Competition between the threatened Bliss Rapids snail, Taylorconcha serpenticola (Hershler et al.) and the invasive, aquatic snail, Potamopyrgus antipodarum (Gray)

Abstract

Due primarily to habitat loss and invasive species, extinction rates for North American mollusk taxa are among the highest for any taxonomic group in the world. Competition between invasive and native species often leads to decreases in native populations. For example, a primary reason for listing the Bliss Rapids snail, Taylorconcha serpenticola as threatened in the Snake River drainage was the perceived impacts of the highly invasive New Zealand mudsnail, Potamopyrgus antipodarum. Despite federal protection of T. serpenticola and the known presence of P. antipodarum in the Snake River drainage for almost 20 years, almost nothing is known about their ecology and competitive interactions. For this dissertation I conducted both field and laboratory studies to determine niche overlaps, spatial patterns, and some life history characteristics of both species. I compared optimal growth temperatures and estimated temperature tolerances for each species, under laboratory conditions; 2) examined stage (size) class fecundity rates and growth rates; and 3) examined photophobic tendencies of both species. I then explored environmental conditions and spatial patterns of both species in Banbury Springs, a tributary of the Snake River, near Hagerman, Idaho, that may have affected their distribution and abundance using regression tree analysis and geostatistical methods. I then conducted several competition experiments between both species under controlled conditions at Banbury Springs, developed competition coefficients, and related their growth rates and competitive outcomes to periphyton abundance and diversity. Finally, I monitored and then modeled seasonal and yearly population density trends of both species in a section of Banbury Springs, where both coexist, using time series analysis. Results of this study show that that both species have niche overlaps (i.e. temperature overlaps, photophobic tendencies, and somewhat similar habitat requirements) and can compete for limited food resources, but may have just enough niche separation or their occupied habitat is heterogeneous enough for them to coexist, at present. It is possible however that not enough time has elapsed for the effects of competition with P. antipodarum to push T. serpenticola to extinction.