A large-scale study of competition of two temperate reef fishes: temperature, functional diversity, and regional differences in dynamics

The dynamics of populations are influenced by competition, both within species and among species. However, the strength of both forms of competition may be modified by environmental factors such as temperature and food availability, and it is not always clear how competition varies throughout a broad geographic range. We examined competition within and between 2 species of temperate reef fish, the black surfperch Embiotoca jacksoni and the striped surfperch E. lateralis. Using data collected by an organization of citizen scientists (Reef Check California), we measured competition by analyzing (co) variation in time-series estimates of densities for both species at many locations along the California coast (86 sites spread across similar to 1050 km). We examined whether competition varied between Northern California (a region characterized by cold water and high food availability) and Southern California (a region characterized by warm water and low food availability). Strength of competition varied between regions, but not as expected. Rather than an intensification of competition with temperature, regional differences in competition resulted from differences in how populations of each species used resources. For black surfperch, intraspecific competition was greater in the north, where populations exhibited greater spatial crowding and ate a narrower variety of prey items than populations in the south. Striped surfperch displayed the opposite pattern. Northern populations experienced weaker intraspecific competition, exhibited less crowding, and ate a greater diversity of prey items than southern populations. Competition and the resulting dynamics of these populations may be more sensitive to functional diversity than to external factors.