Theoretical Ecology Lab Tea

Fall 2017 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

Wed. September 13^th – 12:30pm

Heterogeneous aspiration dynamics in well-mixed and structured populationsLei Zhou

Understanding how individuals update their strategies in social dilemmas is the basis to the evolutionary dynamics of cooperation. Previous models usually assume that all the individuals use the same updating rule and the rule is comparison-based. As it is well known, evolutionary outcomes induced by comparison-based updating rules are sensitive to the model details, e.g., whether cooperation emerges or not depends on the structure of the interaction network and which rule is used. However, recent experiments on human subjects suggest that the level of cooperation does not change qualitatively on different networks, and incorporating heterogeneity of personal decision-making into the evolutionary models is crucial to reproduce the experimental results. Here, we propose the use of heterogeneous aspiration dynamics in which individuals use their endogenous aspirations as a baseline for satisfaction; then based on the extent of their own satisfaction and decision-making rule, they revise their behavior. We found that, under the limit of weak selection, network structure does not change the average abundance of a certain strategy and the criterion for a strategy to outcompete the other in abundance is the same as that under a single updating function. Our results thus are in line with experimental findings in a more realistic scenario of heterogeneous criteria for changing of behavior. They also show that, when considering a given problem of cooperation, it may suffice to study the dynamics of homogeneous populations of individuals using an aspiration-based update, which then can easily be extrapolated to the heterogeneous case.

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Wed. September 20^th – 12:30pm

Carrot or Stick? Institutional Incentives in Games with Continuous StrategiesDaniel Cooney

This talk will provide an introduction to several approaches for studying evolutionary games with continuous strategy spaces, using the problem of overfishing and sustainable management of fisheries as a motivating example. We will consider the role of both peer-to-peer and institutional mechanisms of reward and punishment to incentivize sustainable fishing practices. Mathematically, we can analyze these social-ecological models using measure-valued ODEs and nonlocal transport equations.

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Wed. September 27^th – 12:30pm

Population dynamics constrain immune escape in human influenza A virusesDylan Morris

Seasonal influenza A H3N2 viruses are temporarily immunizing pathogens. When the virus's antigenic epitopes change, population immunity decreases. The genetic evolution of the virus is continuous, but major antigenic change is punctuated: substantially different new variants emerge every 2-6 years. These ''cluster transitions'' cause dramatic decreases in effective population immunity. Recent work has shown that a single substitution in one of 7 key amino acid positions can result in a cluster transition. Why then are immune escape events so infrequent? Models of influenza immune escape have either focused on within-host viral and immune kinetics or on population-level processes, but little attention has been given to the cross-scale interactions among within-host de novo mutant generation, host-to-host transmission bottlenecks, and population immunity. I explicitly model these factors and assess their impact on the tempo and pattern of influenza immune escape.

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Wed. October 4^th – 12:30pm

Voluntary vaccination dilemma with evolving psychological perceptionsXue Feng

Voluntary vaccination is a universal control protocol for infectious diseases. Yet there exists a social dilemma between individual benefits and public health: non-vaccinators free ride via the herd immunity from adequate vaccinators. This is due to the interplay between disease prevalence and individual vaccinating behavior. To complicate matters further, individual vaccinating behavior depends on the perceived vaccination cost rather than the actual one. The perception of vaccination cost is an individual trait, which varies from person to person, and evolves in response to the disease prevalence and vaccination coverage. To explore how evolving perceptions shape individual vaccinating behavior and thus the vaccination dynamics, we provide a model combining epidemic dynamics with evolutionary game theory which captures the voluntary vaccination dilemma. In particular, individuals adjust their perceptions based on the inertia effect in psychology and then update their vaccinating behaviors through imitating the behavior of a more successful peer. We find that i) vaccination is acceptable when the expected vaccination cost considering perception and actual vaccination cost is less than the maximum of the expected non-vaccination cost; ii) the evolution of perception is a “double-edged sword” for the vaccination dynamics: it improves vaccination coverage when most individuals perceive exaggerated vaccination cost, and it inhibits vaccination coverage in the other cases.

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Wed. October 11^th – 12:30pm

Jet Lag: Synchronization of Circadian Oscillators as a Mean Field GameChristy Graves

The Suprachiasmatic Nucleus (SCN) is a region in the brain that is responsible for controlling circadian rhythms. The SCN contains on the order of 10^4 neuronal oscillators which try to synchronize with each other as well as responding to external stimuli such as sunlight exposure. A mean field game model for these neuronal oscillators is formulated with two goals in mind: 1) to understand the long time behavior of the oscillators when an individual remains in the same time zone, and 2) to understand how the oscillators recover from jet lag when the individual has traveled multiple time zones. In particular, we would like to study the supposed east-west asymmetry of jet lag. Finite difference schemes are used to find numerical approximations to the mean field game solutions.

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Wed. October 18^th – 12:30pm

A matching mechanism for international environmental agreementsChai Molina

Matching contribution agreements --- similar to schemes to encourage donations in fundraising campaigns --- have been suggested as a mechanism to reduce global greenhouse gas emissions. In these agreements, countries first commit to rates at which they will match one another’s unconditional emissions abatements, over and above their own unconditional abatements. Based on these matching rates, each country then determines its own unconditional abatement. Boadway et al. (2011) showed that such agreements yield an efficient equilibrium, but assumed that at the business as usual (BAU) emissions levels, at least one country has an incentive to abate its emissions. This assumption contains a contradiction in terms, because BAU emissions levels are those that countries independently decide upon in the absence of an agreement.

We characterize when these agreements can still reduce emissions even if countries will not abate without an agreement. We show that when the functions describing countries' benefits from their own emissions are equal (but damages from global emissions may differ), these agreements are effective if small unilateral decreases in emissions from BAU are globally beneficial (even if they are detrimental to the abating country) --- that is, they work whenever we would like them to.

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Mon. October 23^rd – 04:30pm

Chemical mimicry or chemical crypsis–the evolutionary game played by parasitic ants invading other coloniesShinsuke Satoi

Certain ant species are specialised parasites that invade the nests of other ants and steal their food, larvae and eggs. To be successful, they must evade detection by patrolling hosts who attack invaders. Ants distinguish invaders from individuals of their own nest through the cuticular hydrocarbon profile, as their nestmates have a similar mixture of coating chemicals. To circumvent this, certain parasites adopt mimicry, using a mixture of chemicals that has a similar composition to that of their hosts, while others adopt crypsis, with a reduced amount of chemicals. Here we developed a mathematical model to describe the conditions under which each of these strategies evolves, assuming that the parasites and hosts are ants with their own colonies. Host ants distinguish their nestmates from parasites through differences in their chemical traits, which are represented in multi-dimensional space. Parasitic ants engage in competition with other conspecific colonies, which is more intense between colonies with similar chemical traits, jeopardising the advantage of cryptic parasites. We then define parasites’ fitness with respect to chemical profiles and discuss the evolution of their chemical strategies. Cryptic parasites evolve when competition among colonies is weak, when many types of host colonies exist and when host recognition accuracy is high. Mimetic parasites evolve under the opposite conditions.

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Wed. October 25^th – 12:30pm

Granger causality and time series: usefulness, an application, and limitationsBernat Guillen

I'll explain some concepts behind Granger causality and how to compute these concepts from data. I will also cover some issues coming from the data selection and processing pipeline.

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Wed. November 1^st – 12:30pm

Fall break - no Lab Tea

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Wed. November 8^th – 12:30pm

The endogenous origin of institutionsVitor Vasconcelos

The provision of institutions is a crucial mechanism to ensure cooperative behavior. However, they often appear as an exogenous mechanism when, in reality, they are ultimately created and sustained by the agents they act upon. In this talk, I will discuss ongoing work that explores the origin of institutions as an endogenous process and aims at unifying their different setups and explain the panoply of results on their efficacy and sustainability. Based both on the recent experimental designs and on an evolutionary game-theoretical model with learning, we propose a classification of the different setups according to two axes of variation: the level at which the choice of the institution is made, from individual to collective, and the range of the dilemma, from local to global. I will discuss which scenarios are more propense to cooperative behavior and what changes in both the way we approach the valuation of goods and decision making would lead to a change between scenarios.

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Wed. November 15^th – 12:30pm

Modeling Microbial Dynamics: Effects on Environmental and Human HealthJude Kong

In this talk, I will present two nonlinear models for microbial dynamics vis-a-vis human and environmental health that I formulated and the results I obtained from analyzing them. Firstly, I will present a stoichiometric organic matter decomposition model in a chemostat culture that incorporates the dynamics of grazers. This mechanistic biodegradation model leads to reliable and suggestive ecological insights in the preservation and restoration of our fragile ecosystems. Using the model, I answer the following research questions: (i) What mechanisms allow microbes and resources to persist uniformly or go extinct? (ii) How do grazing and dead microbial residues affect decomposition? Secondly, I will present a cholera transmission model that includes the dynamics of bacteriophage and bacteria (V. cholerae), and also contains an indirect infection term which accounts for a minimum infectious dose of the bacteria. Using this model, I determine what drives cyclical outbreaks of cholera in endemic regions and suggest ways by which such outbreaks can be prevented.

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Wed. November 22^nd – 12:30pm

Thanksgiving break - no Lab Tea

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Wed. November 29^th – 12:30pm

Evolution of public-goods provider in forests - symbiotic nitrogen-fixing treesWenying Liao

Plants' competition for the common-pool resource in soil, such as nitrogen, has significant impact on community assembly and ecosystem properties. Symbiotic nitrogen-fixing trees, capable of acquiring nitrogen from atmosphere instead of solely relying on soil pool, can contribute significantly to the public goods by their foliage input. In nitrogen-limited environment, plants tend to re-absorb leaf nitrogen during the process of leaf senescence to recycle the essential limiting nutrient. However, symbiotic nitrogen-fixing trees tend to re-absorb less leaf nitrogen compared to other non-fixing plants. Why do public-goods provider evolve, when their ''altruistic'' behavior benefit the fitness gain of competitors? Here I use a mechanistic and analytically tractable model to investigate the evolution of low leaf nitrogen re-absorption in symbiotic nitrogen-fixing trees.

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Wed. December 6^th – 12:30pm

Evolutionary Dynamics in a Group Population StructureOlivia Chu

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Wed. December 13^th – 12:30pm

Dynamics in Urban GardensTheresa Ong

Urban gardens are temporally dynamic systems, with many systems (e.g., the Victory Gardens in the U.S. following WWII), sprouting, then quickly disappearing in history, while others (e.g. the urban and periurban gardens of Cuba and Germany) seem more resistant to change. To understand why transitions to urban gardens are more permanent in some cases than others, I am developing a replicator-mutator model of land-use transitions where agents choose between a given set of land-use strategies (vacant lots, gardens and developed land) based off of rates of change (mutation) and perceptions of utility. I will present preliminary work on how relaxing static impressions of utility so that they can change with economic conditions lead to interesting dynamic consequences for urban gardens similar to those we see across different socio-political contexts.

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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
30. Spring 2016
31. Fall 2016
32. Spring 2017