No abstract

Understanding and predicting parasite strategies is of interest not only for parasitologists, but also for anyone interested in epidemiology, control strategies and evolutionary medicine. From an ecological and evolutionary perspective, parasites are an important feature of their hosts' selective environment, and may have diverse roles, ranging from the evolution of host sex to host-sexual selection behavior. Generally, it is the hosts and their biology that have been the focus of these evolutionary investigations, but we approach the subject from the parasites' perspective, illustrating the sophistication of parasite strategies in dealing with contrasting and unpredictable environments.

Relatively few published food webs have included parasites, and in this study we examined the animal community in a stream across eight contiguous seasons to test how inclusion of helminth parasites alters the topology or structure of the food web. Food webs constructed for each season and analyzed using common binary matrix measures show that species richness, linkage density, and the number of observed and possible links increased when parasites were included as individual species nodes. With parasite-parasite and predator-parasite links omitted, measures of community complexity, such as connectance (C), generally increased over multiple seasons. However, relative nestedness (n*) decreased when parasites were included, which may be a result of low resolution of basal resources inflating specialist-to-specialist links. Overall, adding parasites resulted in moderate changes in food web measures when compared to those of four other published food webs representing different ecosystems. In addition, including parasites in the food web revealed consistent pathways of energy flow, and the association of parasite life histories along these pathways suggest stable evolutionary groups of interacting species within the community.

BackgroundParasites significantly alter topological metrics describing food web structure, yet few studies have explored the relationship between food web topology and parasite diversity.Methods/Principal FindingsThis study uses quantitative metrics describing network structure to investigate the relationship between the topology of the host food web and parasite diversity. Food webs were constructed for four restored brackish marshes that vary in species diversity, time post restoration and levels of parasitism. Our results show that the topology of the food web in each brackish marsh is highly nested, with clusters of generalists forming a distinct modular structure. The most consistent predictors of parasite diversity within a host were: trophic generality, and eigenvector centrality. These metrics indicate that parasites preferentially colonise host species that are highly connected, and within modules of tightly interacting species in the food web network.Conclusions/SignificanceThese results suggest that highly connected free-living species within the food web may represent stable trophic relationships that allow for the persistence of complex parasite life cycles. Our data demonstrate that the structure of host food webs can have a significant effect on the establishment of parasites, and on the potential for evolution of complex parasite life cycles.

Grooming behavior may play a part in the transmission of the gastrointestinal nematode, Heligomosomoides polygyrus in the mouse host. After infective larvae are placed on individually housed mice, significantly higher numbers of adult worms were recovered from the small intestine of mice that were allowed to self-groom when compared to infection levels in mice that had been fitted with Elizabethan collars to prevent self-grooming. Larvae placed on a single mouse housed with 3 other untreated mice resulted in all mice in the group becoming infected, suggesting that allogrooming may also be important in parasite transmission. A significantly higher percentage of larvae nictate on rough surfaces such as damp peat moss substrate when compared to smooth surfaces such as 0.5% agarose. Mice exposed to larvae placed on peat moss substrate have significantly higher infection levels when compared to mice exposed to larvae on a 0.5% agarose substratum, suggesting that natural transmission of infective L3 larvae in mice may be dependent on a substratum type that allows nictation behavior. A significantly higher percentage of worms were attracted to mouse urine and mouse and rat epidermal lipids when compared to deionized water controls in an in vitro preference assay, suggesting an attraction to host-specific signals. These results support the hypothesis that transmission of this parasite is an active process involving movement of the infective larvae of H. polygyrus into the host's active space where they are ingested during grooming behavior.