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 Wednesdays at 1.30 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 Adrian de Froment and Jeanne DeNoyer. Please contact adriande[at]princeton[dot]edu or jdenoyer[at]princeton[dot]edu 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
    Fall 2005     Spring 2006     Summer 2006
    Fall 2006     Spring 2007

Fall 2007
 

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

Carey Nadell

The evolution of quorum sensing in bacterial biofilms

Microbes have fascinating and diverse social lives. They display coordinated group behaviors regulated by quorum-sensing systems that detect the density of other bacteria around them. A key example of bacterial group behavior is biofilm formation, in which communities of cells attach to a surface and envelope themselves in secreted polymers. Interestingly, some bacterial species activate polymer secretion at high cell density, while others do the opposite. Here we use detailed individual-based simulations to investigate evolutionary competitions between strains that differ in their polymer production and quorum-sensing phenotypes. We focus upon the counterintuitive case: why would bacteria that secrete polymer in biofilms terminate its production at high cell densities, when high cell densities are a characteristic feature of the biofilm phenotype? The model shows that this behavior can yield an evolutionary advantage by redirecting resources into growth and away from polymers that help the bacterium to compete with other strains, but this advantage occurs in a limited time-window. Down-regulation of polymer secretion at high cell density can evolve to afford bacteria a greater capacity for dispersal, but this is disfavored in stable, long-lived biofilms. Our model suggests that the observed differences in quorum sensing behavior can be explained by the differing requirements of bacteria in chronic versus acute biofilm infections.

[back to schedule]

Charlie Stock

A Mechanistic Model for the Biomass Spectrum in Aquatic Ecosystems

The effect of variations in bottom-up and top-down forcing upon the biomass spectrum (i.e., the distribution of biomass when organisms are grouped by size) is analyzed using a size-structured aquatic ecosystem model. The biomass spectrum is closely linked to the flow of energy from primary producers to fish. The parameters and mechanisms controlling steady-state model results are first diagnosed. It is shown that model solutions using parameters within empirical ranges can recreate the slopes and mean biomass of observed biomass spectra from three ecosystems spanning a broad range of productivity. The zooplankton gross growth efficiency is critical for matching the steady-state slopes of the spectra. Variability in export sources and zooplankton half-saturation constants both provide ways of matching the mean biomass. After understanding the basic model dynamics, simulations in a 1-D water column are carried out to understand the relative impacts of decadal changes in hydrography, wind patterns, and fish populations on the structure and function of the Eastern Scotian Shelf ecosystem. Plans for analysis within 3-dimensional hydrodynamic simulations will then be described.

[back to schedule]

Elizabeth Borer

Disease and Invasion: A case study in California grasslands

Elizabeth Borer is faculty in the Dept. of Zoology at Oregon State University and visiting the Dobson Lab.
Most pathogens exist within complicated food webs of hosts and their resources, competitors, and consumers. While direct pathogen-host interactions are often well-described, infection rates also may be indirectly determined by the host’s consumers and competitors or supply of the host’s resources. For vector-borne pathogens, the abiotic and community context may directly or indirectly alter host infection prevalence by controlling either host or vector species composition and population dynamics. I will discuss results from work on a model system, the aphid-vectored barley yellow dwarf virus in California grasslands. I combine a dynamic model with field and greenhouse experiments to examine the role of host composition and host quality on host infection rates and vector preference and performance. Several factors (i.e. disease, fertilization, and grazing) drive the host composition toward a community dominated by plant species that maximize aphid population growth, thereby increasing local pathogen prevalence. Infection prevalence in this host community could not be predicted in the absence of the broader biotic and abiotic community interactions. These results suggest that predicting infection rates in natural host populations may require a more holistic approach extending to indirect interactions within a whole-community context.

[back to schedule]

Michael Raghib

From consensus in swarms to flow in porous media: A `memory-free' approach

Collective decision making in swarms has been explored within the context of self-propelled particle models of `boids' comprising naive and informed individuals. Several measures of decision making for the collective have been suggested, mainly within the context of conflicting directions for the informed sub-population. Here we propose an analogy with flow in porous media that leads to a macroscopic model for an ensemble of swarm centroids with only one preferential direction. This model is based on the 2D Advection-Diffusion Equation with Memory (ADEM). The parameters of the ADEM can be estimated from a single simulation run, conceived as a `computational experiment'. This allows quick numerical solution for the position pdf in comparison to individual-based simulations. The memory can be thought of as a population-level measure of consensus, since it can be shown that it controls whether the swarm moves diffusively (no-consensus), purely advectively (full consensus) and the intermediate regimes between these two extremes. Although the mathematics used is by no means new, the biological interpretations seem novel.

[back to schedule]

Michael Desai

Detecting selection in bacterial and viral populations using polymorphism data.

The distribution of genetic diversity in a population contains information about natural selection, demographic structure, geography, recombination, and mutation rates in the population. There are a wide array of population genetic techniques used to infer these parameters from various aspects of sequence data. I'll briefly outline one such method, the Poisson Random Field (PRF) which uses the polymorphism frequency spectrum to infer selective pressures. I'll describe how this suffers from systematic biases that cause it to tend to erroneously infer positive selection, particularly for bacterial and viral populations. I'll present several new methods to correct these problems. I'll also describe how all existing inference techniques leave a wide gap in our understanding of genetic variation in an asexual population under selection. Depending on time and interest, I'll describe some very preliminary ideas I have for addressing this question.

[back to schedule]

Ryan Chisholm

Why are there no trees in the Fynbos?

Fynbos is a fire-prone shrubland that occurs on infertile sandy soils within South Africa's Cape Floristic Region. With over 7,000 plant species in 46,000 km2, the Fynbos has a floristic diversity that is greater than that of many tropical rainforests. Invasion by alien trees (mostly Acacia, Hakea and Pinus species) is altering the typical treeless character of Fynbos and threatens native biodiversity. The success of these invading trees makes the question of why native Fynbos trees never evolved particularly compelling. I will present my current set of hypotheses that seek to address this question, and I will discuss possible theoretical frameworks within which to test these hypotheses.

[back to schedule]

Duncan Menge

Emergence and maintenance of nutrient limitation over multiple time scales in terrestrial ecosystems

Nutrient availability limits primary production in ecosystems worldwide, yet the processes governing the dynamics of nutrient limitation are not well understood. Here we examine processes controlling steady state and transient nutrient dynamics at three separate time scales in a simple model of a single nutrient cycling through plants and soil. When the only losses are from the plant-available nutrient pool (e.g., nitrate or ammonium), nutrient limitation at steady state is impossible under a wide class of conditions. However, plant biomass will appear to saturate on a time scale controlled by plant nutrient turnover (years in grasslands or decades to centuries in forests), even though it grows slowly forever. In contrast, primary production can be nutrient-limited at steady state when there are losses from the plant-unavailable nutrient pool (e.g., dissolved organic nitrogen). The long time scale of the model (nutrient buildup in soil organic nutrients and the ent ire system) is controlled by the plant-unavailable loss rate, which is typically multiple millennia in grasslands and forests. The short time scale of the model, at which available nutrients in the soil equilibrate in hours to days, is controlled by biotic uptake. These insights into the different time scales of terrestrial ecosystems can help guide empirical and experimental studies.

[back to schedule]

Jack Brookshire

Climate sensitivity of forest nitrogen cycles revealed with long-term watershed records

Nitrogen cycling in northern hemisphere temperate forests has been dramatically altered by human activity, particularly over the last century. Cutting practices and atmospheric pollution have driven many forests far from long-term N equilibrium, evidenced by widespread and sustained N accumulation but also increased losses of plant available N in stream waters. Such conditions have made it difficult to separate and examine predicted effects of climate variation on forest N cycles. Here we use long-term (32 yr) climatalogical and hydrochemical records from small watersheds and a general theory of nutrient limitation to examine the internal vital rates of the N cycle and the relative influence of climate and atmospheric deposition. We develop a simple biogeochemical model that captures the basic aspects of the terrestrial N cycle and parameterize it within a maximum likelihood framework using observed monthly N inputs and losses. We make the assumption that internal cycling (mineralization and plant uptake) is at or near steady state despite aggrading forest biomass and atmospheric N inputs in great excess of losses. We find our quasi-equilibrium model produces realistic estimates of cycling rates and reasonable reproduction of observed losses without explicit representation of space, hydrologic dynamics, soil compartments, or species effects. Our analysis revealed that N mineralization and its temperature sensitivity, not N deposition, control most of the observed variation in N losses. Intra-annual variation in plant available N concentrations was driven largely by temperature with little influence of moisture while inter-annual dynamics were better captured with fluxes, a pattern explained by precipitation forcing and rapid equilibration of plant available N pools. Our results further suggest that N mineralization has increased over the last 30 years due to increasing temperature, which, in combination with increasing deposition, would accelerate N cycling and thereby increase N availability to plants but also potentially losses to stream waters. We suggest that such a simple theoretical approach to examining historical input-output records can provide critical insight into the climate sensitivity of terrestrial nutrient cycles.

[back to schedule]

Eili Klein

Clinically-immune hosts as a refuge for drug-sensitive malaria parasites

Mutations in Plasmodium falciparum conferring resistance to first-line antimalarial drugs have spread throughout the world from a few independent foci, all located in in areas of unstable malaria transmission in Southeast Asia and South America. Quantitative population genetics models have suggested a number of mechanisms for this phenomenon including the existence of high levels of clinical immunity in sub-Saharan African populations. However, epidemiological models have concluded that immunity does not qualitatively impact the evolution of resistance. Here, we reconsider the role of immunity in the dynamics of malaria transmission and its role in the evolution of antimalarial drug resistance under the assumption that immune individuals are infectious. We find immunity can reduce the effective reproductive rate of the resistant parasite as transmission intensity increases. At high transmission levels the effective reproductive rate of the resistant parasite may fall below the reproductive rate of the sensitive parasite, even though the resistant parasite has a higher basic reproductive rate, R0. Thus, immunity creates a natural ecological refuge. Our results suggest an epidemiological rationale for the more likely emergence of resistance in low transmission settings. This finding has implications for the design of drug policies and the formulation of malaria control strategies, especially those that lower malaria transmission intensity.

[back to schedule]

Henry Horn (with material provided by J. Chester Farnsworth, et al.)

A Special Holiday Lab Tea:

Saint Chester and the Shroud of Pinocchio

The 1985 rediscovery of The Shroud of Pinocchio is one of the major historical events of the 20th Century. The implications of The Shroud force a reinterpretation of almost every field of human intellectual endeavor, from history itself to religion, the psychiatry of dreams, and millimetric radar technology. Yet all of the purported contemporary facts surrounding The Shroud trace back to the deservedly obscure Saint Chester (fl. ca. 800 C.E.). So a prerequisite to understanding The Shroud is to discover as much as possible about the Saint. In turn, many of the facts about Saint Chester have been filtered through the unfettered imagination of J. Chester Farnsworth. The physical documentation of these facts rests on a series of relics discovered by Farnsworth in his grandmother’s attic. These include, int. al.: … The Shroud itself, … the Pinocchian Coins that were placed over the deceased’s eyes as subway tokens to the afterworld, … and the unrelated but equally important Collective Reliquary of Snow White and the Seven Dwarfs. I hope that the presentation and analysis of these facts and artifacts will make it clear that The Shroud of Pinocchio can stand proudly with all the other famous Shrouds.

[back to schedule]

Last updated 02.01.08
adriande [at] princeton [dot] edu