Spring 2010

Titles and abstracts

Wednesday February 3rd at 12.30pm

Foggy forests and (positive) feedbacks
Daniel Stanton
Fog can be an important and often overlooked source of water for many ecosystems. Unlike rainfall, fog water collection involves suspended droplets impacting a surface, and therefore, inputs will be dependent on vegetation structure, leading to potential for interesting feedbacks as vegetation grows from the water it collects. I will be presenting some of my still very rough ideas on modeling this system, particularly with an eye on how epiphytes (plants growing on other plants) might affect the dynamics of this interaction.

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Wednesday February 16th at 12.30pm

Marrying neuroscience and evolutionary biology to model social learning
Adrian de Froment
Social learning - copying the behavior of others - is common throughout the animal world. The ability to copy others, whether directly or indirectly, is particularly important because it underpins the emergence of behavioral traditions and of culture (many of our own problems as a species stem directly from our inability to control our own behavioral traditions and culture). Social learning provides a cheaper alternative to individual learning - exploring the environment from scratch - but this doesn't mean that animals should copy wherever they have the chance (the information implicit in the behavior of others may be inaccurate or out of date). Experimental work in the last five years has uncovered strategies that animals use when deciding whether to copy or not - two of the simplest are "copy when individual learning is expensive" and "copy when you lack reliable prior knowledge". However, we have no general theory to explain how and why animals employ these strategies. What we'd really like is a model of a "meta-strategy" for social learning that encompasses these particular rules within its parameter space, but we currently lack a good framework within which to create one. A fusion of statistical decision-making models used in neuroscience and evolutionary models from our own discipline may offer a productive (and unifying, and testable) way forward. I've begun modeling social learning in this framework, and I'd be very glad of your feedback on these initial efforts.

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Wednesday February 24th at 12.30pm

Cellular tropism in respiratory tract determines influenza virus fitness
Leslie Reperant
Pandemic and seasonal influenza A viruses display diverse virulence phenotypes. Pandemic influenza viruses replicate in bronchiolar ciliated epithelial cells and pneumocytes, leading to viral bronchiolitis and pneumonia, a potentially fatal illness; in contrast, seasonal influenza viruses infect tracheal and bronchial ciliated epithelial cells, causing tracheo-bronchitis. Weak or lack of immunity against influenza viruses is considered the main risk factor for developing severe disease. However, it remains unknown whether infecting cells of specific regions of the respiratory tract contributes to the reproductive fitness of influenza viruses at the population level. I explore the contribution of pathogenesis to influenza virus reproductive fitness using a mathematical model capturing the within-host dynamics of infection and immune response in human respiratory tract. I find that the reproductive fitness of pandemic influenza viruses is maximized by high infectivity for bronchiolar ciliated epithelial cells while that of seasonal influenza viruses is maximized by high infectivity for tracheal and bronchial ciliated epithelial cells. Furthermore, high infectivity for bronchiolar non-ciliated epithelial cells and type II pneumocytes, which are targeted by avian influenza viruses, hinders the reproductive fitness of highly pathogenic avian influenza virus H5N1 in humans and maintains it below the threshold for sustained transmission. Integrating spatially-explicit pathogenesis into mathematical models appears a promising venue to unravel the impact of pathology on pathogen dynamics, and I would love to hear your comments.

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Wednesday March 3rd at 12.30pm

Individual rules underlying collective animal behaviour
KolBjorn Tunstrom
The collective behaviour of an animal group, as e.g. a flock of birds, seems to emerge on the group level through local interactions between the individuals. This is efficiently used in agent based simulations, where complex group behaviour results from individual rules. An important question in this context is: Can we decide from observations what the individual interaction rules are in a animal group? In this talk I will present a methodology that possibly can be applied to test hypotheses about the individual interaction rules underlying collective behaviour.

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Wednesday March 10th at 12.30pm

Asymptotic equivalence of niche and neutral theories in ecology
Ryan Chisholm
A fundamental challenge in ecology is to understand the mechanisms that govern species diversity. Previous numerical simulations have suggested that complex niche-structured models produce species abundance distributions (SADs) that are qualitatively similar to those of very simple neutral models that ignore differences between species. However, in the absence of an analytical treatment of niche models, we cannot tell whether the two classes of model produce the same patterns via similar or different mechanisms. I present an analytical proof that, in the limit as diversity becomes large, a strong niche model give rises to exactly the same asymptotic form of SAD as the neutral model, indicating that neutral processes drive patterns of biodiversity even when strong niche structure exists. I demonstrate the analytical results by showing that the SAD predicted by the neutral model for a Panamanian tropical forest is extremely similar to the SAD predicted by a niche-structured model with up to 16 non-overlapping niches. These results strongly suggest that neutral theory is a parsimonious and mathematically valid model for predicting and understanding macroscopic patterns of biodiversity in high-diversity communities, but that it cannot be used to infer an absence of niche structure or to explain ecosystem function and that it may not be valid in low-diversity communities.

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Wednesday March 24th at 12.30pm

Modeling the response of leatherback turtles to climate change in the eastern Pacific Ocean
Vincent S. Saba
Eastern Pacific leatherback turtles (Dermochelys coriacea) are critically endangered and nesting populations have been precipitously declining since the start of monitoring in the late 1980’s. Nesting females in Pacific Mexico and Costa Rica respond to primary productivity transitions in the eastern equatorial Pacific as indicated by the El Niño Southern Oscillation (ENSO) events La Niña and El Niño. Sea surface temperature anomalies in the eastern equatorial Pacific can be used as a proxy for ENSO events such that cool, highly productive La Niña events increase leatherback nesting probabilities while the opposite holds true for warm El Niño events. Presently, the effect of global warming on the natural ENSO cycle is unknown. Using GFDL's Earth System Model 2.1, I simulated the response of the leatherback migration area in the eastern Pacific to anthropogenic climate warming and found that the relationship between temperature and productivity is not consistent over the next 300 years. This presents a challenge to the current leatherback population model thus I am currently working on a nesting population projection model that will account for the changing dynamics between physical and biological fields as forced by global warming.

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Wednesday March 31st at 12.30pm

To breed or not to breed: a model of partial migration
Allison Shaw
Animals migrate for several reasons: to avoid seasonally unfavorable climate, to track changing food distributions, and to reproduce. In organisms with the third type (breeding migrations) an individual spends the majority of its life in one habitat but must make a costly migration to another location in order to reproduce. In these species, sexually mature individuals will often skip migration and forgo breeding opportunities in certain years, suggesting that there are tradeoffs between current and future reproduction and survival. I have built a matrix population model for this system and use adaptive dynamics to determine the evolutionarily stable strategy (ESS) for breeding frequency. I find that in most cases there is a pure ESS, but that variability (via environmental stochasticity or a fluctuating population size) can lead to a mixed ESS. I am in the process of applying this model to different species with breeding migrations (e.g. land crabs, sea turtles, salmon) in order to make system-specific predictions about migration frequency.

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Wednesday April 7th at 12.30pm

Predicting Reachable Outcomes in Malaria Control
Yael Marshall
I am predicting and exploring the function of {antimalarial intervention package} --> {malaria burden}, accounting for drug resistance and as necessary human acquired immunity and eventually also insecticide resistance. Intervention packages consist of the currently available interventions (drugs, insecticides, bednets, larvicides, etc.) which each can be used to different levels or coverages. In this presentation, I will focus on compartmental models I've developed and used to predict reachable outcomes in malaria control in low transmission settings. I use field data (mainly the 1970s Garki, Nigeria dataset) to tune and validate my model predictions. I will also mention my work on compartmental models for all transmission settings and my microsimulation model. I am also addressing specific questions and patterns including: What are the tradeoffs between various interventions? Is it possible to control malaria sustainably using the currently available tools? By combining interventions, can we reduce pressure on each intervention thereby reducing the rate of drug and insecticide resistance?

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Wednesday April 13th at 1.30pm

Generalized Population Processes
Ricky Der
The Wright-Fisher model has long attained a monolithic status in population genetics, and serves as the reference for an enormous number of calculations in the theory. In the 1970's, C. Cannings introduced a more general and flexible set of processes subsuming the Wright-Fisher process, based on the concept of reproductive exchangeability. Despite the fact that these systems are in many ways more biologically plausible than the prevailing gold standard, their acceptance has been slow, mainly due to the great difficulty in analyzing their dynamics. In this talk I will introduce a new continuum approximation theory for Markov chains that enables an analysis of the forward-time behavior of Cannings processes, among others; this approximation theory generalizes the standard Kimura diffusion equation approach, in that processes with discontinuous sample paths can be accommodated. We obtain bounds over the generalized class for important genetic quantities: expected fixation times, probabilities of fixation, and the forms of the equilibrium measure under mutation. Some Cannings models are found to have remarkable properties quite different from those of the Wright-Fisher model; other features are found to be very stable and robust across the entire generalized class. I will also discuss connections to the recent non-Kingman coalescent theory, and some of the implications of our research for statistical inference of selective and mutational parameters.

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Wednesday April 21st at 12.30pm

Economics of disease control and implications for eradication
Petra Keplac
In the classical epidemiological theory, the fraction of the population that needs to be vaccinated in order to eliminate the disease from the population is equal to 1-1/R0, where R0 is the basic reproductive number defined as the expected number of secondary cases caused when a single infected individual is introduced in a completely naive population. From the economic perspective, it makes sense to eliminate the disease only of the costs of vaccination are sufficiently low, and the benefits of lower level of infection are sufficiently large. What is the optimal, cost-minimizing vaccination coverage when there is also infection from the surrounding areas? How does the local optimal strategy compare to the global one, and what are the implications of these (extremely preliminary) results for regional elimination of diseases?

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Wednesday April 28th at 12.30pm

Are forest and savanna alternative stable states in sub-Saharan Africa?
Carla Staver
Savannas are known as ecosystems with tree cover below their climate-defined equilibrium. However, a predictive framework for understanding constraints on tree cover is lacking. I will start by presenting a spatially extensive analysis of tree cover demonstrating that savanna and forest may be alternative states in Africa, with implications for understanding savanna distributions. Tree cover does not increase continuously with rainfall, but rather is constrained to low (<50%, "savanna") or high tree cover (>75%, "forest"). Intermediate tree cover rarely occurs. The pattern is evident especially in areas with rainfall between 1000mm and 2000mm, where fire - which prevents trees from establishing - differentiates high and low tree cover. This pattern suggests that complex interactions between climate and disturbance produce emergent alternative states in tree cover. I will continue with ongoing modeling work investigating the potential for known drivers to actually produce these apparently bi-stable patterns in tree cover distribution observed in our empirical analysis.

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Wednesday May 5th at 12.30pm

Plant allocation strategies: predictions from adaptive dynamics analyses
Caroline Farrior
All trees and shrubs have a few basic organs: leaves, fine roots, and structural biomass (wood). Across species and environments the proportional allocation to each of these structures varies. Wood is a tool plants use in competition for light while leaves soak up light and carbon dioxide for photosynthesis and fine roots supply the leaves with the water and nutrients they need. We have put together a model of trees which incorporates height-structured competition for light and competition for water in well-mixed soil. This model predicts an unintuitive dependence of allocation strategies on water-availability. Root investment increases with increasing water availability, while water is limiting. The mechanism behind this prediction is a restrained tragedy of the commons for resource use. Individual plants take up more water than they would if grown in isolation because the effect of the draw-down is spread across many individuals. I will discuss these patterns as well as my plans to begin testing these predictions with observations and experiments in the field.

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Wednesday May 12th at 12.30pm

Learning, foraging, information and risk in a dynamic environment
Liliana Salvador
Foraging animals move to explore their environment to constantly learning, risking and making decisions about when and where to go. The success of these routines depends on the searching strategies that animals adopt and on the amount of information they have about the environment. Some important questions to ask are: Why do individuals have the movement strategies they do? How much information do they need to perform these decisions? What is the best tradeoff between using a specific resource site and avoiding hostile neighbors? During labtea, I would like to discuss some ideas in how to incorporate learning and memory in evolutionary foraging models.

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Wednesday May 19th at 12.30pm

Conservation in the UGC depicted as a repeated game
Sara Avila
The environment is the Upper Gulf of California, an incredibly rich ocean. The problem is overharvest and the extinction of vaquita marina, a very charismatic porpoise. The setting is a community lacking institutional arrangements, with complex interlinked incentives and above all, uncertainty about the future. NGO's and the Mexican Government have made an expensive effort to buyout gillnets that kill vaquita marina. However, the results have been quite skimpy. Not quite enough to save vaquita from extinction and not quite enough to avoid overharvest of shrimp. I depict the problem as a repeated game. Given the current situation, is there room to reach a pareto dominant outcome where vaquita is saved from extinction while fishermen don't lose their income?

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Links to previous schedules

    Fall 2000    Spring 2001
    Fall 2001    Spring 2002
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    Fall 2003    Spring 2004
    Fall 2004    Spring 2005
    Fall 2005    Spring 2007
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    Fall 2008    Spring 2009
    Fall 2009