Parasite-host elemental content and the effects of a parasite on host-consumer-driven nutrient recycling

Parasites are ubiquitous members of ecological communities but have only recently been recognized as key players in broader interactions and ecosystem dynamics, such as foodweb structure and energy flow. Ecological stoichiometry provides a framework for placing parasites in an ecosystem perspective by considering elemental imbalances and their consequences. I measured the elemental content of trematodes and their gastropod hosts and then estimated the nutrient-recycling effects of parasitism. N:P of all tissue types in trematodes of the freshwater pulmonate snail, Physa acuta, was similar to N:P of gastropod gonadal tissue. However, N:P of trematodes and gastropod gonads was lower than N:P of other gastropod tissues, a result suggesting an elemental imbalance between parasite and host. N:P of excreta of P. acuta increased with the N:P content of their algal food, consistent with consumer-driven nutrient recycling theory. However, gastropods with patent infections of Trichobilharzia physellae excreted greater N:P than did uninfected snails, a result indicating that infected and uninfected gastropods were functionally different. Overall, these results suggest a significant role for parasites in nutrient recycling.