Searching for general patterns in parasite ecology: host identity versus environmental influence on gamasid mite assemblages in small mammals

Abstract

The abundance and diversity of parasites vary among different populations of host species. In some host-parasite associations, much of the variation seems to depend on the identity of the host species, whereas in other cases it is better explained by local environmental conditions. The few parasite taxa investigated to date make it difficult to discern any general pattern governing large-scale variation in abundance or diversity. Here, we test whether the abundance and diversity of gamasid mites parasitic on small mammals across different regions of the Palaearctic are determined mainly by host identity or by parameters of the abiotic environment. Using data from 42 host species from 26 distinct regions, we found that mite abundances on different populations of the same host species were more similar to each other than expected by chance, and varied significantly among host species, with half of the variance among samples explained by differences between host species. A similar but less pronounced pattern was observed for mite diversity, measured both as species richness and as the taxonomic distinctness of mite species within an assemblage. Strong environmental effects were also observed, with local temperature and precipitation correlating with mite abundance and species richness, respectively, across populations of the same host species, for many of the host species examined. These results are compared to those obtained for other groups of parasites, notably fleas, and discussed in light of attempts to find general rules governing the geographical variation in the abundance and diversity of parasite assemblages.

Ectoparasite Burdens of the Common Mole-Rat (Cryptomys hottentotus hottentotus) From the Cape Provinces of South Africa

“…In contrast, in the highveld mole-rat, ectoparasite burdens were greatest during the wet summer (Viljoen et al, 2011), suggesting that changes in humidity rather than temperature are responsible for the observed pattern. In fact, the importance of rainfall for ectoparasites have been repeatedly shown for various taxa (Vinarski et al, 2007;Krasnov et al, 2008;Korallo-Vinarskaya et al, 2009). The winter rainfall coincides with the reproductive activity of the common mole-rat (Spinks et al, 1999); hence, the seasonal patterns observed may also be a result of a synchronized reproductive cycle between ectoparasite species and their hosts (Rothschild and Ford, 1964;Christe et al, 2000;Louren¸co and Palmeirim, 2008).…”

“…Such aggregation could have implications for the sex ratio of ectoparasite species due to the isolation of infrapopulations on individual hosts (Marshall, 1981;Poulin, 2007). The increasing isolation of such infrapopulations with decreasing ectoparasite prevalence and a resulting increased risk of inbreeding has been invoked to explain the commonly observed female bias of ectoparasites (Marshall, 1981;Matthee et al, 2007;Krasnov, 2008). In such populations, only a small number of males are required to fertilize all females, and females exhibit female-biased offspring sex ratios, resulting in reduced local mate competition (Hamilton, 1967).…”

“…However, a marked female bias was only observed in the most prevalent ectoparasite species in the current study. Alternatively, sex biases in ectoparasite species have been explained with sex differences in mobility, longevity, or susceptibility to abiotic condition or host grooming behavior (Dick and Patterson, 2008;Krasnov, 2008;Pap et al, 2013). Although the balanced sex ratio in E. hilli could be linked to low mobility of lice and the high dependence of both sexes on the host for nutrition, the pronounced male bias in the remaining species runs contrary to the common female bias in ectoparasite populations.…”

“…Seasonal variation in parasite burdens is a particularly common pattern in both endo-and ectoparasites (Wilson et al, 2001;Altizer et al, 2006). However, the influences of the environment on parasite populations tend to be more pronounced in ectoparasites such as fleas and mesostigmatid mites (Krasnov, 2008;Korallo-Vinarskaya et al, 2009). Ectoparasites that have developmental stages that live off the host are thought to be particularly susceptible to effects associated with the surrounding habitat (Vinarski et al, 2007;Krasnov, 2008).…”

“…However, the influences of the environment on parasite populations tend to be more pronounced in ectoparasites such as fleas and mesostigmatid mites (Krasnov, 2008;Korallo-Vinarskaya et al, 2009). Ectoparasites that have developmental stages that live off the host are thought to be particularly susceptible to effects associated with the surrounding habitat (Vinarski et al, 2007;Krasnov, 2008). However, no taxon-specific pattern is apparent, and although some studies have shown that lower levels of humidity can reduce the abundances (Krasnov, 2008;Viljoen et al, 2011), others have found the opposite pattern (Fabiyi, 1996;Matthee et al, 2007).…”