Demographic trade-offs in comparative ecology

Background/Question/Methods

Generalizing the effect of traits on performance across species may be achievable if traits explain variation in population fitness. However, testing relationships between traits and individual vital rates to infer effects on fitness can be misleading. Demographic trade-offs can generate variation in vital rates that yield equal population growth rates, thereby obscuring the net effect of traits on fitness. To address this problem, I will describe a diversity of approaches to quantify intrinsic growth rates of plant populations, including experiments beyond range boundaries, density-dependent population models built from long-term demographic data, theoretical models, and methods that leverage widely available monitoring data. I will provide a specific example using a long-term dataset from the Colorado shortgrass steppe, where we analyzed the relationships between leaf turgor loss point (TLP) and both survival and growth rates across dry and wet years.

Results/Conclusions

After accounting for the effects of individual size and local neighborhood competition, we found a significant trait-by-environment interaction driving variation in survival and growth rates. In dry years species with low leaf TLP had higher survival rates than species with high TLP; whereas in wet years species with higher leaf turgor loss point (TLP) had higher survival rates than species with low TLP. As predicted from demographic trade-off theory, the trait effects on growth rates were reversed. In dry years, species with low TLP grew less than species with high TLP; whereas in wet years species with low TLP grew more than species with high TLP. These non-intuitive results highlight the importance of accounting for demographic trade-offs when analyzing the effects of traits on fitness in contrasting environments.