Date of Final Oral Examination (Defense)


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

Matthew R. Dare


I conducted a three-year field study to gather information about the Bliss Rapids snail, Taylorconcha serpenticola, a rare aquatic mollusk endemic to the Snake River drainage of southwestern Idaho. The goals of my study were to 1) gain an understanding of the species’ distribution and dispersion in the Snake River, 2) characterize habitat relationships, and 3) evaluate methods for monitoring the species’ abundance. When monitoring at-risk species, it is important to understand their spatial distribution and habitat requirements in order to design a study that will provide reliable data with good statistical power. My study suggests the species is not limited to a small number of densely-populated colonies within specific habitat types as previously thought. Instead, the Bliss Rapids snail is patchily distributed throughout the study area. Bliss Rapids snails were found at the majority of my sample sites, but only occurred in 5-13% of the cobbles I sampled. The species exhibits contagious dispersion: the variance-to-mean ratio was greater than 1 for all four spatial scales I examined. When sites containing Bliss Rapids snails were paired between years, abundance was significantly correlated at three spatial scales when compared with a Spearman rank order correlation test. Bliss Rapids snail abundance was positively correlated with bed shear stress, and negatively correlated with distance from the nearest upstream rapid and bank slope (angle), but correlations were weak in both cases. The species was more abundant in the deeper (0.5-1.5 m) transects compared to shallow transects (0-0.5 m) as well as north-facing aspects compared to south-facing aspects. I used a bootstrap method to simulate the probability vi of not detecting the species at a site (bank-section) when occurrence rates were low. This simulation revealed that increasing the bank-section sample size from 40 to 100 cobbles would reduce the probability of not detecting the species when they were present from 0.39 to 0.08 when the occurrence rate was 0.025. I also performed a Monte Carlo simulation-based power analysis to determine the sample size needed to identify 10, 20, 25, 35, and 50% declines in Bliss Rapids snail abundance over a five-year period using data from over 15,000 cobble counts. The analysis indicated that declines in abundance of 10-50% could be detected with statistical power of at least 0.8 over a five-year period (with α=0.1). I recommend a protocol to detect a 25% decline in abundance over a fiveyear period, which would require sampling 6,000 cobbles annually.