LAG TIMES AND BOOM-BUST DYNAMICS IN POPULATIONS OF EXOTIC SPECIES

• Sponsoring Institution: National Institute of Food and Agriculture
• Project Status: COMPLETE
• Funding Source: HATCH
• Reporting Frequency: Annual
• Accession No.: 0232633
• Project Start Date: Feb 1, 2013
• Project End Date: Jan 31, 2016
• Program Code: [(N/A)]- (N/A)

Recipient Organization
RUTGERS, THE STATE UNIVERSITY OF NEW JERSEY
3 RUTGERS PLZA
NEW BRUNSWICK,NJ 08901-8559

Performing Department
Ecology, Evolution & Natural Resources

Non Technical Summary
Under human influence, species are being transferred between regions faster and farther than at any other time in Earths history and, in some cases, have caused enormous changes to recipient ecosystems. Despite considerable advances in our understanding of how exotic species spread and establish self-sustaining populations, our ability to forecast when and where they will exert strong ecological impacts remains weak. Given that the consequences of exotic species invasions provide the impetus for management, and effective prioritization of management resources depends on accurate forecasts of the potential damage of an invasion, a predictive understanding of impact should be a central goal of invasion ecology. However, predicting which exotic species will produce impacts and under what circumstances has proven difficult. Part of the difficulty in prediction is the complicated underlying dynamics of the exotic population itself, and how these dynamics relate to the species impacts. This proposal seeks to explore the prevalence of complex population dynamics in exotic species, and in the process develop new statistical tools for identifying these dynamics. It also seeks to explore the mechanisms behind some of these dynamics using exotic birds as a model system.

Animal Health Component

(N/A)

Research Effort Categories

Basic

(N/A)

Applied

(N/A)

Developmental

(N/A)

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
136	0820	1070	70%
135	0830	1070	30%

Knowledge Area
135 - Aquatic and Terrestrial Wildlife; 136 - Conservation of Biological Diversity;

Subject Of Investigation
0820 - Wild birds; 0830 - Wild animals;

Field Of Science
1070 - Ecology;

Keywords

invasive species

populations

hawaii

birds

lag time

boom and bust

Goals / Objectives
From a societal perspective, invasive species impacts are often measured in terms of economic costs and benefits, but such evaluations are strongly dependent on stakeholder perceptions and may fail to capture the broad range of environmental effects caused by a species invasion. Here I focus on ecological impact, which I define as a measurable change to the properties of an ecosystem by an exotic species. The logical implications of this definition are that every exotic species has an impact simply by becoming integrated into the system; such impacts may be positive or negative in direction, and vary in magnitude on a continuous scale ; and impacts can be compared through time and across space. The complexity and idiosyncrasy of impact is well recognized. One of the main sources of confusion in measuring the impact of invasive species comes from the frequently long periods of time between when an exotic population becomes established and when its range expansion becomes obvious. The time from initial establishment to subsequent spread is called the lag times. Sometimes lag times are very short, lasting only a couple of generations, just enough time to be noticeable. There are other examples of lag times that span 100 or more years. Finally there is the possibility of boom and bust population dynamics among exotic species. As the name implies, sometimes exotic populations will increase dramatically in their numbers and geographic extent after initial establishment. And then, just as dramatically, these populations decline in numbers and extent although not often down to extinction. Objectives: 1. Develop methods to quantitatively measure lag phases and boom-bust dynamics within exotic bird populations. These methods will be tailored for use with regularly collected abundance or occupancy data. 2. Evaluate the prevalence and length of lag phases for exotic bird populations established on the Hawaiian Islands. 3. Evaluate the ability of various hypothesized mechanisms for lag phases to explain the observed lags in exotic Hawaiian birds. 4. Determine the prevalence and magnitude of boom-bust population dynamics within exotic Hawaiian birds.

Project Methods
I will utilize the rich and diverse information base on the exotic birds of Hawaii as a model system to explore my objectives. Over 140 avian species from 14 taxonomic orders have been introduced to the Hawaiian Islands with nearly half of these species now successfully established on one or more of the six main islands. For nearly all of these species, the historical written record provides the year of first release on each island, the source location from which the released individuals were obtained, and the number of individuals initially released. There is also a wealth of information on birds related to their niche breadth, life history, and behavior. These existing data sources can be used to craft covariates to explain any observed lag phases in exotic Hawaiian birds. Importantly given my objectives, I also have quantitative annual estimates of population sizes taken between 1938 to the present from the Audubon Christmas Bird Counts. These annual counts are standardized by survey effort and thus represent a long-term record of each species relative abundance. This data is freely available from the CBC website, and includes detailed information on the species abundance on each of the six main islands. This rich datasource allows me to explore how, for example, the expansion of an existing exotic bird population from one island to another affected its overall population growth rate. To detect lag times among these 25 species, I will compare each exotic populations growth trajectory to piecewise statistical models following the methods of Aikio and colleagues.

Progress 02/01/13 to 01/31/16

Outputs
Target Audience:Academic biologists Research ecologists at Non-Profit, International, Federal and State Agencies Land managers at Non-Profit, International, Federal, and State Agencies Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?The project served as the basis of a PhD student dissertation. Through this research the student became proficient in analyzing long-term population data, building statistical models in a Bayesian environment, and communication their research to technican and non-technical audiences. The student is now a post-doctoral research associate with the USGS. This project also served as the basis for one undergraduate student research experience. The student became proficient in performing and interpreting complex statistical analyses, and communicating these results to their peer. How have the results been disseminated to communities of interest?The results have been published in peer-reviewed manuscripts. The results were also presented to research and extension faculty at the University of Kentucky and to non-profit and agency land mangers in Georgia. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? 1. Develop methods to quantitatively measure lag phases and boom-bust dynamics within exotic bird populations. We developed quantitative methods to evaluate both lag phase and population collapses using Bayesian approaches. The methods are useful in the context of invasive species ecology and management, but also useful for other natural resource situations such as disease outbreak and fisheries collapse. We published these methods within three peer-reviewed manuscripts, and presented our results at several academic and professional meetings. 2. Evaluate the prevalence and length of lag phases for exotic bird populations established on the Hawaiian Islands. We completed this analysis and published results in peer-reviewed manuscripts for over 30 exotic bird populations established on the Hawaiian Islands. We also presented these methods and results at a variety of academic and professional conferences. 3. Evaluate the ability of various hypothesized mechanisms for lag phases to explain the observed lags in exotic Hawaiian birds. We evaluated a small set of mechanisms likely to explain lag phases, and found none to be helpful in explaining why some exotic populations show lags and others do not. These results were published in one peer-reviewed manuscript. 4. Determine the prevalence and magnitude of boom-bust population dynamics within exotic Hawaiian birds. We evaluated how many of 30 established exotic Hawaiian birds show population collapses, and published this information in one peer-reviewed manuscript. We also presented these results at academic and professional meetings.

Publications

Progress 10/01/14 to 09/30/15

Outputs
Target Audience:Invasion ecologist Conservation biologists Natural resource managers Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?This project served as the basis of research for a PhD student, who has now graduated and is employed as a post-doctoral research asociate with the United States Geological Survey. How have the results been disseminated to communities of interest? Nothing Reported What do you plan to do during the next reporting period to accomplish the goals?Our final step is to ensure that the submitted journal articles are published in high-quality peer-reviewed outlets.

Impacts
What was accomplished under these goals? Detecting rapid and substantial population declines (collapses) is of considerable importance to many applied ecological fields. Published definitions of a population collapse describe a decline in abundance over time (e.g., 90% decline within 10 years or less). We developed a flexible, rigorous method to account for uncertainty in the magnitude and period of a collapse, and provide a way to estimate the probability of a collapse having occurred. Using Bayesian approaches we quantified uncertainty in the maximum abundance obtained in a time series and the time step in which this maximum is realized. We then use this estimate of uncertainty as a way to set a confidence interval around a specified percentage decline from the maximum, and as a way to acknowledge uncertainty in how many time steps it took for the decline to occur. We use this method to evaluate the prevalence of collapses among declining native Hawaiian birds, and show a high probability that six of 12 have declined by >90% within 10 years. Our procedure advances current methods for identifying collapses within time series of abundance data by explicitly and transparently accounting for uncertainty in the key component of any definition of a collapse; the maximum abundance. A particularly vexing phenomenon within invasion ecology is the occurrence of spontaneous collapses within seemingly well-established exotic populations. We assessed the frequency and degree of collapses in 68 exotic bird populations in North America. Following other published definitions, we define a 'collapse' if these populations have declined in abundance by 90% or more within the span of 10 years. We show that 44 of these 68 exotic bird populations have exhibited declines within their time series, with 24 of these 44 having declined into a collapsed state. Additionally, 17 of these 24 species have declined to near extinction (percent decline > 99%). We compared the severity and duration of declines across all 44 declining populations according to taxonomic Order and geographic region. Neither variable explained substantial variation in these metrics. Our results indicate that collapses may be more common among exotic species than expectations suggest, and that incorporating the probability of collapse into management considerations can transform decisions regarding when to enact control or eradication measures. We also suggest that applying our approach to other taxa and locations is crucial for improving understanding of when and where collapses are likely to occur.

Publications

• Type: Journal Articles Status: Submitted Year Published: 2015 Citation: Aagaard, K. and J.L. Lockwood. Severe and rapid population declines in exotic birds. Biological Invasions.
• Type: Journal Articles Status: Submitted Year Published: 2015 Citation: Aagaard, K., J.L. Lockwood, and E.J. Green. Accounting for uncertainty when declaring population collapses. Ecological Modelling

Progress 10/01/13 to 09/30/14

Outputs
Target Audience: Natural resource managers Biosecurity policy-makers Land managers Scientific colleagues Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? This project served as the basis for the disser tation for a PhD student. The student developed statistical methods and conceptual models that enabled them to compete effectively for a post-doctoral research position with the US Geological Survey. The student also presented his work at local and national meetings of professional socieities. How have the results been disseminated to communities of interest? Our results have been disseminated to a variety of audiences via oral presentations to local and national academic groups; state extension agents; and federal agency scientists. What do you plan to do during the next reporting period to accomplish the goals? The main goal for the coming year is to publish our detailed research in top-ranked peer-reviewed journals. We will also endeavor to expand the reach of this work via oral presentations to national and international audiences of scientists and natural resource practitioners, and by publishing peer-reviewed articles designed to reach broad international audiences made up of scientists and policy-makers. Finally, we hope to employ the methods we developed to investigate population lags and collapses within other exotic species groups (e.g., freshwater fish) or exotic birds in other regions (e.g., Britain).

Impacts
What was accomplished under these goals? Exotic species have had undeniably profound effects both ecologically and economically, yet we lack fundamental knowledge about their population dynamics. A particularly vexing phenomenon is spontaneous collapses among populations of established exotic species. Based on criteria used by the International Union for the Conservation of Nature to classify species as threatened with extinction, we consider a population to have collapsed if it has experienced a ≥90% reduction in abundance within 10 years or three generations, whichever metric is greater. We developed a flexible, rigorous method to account for uncertainty in the two components of this definition (percent decline and duration of decline) and provide an estimate for the probability of a collapse having occurred. We developed a Bayesian approach to account for uncertainty in observed maximum abundance, which is a necessary step when defining collapses as a percentage drop from this value. This uncertainty is then translated into confidence limits around the magnitude of decline that should be considered a collapse. We apply the same method to assess the uncertainty about the period of potential collapses. There are 54 exotic bird species on Hawaii, 33 of which have exhibited noticeable declines at some point in their time series. We show that 17 of 54 established exotic bird species on Hawaii experienced probable collapses, many leading to near extirpation. Declines from maximum abundances (ranging from 135.47 to 0.11 individuals) into a zone of possible collapse took on average 7 years. Collapse of exotic populations has been under-studied empirically and theoretically, yet its implications for the management of invasive species are profound. We show that collapses may be more common among exotic species than previously expected. Applying these methods to other taxa and locations is crucial for improving our understanding of exotic species population dynamics and management of invasive species.

Publications

• Type: Conference Papers and Presentations Status: Other Year Published: 2013 Citation: Lockwood, J.L. The population biology of exotic species. Invited, Natural Resources Seminar, University of Kentucky, Lexington, Kentucky
• Type: Book Chapters Status: Accepted Year Published: 2015 Citation: Lockwood, J.L. and J.C. Burkhalter. 2015. The impact of invasive species on wildlife. In Habitats in peril: concepts, challenges and solutions (M. Morrison, ed.). Island Press.
• Type: Conference Papers and Presentations Status: Other Year Published: 2014 Citation: Aagaard, K. Exotic Birds, Atlantic Salmon or Chinese Fir Trees: Understanding the Population Dynamics of Lag Phases and Collapses. U.S. Geological Survey, Upper Midwest Environmental Sciences Center Invited Seminar.
• Type: Conference Papers and Presentations Status: Other Year Published: 2013 Citation: Aagaard, K., and Lockwood, J. L. Exotic birds show lags in population growth. 98th Meeting of the Ecological Society of America. Minneapolis, MN. August 4-9.
• Type: Conference Papers and Presentations Status: Other Year Published: 2013 Citation: Aagaard, K., and Lockwood, J. L. Exotic birds show lags in population growth. Rutgers-Princeton-Penn-Columbia Symposium. Columbia University; New York, NY.
• Type: Conference Papers and Presentations Status: Other Year Published: 2013 Citation: Lockwood, J.L. The population biology of exotic species: wheres the management message? Keynote Address, Georgia Exotic Pest Plant Council. Macon, Georgia.

Progress 02/01/13 to 09/30/13

Outputs
Target Audience: Academic, non-profit, and governmental scientists. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided? The project has provided the basis for the dissertation of a PhD student at Rutgers, School of Environmental and Biological Sciences. The output reported here includes this student's first dissertation chapter. The production of this work required the student to deeply engage in data analysis, statistical modelling, and communicating results to a broad audience. How have the results been disseminated to communities of interest? The results have been published in a highly regarded peer-reviewed publication. What do you plan to do during the next reporting period to accomplish the goals? Despite extensive research regarding exotic species, we still lack fundamental knowledge about their population dynamics. A particularly vexing phenomenon is the occurrence of spontaneous population collapses within seemingly well-established exotic populations. Collapses have been under-studied both empirically and theoretically, yet their implications for societal response to invaders are large and their existence evokes a variety of ecological mechanisms as their cause. Nevertheless, there are no systematic quantitative surveys across large taxonomic groups for collapses among exotic species. Next we will use IUCN criteria of a collapsed population as applied in fisheries to evaluate whether 33 Hawaiian exotic birds have declined in abundance sufficiently to be considered ‘collapsed’.

Impacts
What was accomplished under these goals? A key aspect of the ecology and management of biological invasions is the prevalence and duration of lag phases in population growth. Here, we explore the occurrence of lag phases in exotic bird populations using the Audubon Christmas Bird Count database for the Hawaiian Island archipelago. We expand on the use of piece-wise model fitting techniques to detect lags in exotic bird populations on Hawaii. We searched for explanations as to the occurrence of these lags using five possible mechanisms (body size, niche breadth, propagule pressure, length of record, and lag phase growth rate). We found evidence of lag phases for 14 of 17 species we evaluated (range: 10 - 38 years, mean using observed data = 16 + 12), and we discovered very rapid growth to maximum abundance following the end of the lag phase (mean using observed data= 8 + 6 years). We found no evidence for any association between the possible mechanisms influencing the occurrence and duration of the lag phases. Our results are the first to rigorously quantify lags in exotic animal populations; most existing evidence comes from plants. We show that lags are as common in birds as in plants, although we provide preliminary evidence that the duration of lags in birds is shorter than in plants. We highlight the need for continued efforts to elucidate lag phase occurrence and duration in biological invasions, and we demonstrate the expanded utility of piecewise model fitting approaches to quantify these lags using count data.

Publications

• Type: Journal Articles Status: Accepted Year Published: 2014 Citation: Aagaard, K. and J.L. Lockwood. In Press. Exotic birds show lag phases in population growth. Diversity and Distributions.