Trophic resource partitioning among nine sympatric congeners in an ancient tropical lake (Mexico)

The maintenance of species diversity relies on factors such as competition, factors related to morphology, and the partitioning of spatial, temporal and trophic resources. Central Mexico's tropical Lake Chapala provides a rich diversity of nine sympatric congeners of Chirostoma which allows for studies related to the maintenance of species integrity. This research investigated factors of trophic partitioning, spatial distribution and the relationship of morphology to diet to reveal potential forces underlying the maintenance of species integrity. I investigated (1) correlations between dietary composition and morphology; (2) differences and partitioning of dietary resources; (3) ontogenetic influences on resource use; and (4) aspects of distribution and abundance. Results indicated that Jordani Group chirostomids are evenly distributed in Lake Chapala, while Arge Group species are skewed towards the eastern region. Morphologies have remained consistent over a 100 year period. Correlations between single morphological characteristics and dietary composition were not significant. Differences in dietary composition and the amounts of prey consumed were not observed on the basis of gender. Twelve major food categories were identified, all of which were shared by the nine chirostomid species. Initial analyses indicated that the diets were broad with extensive overlap. More rigorous discriminant function analyses identified significant differences among the diets of the nine species. Although significant differences among the diets of the nine chirostomid congeners were detected, competition appeared to be dampened. High productivity, both primary and bacterial, combined with anthropogenic stresses such as over-fishing, human-induced declines in water quality and increased water demands have dampened competition for trophic resources among the nine sympatric chirostomid congeners.