Are abomasal parasite communities of alpine ruminants really isolationist? A classification through a quantitative approach

Introduction: The analysis of the determinants of parasite community structure is a key topic in parasite ecology, and there is a growing interest on the influence of interaction between co-occuring species. The idea that communities may be something more than the simple result of a current assemblage, and that they may reflect an evolutionary process, has led to the proposal of the interactive/isolationist classification of parasite communities (Holmes J.C., Price P.W., 1986, Community Ecology, Blackwell, 185-213). According to this classification the former communities, structured by the parasite responses to their interspecific interactions, do not show vacant niches and tend toward an equilibrium, while the latter, structured by the single species responses, show vacant niches and a distribution of the parasite species in resource space independent by other species. A limitation of this vision is that communities are allocated, mainly through qualitative assessment, to one or other of these two extreme classes (“interactive” or “isolationist”), thereby missing the continuum that occurs in Nature. Actually, only few attempts to quantify the degree of isolationism/interactivity have been proposed, and these have been addressed mainly at a host population level (Dove A., 1999, Int. J. Parasitol, 29, 915-920; Poulin R, Luque J.L, 2003, Int. J Parasitol, 33, 1623-1630). However, parasite interactions occur within individual hosts, which can show even high variability regarding parasite infection intensity, both on the whole and for every single parasite species. Host individuals may thus register differently on the isolationist-interactive scale. It follows that analysis at a population level may miss the role of intrinsic and extrinsic factors which lead to these differences. Material & Methods: In order to quantitatively assess the degree of isolationism/interactivity ,we developed a measure at a single host level, based on the concept of crowding, which expresses the number of contacts with other individual parasite experienced by each individual parasite. Moreover, we compared these patterns with the results from analyses considering the relationship between dominance, richness, and total parasite burden. We applied the analyses described above to an host-parasite system composed by an alpine ruminant (alpine chamois: Rupicapra rupicapra) and its abomasal parasite community that was previously classified as isolationist (Citterio et al. 2007, J. Parasitol. 95, 918-927). The database was then extended including other host species, and it was finally composed by the abomasal parasite communities about 261 alpine chamois, 127 roe deer (Capreolus capreolus) and 124 alpine ibex (Capra ibex). Results & Discussion: The analyses evidenced high variability between species and individual hosts in the degree of crowding, showing that the opportunity to interact greatly varies and do not tend toward an equilibrium. Moreover, the degree of dominance appeared to be independent by the parasite community richness and the total load, confirming the individualistic change of parasite intensities of each species. These findings further confirm previous hypothesis about the isolationist feature of the abomasal parasite communities of alpine ruminants. Moreover, the degree of crowding in the three host species appeared to be influenced differently by host factors (I.e. age and sex), and extrinsic factors such sampling year, month and sampling site.