How well can body size represent effects of the environment on demographic rates? Disentangling collinear explanatory variables

Background/Question/Methods

Demographic rates are shaped by the interaction of past and current environments that individuals in a population experience. Past environments shape individual states via selection and plasticity. Easily observed fitness-related traits (e.g., individual size or mass) are commonly used in demographic analyses to represent the effect of past environments on demographic rates.  Here we tested this assumption and quantified how well the size of individuals captures the effects of a population’s past and current environments on demographic rates in a well-studied experimental system of soil mites. The interdependencies of individual size and environmental quality cause explanatory variables to be collinear. We decompose these interrelated sources of variation with a novel method of multiple regression and plot the results with area-proportional Venn diagrams.

Results/Conclusions

We show that body size encompassed almost all of the explanatory power of the environment for growth, a quarter for stage transition, a third for probability of reproducing, and half for fecundity. In the soil mites’ income breeding life-history, reproduction depended more on the environment than on body size. This demonstrates that the strength of size as a proxy for the past environment can vary widely among life-history processes within a species.