Spatial variation in the abundance, trophic ecology, and role of semi-aquatic salamanders in headwater streams
Phillip Robert Gould, Ph.D.
Advisor: William E. Peterman
Headwater streams are common features of forested landscape and provide critical resources to near-stream riparian forests and downstream aquatic habitat. In many small streams in the southern Appalachians, often narrow and heavily shaded, these systems are almost entirely dependent on external input of nutrients. The volume and quality of nutrients available to headwater ecosystems varies through space and time, depending on the species of trees present in riparian forests, the time of year, and the rate of litter-processing by invertebrates. Additionally, nutrients are not distributed uniformly through aquatic ecosystems, and may be moved or aggregated by both physical and biological processes. Despite the perceived greater reliance on allochthonous input, there has been little work to identify how wildlife inhabiting smaller streams are facilitating stream nutrient dynamics both within streams and riparian systems.
To increase our understanding of the role of animals in headwater streams, my research investigates the patterns of density, biomass, foraging, and excretion in the Black-bellied salamander (Desmognathus quadramaculatus), the largest, most-aquatic species of salamander occurring in fishless headwaters of the southern Appalachians. Desmognathus quadramaculatus forage both within and directly around streams, spending significant amounts of non-foraging time in stream banks or under partially submerged cover objects. Additionally, D. quadramaculatus have a 3-year larval period in this region and require permanent aquatic habitat for larval development. These traits suggest D. quadramaculatus may play a substantial role in the transfer of nutrients between riparian and aquatic ecosystems. The primary objective of my research was to (1) identify spatial patterns in stream-associated salamander density and (2) evaluate the extent to which D. quadramaculatus move nutrients between aquatic and terrestrial habitat through patterns in foraging and excretion.
As a guild, semi-aquatic salamanders require aquatic habitat for larval development and adult activity, however, considerable variation in the degree to which species inhabit aquatic habitat exists. Communities of salamanders of the genus Desmognathus are thought to be structured via competition and predation, and evidence suggests coevolution has resulted in adults of smaller species occupying terrestrial habitat and larger species occurring in aquatic habitat. My research extends our understanding of how abundance varies along environmental gradients and quantifies the role of competition in observed patterns. I found that more terrestrial D. ocoee and D. monticola abundance responded more strongly to variation in variables describing terrestrial environmental gradients. Conversely, I found D. quadramaculatus abundance was described by aquatic and terrestrial variables equally. Subsequently, I found no evidence for species associations, either positive or negative, and conclude competition does not play a significant role in observed occurrence or relative abundance patterns. I then investigate how individual and population-level characteristics influenced D. quadramaculatus diet and excretion patterns. I found adult D. quadramaculatus consumed more terrestrial or flying prey than aquatic, with the terrestrial proportion increasing with salamander SVL. I also found salamanders within the highest elevation sites had more terrestrial diets, suggesting physiological limitations influence terrestrial foraging in this species. Further, I found D. quadramaculatus excretion of NH+ and soluble reactive phosphorous is strongly linked to individual size, and that adult salamanders contribute ~1/3 of the total predicted excretion for the species, despite representing 1/64th of the total biomass. Together, my research extends our current understanding of the role of salamanders in headwater and riparian ecosystems, and how this varies across the landscape.