Predators Lack Complementarity in a Degraded Stream

Anthropogenic disturbance has led to the loss of biodiversity, altering ecosystem processes and decreasing stability. Top predators have been disproportionately affected by this degradation. Functional complementarity via niche overlap is one mechanism by which ecosystem processes may be maintained in the absence of a top predator. Aquatic ecosystems have shown a decline in top predators such as salmonids, but few studies have addressed the functional complementarity of alternative predators. In beaver ponds in the western U.S., Western Tiger Salamanders ( Ambystoma mavortium ) often become the top aquatic predator in the absence of fish, yet no previous studies have explored their trophic ecology. We evaluated this knowledge gap and used criteria including population size structure, diet, and stable isotope analysis to investigate the functional complementarity of a degradation tolerant species, the Arizona Tiger Salamander ( A. m. nebulosum ) , compared to the native but extirpated salmonid top predator, Colorado River Cutthroat Trout ( Oncorhynchus clarkii pleuriticus ). Field data for tiger salamanders and published accounts of various salmonid species suggested that, although both species are characterized by size-structured populations with ontogenetic shifts in a generalist diet, the trophic position of A. m. nebulosum is lower than reported values for large salmonids. This lower trophic positioning suggests that salamanders are likely to be functionally complementary with only the smallest size classes of salmonids. These results support previous work suggesting that functional complementarity exists under a narrow range of environmental conditions, which may limit the degree to which degradation-tolerant species can maintain communities.