Different aspects of single traits structure diversity differently: An example using annual plant germination
Temporal environmental variability is pervasive in nature and plays an important role in structuring diversity. Environmental variability maintains diversity in competitive communities if species acquire resources differentially in time. This temporal variability is also likely to shape species’ life histories and competitive traits. For some, like annual plants, their activity is largely determined by a single life history event, germination, which is closely tied to the environment. But germination is a complex trait that reflects myriad factors associated with species biology. Such complex traits can serve to differentiate competitors in multiple ways. In this study, we ask whether multiple avenues of differentiation with respect to the environment are comparable in their effects on coexistence. To answer this question, we model population growth for two species of annual plants whose germination rates are Gaussian with respect to temperature at rainfall, which is variable across growing seasons. Species may differ from one another in two distinct and independent ways: the temperature at which a species germinates most readily and the relative sensitivity to temperature variation. To measure the relative effect of each germination component on coexistence, we approximated the invader growth rate and partitioned effects into equalizing and stabilizing components.
Differences between species in both the optimal temperature to germinate and sensitivity to temperature variation play a role in both the relative performance of species on average and density-dependent processes. Surprisingly, differences between species in the sensitivity to temperature variation had the strongest effect on average competitiveness. For example, adopting a strategy of increased sensitivity is generally only favorable when a species has an optimal germination temperature that is mismatched with the average temperature across years. This is likely because sensitivity to temperature variation trades off with germination fraction under optimal germinating conditions. Differences between species in optimal germination temperatures largely affect the storage effect. This occurs largely through changes in the magnitude of the covariance between environment and competition. These results provide insight for possible adaptive changes in germination biology. Based on this analysis, species that differentiate in sensitivity to the environment will tend not to increase diversity in a community. In fact, superior competitors that continue to differentiate may exclude other species. In opposition, adaptive differences in optimal germination temperature are expected to promote diversity in communities. Such results serve as a first step into predicting adaptive evolution in traits affecting resource use through environmental variation.