Snails at three scales: Interplay of stream hydrology and hydraulics with the morphology, dispersal and distribution of Elimia proxima

Strauss, Ben

This thesis quantifies effects of hydraulic environment and flooding disturbance on the ecology and evolution of benthic stream macroinvertebrates from organismal to landscape scales, utilizing the snail Elimia proxima as a model species. Recent research has complicated views about the nature of flow environments near stream beds and of the organismal shapes best equipped to cope with them. Analysis of intraspecific morphological variation in field populations exposed to contrasting environments is a largely untapped source of potential insight. I measured shell shape and size in E. proxima sampled from hydraulically diverse locations. At every site, small shells were stubbier than large shells, consistent with the changing hydrodynamic demands snails likely face as they grow, and providing what may be the first evidence for this theoretically predicted ontogenetic shift. Sites subject to greater hydraulic stress were associated with smaller mature snails; and evidence also suggests that snails at stressful sites begin with bluffer shells but grow more streamlined when compared to snails in gentler locations. All of these findings are consistent with adaptive plastic or genetic variation to reduce exposure to hydrodynamic forces. At the next scale, I explored the consequences of hydraulic environment and flood history for snail dispersal by conducting long-term mark-release-resurvey experiments at multiple sites with flow records, and developing maximum likelihood models to fit subsequent observations. Findings suggested that hydrodynamically stressful environments impede upstream migration and enhance the downstream transport caused by floods. Previous research has demonstrated transport of benthic invertebrates by floods, but appears to lack quantitative estimates of relationships between environment, flood size and dispersal impacts, as provided here. Finally, I examined the relationship between stream hydrology and hydraulics and the landscape-scale distribution and abundance of E. proxima. In a geographic survey, the probability of site occupancy varied negatively with indicators of hydraulic stressfulness and flooding likelihood. A negative relationship between these factors and density was evident only in the aftermath of a major regional flood event. Several lines of evidence suggested, however, that hydraulic resistance to upstream migration plays a more important role in setting distributional limits than floods.