The Correlation of Topographically-Derived Relative Wetness with Terrestrial Mollusk Presence and Abundance

Abstract

ABSTRACT Many species of terrestrial mollusks are small and difficult to find, with poorly known ranges and habitat preferences. Because desiccation is a primary cause of mortality for many terrestrial mollusks, incorporating wetness as a habitat variable may improve survey results for different species. We compared presence and abundance data from terrestrial mollusk surveys in Tillamook Resource Area to two measures of relative wetness: topographic wetness index (TWI) and geomorphic features (landslides, debris-flow channels, etc.). Hurdle Model regression revealed a positive correlation between increased TWI and likelihood of presence or abundance for four species, and a negative correlation for six species. Overall species diversity and total mollusk count were negatively correlated with increased TWI, but the effect size was small (p = 0.02, R2= -0.03). Our Kruskal-Wallis Analysis of Variance of TWI between species was significant (p < 0.001), indicating terrestrial mollusks occupy significantly different wetness regimes – but this relationship was driven entirely by the wetness specialization of Hemphillia glandulosa. Our chi-square analysis of topographic features found significant preferences of different species for different topographic types, which correlated loosely but not precisely to the preferences indicated by TWI. These results show that altering current terrestrial mollusk survey protocol to include geomorphic features, which are simpler and less time-intensive than calculating TWI, would increase detection likelihood of certain species, including Hemphillia glandulosa, a species protected under the Survey and Manage guidelines of the Northwest Forest Plan.