On the relevance of abundance and spatial pattern for interpretations of host-parasite association data.

The quantification of host-parasite associations from field data is a fundamental step towards understanding host-parasite and host-parasite-pathogen dynamics. For parasites that are not rigid host specialists, exemplified in this paper by ticks, the interpretation of host-parasite association data is difficult. Interpretations of tick collection records have largely assumed that off-host collection records offer a valid basis from which to make claims about the host specificity or generality of tick species. A simple simulation analysis of rudimentary tick-host interactions in a hypothetical 50 x 50-cell habitat demonstrates that perceptions of tick-host relationships can be strongly biased by spatial patterns. Regardless of their true level of host specificity or generality, it seems that: (i) more abundant ticks will be perceived as generalists, while rarer species will be considered specialists; and (ii) tick species that have patchy, strongly aggregated distributions will be more likely to be perceived as host specialists than species that have more dispersed or uniform distributions. Since all available evidence suggests that abundances and spatial patterns vary between tick species, there is no way of assessing the true validity of claims about host specificity without first undertaking detailed research on the relative abundances and spatial and temporal patterns of both tick and host distributions.