Ecology of tropical freshwater fish assemblages in the Osa Peninsula and southwestern Costa Rica

Background/Question/Methods           The Osa Peninsula, Costa Rica’s last Wild-Land Frontier, has become one of the most threatened regions in Central America because of increasing anthropogenic changes, such as clearing forests for agriculture, ranching, logging, or gold mining. These processes alter the common land-use patterns surrounding tropical streams and each is resulting in the subsequent loss of biodiversity. In this study we examined the effects of both these anthropogenic changes and geographic distance between drainage basins to determine which factor(s) influence freshwater fish community composition across the Osa Peninsula. In addition, the fish community of the peninsula was compared with the community of neighboring mainland Costa Rica to elucidate differences in fish fauna between these two landmasses.
Fishes were sampled from 26 streams composing 16 different drainage basins, each influenced by one of three surrounding land-use patterns (pristine forest, agriculture/ranching, or tourist/rural communities). A sampling site was classified as pristine if over 70% of the surroundings were forested (both at sampling site and upstream). Over 6,000 individuals were observed/collected during the dry seasons of 2009-10, where both visual surveys and fishing events were used. Abiotic stream variables and GPS coordinates were recorded at each site. Satellite images were used to characterize land-use patterns.

Results/Conclusions

We found 28 total species in 13 families across all streams. Our results show that fish community composition differs across both geographical areas and different land-use patterns (especially agricultural areas vs. pristine forests). Pristine forests had the highest species richness values with an average of 8.5 species while agricultural streams averaged only four species (F = 7.26, p < .05). Our data also show that certain species were less common in agricultural streams than in pristine forests or rural/tourist communities. Multivariate analyses revealed that physical characteristics of streams, particularly the quality of riparian buffer zones, are very important determinants of fish community composition. Comparisons of community composition between drainages using Jaccard’s similarity index indicate that streams within a drainage are more similar to each other than to streams in different drainages. Our data also show that fish species composition in streams of the Osa Peninsula are more similar to each other than to communities in the mainland. Our results show that geographic separation and land-use patterns both play a role in determining community composition of streams, and for these reasons we found unique community assemblages in the Osa Peninsula, a threatened and rapidly changing region.