On mutualism, models, and masting: The effects of seed‐dispersing animals on the plants they disperse

Species interactions are context dependent, in that their direction and magnitude can vary across ecological conditions. For seed dispersal interactions—especially interactions between plants and seed‐caching animals—the direction of the interactions is often obscured because of seed mortality inherent in seed handling and the delayed effects of fitness benefits received by plants. It is, therefore, an open question in ecology to understand the ecological contexts under which seed dispersal interactions are facilitative, antagonistic or null. We review the fitness benefits of animal‐mediated seed dispersal, extend a recently published model to include negative density‐dependent effects, and review the feedback between seed production (with a focus on masting) and seed‐caching animal populations. Negative density‐dependent effects are pervasive and strongly affect the direction of plant‐seed‐disperser interactions, and including them into models will give a more accurate understanding of the direction of the interaction. Including negative density‐dependent effects also makes the interpretation of interaction more mutualistic since seed dispersers decrease seed densities. Additionally, there is substantial interannual variability in seed production in most nut‐producing plant species, and the lags between seed production and seed‐disperser population sizes complicate and limit inferences made about the direction of interactions in any given short‐term study. Synthesis. If we wish to know the direction of species interactions in real ecological communities, we need models that contain a minimum level of biological realism. For complex and long‐term phenomena such as context‐dependent species interactions we should embrace a multifaceted approach of short‐term field research, long‐term field research, simple models, and complex models to form a more robust understanding of the ecological problem of context dependency.