- Habitat fragmentation is one of the major causes of local and regional species extinctions in freshwater ecosystems. To predict future trends in community composition, and the potential sequence of extinctions due to fragmentation of the river continuum, it is important to understand how habitat size and isolation affect the dynamics of species immigrations and extinctions and patterns of abundance and occupancy.
- We examined fish immigration and extinction rates, and abundance and occupancy patterns, in relation to habitat isolation, size and variability in the Rio Grande and its tributaries in the Trans-Pecos region of Texas, U.S.A.
- Our results indicated that as habitat isolation increased in tributaries, fish immigration and extinction rates and riverine species abundances decreased. Fish assemblages showed significant nested subset patterns across the study sites, and the influence of habitat size and isolation on the nested subset patterns varied with spatial scale. Certain non-native fish taxa were idiosyncratic, reducing the strength of the nested subset pattern.
- The high temporal species turnover in connected habitats was related to the movement of riverine fish species between the mainstem and mouths of tributaries. High persistence of spring-adapted species decreased extinction rate in isolated habitats.
- Increasing the spatial scale of the study system probably resulted in an increased ability to detect dispersal limitation between the mainstem and its tributaries. Large, deep pools decreased local extinction of certain native fish taxa in upstream tributaries. The deviation from the nested subset pattern was attributed to the unique regional occupancy of non-native fish taxa that could be associated with their artificial introduction into this system.
- Our results suggest that increased habitat fragmentation by human activities can accelerate the regional extinction of certain native fish taxa and the dominance of ecologically tolerant, possibly non-native fish, leading to a decline in regional diversity.