We analyzed the current status and historical trends of environmental quality in freshwater ecosystems in Central West Mexico (Colima and Jalisco states) using a fish-based index of biotic integrity (IBI) as framework. The IBI is a bioassessment tool that analyzes compositional, structural, and functional data from a biological community to infer ecosystem health. Using data from over 230 collections made between 1955 and 2008 at more than 140 sites in both lotic and lentic environments, and ecological guild information for more than 100 primary, secondary and peripheral species, we were able to describe how ecosystem health has changed through time, and define the current status of freshwater ecosystems in these two Mexican states.
We detected a significant increase in the number and geographic spread of exotic species through time, and a decrease in the number and abundance of native species, especially carnivores and benthic species. The most frequently encountered exotics in the area were tilapias of the genus Oreochromis. These changes translated into losses of biological integrity for many sites, but other sites have maintained their environmental quality through time. Regionally, sites located near or in urban and agricultural areas are much more likely to have had substantially important declines in quality, than those located in protected and/or mountainous regions. Specifically, sites located in relatively high gradient streams and rivers draining directly to the Pacific Ocean had a comparatively better environmental quality that other sites in the region.