Competition – colonization tradeoffs promote coexistence in salt marsh trematode parasites

Background/Question/Methods

Understanding the mechanisms that maintain coexistence of competing species is a fundamental goal of community ecology. Parasite communities are often both diverse and intensely competitive for host resources. In California salt marshes, over a dozen trematode species coexist at local spatial scales, despite a competitive dominance hierarchy within their shared first-intermediate host. These parasites obligately infect the California horn snail, Cerithidea californica, as a first-immediate host, before infecting a range of invertebrates and fishes as second-intermediate hosts and birds as final hosts. We tested the hypothesis that a tradeoff between competition within the snail and colonization of uninfected snails maintains coexistence in this trematode community. To do so, we used a field experiment to parameterize a mathematical model. The snail trematode system is particularly well-suited for testing the predictions of the competition – colonization tradeoff model because competition is asymmetric (i.e. follows a dominance hierarchy) and dynamics in second-intermediate and final hosts could cause variation in colonization success in the snail. We measured the colonization rates of trematodes by placing uninfected snails in cages in Carpinteria salt marsh in Carpinteria, California, and monitoring their infection status over time using a non-destructive method called shedding.

Results/Conclusions

One year after the uninfected snails were deployed, the overall trematode infection prevalence was 12.3% in 1,141 snails. Colonization rates increased with declining competitive ability (i.e. the colonization hierarchy was the inverse of the competitive hierarchy) for the six most common trematode species. By comparing observed colonization rates with model predictions, we show that competition – colonization tradeoffs are stronger than necessary to maintain coexistence for most of the trematode species in the study. This suggests that variation in aspects of parasite life cycle outside the snail, such as availability of subsequent hosts, parasite fecundity in the final host, and cercarial production, can prevent competitive exclusion in the first-intermediate host.