Rathert D., Denis White, J. Sifneos, and R.M. Hughes. 1999. Environmental associations of species richness in Oregon freshwater fishes. Journal of Biogeography.26(2):257-274.
The purpose of this study was to better understand how environmental factors affect fish species richness across the state of Oregon, USA. A database showing collection locations of 4,911 fish specimens in the Oregon State University museum was modified by the Oregon Natural Heritage Program to include probable occurrences, and mapped within a grid of 375 hexagons that cover the state. The individual species maps of freshwater fish in Oregon were reviewed and revised by 30 regional fish biologists and then synthesized into a single map of native species richness. We used regression tree analysis (RTA) and multiple linear regression (MLR) to assess patterns of fish species richness with 20 environmental, 3 anthropogenic, and 2 historical variables. RTA explained 66% of the variation in native species richness, associating richness with annual air temperature range, minimum January temperature, introduced species richness, and stream density. MLR explained 68% of native species richness variation and associated richness with maximum July temperature, air temperature range, standard deviation of monthly temperature, stream density, introduced species richness, and basin connectivity. We conclude that for these data and at this scale, native fish species richness in Oregon is associated with annual climatic extremes, spatial variability of climate, stream density, basin connectivity, and introduced fishes.