for the next reporting period. Our hypothesis is that ERODIUM CICUTARIUM is being favored by the changed weather patterns of the last decade,
which may be predictive of the direction the system will take under global climate change. Important advances in the theory of diversity maintenance in variable environments, which are relevant to the field system, were made. A general mathematical model of competitive interactions in a spatially varying environment was developed. This model quantifies the strength of species coexistence by the rate of recovery from low density and allows examination of the interaction between local environmental factors and local competition in the performance of species in any natural community. However, this model is especially useful for understanding the maintenance of species diversity in plant communities. The results of this model were compared with previous results on competition in a temporally varying environment, and it was shown generally that spatial variation is a more powerful mechanism of species coexistence than temporal variation. Local dispersal has an important role in the
maintenance of species diversity in a spatially variable environment because it means that species' densities build up in locations that are favorable to a species, which has a reinforcing effect on coexistence when different species tend to be favored by different environmental conditions. Although spatial variation may be more important than temporal variation in the maintenance of species diversity, temporal variation is nevertheless likely to have a significant role. The different plant species in any one system often differ in their times of peak growth activity, and theoretical models developed as part of this project show that such differences do promote species coexistence. Results obtained from this model over the last year show also that such differences have the effect of allowing the community as a whole to respond to variation in the weather in such a way that provision of ecosystem services (primary production, evapotranspiration and nutrient recycling) is maintained at
high levels in the presence of variable weather patterns. Loss of species diversity may be a significant environmental problem because it leads to systems with less reliable performance of these ecosystem services.
Impacts
Unusual weather of the last decade may be responsible for increase of an invasive weed and its displacement of native plants in the Chihuahuan Desert. Theoretical models indicate diversity of physical environment may be essential to maintenance of species diversity of natural communities, and species diversity is important to maintenance of ecosystem services as the environment changes over time.
Publications
- No publications reported this period
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