Theoretical Ecology Lab Tea

The Theoretical Ecology Lab Teas are designed to be informal meetings for members of the research groups of Simon Levin, Steve Pacala, Henry Horn, and Andy Dobson to give talks on their current research and receive feedback from their audience. The talks are usually 30 minutes, including the question and answer sessions, scheduled on Tuesdays at 2:00 PM. Additionally, other members of the Princeton University community and visitors are welcome to attend and to give presentations.

Talk schedules and email lists are maintained by Juliet Pulliam and Duncan Menge. Please contact or to have your name added to the labtea email list so that you can receive reminders about upcoming lab teas.

To view previous schedules and summaries, go to:
    Fall 2002     Spring 2003
    Fall 2003     Spring 2004
    Fall 2004


Spring 2005
Tuesday, February 1, at 2:00 PM
John McNamara
Tuesday, February 8, at 2:00 PM
Heather Leslie
Tuesday, February 15, at 2:00 PM
Erik Rauch
Tuesday, February 22, at 2:00 PM
Jeremy Lichstein
Tuesday, March 1, at 2:00 PM
David Goehring
Tuesday, March 8, at 2:00 PM
Christian Wirth
Tuesday, March 15, at 2:00 PM
(Spring break)
Tuesday, March 22, at 2:00 PM
Ben Strauss
Tuesday, March 29, at 2:00 PM
Mike Morelli
Tuesday, April 5, at 2:00 PM
Adi Livnat
Tuesday, April 12, at 2:00 PM
Annette Ostling
Tuesday, April 19, at 2:00 PM
Katie Hampson
Tuesday, April 26, at 2:00 PM
Stephen Pratt
Tuesday, May 3, at 2:00 PM
Kelly Caylor
Tuesday, May 10, at 2:00 PM
Juliet Pulliam
Tuesday, May 17, at 2:00 PM
Nikolay Strigul

Titles and abstracts
(posted approximately one week before the talk):

Tuesday, May 10 @ 2:00 PM

Juliet Pulliam

Virus Emergence: What factors allow host jumps?

Considering the broad public and scientific interest in infectious disease emergence, surprisingly little work has been done to quantitatively describe broad-scale patterns of emergence in light of the mechanisms that we believe drive them. Specific ecological processes and pathogen characteristics must affect the probability of emergence, and until these factors are recognized we will have little predictive power regarding what pathogens are likely to infect species other than their current host/s. Focusing specifically on viruses, I will first describe the steps involved in the emergence process and discuss several molecular properties that may facilitate spillover. I will then give an example of how these ideas can be tested and used to formulate predictive models regarding spillover potential. Finally, I will describe the structure of a database that I am building to address questions of emergence potential and, more broadly, of the determinants of virus host-range.

Tuesday, May 3 @ 2:00 PM

Kelly Caylor

Horizontal and vertical variability of soil moisture in semi-arid ecosystems

Soil moisture is a key hydrological variable that mediates the interactions between climate, soil, and vegetation dynamics in water-limited ecosystems. Because of the importance of water limitation in savannas, a number of theoretical models of tree-grass coexistence have been developed which differ in their underlying assumptions about the ways in which trees and grasses access and use soil moisture. However, clarification of the mechanisms that determine structure of savanna vegetation remains a vexing problem in both hydrological and vegetation science. A particular challenge is the fact that the spatial pattern of vegetation is both a cause and effect of variation in water availability in semiarid ecosystems. In particular, at local to landscape scales, the patchy vegetation structural mosaic serves to redistribute the availability of soil moisture in ways that have important consequences for structural dynamics and community composition. Here I address the role of patchy vegetation structure through the derivation of a landscape-scale model of soil moisture based on the statistics of an underlying poisson distribution of individual tree canopies and their accompanying root systems. The coupling of individual pattern to landscape-scale distribution of soil moisture allows for model investigations into the role of tree density, tree size, and canopy root system structure on the spatio-temporal patterns of soil moisture dynamics in savanna ecosystems.

Tuesday, April 26 @ 2:00 PM

Stephen Pratt

Can ants count? Drift-diffusion models of quorum sensing by house-moving ants

In performing collective tasks, social insects often respond to the local number or density of their nest mates, but little is known of how they ~Scount~T one another. When ants of the genus Temnothoraxchoose a new home, their decision relies on scouts who respond to the number of nest mates already at a promising site. At low numbers, they slowly recruit other scouts to the site, but at high numbers they switch to rapid recruitment of the colony~Rs non-scouting majority. This quorum detection depends on a scout~Rs rate of encounter with nest mates at the site. To explore how a simple cognitive mechanism might use this information, I have adapted a model of two-choice decision-making in humans. Quorum sensing by an individual ant is modeled as a drift-diffusion process with two absorbing boundaries, corresponding to the decisions that a quorum has, or has not, been attained. When the drift rate is modeled as a saturating function of population, the model effectively replicates the population-dependent recruitment decisions of real ants, as well as the distribution of decision times for both kinds of recruitment. Next steps are to fit the model to direct measurements of encounter rates, rather than population, and to determine whether it can predict changes in quorum size as nest area (and so encounter rate) is altered.

Tuesday, April 19 @ 2:00 PM

Katie Hampson

Multihost-epidemics: empirical patterns and implications for theory and control

Disease dynamics of multi-host systems have important implications for public health, livestock husbandry, wildlife conservation and ecological processes. I explore the infection dynamics of rabies, a generalist pathogen, during an ongoing outbreak in multi-host communities of the Serengeti ecosystem. During the last 3 years, cases of rabies have been recorded in more than 10 species in this region. Although the vast majority of cases have been domestic dogs, spatial and temporal clusters of infection in other carnivores indicate interspecific transmission followed by transient chains of intraspecific infection. Cross-infection of generalist pathogens such as rabies is generally assumed to be of little importance to disease dynamics. However, alternative hosts increase the size of the overall effective susceptible population and may alter transmission patterns as well as spatial coupling. Preliminary analysis suggests that spillover epidemics may act as short-term reservoirs of re-infection, which can prolong rabies persistence, increase human exposures and hamper control efforts that target only domestic dog populations.

Tuesday, April 12 @ 2:00 PM

Annette Ostling

Communities under a recruitment-survival tradeoff: building on a dispersal-assembly theory

Some ecologists argue that communities are shaped primarily by dispersal and stochastic birth and death events, rather than by niche differences between species. In particular, the recently developed "neutral theory" of ecology has had some success at predicting the properties of communities from the starting assumption that individuals of different species are ecologically equivalent. It's been proposed that this theory works even if individuals differ in their traits, as long as they differ along a life-history tradeoff that equalizes fitness across individuals of different species. I test this proposal for the simple case of a recruitment-survival tradeoff, and find this tradeoff leads to a positive correlation between a species' abundance and its per capita recruitment rate (or mortality rate, since they are equal under the tradeoff), and to higher diversity, more rarity and less dominance than the case of ecological equivalence. It also leads to increased variability in a species' abundance, and ultimately a shorter time to extinction, with increasing vital rates. I conclude that there are important differences between fitness-equalization and ecological equivalence, but that the neutral theory of ecology provides a useful framework to build on and elucidate how species differences shape stochastic variation in the community. The results presented here are also important because the recruitment-survival tradeoff provides an alternative to the widely studied competition-colonization tradeoff, an alternative that is more realistic for many communities and yields the opposite relative abundance predictions.

Tuesday, April 5 @ 2:00 PM

Adi Livnat


I'll be talking about the emergence of conflict within a system that is selected as a collective (and only as a collective). The work is motivated by some unexplained observations of animal and human behavior, but it is done on an abstract level that makes it applicable in general (like classical game theory is). The work has some interesting implications. For example, it follows that we can't say, simply by the observation of conflict, that the observed system involves independently-selected units (although in many cases of course it does, this inference is not necessarily always valid).

Tuesday, March 29 @ 2:00 PM

Michael Morelli

The Impulsive Beverton-Holt Model

We will be looking at how impulses affect the following discrete Beverton-Holt fisheries model.

X(n+1) = AX(n)/(1+BX(n))

Usually, the above difference equation converges very quickly to its equilibrium. In our version of the model, we allow periodic harvesting. Impulses model this harvesting of the fish.

X(1) = AX(0)/(1+BX(0))

X(2) = AX(1)/(1+BX(1))

X(3) = AX(2)/(1+BX(2))

X(4) = AX(3)/(1+BX(3))

Every fourth year we allow, say, sixty percent of the fish to be harvested with an additional 10 sent to the EPA for testing. Hence we have

X_plus(4) = (1 - 0.60)X(4) - 10

We use the impulsed population to determine the fish population the next year.

X(5) = AX_plus(4)/(1 + BX_plus(4))

We continue the model in this fashion.

Some questions that arise include, "How much fishing should we allow and how often to avoid killing off all the fish in the pond?" and "How much time is required to allow the fish population to reach equilibrium?"

Also, if there is time, we shall look at the impulsive version of E.C. Pielou's discrete delay equation

X(n+1) = AX(n)/(1 + BX(n-k))

The delay of "k" years in the model allows the possibility of oscillations and periodic behavior to occur.

Tuesday, March 22 @ 2:00 PM

Ben Strauss

Tracking and modeling dispersal in snails

I will present data and two unconventional models of snail dispersal across a range of different stream environments. The first model is coarse and hinges on dispersal kernels which change as a function of environment (dispersal mean, variance and tail length depend on the average force of the stream). The second model is individual-based and incorporates a finer level of environmental detail. Work in progress.

Tuesday, March 8 @ 2:00 PM

Christian Wirth

Wildfires up north: Detecting and modeling the species imprint on fire regimes and succession in boreal forests

In the first part of the talk I will present results from a meta-analysis where I show (1) that there are striking differences in the fire regimes between the boreal forests of North America and Eurasia (i.e. it burns more often in Eurasia and fires are less severe), (2) that this can be explained by the fire adaptation strategies of the dominating tree species, and (3) why this has important consequences for biome-scale carbon dynamics. In the second part I will focus on fire-vegetation interactions in Siberian forests and ask how changes in burn frequency may alter successional pathways and cause a switch between alternate stable states (light versus dark taiga). To answer this question I have parameterized the forest simulator SORTIE for Siberian tree species and added a fire and fuel production model. I will briefly introduce the main model components and how they were parameterized from a wide array of field data. Although this is work in progress I might dare to show some premature first model results.

Tuesday, March 1 @ 2:00 PM

David Goehring

Goal-Oriented Reserve Management -- Ecosystem Service Haloes: Patterns of Propagation

As we enter the twilight of de novo reserve creation, the need is intensifying for theory to help optimize our ecosystem management and restoration efforts. I present preliminary work and early thinking in this area, emphasizing spatial and dynamic contexts. The challenge is to understand the interaction among diverse, sometimes conflicting, objectives that must be considered in an effective management program. The goals which I have begun pondering are: population/species persistence, maintenance of ecosystem services,, learning/information, and resilience. I will address some inchoate thoughts on these goals, and present the model described below.

Researchers have begun asking how to optimally manage ecosystems with the goal of maintaining their economically tractable services. There have been some recent stimulating studies that have demonstrated interesting empirical properties of service haloes around patches of natural landscape. If proximity to natural land increases the benefits of certain services, we'll see some interesting landscape patterns among stakeholders and their land-use strategies. The situation is further complicated by the fact that the diverse suite of services will likely have different propagation (e.g., that of erosion, pollination, or water quality). The challenge is to properly incentivize conservation for stakeholders, given their spatially contingent interactions with resources - a dilemma with significant consequences for the fate of comanagement and community management. I have formulated a simple toy model which considers static, spatial incentives for stakeholders in artificial landscapes, in the form of service haloes around natural landscape patches. I will present preliminary descriptive results regarding the role of the pattern of fragmentation and the scale of property rights, with the goal of getting input on future directions.

Tuesday, February 22 @ 2:00 PM

Jeremy Lichstein

Drift, determinism, and intraspecific variation in a simple model of plant competition

Interspecific tradeoffs lead to multi-species coexistence in a variety of models, and empirical studies support the existence of such tradeoffs. Jim Clark et al. (2003) used super-duper Bayesian hierarchical modeling to show that traditional statistical analyses underestimate intraspecific variation, and, therefore, overestimate interspecific differences in growth rates. They conclude that (1) species are not as different from each other as we typically assume, and (2) the broad overlap in growth rates between competing species implies a more prominent role for stochastic drift and a less prominent role for interespecific tradeoffs than most people, besides Steve Hubbell, presently believe. Using a simple two-species model of plant competition, I will show that these conclusions are not generally correct. I examine a model in which two species have the same fecundity (per-adult seed production) but differ in their distributions of seed quality. When fecundity is high, the species with the highest potential seed quality always wins (i.e., it always persists, while the other species always goes extinct), even if the two distributions of seed quality are nearly identical. In contrast, when fecundity is low, the two species have about the same chance of winning, even if the distributions of seed quality are non-overlapping. I will also show that, depending on the fecundity, a species whose distribution of seed qualities has a relatively high mean but low variance can beat, get beaten by, or coexist with a competing species. I apologize to anyone has read this entire abstract. I did not intend for it to be so long.

[back to schedule]

Tuesday, February 15 @ 2:00 PM

Erik Rauch

Patterns of genetic diversity

Within-species (genetic) diversity is important in a wide range of biological systems. For species of conservation interest, genetic diversity is necessary for their survival in the face of environmental changes and disease. For pathogens, diversity plays an important role in their population dynamics and control. In this talk I will show that genetic diversity is distributed unevenly. Using simple genealogical models, I show that genetic distinctiveness has a power-law distribution. This property implies that much of the diversity is concentrated in small sub-populations. These theoretical results agree with genetic data on the distribution of diversity in global samples of Pseudomonas bacteria. I will also present a new method of estimating the genetic diversity of a population from a sample. Finally, I show that the total genetic diversity in a population depends strongly on the size of its habitat, much more strongly than does biodiversity as measured by the number of species.

[back to schedule]

Tuesday, February 8 @ 2:00 PM

Heather Leslie

Managing for resilience in coastal social-ecological systems

Humans impact almost every aspect of ecological systems, and threaten the continued production of valued ecosystem services. Forecasting how human activities alter the structure and functioning of ecosystems, and thus the production of ecosystem services, is fundamentally a biological problem. But biology alone will not be sufficient as ecological and social systems are coupled. In particular, we must better understand the responses of coupled social-ecological systems to disturbance. Resilience (i.e. the amount of disturbance that a system can withstand before its structure and functioning change significantly) theory and related concepts (robustness, vulnerability, adaptive capacity) have the potential to contribute to more effective coastal marine management and conservation. Core research questions include: What are the important variables, linkages, and drivers of change that define coupled coastal systems? What are the sources of resilience in these coupled systems? I will introduce how the developing knowledge of resilience can be applied to advance sustainability at the regional scale, by focusing on the coupled social-ecological systems of Chesapeake Bay.

[back to schedule]

Tuesday, February 1 @ 2:00 PM

John McNamara

A critique of how game theory is used; based on models of conflict and cooperation

Game theoretical models in behavioural ecology often make simplifying assumption without thought as to their consequences. It is usually assumed that the choice of action by contestants is simultaneous. In other words individuals ignore others in making their choice. This assumption not only conflicts with empirical evidence, but crucially affects the outcome of the game. I argue that we need instead to carefully model the process (over time) by which decisions are reached. Model often ignore differences between individuals. It may seem that this simplifying assumption is innocuous when differences are not large, but differences can fundamentally change the nature of a game. Finally, theoretical analyses of extensive form games usually assume that individuals are completely flexible and can always take the best action given circumstance (subgame perfection). Restricting flexibility can again fundamentally change predictions. All these three point are inter-related and will be illustrated by simple games involving conflict and cooperation.

[back to schedule]

Last updated 01/05/05