Three main projects are the focus of our work now:

Climate change and community structure: Understanding the interplay between species age and community age is important for determining what processes structure communities. In many cases, communities have been assembled much more recently than the speciation events which have generated the species filling the communities. Although species age sets an upper limit on how long species could have interacted, how long communities have actually been assembled is key for determining the extent to which local interspecific interactions have shaped the evolution of phenotypic differences allowing species to coexist. Latitude is an important determinant of how long communities have been assembled. Because of glaciation, northern communities are often more recently assembled than their southern counterparts. As a result, species in these communities have had less time to locally interact and niche differences evolve allowing for coexistence. This project is therefore testing the hypothesis that northern communities are structured via neutral dynamics, whereas southern communities show the signature of niche-based coexistence. Understanding how past climate change shaped the structures of communities with different evolutionary histories will allow us to better understand and predict responses to similar changes in the future. This work is funded by the NSF.

Temporal variation in niches and the maintenance of species diversity: Insight into the problem of the maintenance of species diversity has centered on disentangling niche from neutrally structured communities. Temporal variation in the extent to which niches and neutrality may jointly structure communities makes the problem even more daunting and is a largely unresolved problem. We are gaining insight into the signature of such a temporally variable system by developing mathematical models to understand how rare niches affect species diversity within communities. Preliminary results indicate that although increasing the occurrence of rare niches increases coexistence, rare niches can also cause competitive exclusion. These contrasting outcomes appear to depend largely on the relative abundances of species and the timing between the occurrence and magnitude of resources generating niche differences. We are also extending this modeling approach to gain insight into how spatial variation in rare niches may affect community structure.This work is funded by the NSF.

The ecology of natural selection: A substantial body of empirical work has shown that natural selection is common in nature. However, general features shaping the temporal and spatial dynamics of selection and the role of ecology in determining these dynamics have yet to be identified. I am leading collaborative projects where we have assembled a database of over 9,000 selection coefficients from temporally and spatially replicated studies of selection in the wild. Our approach is to use Bayesian hierarchical modeling methods combined with climate databases to uncover general patterns in the ecological determinants of selection. Results to date show that temporal and spatial variation in selection is common, however, surprisingly this variation may have little impact on adaptive evolution.  In principal, these findings will allow us to develop a predictive framework for how suites of species may experience changes in natural selection in response to future changes in climate. This work is funded by a NESCent working group I co-organized.

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