Spatial relationships and mesoscale habitat variance in co-occurring populations of Church’s sideband and Trinity bristle snail in the Greater Trinity Basin, northern California

Abstract

In resource management, the kind and extent of ecological co-occurrence between closely related species frequently requires assessment of the spatial relationship among taxa. In my study, analysis of inter-species pair-wise distances revealed no syntopic overlap between Church’s sideband (Monadenia churchi) and Trinity bristle snails (M. setosa). No pair of samples had the same geographic coordinates and no parapatric boundary in environmental covariates was evident between species. This “microsympatric” spatial relationship resembled a metapopulation structure with no high degree of overlap, as co-occurrence was rare and small in geographic scope. Fifteen forest cover-types and 82 soil-types were identified between species. The most common forest-type for M. churchi was Sierra Mixed Conifer (39.9%) and Douglas fir (28.9%). In M. setosa the most common forest-types were the same but in much different percentages (78.8% and 14.8%, respectively). Sixty-one and 39 soil-types were associated with samples of M. churchi and M. setosa, respectively. The Hohmann-Neuns family complex was the most common (22.5%) soil-type for M. churchi and the Holland Deep-Hugo family complex was the most (50.6%) dominant for M. setosa. There were significant differences between species in all environmental attributes and in values of monthly temperature and precipitation, which reflected variance in the mesoclimatic regime seasonally. Principal Components Analysis (PCA) accounted for 57.8% of the dispersion contained in environmental variables on the first 3-eigenvectors. Evapotranspiration and Summer and Winter Temperatures loaded positively while Summer and Winter Precipitation and Elevation loaded negatively along PC I (26.2%). Given significant inter-species differences in ecological occupancy, it seems plausible that microsympatry is based in part on both mesoscale habitat variance and subtle differences in mesoclimate defined by seasonal variation in temperature and precipitation. The hypothesis that M. setosa is adapted to cool habitats and M. churchi to warmer more arid environs in microsympatry was substantiated at a macroscale level.