Anthropogenic disturbances in the riparian zone influence exploitative competition between a generalist and a specialist species of stream-dwelling salamander

Background/Question/Methods

Species assemblage structure in a given ecosystem can be influenced by community interactions and disturbances. Ecological perturbations in the watershed and riparian zone alter the habitat structure, microhabitat availability, environmental stress, and ecosystem processes in the stream channel; all of which affect the stream biota. We experimentally investigated competition between two species of stream-associated salamanders with different body sizes, natural histories, and sensitivity for habitat alterations; Black-bellied salamander (large-bodied habitat specialists) and northern dusky salamander (small-bodied habitat generalists), in the context of differential riparian land-uses (forested, agricultural and urban). We simulated three riparian land-uses in artificial streams and observed the microhabitat use and area of occupancy of focal species in isolation and co-occurrence.

Results/Conclusions

Black-bellied salamanders competitively dominated microhabitat use and area of occupancy over northern dusky salamanders in forested and agricultural streams where they displaced the heterospecific from the wetted channel. Northern dusky salamanders dominated the same metrics over black-bellied salamanders in the urban stream; the former displaced the latter from bank crevices. We also noted that the habitat preference by the focal species changed substantially among land-uses: wetted channel in the forest stream; upland banks in the non-forest streams. Riparian land-uses resulted loss of the microhabitats of the wet channel (due to sedimentation and reduced supply of woody debris) in the non-forest streams forcing large-bodied habitat specialists to seek alternatives, who were less-adapted for xeric uplands, providing a competitive advantage for the small-bodied generalists well-adapted to occupy banks. The interstices available in uplands of human-impacted streams are size-limited which precluded the access to large-bodied species. Northern Dusky salamanders, who selected stream banks in occurrence at forested streams had the energetic advantage of being the residents. In addition, the environmental stress induced by the non-forest streams might have incapacitated the heterospecific signals of dominance and energetically-costly aggression of larger species. The distance to nearest heterospecific neighbor was highest in forested streams and lowest in non-forested streams which also indicated the loss of competitive superiority for microhabitats and area of occupancy from black-bellied salamanders. We showed that riparian anthropogenic disturbances negatively affected large-bodies species with highly specific habitat requirements that were competitively dominant under historical disturbance regimes and favored subordinate, small-bodied habitat generalists. This study illuminated a behavioral mechanism for how habitat degradation can influence species persistence in urbanizing environments.  Upland anthropogenic disturbances can modify interspecific competition among in-stream fauna, resulting-in exclusion of disturbance-sensitive species, ultimately leading to biotic homogenization.