Theoretical Ecology Lab Tea

The Theoretical Ecology Lab Teas are informal meetings where members of affiliated lab groups give talks on their current research and receive feedback from their audience. The talks are 30 minutes (20 minutes of presentation and 10 minutes of questions) and are scheduled generally on Wednesdays at 12:30 pm. All talks this semester will be held in Eno 209 unless stated otherwise.

This semester, talk schedules and email lists will be maintained by Rutwik Kharkar and Olivia Guayasamin. Please contact one of us to have your name added to the labtea email list so that you can receive reminders about upcoming meetings.

Spring 2016 schedule

Date and time Speaker
Matthieu R. Barbier
Elise M. Myers
Sarah Drohan
Michael Price
Olivia Guayasamin
Charlotte H. Chang
Matt Grobis
Anieke van Leeuwen
Chai Molina
George W. Constable
Flavia D. Marquitti
George Hagstrom

Note: Priority is given to graduate students. A symbol next to the speaker's name means that approval is pending for a week and graduate students can still claim the slot.

Titles and abstracts

The Pig, the Fish, and the Capital Matthieu R. Barbier

This lab tea is about human economic attitudes towards non-humans, how they are constructed and transformed, and why Papuan pig exchanges have a lot to tell us about how to theorize it all.

Many empirical facts, especially of the "soft" socio-psychological kind, appear fairly regular after a change of modeling mindset. To overstate a little: agents have no preferences, norms are not meant to curb human nature, and many of the cognitive biases in behavioral economics are actually not biases at all. I have attempted to elaborate this framework-in-progress around stylized examples from Melanesia and the North Atlantic. I would appreciate some critical input, especially as it slowly evolves toward applicability.

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Exploring Syntrophic Relationships between Sulfate Reducing Bacteria and Methanogens Elise M. Myers

Cooperation and competition among microorganisms in microbial communities is essential to study, especially in order to improve our understanding of how microbes act in natural systems, function as a unit, and respond concurrently to environmental changes. Microbial flexibility, the ablitiy of microorganisms to utilize different metabolic pathways than their "preferred" pathway (espectially in anoxygenic environments), can support cooperation as microbes shift to more energetically favorable, cross-microbial redox coupling for their metabolism. In particular environmental conditions, these microbes enter into syntrophy, an obligate dependency where the metabolism of each microbial type relies on the metabolites produced by the other microbial type.

In my current work, I am exploring the relationship between methanogens, which release hydrogen, and sulfate reducing bacteria (SRB), that can use the hydrogen as an electron donor, resulting in overall exergonic (energy releasing and, thereby more energetically favorable) reactions for both microbial types. I have created a basic framework for a cooperation based mathematical model of the differential syntrophic relationships that are possible between SRB and methanogens. The different cases I am simulating are defined by the following environmental conditions: 1) abundant resources for both microbial types, 2) abundant resources for methanogens only, 3) scarcity of resources for both microbial types. By examining these three test cases, I hope to help elucidate how complex dependencies (including syntrophy) can develop, what environmental conditions trigger these dependencies, and how different levels of co-dependence between microbes impact the population dynamics of these communities. Initial simulation results suggest this model can generally predict population dynamics, though I hope to increase precision by modeling the shuttling of metabolites between the microbial types.

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Intergenerational Resource Transfers with Heterogeneous Rates of Return Sarah Drohan

In both economics and ecology there has been substantial research on familial resource sharing and the distribution of wealth in a population. In 2009, Arrow and Levin considered the probability distribution of wealth resulting from parents allocating an amount of resources to their children, which maximizes total welfare, and then consuming the rest. Assuming that the number of offspring is a random variable and resources grow over time, they showed that the ultimate distribution will be log-normal. However, for at lest the richest portion of society, the data on the distribution of wealth appears to obey a power law. Motivated by this discrepancy, we present a modification of Arrow and Levin's model with the additional assumption that the rate at which resources grow is dependent on an individual's wealth. The set of wealth distributions generated by this new requirement are more likely to have characetristics associated with wealth inequality.

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The Evolution of Mammalian Life Histories in Non-Stationary Populations Michael Price

We describe a novel allometric model of the evolution of mammalian life histories that extends work by Charnov and colleagues. Charnov's model combines three basic components: (1) a growth production function that links growth in body size to body size, (2) natural selection on age at first reproduction, and (3) stationary demography (i.e., a constant population size). Our extension combines four basic components: (1) an arbitrary growth function, (2) natural selection on age at first reproduction, (3) non-stationary demography, and (4) natural selection on size at independence. The three key differences are thus (a) an arbitrary growth production function, (b) non-stationary demography, and (c) optimization over size at independence (in addition to age at first reproduction). Charnov's model fixes the ratio of size at independence to size at first reproduction, but work by Purvis and Harvey shows that there exists considerable variability in this ratio across mammal species. Furthermore, the ratio correlates with other life-history traits, which our extension can explain since we model natural selection on size at independence.

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Estimating the Difficulty of a Search Task using Movement Trajectories Olivia Guayasamin

Humans and non-human species spend a remarkable amount of their time engaged in the act of search. To successfully find a resource or goal, individuals must adapt their search strategies in response to current environmental conditions by deciding whether to spend their time and energy exploring new parts of their environment or exploiting a current location. For members of social species, search has an extra complication. In addition to choosing when and where to explore and exploit, social individuals must decide whether to use costly and reliable personal information, or cheap and potentially unreliable social information.

So, how and when do individuals decide whether to rely on presonal information gathered directly from the environment or social information communicated by others? One prediction comes from the field of animal social learning, "copy others when acquiring personal information is costly". This heuristic refers to the fact that search can be very costly to an individual, and then it is best to offset the costs of search onto another individual when possible. In studies of animal social learning, search "costs" often refer to factors that reduce reproductive potential. Lost time, missed calories, and increased predation risk are commonly explored costs. But costs can also be cognitive in character. Attention and mental processing abilities are limited resources, and if a search task is particularly difficult, it will pull more of these resources away from other important tasks, such as predator vigilance. Quantifying and controlling the difficulty of search tasks is nearly impossible for studies involving non-human animals, making this an enexplored area of research. My work aims to fill this gap by using humans to study how individuals use social information during search tasks of varying cognitive difficulty. Using eye movement and behavioral data collected during novel visual search paradigm, I am currently trying to quantify the cognitive difficulty of the visual search paradigm, so that I can reliably manipulate cognitive difficulty in later studies.

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Exploring Time Allocation to Hunting and Developing a Metric for Measuring Opportunistic Exploitation Risk under Different Reward Regimes Charlotte H. Chang

Pecuniary instruments are a prominent form of managing (illegal) hunting in the developing world. However, this assumes that hunters are driven largely by subsistence or financial reward. My research in Southwest China demonstrates that hunters are willing to continue harvesting wildlife even when catch rates fall dramatically, challenging the assumption that hunting rates should respond to monetary regulations. As such, I am adapting existing agricultural household models to explore how individual villagers allocate time to farming versus hunting. I also seek to derive an indicator that can be used in data-poor systems to highlight species at risk of opportunistic exploitation under different harvester motivations.

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Minority Influence in Collective Behavior Matt Grobis

As a starting point, assuming homogeneity of group membership has yielded useful insights into group dynamics, such as the emergence of group behavioral states from individuals following simple rules. However, there is growing evidence that individual variation may have paramount importance in long-assumed egalitarian group behavior. I will present preliminary results from an experiment in which I administered a predator cue to fish schools composed of a majority that is habituated to the the cue and a minority that is sensitive. My goal is to create a discussion on how best to reveal how sensitive individuals respond to the conflict of personal and social information, and whether they influence their neighbors in turn.

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When Fishing Stabilizes Population Fluctuations Anieke van Leeuwen

Human exploitation of marine ecosystems has led to fish population collapses worldwide. The current challenge to manage fish stocks and the imposed harvesting levels is complicated by the dynamic linkage of both those levels: ecological dynamics interacting with social dynamics.

Fisheries management focuses increasingly on the ecological dynamics within ecosystems, a view promoted by the Ecosystem-based management approach. In this view, the details involved with fishermen, such as choices of species to target, choices of effort expended, and choices of gear selectivity are mostly ignored. On the other hand, the economic perspective on exploitation of shared resources is usually translated into a description of strategy space in which actors make choices based on profitability of the specific resources. This perspective is characterized by a simplified representation of the exploited species, i.e. the fish species population dynamics. Commonly these resources are represented with a single variable in the system, using a dynamic equation such as logistic growth.

The implications of linking these two dynamic levels are currently unknown but present a necessary basis for understanding and managing exploited ecosystems.

I present a framework to analyze the impacts on population stability and species persistence potential from fishing pressure as resulting from the dynamics of fishermen strategy choices.

In the models I analyze, increasing harvesting pressure stabilizes fish population dynamics. This outcome contrasts the empirical evidence for increasing abundance fluctuations as a result from fishing. I will discuss this discrepancy and some theoretical insights or speculations, with implications for early warning theory.

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On Evolutionary Stability and Instability in Public Goods Games Chai Molina

The evolution of cooperation is frequently investigated using public goods games. A classic example is the n-player snowdrift game, in which each player incurs a cost from contributing to a common good but benefits from the pooled contributions of all group members. Such games arise in many biological contexts, from bacterial communities to human societies. With a continuum of contribution strategies (e.g., time devoted to a task benefiting the community), analyses to date have typically assumed---for mathematical convenience---that groups are drawn from an infinite population. Here, we rigorously analyze the continuous n-player snowdrift game in finite populations and compare the evolutionary outcomes with those in infinite populations. We show that evolutionarily stable strategies (ESSs) in infinite populations are always unstable when played in finite populations: selection favours invasion and fixation by less cooperative mutants. We demonstrate that in a large class of snowdrift games that always have a cooperative ESS in infinite populations, there may be no cooperative ESS in a finite population, even for arbitrarily large population size. We show that in such cases, not contributing is a globally convergently stable finite-population ESS, implying that apparent evolution of cooperation in such games is an artifact of the infinite population approximation. In addition, we find that in finite-population snowdrift games in which cooperation can evolve, a large population size is often required. Our results are robust to the underlying selection process.

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Strength in Numbers: Demographic Noise can Reverse the Direction of Selection George W. Constable

Demographic stochasticity, the population-level randomness that emerges when the timing of birth, death and intearction events is unpredictable, can profoundly alter the dynamics of a system. In this talk I will show that phenotypes that pay a cost to their birth rate in order to modify the environment by increasing the global carrying capacity can be stochastically selected for, where they would otherwise be deterministically disfavored. The results hold for a general class of mathematical models but I will use a model of public good production for illustration. In this case, demographic stochasticity is exploited by populations of cooperators to turn selection in their favor; it therefore operates as a mechanism that supports evolution of public good production.

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The Role of Reproduction and Genetics in Nitrogen Fixation Stability Flavia Maria Darcie Marquitti

Many species of bacteria within the Rhizobia group make nodules in plant roots and fix nitrogen in exchange for photosynthates. Non-cooperative rhizobia (non-fixers and non-nodulation strains) should be spread across this group since nitrogen fixation demands high energy. The nodulation and the fixation processes are important factors determining this mutualistic interaction. In the project I will develop here, I aim to understand the importance of different reproduction mechanisms in these bacteria and how the genes involved in the nodulation and fixation in given genetic structures of bacteria can affect the stability of cooperative strains.

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Variable Stoichiometry in a Coupled Food Web-Biogeochemical Model George Hagstrom

Phytoplankton stoichiometry couples the dynamics of marine ecosystems to those of the Earth's biogeochemical cycles. The majority of global models assume that phytoplankton use nutrients at the Redfield ratio, but recent observational work demonstrates that phytoplankton nutrient demand ratios vary substantially by ocean ecosystem type. In order to study the implications of these variations, we modified the COBALT model to incorporate variable phytoplankton stoichiometry. COBALT is a global model that resolves the cycling of multiple important nutrients as well as the dynamics of marine food webs. We present simple results from our modified modelling framework, exploring the implications of variations in upwelling rate and temperature on ecosystem stoichiometry.

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Links to previous schedules

  1. Fall 2000
  2. Spring 2001
  3. Fall 2001
  4. Spring 2002
  5. Fall 2002
  6. Spring 2003
  7. Fall 2003
  8. Spring 2004
  9. Fall 2004
  10. Spring 2005
  11. Fall 2005
  12. Spring 2007
  13. Fall 2007
  14. Spring 2008
  15. Fall 2008
  16. Spring 2009
  17. Fall 2009
  18. Spring 2010
  19. Fall 2010
  20. Spring 2011
  21. Fall 2011
  22. Spring 2012
  23. Fall 2012
  24. Spring 2013
  25. Fall 2013
  26. Spring 2014
  27. Fall 2014
  28. Spring 2015
  29. Fall 2015