Bassar Lab

This first paper provides an overview of theory and empirical research

Bassar, R. D., et al. 2010. Bridging the gap between ecology and evolution: integrating density regulation and life history evolution. The Year in Evolutionary Biology.

Early demographic models of life history evolution were formulated in a density-independent framework and saw extrinsic sources of mortality, such as predation, as the primary driving force that shaped the evolution of life history traits. The evidence for density-dependence in nature, motivated theoreticians to build models that incorporated population regulation. These later generations of models acknowledge that demographic mechanisms of population regulation and extrinsic mortality interact with one another, and predict a wide variety of life-history evolutionary responses. Such ecologically realistic models require knowledge of the demographic traits and life-stages most affected by density, and their effect back on the demography. Despite the vast empirical literature characterizing population regulation, and a wealth of methods to analyze it, such mechanistic understanding is rare. Ecological experiments whereby density is manipulated can be a powerful tool to disentangle the life-history determinants of population regulation. Here we review published density-manipulation experiments and highlight how they can be coupled with existing analytical tools to extract the mechanistic information needed for evolutionary models of life histories.

Bassar, R. D., et al. 2013. Experimental evidence for density-dependent regulation and selection on Trinidadian guppy life histories. The American Naturalist.

Recent study of feedbacks between ecological and evolutionary processes has renewed interest in population regulation and density-dependent selection because they represent black-box descriptions of these feedbacks. The roles of population regulation and density-dependent selection in life history evolution have received a significant amount of theoretical attention, but there are few empirical examples demonstrating their importance. We address this challenge in natural populations of the Trinidadian guppy (Poecilia reticulata) that differ in their predation regimes. First, we tested whether natural populations of guppies are regulated by density-dependence and quantified in which phases of the life cycle the effects of density are important. We found guppies from low predation (LP) environments are tightly regulated and that the density-dependent responses disproportionately affected some size classes. Second, we tested whether there are differences in density-dependent selection between guppies from LP or high predation (HP) environments. We found that the fitness of HP guppies is more sensitive to the depressant effects of density than the fitness of LP guppies. Finally, we used an evolutionary invasion analysis to show that depending on the effect of density on survival of the HP phenotype, this greater sensitivity of the HP phenotype to density can partially explain the evolution of the LP phenotype. We discuss the relevance of these findings to the study of feedbacks between ecology and evolution.

Bassar, R. D., et al. 2015. Population size-structure-dependent fitness and ecosystem consequences in Trinidadian guppies. Journal of Animal Ecology.

The vast majority of theory and empirical research on these eco-evolutionary feedbacks has focused on interactions among population size and mean traits of populations. However, numbers and mean traits represent only a fraction of the possible feedback dimensions. Populations of many organisms consist of different size classes that differ in their impact on the environment and each other. The goal of this study was to test the role of size structure in eco-evolutionary feedbacks of Trinidadian guppies. We used a factorial experiment in mesocosms wherein we crossed high and low predation guppy phenotypes with population size structure. We tested the ability of changes in size structure to generate selection on the demographic rates of guppies using an integral projection model (IPM). We found a significant interaction between guppy phenotype and the size structure treatments for absolute fitness. These results indicate an important role for size structure partially driving eco-evolutionary feedbacks in guppies. Overall, these results demonstrate size structure as a possible dimension through which eco-evolutionary feedbacks may occur in natural populations.

El-Sabaawi, R. W. and R. D. Bassar, et al. 2015. Intraspecific phenotypic differences in fish affect ecosystem processes as much as bottom–up factors. Oikos.

Whereas several studies have shown that evolution can affect ecological processes as much as commonly studied biotic (top-down) ecological variables, currently we do not know how the effects of evolution compare to bottom-up (abiotic) factors, or whether the effects of evolution are sensitive to abiotic conditions. Using a factorial mesocosm experiment we compared the ecosystem effects of guppy (Poecilia reticulata) life history phenotypes in two light treatments representing a three-fold difference in light levels, which is comparable to upstream-downstream differences in light availability in Trinidadian streams.  Light had a stronger effect on most guppy life history traits, somatic growth rates and guppy fitness than differences between phenotypes. The effect of light on these traits was consistent with higher availability of guppy food resources in the high light treatments.

Auer, S. K., A. Lopez-Sepulcre, T. I. Heatherly, T. Kohler, R. D. Bassar, S. A. Thomas, and D. N. Reznick. 2012. Life histories have a history: effects of past and present conditions on adult somatic growth rates in wild Trinidadian guppies. Journal of Animal Ecology.

Walsh, M. R., D. F. Fraser, R. D. Bassar, and D. N. Reznick. 2011. The direct and indirect effects of guppies: implications for life-history evolution in Rivulus hartii. Functional Ecology.

Predators reduce prey abundance and increase food to survivors.  Such indirect effects may also cause evolution.  Predictions from theory that model how life histories evolve in response to increased mortality rates often change when they include indirect effects.  Thus, indirect effects need to be evaluated to couple theory with natural systems.  Trinidadian killifish, Rivulus hartii, are found in communities with and without guppies (Poecilia reticulata).  Rivulus densities decline when guppies are present, which may be due to size-specific competitive or predatory interactions with guppies that increase Rivulus mortality rates.  We previously showed that Rivulus from sites with guppies begin reproduction at a smaller size and have increased reproductive allotment compared to Rivulus from sites with just Rivulus.  Such divergence is inconsistent with theory that considers increased juvenile mortality, but is consistent with frameworks that incorporate potential indirect effects of guppies.  We explored the mechanism of divergence with mark-recapture studies that compared the population biology of Rivulus between communities that are and are not sympatric with guppies.  Rivulus were 50% less abundant when guppies were present but guppies were not associated with differences in adult survival.  Related experiments show that the declines in density are likely due to guppy predation on young/small Rivulus.  Rivulus with guppies grow >3x faster than those upstream, above waterfalls that excludes guppies.  When Rivulus are transplanted from above to below this barrier, their growth accelerates to match the residents.  This response argues that the differences in growth are mediated by an environmental factor, likely lower population densities, which allow Rivulus from sites with guppies to grow faster.  These results imply that an indirect effect of the presumed agent of life history selection can improve the fit of empirical findings with theoretical predictions.