Beta-specificity: the turnover of host species in space and another way to measure host specificity.

Host specificity is often measured as the number of host species used by a parasite, or as their phylogenetic diversity; both of these measures ignore the larger scale component of host use by parasites. A parasite may exploit very few host species in one locality but these hosts may be substituted for completely different species elsewhere; in contrast, another parasite may exploit many host species in one locality, with the identity of these hosts remaining the same throughout the parasite's geographical range. To capture these spatial nuances of host specificity, we propose to use an index for host species turnover across localities, or beta-specificity (β(SPF)), that is derived from studies of spatial patterns in plant and animal diversity. We apply this index to fleas parasitic on small mammals to show that: (i) it is statistically independent of traditional or "local" measures of host specificity as well as of "global" measures of host specificity, and (ii) it is also independent of the size of the geographical area studied or the sampling effort put into collecting hosts and parasites. Furthermore, the distribution of β(SPF) values among flea species shows a significant phylogenetic signal, i.e. related flea species have more similar β(SPF) values than expected by chance. Nevertheless, most possible combinations of either local specificity (alpha-specificity) or global (gamma-specificity) and beta-specificity are observed among flea species, suggesting that adding a spatial component to studies of host use reveals a new facet of specificity. The measure presented here provides a new perspective on host specificity on a scale relevant to studies on topics ranging from biogeography to evolution and may underlie the rate and extent of disease transmission and population dynamics.