Theoretical Ecology Lab Tea

Spring 2021

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

Wed. January 27^th – 12:30pm

Coevolution of actions, personal norms, and beliefs about others in social dilemmasSergey Gavrilets

Human decision-making is affected by a diversity of factors including material cost-benefit considerations, normative and cultural influences, learning, and conformity with peers and external authorities (e.g., cultural, religious, political, organizational). Also important are their dynamically changing personal perception of the situation and beliefs about actions and expectations of others as well as psychological phenomena such as cognitive dissonance and social projection. To better understand these processes, I develop a modeling framework describing the joint dynamics of actions and attitudes of individuals and their beliefs about actions and attitudes of their groupmates. I will discuss some results as well as implications for individual and group behavior.

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Wed. February 3^rd – 12:30pm

Coupled metapopulation dynamics with patch memory and modificationZach Miller

All organisms alter their abiotic environment, and these modifications impact other members of the biotic community. By shaping, for example, the spatial structure, resource availability, or pathogen prevalence in its local habitat, an individual of one species might enhance or diminish the future success of others. What will be the dynamics and outcomes when these environmentally-mediated interactions play out across a landscape? We introduce a simple yet flexible model to capture the dynamics of ecological communities where the state of a local patch is determined by its last occupant, and the ability of species to colonize a patch is determined by the patch state. Based in the metapopulation framework, our model can be used to describe a wide range of ecological scenarios, from the coexistence of tropical trees to immune-mediated pathogen interactions. We show that this model can exhibit diverse behaviors, including the robust coexistence of many "ecosystem engineers", and demonstrate some general conditions for coexistence to occur.

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Wed. February 10^th – 12:30pm

Works-in-progress: parasites, sex ratios, and other rapidly reproducing ideas Christie Riehl

My work is inspired by the diversity of reproductive and life-history strategies exhibited by birds. My lab primarily uses empirical approaches (lots of field work!) but all of our research is grounded in evolutionary theory, and our empirical results often lead to new conceptual questions. In this talk I'll discuss two recent projects in which our empirical work has tested theoretical predictions and, in turn, highlighted gaps that only theory can fill. I'll first present work on an obligate brood parasite, the Screaming Cowbird, a South American species that lays its eggs in the nests of one primary host species, the Baywing. Cynthia Ursino, a postdoc in my lab, tested the hypothesis that female cowbirds should avoid repeatedly parasitizing the same host nest (in order to reduce competition among nestlings). Empirical data supported this, but the observed patterns raise new questions about what the optimal pattern of parasitism really "should" be under different scenarios of competition among hosts and parasites. Second, I'll discuss an ongoing project on sex ratio bias in a cooperatively breeding bird, the Greater Ani. Trey Hendrix, a graduate student in my group, is using long-term data to test the hypothesis that cooperatively breeding birds should bias the sex of their offspring to produce more "helpers" (birds that help their natal group raise more offspring). Although Trey has found no support for this hypothesis, other intriguing patterns have emerged that suggest more nuanced adjustments of sex ratios. When it comes to sex allocation and cooperative breeding, the literature is rife with sloppy thinking and it could benefit from a theoretical reevaluation. My hope is that this talk will illustrate a few unsolved problems in behavioral ecology and inspire new connections between theory and empirical work in EEB.

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Wed. February 17^th – 12:30pm

Social matching algorithms and dynamics of polarization in online social networksFernando Santos

The level of antagonism between political groups has risen in the past years. Supporters of a given party increasingly dislike members of the opposing group, a phenomenon known as affective polarization. As a result, individuals may avoid inter-group interactions, leading to homophilic social networks. In online social platforms, however, the choice to connect with different individuals is often intermediated by social matching algorithms. These algorithms recommend new connections to users (e.g., “People you may know” or “Whom to follow”). The long-term impact of link recommendation is not trivial, particularly as exposure to opposing viewpoints was shown to have a dual effect: connections with out-group members can 1) lead to opinion convergence and prevent group polarization or 2) further cleave opinions. In this presentation, I will discuss a model of social influence over dynamic networks and speculate about the potential impacts of link rewiring based on structural similarity (i.e., sharing many neighbors). We will see that preferentially establishing links with structurally similar nodes results in network topologies that are amenable to opinion polarization. When networks are composed of nodes that react differently to out-group contacts, either converging or polarizing, we find that connecting structurally dissimilar nodes contributes to moderate opinions.

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Wed. February 24^th – 12:30pm

Investigating future SARS-CoV-2 infection dynamics and landscapes of immunityCaroline Wagner & Chadi Saad-Roy

The ongoing SARS-CoV-2 pandemic has caused at least 100 million confirmed infections and 2 million deaths so far. Vaccines are currently being deployed, and they will undoubtedly be key for disease control and mitigation. However, numerous uncertainties remain surrounding the strength and duration of natural and vaccinal immunity. In this talk, we examine possible future SARS-CoV-2 infection dynamics and `landscapes of immunity' under different assumptions related to the robustness of adaptive immune responses. First, we use a simple immuno-epidemiological model that tracks multiple immunity phenotypes, and find that longer-term disease dynamics are critically shaped by the strength and duration of natural and vaccinal immunity. Thus, clinical measurements of the strength and duration of host immunity are crucial for accurate longer-term projections and policy recommendations. Then, we refine the underlying framework and couple it to a simple phylodynamic model for antigenic evolution in order to investigate the epidemiological and evolutionary consequences of vaccine dosing regimes. In the short-term, we find that focusing on single doses is beneficial and decreases infections. In the longer-term, however, the epidemiological and evolutionary outcomes depend upon the robustness of vaccinal immunity conferred by a single dose. If this immunity is poor, widely spaced doses may lead to higher infection burdens and increased potential for antigenic evolution.

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Wed. March 3^rd – 12:30pm

How do substitutability and effort asymmetry change resource management in coupled natural-human systems?Woi Sok Oh

Our world is composed of diverse components that interact with each other and co-evolve. Therefore, it is required to couple dynamics of humans, natural resources, and infrastructure in managing natural resources. Many resource management models often start based on a single-resource condition due to the simplification. However, humans may have another option to substitute insufficient resources for their wellbeing. Once humans have multiple resources, they need to assign a different amount of their efforts to each resource. Assuming that multiple resources exist, this research focused on how substitutability and effort asymmetry would influence system responses, resource management, and system sustainability. I first expanded the existing conceptual framework and developed a stylized model with two resources, resource users, public infrastructure provider, and public infrastructure. Substitutability is mathematically incorporated into the model using constant elasticity of substitution (CES) production function from economics. Different levels of disturbance were also tested to understand system sustainability. The model showed that substitutability and effort asymmetry significantly affect policy flexibility, system performance, and sustainability in nonlinear ways. The results and analysis highlighted the challenges of coupled systems and provided insights on managing multiple resources.

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Wed. March 10^th – 12:30pm

Noise induced transitions and dynamics of forest-savanna ecosystemsDenis Patterson

Due to their tractability and relative ease of analysis, deterministic models based on ordinary differential equations are popular modeling tools in the applied sciences. More realistic models incorporate randomness, but this poses additional challenges in characterizing system behavior, particularly in systems with multiple stable states. For example, adding noise into a system’s dynamics may lead to noise-induced attractor switching or even create new stable states. I will discuss some recent work on the so-called landscape and flux theory from non-equilibrium statistical mechanics addressing these problems. This approach can serve as a useful tool in understanding stochastic dynamics in ecology and linking them with empirical data. The origin and nature of noise-induced responses may differ based on the modeling framework employed for a given problem. I will highlight some potential subtleties for spatially extended stochastic systems and show that the addition of different types of noise can produce quite different system responses. Throughout, the Staver-Levin model of tropical forest-savanna ecosystems will serve as our reference example to illustrate techniques and ground the discussion.

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Wed. March 24^th – 12:30pm

Ani Games: Using Agent-Based Modeling to Explore Alternative Reproductive Tactics in a Cooperatively Breeding BirdTrey Hendrix

Understanding how cooperative behaviors evolve despite their vulnerability to cheating is a fundamental question in evolutionary biology. Among cooperatively breeding birds, one of the most commonly observed instances of cheating is conspecific brood parasitism, where a female lays an egg in another female’s nest without providing parental care. A large body of research on conspecific brood parasitism has focused on the co-evolutionary arms race between host and parasitic females. However, why parasitic tactics emerge in the first place and how they coexist with non-parasitic tactics is not fully understood. For more than a decade, the Riehl lab has studied cooperatively breeding Greater Anis (Crotophaga major) and has recently documented two distinct reproductive tactics among females. One type of female – called a “pure cooperator” – does not parasitize other females’ nests while another type of female employs a “mixed” strategy in which she begins a breeding season by cooperatively breeding but, if her nest fails, she switches to laying eggs parasitically. Our empirical data suggest that these two strategies are highly repeatable among individuals and have approximately equal fitness payoffs. In this talk, I will discuss ongoing work to develop an individual-based model that captures the dynamics of the ani system and present preliminary simulation results. The goals of this research project are (1) to explore how different parameter values (particularly rates of nest predation) impact the fitness of each reproductive tactic and (2) to use evolutionary invasion analyses to provide insights into the origins of the two tactics. Specifically, I will discuss the impacts of egg costs, predator memory, and life-history tradeoffs on the dynamics of the model. Since this project is still in its early stages, any and all feedback (from modeling methodology to project rationale to next steps) is appreciated.

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Wed. March 31^st – 12:30pm

Resilience offers escape from trapped thinking on poverty alleviation Steven Lade

The poverty trap concept strongly influences current research and policy on poverty alleviation. Financial or technological inputs intended to “push” the rural poor out of a poverty trap have had many successes but have also failed unexpectedly with serious ecological and social consequences that can reinforce poverty. First, we review commonly observed or assumed social-ecological relationships in rural development contexts, focusing on economic, biophysical, and cultural aspects of poverty. Second, we develop a classification of poverty alleviation strategies using insights from resilience research on social-ecological change. Last, we use these advances to develop stylized, multidimensional poverty trap models. The models show that (i) interventions that ignore nature and culture can reinforce poverty (particularly in agrobiodiverse landscapes), (ii) transformative change can instead open new pathways for poverty alleviation, and (iii) asset inputs may be effective in other contexts (for example, where resource degradation and poverty are tightly interlinked). One goal of our model-based approach is to legitimise within economic modelling communities underrepresented poverty alleviation pathways such as food sovereignty.

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Wed. April 7^th – 12:30pm

Interaction of immune boosting with short- and long-term antibody kinetics of childhood infectionsLuojun Yang

Duration of immunity is central to understanding infectious disease dynamics and vaccine success, but the drivers of immune kinetics remain largely unknown. While both infection and vaccine-induced immunity to measles and rubella have been considered lifelong, reported infections among vaccinated individuals and observed decline in antibody titers raise questions about the durability of measles and rubella immunity in a post-endemic world. By unifying serological and epidemiological data across individual and population scales, we show that natural immune boosting by re-exposure to circulating viruses is likely an important driver of long-term humoral immunity to measles and rubella. We use statistical and mechanistic models to estimate the duration of immune boosting and to predict long-term patterns of population immunity, which allows us to establish that the model outputs are consistent with published experimental and serological studies. Our findings suggest that an immunity gap among older individuals could be a future barrier to elimination of vaccine-preventable diseases.

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Wed. April 14^th – 12:45pm

Understanding Species Abundance Distributions in Complex Ecosystems of Interacting Species Jim Wu

Niche and neutral theory are two prevailing, yet much debated, ideas in ecology proposed to explain the patterns of biodiversity. Whereas niche theory emphasizes selective differences between species and interspecific interactions in shaping the community, neutral theory supposes functional equivalence between species and points to stochasticity and immigration as primary drivers of ecological dynamics. In this work, we draw a bridge between these two opposing theories and investigate how the shape of species abundance distributions (SAD) is influenced by the relative strength of various niche and neutral processes. Starting from a generalized Lotka-Volterra (gLV) model with demographic noise and random symmetric interactions, we analytically derive the stationary population statistics and SADs using methods from statistical physics. We show that the SADs fall into three distinct classes commonly found in many ecosystems: log-normal, Fisher-log series, and multimodal distributions . When immigration is the primary driver of community structure, we find log-normal SADs. However, when demographic noise dominates over immigration, then the structure of the SAD depends on the strength of competition. Fisher log-series SADs are indicative of ecosystems with large amounts of competition and noise, whereas multimodal distributed SADs occur in ecosystems with low amounts of competition and/or mutualistic interactions. These results explain the prevalence of neutral SADs in complex ecosystems and also suggest that SADs can be predicted based on the primary ecological process driving community assembly and structure.

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Wed. April 21^th – 12:30pm

Cross-Scale Management of Climate Risks among Smallholder Farmers Nicolas Choquette-Levy

Over the next few decades, climate change poses significant risks for many of the world's 500 million smallholder farming households. These risks accentuate the already high uncertainty regarding the viability of traditional farming livelihoods. In this presentation, I will discuss ongoing work to investigate cross-scale interactions between climate risk management strategies - including rural-urban migration, informal risk-sharing mechanisms, and formal insurance policies. I will briefly summarize results from an agent-based model that investigated Nepali farming households' climate adaptation strategies under different climate and policy scenarios. Then, I will introduce new work to further examine the links between risk management strategies across decision-making scales through an evolutionary game theoretic framework. I hope to benefit from the broad expertise of the Lab Tea community on these questions and methods!

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Wed. April 28^th – 12:30pm

Exploring the Influence of Perception, Experience, and Social Context on Learning ProcessesMadeleine Andrews

Learning is a ubiquitous process, extending from single celled organisms to vertebrates. In behavioral ecology, learning generally refers to an “update” in behavior – this simplification ignores the fact that experience and memory not only affect the necessity of learning novel behaviors, but can also cause updates to one’s internal state that may lessen the cost of future learning. Furthermore, experience can affect not only the quality of future decisions, but also the quality of future learning; the environmental context one occupies can affect what needs to be learned, how best to learn, or the reliability of learned information. The ability to flexibly shift between different learning rules based on specific features of the physical and social environment is a potential mechanism by which learning can be more sensitive to a dynamic environment. While experience and learning sensitivity have been included in some models of learning rule evolution, they are largely missing in the context of understanding individual and social learning choice. Incorporating the value of experience in changing environments and learning sensitivity across environments may allow us to understand the dynamics of social and individual learning with more nuance on the behavioral timescale. In this talk, I will begin by discussing a general framework of learning that incorporates more sensitive behavioral and evolutionary feedbacks. Next, I will briefly describe two modeling frameworks looking specifically at the effects of experience and learning sensitivity on learning performance, respectively.

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Wed. May 5^th – 12:30pm

Detecting the origin and following the evolution of cancer: a case study in myeloproliferative neoplasmsMaximilian Nguyen

There is a growing body of evidence that some cancers slowly develop over long periods of time before manifesting as a distinct disease. Determining the precise age when a cancer first arises in the human body had remained previously elusive. Blood cancers known as myeloproliferative neoplasms (MPNs) are thought to originate when a driver mutation is acquired by a hematopoietic stem cell (HSC). In this talk, we will discuss how a mathematical model of stem cell self-renewal based on the Wright-Fisher model can be used to infer both the time-of-origin of the cancer’s driver mutation and the fitness of the cancer mutant. Using approximate Bayesian computation in combination with lineage trees obtained from whole-genome sequencing, we found that the driver mutations occurred in a single HSC several decades before MPN diagnosis—at age 9 ± 2 years in a 34-year-old individual and at age 19 ± 3 years in a 63-year-old individual—and found that mutant HSCs have a significant selective advantage in both individuals.

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Wed. May 12^th – 12:00pm

Stereotypes: The effect of generalizing reputations on cooperative behaviorMari Kawakatsu & Sebastián Michel-Mata

A stereotype is a(n over-)generalized belief about a social group with some shared attributes. While stereotyping might provide a cognitively inexpensive heuristic for social interactions, it can lead to discrimination, prejudice, illusory correlations between group membership and behavior, and inaccurate homogenization of heterogeneous groups. In this talk, we will discuss an ongoing project exploring the effects of stereotyping on cooperative behavior. Our model adopts the framework of indirect reciprocity, where individuals decide how to act based on their perceived reputations of others. We consider a population with two discernible groups and introduce a stereotyping mechanism in which the perceived reputation of one member of a group is generalized to all others in that group. We will present preliminary analysis on the impact of this mechanism on cooperation and reputation dynamics within and between groups.

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Wed. May 19^th – 12:30pm

Dynamic range models for species on the moveAlexa Fredston-Hermann

Understanding why species are found where they are, and what makes them move, is a central challenge in ecology and biogeography. This challenge has taken on new urgency in the Anthropocene as species shift their ranges in response to climate change. To manage and conserve species on the move, stakeholders need accurate, near-term predictions of future range shifts. I will present a dynamic range model — a process-based approach where historical occurrences and abundances are used to estimate latent demographic rates and simulate future states — designed to fill this knowledge gap. This project is a partnership with the Mid-Atlantic Fisheries Management Council to build 1-10 year range forecasts for four species of importance to commercial and recreational fisheries. The model is implemented as a Bayesian hierarchical model in Stan. In this talk, I will describe the model structure, our challenges fitting it to real data, and some preliminary results on summer flounder.

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Links to previous schedules

1. Fall 2000
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