Effects of habitat isolation on species turnover and community nestedness in a desert river system

Background/Question/Methods

Habitat connectivity is a vital and dynamic element for persistence and maintenance of fish communities in desert river systems. The Rio Grande and its tributaries in the Trans-Pecos region of Texas have been impacted by a variety of anthropogenic activities including dewatering and the introduction of non-native species.   Although the tributary systems are naturally fragmented and were historically populated with spring-adapted native fish species, they now have to contend with introduced nonnatives such as the green sunfish and plains killifish.  The river, a shadow of its former self, may provide an important dispersal corridor for native and non-native fishes in the region.  It is imperative to understand how habitat connectivity and non-native species impact fish assemblages in the Rio Grande and its spring-fed tributary habitats. We hypothesize that 1) connectivity between the Rio Grande and tributary habitats can influence colonization and extinction rates of certain fish taxa in the tributaries, and 2) nested patterns of community structure can be disrupted by nonnative species due to differences in their dispersal abilities and ecological tolerances. To understand these processes, we examined the colonization/extinction dynamics and nested subset patterns of fish assemblages in the Rio Grande and its spring-fed tributary habitats, and the relationships with habitat isolation and environmental variability, using univariate and multivariate techniques.

Results/Conclusions

As habitat isolation from the Rio Grande increased, the colonization and extinction rate decreased and temporal environmental variability increased. This result indicated that certain tolerant fish taxa permanently persisted in the isolated refugium habitats with high environmental fluctuation.  Fish assemblages had significant nested subset patterns along the habitat isolation gradient. However, certain nonnative fish taxa were idiosyncratic, reducing the strength of the nested subset pattern. Deviation from the nested subset pattern may be attributed to randomness associated with artificial introduction of non-native fish taxa or difference in response of native and nonnative species to environmental variability. We suggest that reduction of the habitat connectivity by the change of flow regime (e.g., decrease in flow volume and flood frequency) in the Rio Grande and tributaries may cause increased local extinction rates of certain native fish taxa and accelerate the dominance of tolerant species or nonnative fish taxa, leading to altered fish assemblages in the region.