Evaluating the role of regional and local processes in structuring a larval trematode metacommunity of Helisoma trivolvis

Abstract

Metacommunity theory has advanced our understanding of how local and regional processes aff ect the structure of ecological communities. While parasites have largely been omitted from metacommunity research, parasite communities can provide the large sample sizes and discrete boundaries often required for evaluating metacommunity patterns. Here, we used assemblages of fl atworm parasites that infect freshwater snails ( Helisoma trivolvis ) to evaluate three questions: 1) what factors aff ect individual host infections within ponds? 2) Is the parasite metacommunity structured among ponds? And 3) what is the relative role of local versus regional processes in determining metacommunity structure and species richness among ponds? We examined 10 821 snails from 96 sites in fi ve park complexes in the San Francisco Bay area, California, and found 953 infections from six parasite groups. At the within-pond level, infection status of host snails correlated positively with individual snail size and pond infection prevalence for all six parasite groups. Using an ordination method to test for metacommunity structure, we found that the parasite metacommunity was organized in a non-random pattern with species responding individually along an environmental gradient. Based on a model selection approach involving local and regional predictors, parasite species richness and metacommunity structure correlated with both local abiotic (pH and total dissolved nitrogen) and biotic (non-host mollusk density, and H. trivolvis biomass) factors, with little support for regional predictors. Overall, this trematode metacommunity most closely followed the predictions from the species sorting or mass eff ects metacommunity paradigm, in which community diversity is fi ltered by local site characteristics.

Drivers of symbiont diversity in freshwater snails: a comparative analysis of resource availability, community heterogeneity, and colonization opportunities

“…In a study of trematode richness and metacommunity structure within Helisoma trivolvis , Richgels et al (2013) found that the probability of infection in individual hosts associated strongly with snail body size, while trematode richness at the population level correlated negatively with pH and marginally positively with pond size. Our results supported the influential roles of host body size and pond area but also highlighted the effects of free-living richness and colonization opportunities.…”

“…From May to August 2013, we collected snails from freshwater ponds Contra Costa, Alameda, and Santa Clara counties (Johnson et al 2013; Richgels et al 2013). We selected accessible pond ecosystems across grassland, oak woodland, and chaparral biomes to encompass a broad range in resource availability, community composition, surrounding habitat, and physical and spatial characteristics.…”

“…For the ‘heterogeneity model’ we incorporated snail richness and the richness of other free-living aquatic taxa as additional fixed effects (measured as a discrete count of the taxa encountered during dipnet surveys, including macroinvertebrates, fishes, and amphibians), with the rationale that for generalist symbionts, these taxa may function as alternative or additional hosts (Hechinger and Lafferty 2005). Fourth, the ‘colonization model’ included characteristics hypothesized to influence parasite colonization and establishment, such as pond area (log 10 +1 transformed) and the percentage of wetland habitat within a 1 km radial buffer of each pond (derived from the National Land Cover Database using ArcGIS), although we acknowledge that such variables could also have effects on variables such as total host population size (Richgels et al 2013; Zelmer 2014). Finally, we built both an ‘intercept-only’ model and a ‘global model’ containing all of the fixed effects after verifying a lack of collinearity among predictors (Richgels et al 2013).…”

“…Fourth, the ‘colonization model’ included characteristics hypothesized to influence parasite colonization and establishment, such as pond area (log 10 +1 transformed) and the percentage of wetland habitat within a 1 km radial buffer of each pond (derived from the National Land Cover Database using ArcGIS), although we acknowledge that such variables could also have effects on variables such as total host population size (Richgels et al 2013; Zelmer 2014). Finally, we built both an ‘intercept-only’ model and a ‘global model’ containing all of the fixed effects after verifying a lack of collinearity among predictors (Richgels et al 2013). Other individual or host species-level traits were either not measurable (e.g., days alive) or not relevant to this system (e.g., sex, as all snail host species included here are hermaphroditic).…”

Role of Antimicrobial Peptides in Amphibian Defense Against Trematode Infection

“…We collected snails ( Helisoma trivolvis and Physa acuta ) from ponds in the San Francisco Bay Area of California between May and August of 2013 and 2014 (see Richgels et al 2013 Fig. 1 for map).…”