Interaction frequency across the geographical range as a determinant of host specialisation in generalist fleas

The strength of interspecific interactions varies over geographical scales, and can influence patterns of resource specialisation. Even with gene flow preventing local adaptation of a consumer to particular resources, we might expect that across its entire range, the consumer would show some specialisation for the resource types most likely to be encountered across the localities where it occurs. We tested the hypothesis that generalist fleas are more successful at exploiting small mammalian host species with which they co-occur frequently across their geographical range than host species that, though suitable, are encountered less frequently. This hypothesis was tested with data on 121 flea species compiled from field surveys across 35 regions of the Palaearctic. Using abundance (mean number of individual fleas per individual host) as a measure of flea success on a particular host species, positive correlations between flea abundance and the frequency of co-occurrence of a flea with each of its hosts amongst all regions surveyed were found in all but two of the flea species investigated, with one-fifth of these being significant. If overlap in geographical range between flea and host is used as a measure of frequency of encounters instead of the actual proportion of regions where they both occur, similar patterns are observed, though they are much weaker. In a comparative analysis across all flea species, there were significant relationships between the average abundance of fleas and average values of both measures of frequency of encounters (proportion of sites where they co-occur and range overlap), even when correcting for potential phylogenetic influences. The results suggest that for any given flea species, host species more commonly encountered throughout the spatial range of the flea are generally those on which the flea does best. Interaction frequency may be a key determinant of specialisation and abundance in host-parasite systems.