Are functional trait-mediated vital rates contingent on the environment?
Linking the multivariate phenotype to fitness is an essential step toward accurately predicting ecological responses to global change. Trait variation has been shown to be predictable along environmental gradients, but these analyses only indirectly link traits to fitness at one point in time, and assume that a space-for-time substitution will yield good predictions in this rapidly changing world. Quantitatively linking the multivariate phenotype to fitness components, such as probabilities of establishment, growth, survival, and reproduction, is data intensive, but is potentially powerful because it integrates temporal dynamics. However, the few studies that have analysed the relationships between traits and vital rates did not incorporate the environmental context despite the strong likelihood that trait–environment interactions will influence vital rates. In this talk we describe the theory of how trait–environment interactions affect individual fitness, and test the theory on a long-term dataset. Using census data from a network of long-term permanent plots we compiled survivorship data for over 15,000 individuals representing 47 herbaceous perennial species (10 graminoids and 37 forbs). We applied Cox mixed effects models to examine how traits, the environment, and their interaction, influence mortality risk across these individuals.
The risk of mortality was significantly affected with increasing or decreasing values of specific root length (SRL), but this was not the case for leaf nitrogen or plant height. At average values of leaf nitrogen, plant height, and SRL, mortality risk increased with increasing canopy cover and soil moisture. Importantly, significant interaction terms indicated that the relative order of these changes were not consistent across trait values or environments. Interactions plots reveal that dynamic patterns are possible in which trait-mediated vital rates shift along environmental gradients. Linking functional traits to demographic rates is central to the theoretical framework which underlies trait-based models of community assembly. Our results demonstrate that fitness is a function of complex trait-environment interactions. It is through these interactions that the functional trade-offs necessary to promote coexistences are possible. While understanding overall trait-rate relationships is fundamental to advancing trait-based community ecology, untangling the degree to which different traits confer fitness in contrasting environments is vital to refining its theoretical framework.