Community assembly theory is a powerful framework for investigating epidemiological patterns, as it can reveal the processes governing parasite occurrence across time and space. The abundance and distribution of parasites within hosts may be influenced by host-level factors, like host condition and heterogeneity in susceptibility. For parasites with free-living stages, there may also be strong filtering at the habitat-level, with certain habitats containing suitable conditions for parasite environmental stages. In addition, neutral processes, like stochastic colonization events, can strongly influence distributional patterns of parasites. The relative importance of these processes may change over time as hosts develop, and differ at the scale of study. We used a community assembly framework to investigate the temporal assembly of parasite (trematode, fungal, protozoan, and viral) communities infecting amphibian hosts in small ponds of California. We examined patterns of succession both within individual larval amphibian hosts and at the pond-level, and re-sampled ponds over a period of three months to examine how communities changed over time. We quantified turnover at the individual level and across time to see whether hosts converged in their parasite communities. Further, we examined whether deterministic processes increased over time as parasite communities became saturated.
Results/Conclusions
We quantified parasite communities within 584 Pacific chorus frogs (Pseudacris regilla) and 440 California newts (Taricha torosa) across 10 ponds. Individuals contained up to 8 different parasite species, with the majority of individuals harboring more than one parasite. Our analyses pointed to a strong role of stochastic colonization in determining pond parasite communities, with little evidence of host-level factors filtering parasite species. By comparing the relationship between within-host richness and developmental time, we found no evidence of saturation in frogs, meaning each individual accumulated parasites until metamorphosis. Beta diversity (community dissimilarity) among individuals of the same population declined over time, indicating that individuals did not differ in their ability to filter parasites. Surprisingly, ß diversity between the two host species also declined over the summer due to colonization of generalist rather than specialist parasites. Altogether, our results point to strong roles of dispersal in structuring parasite communities. The strong influence of neutral rather than niche-based or deterministic processes may present a challenge for predicting disease outbreaks.