Long-term Changes in Parasite Population and Community Structure: A Case History

Abstract

-A series of studies on the population and community ecology of several digenetic trematodes was begun in 1983 in a small, eutrophic impoundment in the Piedmont area of North Carolina. Initially, efforts were focused on the hemiurid trematode, Halipegus occidualis. Prevalence of this parasite in its snail intermediate host, Helisoma anceps, reached 60% in late June-early July, then dropped precipitously as the mature cohort of the snail host died-off. Over the next 13 yr, this annual pattern consistently repeated itself. Several years later, another pulmonate snail, Physa gyrina, colonized the pond. Either simultaneously, or closely thereafter, Halipegus eccentricus and two other species using P gyrina as an intermediate host also appeared. The trematode infracommunities in both hosts were examined; structuring forces were found not to be affected by competitive interactions among larval trematodes, but by a variety of factors, including snail vagility and size (age), microhabitat heterogeneity, distribution and temporal visitation patterns of definitive hosts, and the sequence of parasite recruitment mechanisms. Very few ( 7000 Helisoma anceps, whereas nearly 19% of all infections in 1181 P gyrina were multiple. Additional studies indicate that Halipegus occidualis cercariocysts do not remain infective for extended periods for second intermediate hosts (ostracods) as suggested in the older literature. Moreover, foraging ecology by odonate naiads is a significant factor in the recruitment and dispersion patterns of H. occidualis metacercariae. The long-term population ecology of H. occidualis and H. eccentricus was examined in mark-release-recapture studies of Rana clamitans, the definitive host. Halipegus occidualis prevalence and density were higher than those of H. eccentricus, although both were contagiously distributed in their frog definitive hosts. Halipegus occidualis exhibited large and rapid changes in density within frog hosts. Increases were probably because of the recruitment of contagiously distributed parasites from their third intermediate host, whereas declines in heavily infected definitive hosts were attributed to inflammation and tissue sloughing at the point of parasite attach-