Ecology Letters (2010) 13: 1160–1171
Species interactions come in a variety of forms, from weak to strong, and negative or positive, each with unique consequences for local community structure. However, interactions depend on several biotic, abiotic and scale-dependent variables that make their magnitude and direction difficult to predict. Here, we quantify the relative impacts of multiple factors on species interactions for a diverse array of intertidal organisms, using our own experiments across a range of environments in New Zealand and North America. Interaction strengths are related to organism body size and trophic level, but are relatively insensitive to environmental conditions associated with tidal height. Although species at higher trophic levels exert stronger per-capita effects on other taxa, their population-level impacts are equivalent to basal trophic groups. This indicates that interaction intensity is largely based on requirements for resources, such as food or space, that follow allometric scaling rules. These results demonstrate the potential to predict interactivity based on simple criteria without detailed information on particular species or communities.