Theoretical Ecology Lab Tea

Fall 2019 schedule

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

Tue. September 10^th – 11:00am

Intraspecific adaptation load: a mechanism for species coexistenceMasato Yamamichi

Selection arising from intraspecific interactions, such as sexual and kin selection, is a major driver of adaptive evolution. However, the role of the intraspecific adaptation in species coexistence is still poorly understood, as community ecologists have paid attention mainly to adaptation to interspecific interactions and/or abiotic environments. Theory and empirical evidence suggest high conspecific density intensifies intraspecific interactions and conflicts among individuals, causing "selfish" adaptation that increases individual’s reproductive success at the expense of population growth (evolutionary tragedy of the commons). We propose that the reduction of population growth due to intraspecific adaptation (intraspecific adaptation load) results in negative density-dependent population growth, thereby enhancing species coexistence through negative frequency-dependence in community dynamics.

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Wed. September 11^th – 12:30pm

Theoretical Challenges in Microbial EcologyJinzhong Zhou

Microorganisms constitute the most abundant life forms on Earth’s biosphere, and play integral and unique roles in ecosystem functioning. Understanding the ecology of microorganisms is one of the most compelling intellectual challenges facing contemporary ecology. Although various theoretical approaches (conceptual, mathematical and computational) have been used to interpret and predict the interactions between microbes, and their physical, chemical and biological environments, the application of theoretical approaches to microbial ecology is currently still very limited due to several reasons. First, because of unique biology, and the nature of molecular biology-based detection approaches, the experimental data in molecular microbial ecology are typically highly noisy, with poor quantitation and low reproducibility. Also, due to technical problems, relative rather than absolute abundance is often used in microbial ecology studies, and hence the data is compositional and not independent. In addition, since relative high cost and extremely high diversity, microbial communities are severely under sampling, and hence the data matrices are highly sparse. As a result, well replicated, long-term temporal dynamic data with appropriate time-intervals required for dynamic modeling are rarely available. All of them present unique challenges to theoretical analyses in microbial ecology. In this informal talk, I will briefly explain the unique characteristics of microbial data, followed by examples of theoretical questions addressed in microbial ecology. I will then highlight some mathematical challenges (e.g. reverse and dimensionality problems) unique to microbial ecology using molecular ecological networks (MENs) as examples. I will present random matrix theory (RMT)-based network approaches to reconstruct relevance networks, with a recent further development of a new framework, called iDIRECT (inference of Direct and Indirect Relationship with Effective Copula-based Transitivity), to disentangle direct from indirect relationships by particularly highlighting various mathematical challenges (e.g., ill conditioning, self-looping, interaction strength overflow) and solutions in detecting spurious network edges. Furthermore, I will introduce another recent study on how to incorporate omics information into ecosystem models to improve predictions of grassland ecosystem’s responses to climate warming. Finally, I will briefly discuss future perspectives from both theoretical and experimental angles on developing dynamical models to simulate and predict microbial interactions under more realistic natural settings.

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Wed. September 18^th – 12:30pm

Liana hydraulic performance as driver of dry season growth advantageHannes de Deurwaerder

Neotropical lianas (woody vines) increase in abundance, assumedly as the result of climate change. The exact driver behind liana proliferation is, however, still unknown. One postulated hypothesis states that lianas have deep roots which provides them access to deep water reserves, enabling them to avoid drought which results in a dry season growth advantage over trees (i.e. ‘the deep root hypothesis’). Unfortunately, comprehensive evidence in support of this hypothesis has not been forthcoming. Moreover, two opposing liana drought strategies emerge from available empirical data, i.e. ‘drought sensitive’ and ‘drought tolerant’. There is need for a coherent theoretical framework which assembles these apparent contrasting observations. We explore and characterize drought sensitivity and hydraulic dynamics in tropical lianas and co-occurring trees using a basic hydraulic plant model. Hydraulic performance responses of both growth form are validated to various soil and atmospheric drought conditions Our model provides a theoretical framework which connects apparent inconsistent observations and related drought strategies, and provides a mechanistic explanation for the dry season growth advantage of lianas. The model shows that the deep root hypothesis is not required to explaining the dry season growth advantage nor the liana proliferation. Models that include even a basic hydrological representation of plants in shallow soil layers can already justify both phenomena.

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Wed. September 25^th – 12:30pm

A Social Dilemma of SocialityDylan Morris and Daniel Cooney

Why are some animals socially gregarious while others keep to themselves? Evolutionary models of gregarious behavior typically treat benefits and costs of social interaction qualitatively. Some of these benefits and costs are contagious: for example, socializing may allow individuals to share useful information with their neighbors, but also expose them to dangerous infectious diseases. In this lab tea, we will present a preliminary model the evolution of sociality in the presence of beneficial and costly social contagion processes. We characterize a socially optimal level of social interaction, and show that evolutionary dynamics produce a social dilemma: individuals maximizing their fitness drive the population to a level of sociality at which all individuals are worse off. In some cases, social behavior can disappear entirely -- even when any level of socializing would be advantageous for the species as a whole.

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Wed. October 2^th – 12:30pm

Exploring the immune response to wild-type and live-attenuated measles virus using within-host data and mathematical modelsCaroline Wagner

Both the wild-type measles virus (MV) and the live-attenuated vaccine induce MV-specific humoral and cellular immune responses, although antibody titers to vaccination are usually lower than those induced by wild-type infection. Consequently, the live-attenuated MV vaccine is recognized as one of the most successful public health interventions, but the biological basis for its attenuation is not well understood. Notably, wild-type MV results in systemic disease associated with extensive virus replication in lymphoid and epithelial tissues, whereas vaccination predominantly results in sub-clinical and localized low-level virus replication in myeloid cells. Further, as a result of its lymphotropic nature, infection with wild-type MV is associated with a marked depletion of non-measles antibodies owing to lymphocyte destruction. To begin to explore these differences, we develop a biologically informed mathematical within-host spatial model of the immune response to both wild-type and live-attenuated MV. We validate this model using a variety of in-vivo data, including cellular data from animal models as well as human antibody titers. This work contributes to the mechanistic understanding of immune responses to vaccine and wild-type viruses, and may prove useful for public health surveillance of MV infection and vaccination.

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Wed. October 9^th – 12:30pm

Abundance-distributions of animal diet: description, variation, and predictionMatthew Hutchinson

Most animals, much of the time, include a variety of resource species in their diets. Importantly, the breadth and evenness of consumers' diets has direct repercussions for both consumer and resource population dynamics, the structure of trophic webs, and the behavior of animals. However, the importance of ecological context for understanding diet composition has been a barrier to a general picture of how consumer populations' diets are structured. In this talk, I will discuss how we have tried to address this issue of context-dependence by thinking of animal diets as modified ecological communities that have been filtered through a consumer's mouth.

With a global dataset of 1200 vertebrate diets, which spans 138 taxonomic families and seven continents, we tested whether populations' dietary-abundance distributions (DADs) tend to take a similar shape and how ecological context alters that shape. I will discuss how abundance distributions of food items tend to follow a steep decline in abundance with few common and many rare species and how we made that assessment. I'll also show how, despite consistency in their general shape, DADs tend to be more specialized in higher-trophic-level consumers and at higher latitudes. Finally, I will demonstrate how the environmental availability of resources is not a great predictor of diet composition and introduce how we are using foraging theory, specifically resource value and a consumer's selectivity, to predict both consumer diets and the DADs that they represent.

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Mon. October 14^th – 1:30pm

Information theory, predictability, and the origin of complex lifeLuis Seoane

Darwinism emphasizes fast replication and large progeny. To this end, it pays off being small and simple. Complex structures might be penalized, yet different levels of complexity have been realized by living systems—thus posing a conundrum. Stephen J Gould proposed that, actually, simple life forms dominate the biosphere. The ‘incidental’ complexity we observe would result from a random drift. Gould insisted that there are not evolutionary drivers pushing evolution toward higher complexity. We argue that such evolutionary pressures do exist. We hypothesize that a tradeoff is in place between i) fast, cheap replication and ii) an organism’s ability to process meaningful information—thus predict its environment. We further hypothesize that this tradeoff sits at the origin of more complex life forms. To study this tradeoff we introduced bit-guessers, a minimal model that brings together information theory and Darwinian dynamics. This toy model allows us to show explicit evolutionary pressures that select for complex organisms. Our latest and most important results examine the role of parasites, revealing yet stronger pressures toward increased complexity; often with outstanding dynamics despite the simplicity of the model. The bit-guessers paradigm turned out to be very rich, suggesting plenty of open research questions about its dynamics. It is also very apt to model an array of biological scenarios at a very fundamental level involving Darwinism and information theory alone.

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Wed. October 16^th – 12:30pm

Speaker's work is unpublished, and prefer her talk contents not displayed here.Olivia Chu

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Wed. October 23^th – 12:30pm

Timing SexFernando Rossine

Sex is almost ubiquitous in Eukaryotes. However, in many branches of the tree of life, sex is facultative, occurring in between many asexual generations. In these taxa, sexual behavior is often triggered by stressful conditions, which is usually explained by invoking a need to generate diversity in those particular instances. Can we explain the timing of sex without relying on such a particular fitness landscape?

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Wed. November 6^th – 12:30pm

The Spandrels of Ecological Networks: rethinking the architecture of mutualistic ecosystems Jordi Pinero

‘Mutualism’ is a type of ecological interaction occurring between pairs of species that benefit each other in a cooperative way. Plant-animal interactions (such as plant-pollinator) are among the most widespread and well-studied mutualistic webs. Networks of mutualistic species are of special interest in ecology, as they manifest universal statistical patterns, i.e., global structural regularities present in different webs of mutualistic species. These have been suggested to emerge from potential selection processes favouring biodiversity. However, here (1) we show that these structural quantitative patterns can be properly explained by means of a very simple dynamical model of speciation and divergence with no selection-driven coevolution of traits. The agreement between observed and modelled networks suggests that the patterns displayed by the studied mutualistic webs might actually represent evolutionary byproducts of the network generative process, which are defined ! as evolutionary spandrels (2).

(1) Sergi Valverde, Jordi Piñero, Bernat Corominas-Murtra, Jose Montoya, Lucas Joppa & Ricard Solé “The architecture of mutualistic networks as an evolutionary spandrel”. Nature Eco. & Evo. 2, 94-99 (2018). (2) Stephen J. Gould & Richard C. Lewontin, “The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme”. Proc. R. Soc. Lond. B 205, 581–598 (1979).

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Mon. November 11^th – 2:30pm

Cooperation and Strategy Choice in the Infinitely Repeated Prisoners’ Dilemma When Players Can CheatAstrid Dannenberg

We study behavior in an experimental infinitely repeated prisoners’ dilemma when players are able to cheat by signaling cooperation while actually playing defection. Players in this game decide whether to cheat or play honestly while nature decides if a cheating attempt succeeds or fails. We test how the possibility to cheat affects strategy choice and cooperation compared to the classic version of the game where players are unable to cheat. We also test how the probability that a cheating attempt succeeds or fails and the co-player’s ability to discover failed cheating attempts change behavior.

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Wed. November 13^th – 12:30pm

Evolutionary dynamics of polarization and cooperation in group-structured populationsMari Kawakatsu

The emergence and maintenance of stable social structures have long fascinated researchers in fields ranging from evolutionary biology to political science. Political theorists as early as James Madison theorized that a diversity of interests should inhibit the formation of factions; when individuals have many political interests, they must cooperate and form alliances across disagreements to achieve common goals. But this hypothesis runs counter to a result established by evolutionary models of cooperation with dynamic population structures: cooperation is often associated with increased connectivity, which in turn makes the system vulnerable to invasion, leading to the fragmentation of population structure. Under what conditions, then, can cooperation and social cohesion evolve simultaneously? In this talk, I will present an evolutionary game-theoretic framework to study this question in the context of multiple, overlapping groups that represent various issues of interest. I will present preliminary results from exploring factors that promote cooperation and cohesion in the population. I will then describe additional analyses that I plan to perform to better understand the mechanisms at work. This is a work in progress, and I look forward to collecting feedback on both the model and the analysis approach.

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Wed. November 20^th – 12:30pm

Large and Small Herbivores should prefer Different Plant SpeciesArjun B. Potter

The size of prey items has been classically invoked as a mechanism of dietary niche partitioning for a wide variety of animal taxa. However, this mechanism has been harder to apply to plant-herbivore interactions, since plant size is difficult to quantify and there is often little connection between plant size and herbivore size. Classical explanations for large herbivore niche partitioning often invoke herbivore body size and plant tissue quality, while more modern DNA-based approaches focus on the plant taxa in the diets of herbivores. We reconcile these two approaches by proposing a simple mechanism by which herbivores of different sizes would prefer to eat different plant species even if they attempt to maximize the same dietary nutrient. Using a simple mathematical model, we show that our mechanism suffices to produce dietary niche partitioning according to herbivore size, reconciling classical (allometric, plant tissue-based) and modern (taxonomic) approaches to studying large herbivore diet.

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Mon. December 2^th – 3:00pm

Games on Graphs and Graphon GamesDan Cooney

In this talk, we will consider game-theoretic models in which strategic interactions occur on large deterministic or random graphs. In the limit of infinitely many individuals, we can use the theory of graphons (a continuum limit of graphs) to describe a limiting game played by a continuum of players. We will first discuss some results for the Nash equilibrium and social optimum strategy profiles of graphon games, and then explore example games motivated by models from the network economics and mean-field games literatures. For special families of graphons with small-world, power-law, and threshold connectivity, we will present explicit solutions of Nash equilibrium and social optimal strategies and compute the price-of-anarchy comparing the social cost of individually and socially optimal outcomes. This is joint work with René Carmona, Christy Graves, and Mathieu Laurière.

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Wed. December 4^th – 12:30pm

The role of expectation bias in public goods provisionFernando Santos and Vitor Vasconcelos

In non-linear public goods games, benefits are only obtained after a certain level of cooperation is achieved. In that context, the assumption about other peoples’ willingness to contribute can be a determinant of cooperation. Several experiments show that individuals who expect that others will cooperate are more inclined to, themselves, cooperate. Despite the link between expectations and cooperation, individuals may not always anticipate the overall levels of cooperation in a population accurately. For example, in the context of mitigating climate change effects, recent results show that both general public and political actors tend to underestimate the pro-climate positions of others, expecting lower support than what is actually verified. Designing effective institutions and mechanisms to sustain cooperation must consider individuals’ expectation biases. In this presentation, we will discuss some work-in-progress on modeling the effect of expectation bias in promoting cooperation in public goods games. We model populations of rational individuals that adapt their behavior through smooth best-response, while possibly incurring in expectation bias— either under- or over-estimating the overall levels of cooperation. We find that expectation bias can fundamentally alter the dynamics of cooperation and either prevent or ease collective success in the context of non-linear public good games.

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Wed. December 11^th – 12:30pm

Spatial ecology of territorial populationsBenjamin Weiner

Many ecosystems, from vegetation to biofilms, are composed of territorial populations that compete for both nutrients and physical space. What are the implications of such spatial organization for biodiversity? To address this question, we developed and analyzed a model of territorial resource competition. In the model, all species obey trade-offs inspired by biophysical constraints on metabolism; the species occupy nonoverlapping territories, while nutrients diffuse in space. We find that the nutrient diffusion time is an important control parameter for both biodiversity and the timescale of population dynamics. Interestingly, fast nutrient diffusion allows the populations of some species to fluctuate to zero, leading to extinctions. Moreover, territorial competition spontaneously gives rise to both multistability and the Allee effect (in which a minimum population is required for survival), so that small perturbations can have major ecological effects. While the assumption of trade-offs allows for the coexistence of more species than the number of nutrients—thus violating the principle of competitive exclusion—overall biodiversity is curbed by the domination of “oligotroph” species. Importantly, in contrast to well-mixed models, spatial structure renders diversity robust to inequalities in metabolic trade-offs. Our results suggest that territorial ecosystems can display high biodiversity and rich dynamics simply due to competition for resources in a spatial community.

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Wed. December 11^th – 01:30pm

A Neutral Theory of Mutualistic Communities: how stochastic fluctuations may drive community assembly in the microbiomeJordi Pinero

Many complex communities display mutualistic interactions: pairs of species help each other to reproduce and thrive. An important example is provided by microbiomes, where a large number of microorganisms living inside a multicellular host operate by breaking down metabolic resources in a distributed fashion, which induces an indirect cooperative scheme. What is the role of these cooperative interactions in maintaining the diversity of such microbial communities? Can there be a stable cooperative community? Here we explore a neutral model that considers all species as ecologically equivalent cooperators. The analysis of the model provides both qualitative and quantitative predictions for the overall statistical properties comparable with those exhibited by these microbial ecosystems. Finally, future work directions will be discussed - involving the role of space or the interplay between microbial community assembly and physiological architectures, competition and network structure.

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Links to previous schedules

1. Fall 2000
2. Spring 2001
3. Fall 2001
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11. Fall 2005
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35. Fall 2018
36. Spring 2019