Understanding species-poor communities: evaluating drivers of within-host parasite assemblages from freshwater snails

Background/Question/Methods

Species-poor communities can result from a variety of potential mechanisms related to either species interactions or species isolation. Specifically, interactions such as competition can exclude species, yet other mechanisms, such as temporal heterogeneity can isolate species utilizing a specific habitat at any one time.  Flatworm parasites within their molluscan hosts typically have single species communities with multiple species communities occuring less often than expected by chance.  The predominant view is that flatworm parasites are antagonistic and single species communities assemble through dominance competition.  However, this hypothesis has rarely been tested explicitly.  Using an extensively sampled metacommunity of flatworm parasites of Helisoma trivolvis compared to theoretical models we asked the questions: 1) Which of the potential mechanisms creating species poor communities best fits flatworm parasite assemblages? 2) When do species interactions become important for regional community composition? We created mathematical models representing potential hypotheses (dominance hierarchy, species traits, invader exclusion, behavioral avoidance, and temporal heterogeneity) that could lead to species-poor communities and optimized each model based on the observed communities, comparing models using a model selection approach.  We then simulated the best fit model over a range of abundances to estimate when species interactions would become important for metacommunity composition.

Results/Conclusions

At a well-studied wetland in Minnesota, we detected five flatworm parasite species within Helisoma trivolvis (in order of average abundance): Ribeiroia ondatrae, Echinostoma sp., Allassostomoides sp., Cephalogonimus sp., and Alaria sp. The best supported model for flatworm parasites within their snail hosts was temporal heterogeneity in combination with invader exclusion.  Additionally, species interactions increased with increasing total abundance.  Thus, the importance of competition between flatworm parasites depends on their regional abundance.  Overall, a combination of isolating (temporal heterogeneity) and species interaction (invader exclusion) mechanisms lead to species poor communities.