The CHESSON Lab

Peter Chesson Post-doctoral Associates Andrea Mathias Jessica Kuang Danielle Ignace Graduate Students Galen Holt Adam Miller Maureen Ryan Recent Graduates Megan Donahue Anna Sears Jessica Kuang Undergraduate Students Tara Woodcock Friends of the Lab Charlotte Lee Brett Melbourne Kendi Davies Scott Stark   Graduation Photos

Our study systems Some of us are fond of studying communities of annual plants. We find these in the deserts of Arizona and Eyre Pennisula (South Australia), in the vernal pools of the Central Valley of California, and in the coastal swales near Bodega Bay. We also study perennial herbs in the understorey of Eucalypt forests in Australia. Some of us are aquatic in orientation studying amphibia in ponds and streams or marine intertidal invertebrates. There are lots of systems that we think about but do not collect data on ourselves, at least not right now. These include coral reef fish communities, tropical forests, and host-parasitoid systems. Our tools For studies of natural systems, we are developing new classes of quantitative tools to test coexistence mechanisms and hypotheses about dynamics of populations and communities subject to spatial and temporal variation.  Then we go out into the field to count, measure, and manipulate. For theoretical studies, we use math and computers. We are fond of probability theory and statistical theory. We also get into differential equations and Fourier transforms.

Our questions Ecological diversity and its maintenance The diversity of life is the wonder of the world. It inspires us, but it challenges us to explain it too. Sadly also, diversity is under threat on many fronts from human activities. To confront this threat we need better understanding of the maintenance of diversity. The diversity of life involves ecological diversity: different organisms do different things in the aid of their lives and their offspring. Some differences are subtle. Some differences are gross. How do these differences arise, and what is their role in maintaining diversity? How do the attributes of species predict community change with environmetal change? General testable community theory To understand diversity maintenance, we need theory for ecological communities. It must be realistic enough to contain the key features behind diversity maintenance in nature, it must be general enough to be robust to variation in minor features, and it must be testable too. Nature has many complexities, not the least of the which are the many different species in any ecological community interacting with one another in many different ways. The environments of organisms are complex too. They vary greatly in space and time interacting with the lives of organisms and their relationships with others. We try to rise the challenge of this complex work, seeking theory that defines general principles, and shows how to test them in specific contexts. We make it quantitative, operational and testable in practice.  Then we go out and test it. Variation and scale Ecological systems are characterized by variability on all scales. Individual organisms vary one from another, their environments vary every which way, populations fluctuate over time and in space, communities vary and change in composition. We ask how variation affects the way ecological systems work. On the individual level, life histories may be adapted to variation, ready to take advantage opportunities, but hedging against uncertainties. Life-history properties, and the variation moulding them, have consequences for population dynamics and species interactions. Population dynamics take different forms on different temporal and spatial scales. Spatio-temporal niche differentiation arises from taking advantage of opportunities in space and time. Species diversity is promoted, interactions are stabilized, ecosystem functioning is affected, and invasion resistance develops. We develop the mathematical theory behind these ideas. We develop the methods to test them, and then we go out and test them.

Research Highlights

Community Processes in Variable Environments

The natural world is inherently variable. The physical environment varies on all scales and so do the organisms inhabiting it. Organisms are adapted to this variability, not merely hedging against uncertainty but taking advantage of opportunities that variability brings. My main interest is how the adaptation of organisms to variability promotes species diversity and affects ecosystem functioning. I do this primarily theoretically by developing mathematical models using probability theory and the methods of theoretical statistics. Recent research topics have involved neighborhood competition models, community assembly, invasion resistance, and the various manifestations of temporal and spatial niches, also known as the storage effect and relative nonlinearity of competiton. In addition, I am involved with field projects on annual plants and herbaceous perennial plants. Other systems that are foci of theorizing are coral reef communities, tropical forests, and communities in mediterranean climates.

Some recent 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, 211-237.

Chesson, P. 2001. Metapopulations. Pp 161-176 in Encyclopedia of Biodiversity, Vol 4, Simon A. Levin, ed, Academic Press.

Shea, K., Chesson, P. 2002. Community ecology theory as a framework for biological invasions. Trends in Ecology and Evolution 17, 170-176.

Chesson, P., Peterson, A.G. 2002. The quantitative assessment of the benefits of physiological integration in clonal plants. Evolutionary Ecology Research 4, 1153–1176.

Chesson, P. 2003. Quantifying and testing coexistence mechanisms arising from recruitment fluctuations.  Theoretical Population Biology 64, 345–357.

Chesson, P., Gebauer, R. L. E., Schwinning, S., Huntly, N., Wiegand, K., Ernest,  S. K. M., Sher, A., Novoplansky, A., and Weltzin, J.F.  2004. Resource pulses, species interactions and diversity maintenance in arid and semi-arid environments.  Oecologia 141, 236 - 253.

Facelli, J.M., Chesson, P., Barnes, N. 2005. Differences in seed biology of annual plants in arid lands: a key ingredient of the storage effect.  Ecology 86, 2998-3006. 

Chesson, P., and Lee, C.T. 2005. Families of discrete kernels for modeling dispersal.  Theoretical Population Biology 67, 241-256.

Chesson, P., Donahue, M., Melbourne, B., Sears, A. 2005. Scale transition theory for understanding mechanisms in metacommunities.  In Holyoak, M, Leibold, M.A., Holt, R.D., eds, Metacommunities: spatial dynamics and ecological communities, pp 279-306

Sears, A.L.W., Chesson, P. 2007. New methods for quantifying the spatial storage effect: an illustration with desert annuals.  Ecology 88, 2240-2247

Chesson, P. 2008. Quantifying and testing species coexistence mechanisms. Pp 119 – 164 in Valladares, F., Camacho, A., Elosegui, A., Estrada, M., Gracia, C., Senar, J.C., & Gili, J.M., ed. “Unity in Diversity: Reflections on Ecology after the Legacy of Ramon Margalef.”

All publications

Andrea Mathias

Non-equilibrium dynamics and species coexistence

Non-equilibrium dynamics of natural ecosystems have a major role in generating and maintaining biodiversity. The main objective of my research is the mechanistic understanding of this role in both ecological and evolutionary context. In particular I am interested in life history evolution with frequency dependent selection in temporally fluctuating and spatially heterogeneous environments.

So far I have studied the conditions for the development of polymorphisms in initially monomorphic populations leading to adaptive speciation, allowed by different regimes of spatial and temporal heterogeneity (Mathias et al., 2001; Cheptou and Mathias, 2001; Mathias and Kisdi, 2002).

Currently I am studying the competitive mechanisms promoting coexistence in a multiple resource competition model of phytoplankton species, in the context of a generalized theory (Chesson, 1994; Chesson and Huntly, 1997; Chesson, 2000a, b). In this model the growth rate of the species is a non-linear function of the limiting factors, which can result in endogenously created oscillations of density. This mechanism has enormous stabilizing potential for the coexistence of a surprisingly high number of species on few resources. My goal is to understand the details of the role of relative nonlinearity in species coexistence, which has not yet been investigated sufficiently in this case.

In the future I would like to extend the theory of species coexistence in competitive systems to life history evolution in order to understand the mechanisms that promote the coexistence of different life history strategies in temporally fluctuating and spatially heterogeneous environments.

Danielle Ignace

Physiological Ecology of Plant Invasions

I am studying the causes of the dramatic irruption of the Eurasian invasive plant, Erodium cicutarium, in the San Simon Valley of southeastern Arizona.  It many areas, it is now 95% or more of the winter annual plant biomass, up from a miniscule presence away from roads in the 1980s.  At the same time, native winter annual plants have become very rare and are showing changes in species composition.  This new monoculture potentially has profound implications for biodiversity, habitat quality, ecosystem functioning and forage quality of the rangeland. My research applies a physiological, ecosystem, community and theoretical approach to understanding the characteristics associated with the irruption and distribution of E. cicutarium and its impacts for biodiversity in the San Simon Valley.

Jessica Kuang

Galen Holt  

Empirical study of ecological concepts

Adam Miller  

Spatial models of species coexistence

Maureen Ryan

I'm interested in how environmental variation promotes diversity within species and communities, and how mechanisms such as the storage effect that function through environmental variation can affect endangered species, in particular. My research focuses on a grassland salamander community in the Central Valley of California that includes the threatened California tiger salamander (Ambystoma californiense), hybrid tiger salamanders (A. californiense crossed with A. tigrinum mavortium, an introduced tiger salamander from the central US), and the California newt (Taricha torosa).

I'm using a combination of artificial pond experiments and field experiments to look at whether and how environmental variation promotes coexistence between the two native species, Ambystoma californiense and Taricha torosa, and how coexistence mechanisms operating in the native community are influenced by the addition of hybrid salamanders. Additionally, I'm looking at how environmental variation influences hybridization dynamics between A. californiense and A. tigrinum mavortium and whether mechanisms associated with environmental variation can contribute to the maintenance of genetic polymorphism within hybridizing populations of tiger salamanders. Besides experiments, I'm using theoretical modeling and data from field surveys/sampling to investigate how dispersal and landscape structure interact with local community dynamics to influence species diversity and hybrid spread on a regional scale.

From a conservation perspective, my work is motivated by an interest in how landscape alteration may influence amphibian diversity and species coexistence within extant communities, and in particular how common pond manipulations and the loss of vernal pools in the California Central Valley might influence California tiger salamander populations.

Anna Sears

Environmental variation and species interactions

Ecology is challenged to find simple ways to understand the messiness of the natural world. I like to work from a metacommunity context, accepting that natural ecosystems contain a variety of different habitat types, and that this environmental variation affects the interactions of species within ecosystems. My PhD research specifically involves using natural annual plant systems to test how variation in the environment affects species coexistence. I am also interested in how environmental variation affects food-web interactions, and generally interacts with other forms of density dependence. My field sites are in the Chihuahuan desert and at the Bodega Marine Reserve.

Megan Donahue

Scale dependence in marine populations

For my dissertation research, I studied how spatial variation in local density-dependent processes influences regional dynamics. In collaboration with Marcel Holyoak, I investigated the effects of local resource distribution, dispersal rate, and the strength of density dependence on regional dynamics in a highly manipulable microcosm system, in which we could track population dynamics over time. Applying the same principles to a field system, I studied Petrolisthes cinctipes, a filter-feeding porcelain crab that lives in intertidal cobblefields along the Pacific coast. Based out of Bodega Marine Laboratory, I parameterized models of local density-dependence in this organism using field and laboratory experiments. For Petrolisthes cinctipes, the primary sources of density dependence are gregarious settlement, competition, and predation. With Leah Akins, I quantified the spatial variation that influences these process at two scales: variation in P. cinctipes density, predator abundance, and larval supply among rocks within sites and among sites. By integrating information about spatial variation into empirically-based models of local dynamics, we can predict what local processes are most important when "scaling up" to regional dynamics. For links to publications and Megan's CV, see her website.

Population dynamics and spatial heterogeneity

My main research interest is population dynamics, especially how small-scale heterogeneity affects dynamics at larger scales. Understanding how spatial and temporal heterogeneity affect the dynamics of populations and communities is fundamental to ecology. Indeed, answers to the most prominent questions in ecology involve the pervasive influence of heterogeneity. Why do species coexist? Why are there so many species? Why do some species cycle whereas others don't? How does habitat fragmentation affect extinction? Why are some species more invasive than others? Spatial and temporal variation in population densities and in the physical environment are essential considerations in these questions. My current research and research to date focuses on the effects of spatial heterogeneity. Whereas empirical studies have typically focused on heterogeneity exclusively, theory demonstrates that population dynamics are altered by spatial heterogeneity only when heterogeneity interacts with nonlinear demographic processes - the pillar for research in the Chesson lab. The aim of my research is to develop empirical tools to measure the interaction of nonlinearity and heterogeneity and to use these tools to probe real populations. Thus, I tackle the important issue of how to integrate theory and models with data from real systems, to understand how the interaction of spatial heterogeneity and nonlinearity alters population dynamics. I use these insights to address important basic and applied questions in ecology.

Kendi Davies

The persistence of species in patchy landscapes

My research focuses on testing and refining theory that makes predictions about the persistence of species in heterogeneous landscapes, including metapopulation and related spatio-temporal theory. So far, I have concentrated on the spatial and temporal dynamics of populations and communities in landscapes that have been modified by humans, through habitat fragmentation and grazing. I am interested in how the life history characteristics of species determine the response of species to landscape heterogeneity. I use quantitative methods to test theory and have approached this work using data from a large-scale, long-term field experiment for beetles and data for a continental scale study of arid zone birds in Australia.

I am currently broadening this focus to explore what this same theory predicts about patterns of biological diversity in naturally patchy landscapes. I am focusing on the grassland plant community in serpentine patches in a non-serpentine matrix (in the University of California’s McLaughlin Reserve). Right now, I am particularly interested in spatial scale and the relationship between diversity and invasibility (Shea and Chesson 2002).

Shea, K., Chesson, P. 2002. Community ecology theory as a framework for biological invasions. Trends in Ecology and Evolution 17, 170-176.

Some recent publications

Davies, K.F., Chesson, P., Harrison, S., Inouye, B.D., Melbourne, B.A., Rice, K.J. 2005. Spatial heterogeneity explains the scale dependence of the native-exotic diversity relationship.  Ecology 86, 1602-1610.

Davies, K. F., C. R. Margules, and J. F. Lawrence. 2004. A synergistic effect puts rare, specialized species at greater risk of extinction. Ecology 85: 265-271.

Davies, K. F., B. A. Melbourne, C. R. Margules, and J. F. Lawrence (in press). Metacommunity structure influences the stability of local beetle communities. In M. Holyoak, M. A. Leibold and R. D. Holt eds. Metacommunities: Spatial Dynamics and Ecological Communities. Chicago, University of Chicago Press.

Davies, K. F., B. A. Melbourne, and C. R. Margules. 2001. Effects of within- and between-patch processes on community dynamics in a fragmentation experiment. Ecology 82: 1830-1846.

Davies, K. F., C. Gascon, and C. R. Margules. 2001. Habitat Fragmentation: consequences, management and future research priorities. Pages 81-97 in M. E. Soulé and G. H. Orians, editors. Conservation Biology: Research Priorities for the Next Decade. Island Press, Washington.

Davies, K. F., C. R. Margules, and J. F. Lawrence. 2000. Which traits of species predict population declines in experimental forest fragments? Ecology 81: 1450-1461.

Davies, K. F., and C. R. Margules. 1998. Effects of habitat fragmentation on carabid beetles: experimental evidence. Journal of Animal Ecology 67: 460-471.

Charlotte Lee

Theory of multispecies interactions; human-environment interactions

I study the theory of multispecies interactions, with specific focus on the matrix of coefficients for all pairwise interactions in a community. Specific projects include: 1) Determining relationships between different patterns of direct and indirect competitive (or facilitative) interactions and the structure of model community matrices; 2) determining patterns of shared resource use that result in specific community matrices; and 3) developing techniques to do 1) and 2). I also study human-environment interactions using theory from ecosystem ecology and demography.

Scott Stark

Diversity maintenance in tropical plant communities