Fragmentation in stream networks: quantification, consequences, and implications to decline of native fish fauna

Abstract

Habitat fragmentation and loss threaten global biodiversity, but organism responses to changing habitat availability are mediated by structural properties of their habitats. In particular, organisms inhabiting dendritic landscapes with hierarchically arranged branches of habitat tend to have limited access to some patches even in the absence of fragmentation. Consequently, organisms inhabiting dendritic landscapes such as streams respond strongly to fragmentation. Using a combination of meta-analysis, field observations, and ecological network modeling I show that stream fishes respond to fragmentation in predictable ways. First, I addressed how dams and stream dewatering have created a mosaic of large river fragments throughout the Great Plains. Using a geographic information system and literature accounts of population status (i.e., stable, declining, extirpated) for eight “pelagic-spawning” fishes, I found stream fragment length predicted population status (ANOVA, F2,21 = 30.14, P < 0.01) and explained 71% of reported extirpations. In a second study, I applied a new measure of habitat connectivity (the Dendritic Connectivity Index; DCI) to 12 stream networks in Kansas to test the DCI as a predictor of fish response to fragmentation by road crossings. Results indicated fish communities in stream segments isolated by road crossings had reduced species richness (alpha diversity) and greater dissimilarity (beta diversity) to segments that maintained connectivity with the network, and the DCI predicted patterns in community similarity among networks (n = 12; F1,10 = 19.05, r2 = 0.66, P < 0.01). Finally, I modeled fish distributions in theoretical riverscapes to test for mechanistic linkages between fragmentation and local extirpations. Results suggested the number of small fragments predicted declines in patch occupancy, and the magnitude of change in occupancy varied with dispersal ability (“high” dispersers responded more strongly than “low” dispersers). Taken together, these works show context-dependencies in fish responses to fragmentation, but a unifying theme is that small fragments contribute to attenuated biodiversity. Moreover, the predictable manner in which stream fish react to fragmentation will aid in biodiversity conservation by revealing potential responses to future scenarios regarding changes to habitat connectivity.