Evaluating Janzen-Connell dynamics as a potential driver of evolutionarily stable coexistence

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Matthew L Bradley, Computational Biology, University of Texas at Austin, Austin, TX, Robin Decker, Damla Cinoglu and Caroline Farrior, Department of Integrative Biology, University of Texas at Austin, Austin, TX

Background/Question/Methods
The Janzen-Connell hypothesis (JCH) posits that specialist enemies can maintain plant diversity by reducing the survival of abundant species. For instance, plant pathogens can increase conspecific rates of mortality near a parent, increasing the likelihood that other species will grow in nearby open space. Evidence has shown that the ingredients for Janzen-Connell do exist for some plant species, but the theoretical question remains whether ecological and evolutionary processes, governed by realistic physiological tradeoffs, are likely to generate species with those ingredients. We use adaptive dynamics to study whether the Janzen-Connell hypothesis can explain the ultimate (and not only proximate) causes of forest community diversity.

We asked, will simulating adaptive dynamics of plant-pathogen interactions governed by realistic physiological tradeoffs result in the coexistence of plant species that undergo Janzen-Connell effects?
Results/Conclusions
We develop a spatial Janzen-Connell model and use adaptive dynamic techniques to examine if the Janzen-Connell Hypothesis can explain both ecologically and evolutionarily stable diversity among traits. As a foundation, we model plant-pathogen interactions to explore the effects of Janzen-Connell on population dynamics, age structure, and density of competing tree species. We have found, in our simulations, that Janzen-Connell dynamics reduce conspecific density among plants in ecosystems with several species, maintaining diversity, as expected. We have also found that Janzen-Connell dynamics can cause a change in age structure, decreasing the proportion of trees that survive to adulthood. Moving forward, we will examine more specifically the fitness landscapes created by the interactions between varying strategies of both plants and their pathogens and how these landscapes evolve over time. We will simulate plant species with a range of physiological tradeoffs such that a plant that is more able to resist pathogens is less able to reproduce or disperse seeds effectively. Similarly, we will also introduce physiological tradeoffs into our enemy species, specifically we will create a range of host-specificity in the pathogens, such that a pathogen that is more host-specific will also be better able to spread among those hosts, whereas a generalist pathogen, while having more potential hosts, will be less efficient at spreading to them. By using adaptive dynamic techniques, we will evaluate whether these tradeoffs can lead to species differentiation that then allows them to coexist via Janzen-Connell dynamics.