Species-specific responses to a variable environment can promote coexistence by the storage effect, relative nonlinearity, and the fitness-density covariance. These mechanisms depend on how the species respond to the environment rather than a direct measurement of the physical environment itself. Therefore, the way organisms experience the environment will interact with the actual physical environment to determine the pattern of environmental responses in space. The spatial pattern of environmental responses can have a profound effect on the strengths of coexistence mechanisms and species distributions. These environmental patterns may not be easily discernable to researchers. A simple model of the environment as a gradient may omit much about what is actually important to the organisms and may overlook large opportunities for coexistence. Here, we explore the implications of different spatial patterns of environmental responses. We use numerical models to measure how different patterns of environmental variation affect population distributions and the strengths of coexistence mechanisms. We compare these results with observations of natural population distributions.
Results/Conclusions
Numerical modeling shows a large increase in the strengths of coexistence mechanisms as the environment becomes less autocorrelated in space. This corresponds to organisms recognizing large variation in the environment between adjacent sites. These results hold provided that dispersal is not uniform and global. Very highly autocorrelated environments (smooth gradients) lead to strongly segregated population distributions and much lower values of variation-dependent coexistence mechanisms. Observations of aquatic insect populations show co-occurrence over large areas, suggesting that these species are responding to relatively uncorrelated environmental variation rather than experiencing the environment as a highly autocorrelated gradient. It appears that the common abstraction of streams as environmental gradients is not how the species themselves experience stream environments. The importance to the organisms of environmental variation occurring at small spatial scales likely has a positive effect on species coexistence.