Host-parasite interactions depend on the extent of ontogenetic overlap between hosts and parasites and therefore on the phenology of both host and parasite species. Furthermore, phenology of both host and parasite species may be idiosyncratically influenced by factors such as shifts in climate. However, relative to other types of ecological interactions, phenological shifts in host-parasite systems and their ecological impacts are understudied. We examined seasonal dynamics of echinostomes, a family of trematode parasites, over a five-year survey of 23 ponds in southeastern Michigan. We focused on echinostome infection in four common snail first intermediate host species and one second intermediate host, larval green frogs (Rana clamitans). Green frogs have a long (2-3 month) breeding season, and variation in parasite phenology in snail hosts likely influences the relationship between breeding time and tadpole parasite exposure.
We found that echinostome prevalence in snails generally peaked in late summer, although the timing and magnitude of peaks differed among host species and across years. Across snail species, the peak in prevalence occurred on the order of a month earlier in the two warmest years of the survey period, suggesting a climatic effect on parasite dynamics. Such shifts will likely influence parasite transmission to subsequent hosts, as infection levels in larval green frog hosts correlated positively with prevalence in snails. Our results indicate that the combination of snail community composition, climate, and parental breeding time affect the developmental stage at which tadpoles are exposed to parasite infective stages (i.e., the ontogenetic overlap between hosts and parasites). Our study thus offers new insights into how both changing climate and diversity influence parasite dynamics.