Effects of disturbance on host-parasite interactions: A case study involving the Pacific giant salamander

Impacts of parasitic diseases in wildlife scale from the individual to the community such that alterations in parasite-host interactions can cascade through the community. Recent studies suggest that anthropogenic disturbance significantly affects parasitic diseases in wildlife that could further exacerbate direct effects of disturbance on free-living species. Few studies rigorously evaluate the impact of disturbance on the incidence or abundance of parasitism in hosts. Even fewer assess mechanisms of disturbance that alter parasitism in wildlife. My work addresses the impacts of clearcut logging on parasitism by a digenetic trematode, Cephalouterina dicamptodoni, of three stream-dwelling hosts. The trematode obligately and sequentially infects the Pacific giant salamander, Dicamptodon tenebrosus, the stonefly, Calineuria californica, and the snail, Juga silicula. My goals were to (1) assess whether parasite density or prevalence in each host responded to logging, (2) evaluate the mechanistic relationship between logging-induced disturbances and parasitism, (3) experimentally evaluate the functional response of transmission to host density between each donor-target host pair, and (4) to relate the functional response of transmission to the biotic changes in parasitism associated with logging-induced disturbances.; In 1998, I conducted censuses of each host in 7 unlogged basins, 7 logged basins and 4 logged basins that also experienced debris flows. All stream basins were located in the McKenzie or Little Fall creek watersheds, of the central Cascade Mountains, Oregon. During each of the 18 censuses, I measured geomorphic and other abiotic features of the creeks that responded to logging-induced disturbances. To evaluate whether parasitism responds to logging, I compared the prevalence and density for each parasite stage and host species between streams in the 3 forest types: unlogged, logged and logged-with-debris flows. I correlated parasite levels with abiotic and biotic features across forest types to assess whether the relationship between parasites and their hosts changed with disturbances. I conducted field experiments during 1995, 1996, and 1997 in which I manipulated donor and target host densities to test for the transmission response of parasitism to densities of each host.; The data reveal a clear and significant effect of logging on parasite population dynamics. Each stage in the life cycle of the parasite is decreased by disturbances associated with logging, but the strength of the effect is dependent upon severity of logging. In addition, the relative roles of abiotic and biotic variables in the parasite-host system are context dependent.; Logging has a significant effect on host-parasite relationships and on the density of each host, which suggests that logging alters the success and functional response of parasite transmission. Based on my experimental study, non-linearities in transmission exist in at least two of the three transmission stages. In addition, transmission is context dependent in that limitations in transmission depend differentially upon reproduction and recruitment but in a predictable manner that is subject to the relative densities of donor and target hosts. According to epidemiological theory, non-linear transmission generates complex model behavior resulting in multiple stable states. The patterns of parasitism and host density that followed logging, coupled with the experimental results that non-linearities exist in transmission of C. dicaniptodoni, suggest that logging causes this parasite-host system to shift into alternate states that depend upon the severity of logging-induced disturbances.