Theory Tea

Fall 2023

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

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Wed. Jan 26^th – 12:30pm

A demographic perspective on diverse problems in theoretical biologyArne Traulsen

A single theoretical biologist is often working on different kinds of problems which for experimental scientists may have little connection to each other. However, theoretical research can often establish such connections by applying the same kind of methods. One such theoretical approach are demographic models, which track the dynamics of an age-structured population. When we think about ages as e.g. cell types in a tissue or positions on a network, we can take new perspectives on such systems and apply established insights from other fields. I will show how such an approach has helped me to learn more about cancer evolution, microbiome research, bacterial life cycle evolution and evolutionary dynamics on networks.

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Wed. Feb 23^rd – 12:30pm

A Diet for the PlanetMarta Tuninetti

The EAT–Lancet Commission has proposed a global benchmark diet to guide the shift towards healthy and sustainable dietary patterns. Yet it is unclear whether consumers’ choices are convergent with those guidelines. Applying an advanced statistical analysis, we mapped the diet gap of 15 essential foods in 172 countries from 1961 to 2018. We found that countries at the highest level of development have an above-optimal consumption of animal products, fats and sugars but a sub-optimal consumption of legumes, nuts and fruits. Countries suffering from limited socio-economic progress primarily rely on carbohydrates and starchy roots. Globally, a gradual change towards healthy and sustainable dietary targets can be observed for seafood, milk products, poultry and vegetable oils. We show that if all countries adopted the EAT–Lancet diet, the water footprint would fall by 12% at a global level but increase for nearly 40% of the world’s population.

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Wed. Mar 23^rd – 12:30pm

Evolution of informed dispersal in trophic meta-populationsYuriy Pichugin

Species inhabiting a meta-population have the opportunity to improve their conditions by migrating away from a poor quality patch to a better one. However, an accurate perception of the patch quality is a trait of critical importance, as migrating away from a high-quality patch will likely be detrimental to an organism. Such considerations are of particular importance for predators, for which the patch quality changes with time due to fluctuations in the amount of the available prey and the number of conspecific competitors. In this work, we propose a model of the evolution of informed dispersal for a predator species living on a meta-population. We investigate what kinds of dispersal decisions evolve depending on the information available to an individual: the number of prey, the number of other predators, or both. We found that in all cases, two classes of decision strategies evolve: discrete switch between zero and high dispersal rates and gradual ramp-like response to the sensed information. If the sensed information is the number of conspecific predators, the evolved dispersal strategy reduces the amplitude of population fluctuations. In other cases, the evolved dispersal behavior features spikes of mass emigration separated by longer periods without migrants being produced.

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Wed. Mar 30^th – 12:30pm

Coexistence in diverse communities with higher-order interactionsTheo Gibbs

A central assumption in most ecological models is that the interactions in a community operate only between pairs of species. However, the interaction between two species may be fundamentally changed by the presence of others. Although interactions among three or more species, called higher-order interactions, have the potential to modify our theoretical understanding of coexistence, ecologists lack clear expectations for how these interactions shape community structure. Here, we analytically predict and numerically confirm how the variability and strength of higher-order interactions affect species coexistence. We found that, as higher-order interaction strengths become more variable across species, fewer species coexist, echoing the behavior of pairwise models. If inter-specific higher-order interactions become too harmful relative to self-regulation, coexistence was destabilized, but coexistence was also lost when these interactions were too weak and mutualistic effects became prevalent. Last, we showed that more species rich communities structured by higher-order interactions lose species more readily than their species poor counterparts, generalizing classic results for community stability. Our work provides needed theoretical expectation for how higher-order interactions impact species coexistence in diverse communities.

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Wed. Apr 6^th – 12:30pm

Assembly archetypes in ecological communities.Marco Tulio Angulo

One instrumental discovery in developmental biology was the existence of design archetypes that synthesize the huge diversity of organisms' body plans into simple and general assembly rules. In this talk, I will discuss if similar "assembly archetypes" exist at the larger organization scale of ecological communities ---that is, a synthesis of what the rules observed when assembling a community across diverse environmental conditions fundamentally share. Importantly, unlike the design archetypes of body plans that can be synthesized by direct inspection using human senses, synthesizing assembly archetypes is fundamentally more challenging because ecological communities lack the necessary structure to allow our senses to perceive them directly. To circumvent this fundamental challenge, we built a mathematical formalism based on Category Theory that replaces our human senses and provides an abstract "synthesis operation" to identify assembly archetypes in ecological communities. Combining this formalism with experimental data shows that assembly archetypes exist in in-vitro and in-vivo microbial communities, like the human gut microbiota. Discovering assembly archetypes could pave the way to understanding general design principles of complex context-dependent ecological systems. This is joint work with Hugo Flores-Arguedas (UNAM Mexico), Omar Antolin-Camarena (UNAM Mexico) and Serguei Saavedra (MIT).

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Mon. May 2^nd – 1:30pm

Dynamic Stoichiometry in COBALT.George Hagstrom

Phytoplankton elemental stoichiometry determines the interactions between the carbon, nitrogen, and phosphorus cycles, but most biogeochemical models approximate these ratios as constants, known as the Redfield ratios, or treat them statically. However, observations now reveal systematic variations of the C:N:P of organic matter, exceeding Redfield in the subtropical gyres and falling below Redfield in high-latitudes. The use of Redfield stoichiometry leads to several shortcomings common across Earth Systems Models, including overexpression of phosphorus limitation and dysequilibrium in the nitrogen cycle. Further, the existence of environmental controls on C:N:P could lead to novel feedbacks coupling nutrients and temperature to export, which might alter the response of the carbon cycle to anthropogenic global warming. Here we developed a robust and computationally efficient extension of the COBALT ocean biogeochemical model that incorporates the dynamic C:N:P of phytoplankton by harmonizing COBALT with the ATOM model, a trait-based model based on optimal resource allocations which captures the primary physiological mechanisms behind C:N:P variation. We first used ATOM-COBALT within a series of global ocean-ice-ecosystem retrospective simulations to study how dynamic stoichiometry impacts patterns of nutrient limitation and biogeochemical cycles. Compared to static stoichiometry, ATOM-COBALT simulations reproduced observed variations in the N:P ratio of particulate organic matter, exhibited substantially reduced areas of phosphorus limitation near high-anthropogenic nitrogen inputs, and yielded enhanced rates of nitrogen-fixation. To understand the implications of the physiological mechanisms that control phytoplankton stoichiometry, we performed simulations using three sub-models each of which alters the strength of a single mechanism. These simulations revealed that both the growth rate hypothesis and phytoplankton frugality are needed to capture the stoichiometry of exported organic matter and patterns of nutrient limitation and nitrogen fixation, with frugality playing an essential role in capturing the response of marine ecosystems to high levels of anthropogenic N-inputs. The translation compensation mechanism had a different effect than expected, leading to altered nutrient utilization patterns and a surprising decrease in export and N:P ratios in certain regions. These results show how important dynamic phytoplankton stoichiometry is to biogeochemistry and support it's incorporation into the next generation of earth system models.

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Wed. May 4^th – 12:30pm

Competition Between Social Norms via Multilevel Selection.Daniel Cooney

Recent work in evolutionary game theory has focused on a variety of mechanisms for allowing the maintenance of cooperative behaviors within social groups when individuals always have an incentive to cheat. Two mechanisms that have frequently been studied are indirect reciprocity, where individuals reward cooperators and punish defectors through the use of social reputations and social norms, and multilevel selection, in which competition between groups can help cooperative groups to overcome the individual-level advantage of defectors. In this talk, we will start with a discussion of the evolution of cooperation via indirect reciprocity, exploring how the stern-judging, simple standing, image scoring, and shunning strategies can help to stabilize cooperation in populations with an initial cooperative cohort. We will then see how incorporating competition between groups can further increase the cooperation achieved by groups following these norms. Applying an approach using a system of partial differential equations, we also study the coevolution of strategies and social norms, allowing groups following different norms to compete with each other. We find the stern-judging and simple standing norms with public evaluation of reputations win out over the other norms under multilevel selection, and that private evaluations of these norms can overcome public scoring and shunning norms as well via multilevel selection. This approach allows us to compare the levels of cooperation achieved by groups following different norms, and may provide an understanding of how norms can compete with each other through their ability to achieve cooperative behavior in social groups.

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Wed. May 11^th – 12:30pm

Linking individual actions and social change.Kelly Finke

In the face of global, existential challenges – such as pandemic or climate change – calls for changes in individual action are often dismissed as trivial or ineffective. However, targeting small, repeated actions may provide an avenue to initiate the large-scale shifts in attitudes necessary to tackle such problems. Psychological theories of cognitive dissonance and self-perception predict that, just as our values, beliefs, and opinions influence our behaviors, the actions we take in turn affect these attitudes. Informed by this literature, we model a link between actions, which are reinforced by habit formation, and opinions, which are influenced by social conformity. Under what conditions can behavioral change be leveraged to break away from undesirable, self-reinforcing social norms?

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