Causes and consequences of seasonal dynamics in the parasite community of red-spotted newts (Notophthalmus viridescens)

by Raffel, Thomas R.

Recent implications of parasites as agents of worldwide amphibian decline make determining the drivers of parasitic infection in amphibians a priority for ecological research. Despite the apparently seasonal nature of these outbreaks, seasonal dynamics remain largely ignored in recent studies of amphibian parasites. In this thesis, I present results of field studies describing seasonal patterns for multiple parasitic infections and testing between potential drivers of parasite infection risk using red-spotted newts (Notophthalmus viridescens) as a model species. Most newt parasites had seasonal dynamics, with significantly more parasite species having peak infection rates during the early and late spring than in other seasons. These similar seasonal patterns appear to have been driven by multiple different factors, many of which may have been driven in turn by bottom-up effects of the spring bloom in pond productivity. Seasonal patterns in white blood cell counts indicated that amphibian immunity has similar responses to temperature in the field as have been recorded in laboratory studies. Moreover, both temperature increases and decreases were associated with low levels of immunity relative to temperature, indicating a negative effect of temperature variability on amphibian immunity. This pattern is probably due to a lag in production of immune cells following temperature increases in spring, and to a lag in seasonal acclimation to winter temperatures causing low immune cell production rates in autumn. The last three chapters focus on the ecology of Ichthyophonus sp., a protist parasite of newts which has caused mass morbidity events in North America. Based on data from the seasonal survey, an additional survey of sixteen populations, and a capturemark-recapture study, several lines of evidence suggest that the amphibian leech Placobdella picta is the vector of Ichthyophonus sp. infection in newts. There was little indication of mortality due to infection despite high apparent morbidity, but Ichthyophonus sp. infection did appear to cause males to stop breeding earlier in the spring and to induce temporary emigration in female and possibly male newts. These results suggest that parasites may influence the the newt life history strategy by driving newts onto land during the summer. iii