The characteristics and impact of macroparasites on wild animal populations.

Abstract

The parasitic helminths and arthropods (ticks and fleas) are usually classified, according to their ecological characteristics, as macroparasites. Reproduction usually occurs via the transmission of free-living infective larval stages and the characteristics of the relationship with the host are a function of the number of parasites harboured by the host. Macroparasites exhibit an aggregated distribution within the host population with a small proportion of the host population carrying a high proportion of the parasite population. Within an individual host, the size of the parasite population is a balance between infection and mortality rates. In many macroparasite systems the number of parasites within a host increases with the age of the host to an asymptote determined by the interaction between transmission and parasite mortality rates. However, some species show a convexity in this age intensity curve with a reduction in numbers probably brought about by parasite-induced host mortality, acquired immunity, or changes in host exposure to infection. Empirical studies have demonstrated a range of impacts of macroparasites on host fitness including reductions in host survival and fecundity, increased susceptibility to predation and the reductions in the ability of individuals to defend resources. At the population level there is evidence that macroparasites may regulate host numbers and that delayed density-dependent effects can lead to large scale changes in host numbers. Such effects may act directly, as in the grouse-Trichostrongylus tenuis system or as a consequence of immuno-suppression.