Scale‐invariance of niche breadth in fleas parasitic on small mammals

The resource specialization or niche breadth of a species is not fixed across populations, but instead varies over geographical space. A species may be a local specialist but a regional generalist, if it uses locally few resources that are substitutable across locations. In contrast, a species is a local generalist and a regional specialist if it uses locally many resources that cannot be substituted from 1 location to the next. Scale‐dependence can thus be a major factor in estimation of niche breadth. Here, we test for relationships between local and global estimates of host specificity (a measure of niche breadth for parasites) in fleas (Siphonaptera) parasitic on small mammals from 49 different regions within the Holarctic. Across all fleas, we found a strong, positive relationship between the number of host species that a flea uses in 1 locality and the number of different host species that can serve as the flea's principal host (i.e. the one supporting the most fleas in a region) among all regions. Also, we observed a strong positive relationship between the taxonomic distinctness of the host species used in 1 locality and that of all known principal hosts among all localities. These relationships held after correcting for potentially confounding phylogenetic influences. We discuss the implications of scale‐independent host specificity and its association with geographical range size and species‐specific patterns of host use.     

Spatial variation in species diversity and composition of flea assemblages in small mammalian hosts: geographical distance or faunal similarity?

Aim Spatial variation in the diversity of fleas parasitic on small mammals was examined to answer three questions. (1) Is the diversity of flea assemblages repeatable among populations of the same host species? (2) Does similarity in the composition of flea assemblages among populations of the same host species decay with geographical distance, with decreasing similarity in the composition of local host faunas, or with both? (3) Does the diversity of flea assemblages correlate with climatic variables? Location The study used previously published data on 69 species of small mammals and their fleas from 24 different regions of the Holarctic. Methods The diversity of flea assemblages was measured as both species richness and the average taxonomic distinctness of their component species. Similarity between flea assemblages was measured using both the Jaccard and Morisita–Horn indices, whereas similarity in the composition of host faunas between regions (host ‘faunal’ distance) was quantified using the Jaccard index. Where appropriate, a correction was made for the potentially confounding influence of phylogeny using the independent contrasts method. Results Flea species richness varied less within than among host species, and is thus a repeatable host species character; the same was not true of the taxonomic distinctness of flea assemblages. In almost all host species found in at least five regions, similarity in flea assemblages decreased with increases in either or both geographical and faunal distance. In most host species, the diversity of flea assemblages correlated with one or more climatic variable, in particular mean winter temperature. Main conclusions Spatial variation in flea diversity among populations of the same mammal species is constrained by the fact that it appears to be a species character, but is also driven by local climatic conditions. The results highlight how ecological processes interact with co‐evolutionary history to determine local parasite biodiversity.     

Latitudinal gradients in niche breadth: empirical evidence from haematophagous ectoparasites

Aim  We searched for relationships between latitude and both the geographic range size and host specificity of fleas parasitic on small mammals. This provided a test for the hypothesis that specialization is lower, and thus niche breadth is wider, in high-latitude species than in their counterparts at lower latitudes.
Location  We used data on the host specificity and geographic range size of 120 Palaearctic flea species (Siphonaptera) parasitic on small mammals (Soricomorpha, Lagomorpha and Rodentia). Data on host specificity were taken from 33 regions, whereas data on geographic ranges covered the entire distribution of the 120 species.
Methods  Our analyses controlled for the potentially confounding effects of phylogenetic relationships among flea species by means of the independent-contrasts method. We used regressions and structural equation modelling to determine whether the latitudinal position of the geographic range of a flea covaried with either the size of its range or its host specificity. The latter was measured as the number of host species used, as well as by an index providing the average (and variance in) taxonomic distinctness among the host species used by a flea.
Results  Geographic range size was positively correlated with the position of the centre of the range; in other words, fleas with more northerly distributions had larger geographic ranges. Although the number of host species used by a flea did not vary with latitude, both the mean taxonomic distinctness among host species used and its variance increased significantly towards higher latitudes.
Main conclusions  The results indicate that niche breadth in fleas, measured in terms of both its spatial (geographic range size) and biological (host specificity) components, increases at higher latitudes. These findings are compatible with the predictions of recent hypotheses about latitudinal gradients.     

Ectoparasitic "jacks-of-all-trades": relationship between abundance and host specificity in fleas (Siphonaptera) parasitic on small mammals

Animal species with larger local populations tend to be widespread across many localities, whereas species with smaller local populations occur in fewer localities. This pattern is well documented for free-living species and can be explained by the resource breadth hypothesis: the attributes that enable a species to exploit a diversity of resources allow it to attain a broad distribution and high local density. In contrast, for parasitic organisms, the trade-off hypothesis predicts that parasites exploiting many host species will achieve lower mean abundance on those hosts than more host-specific parasites because of the costs of adaptations against multiple defense systems. We test these alternative hypotheses with data on host specificity and abundance of fleas parasitic on small mammals from 20 different regions. Our analyses controlled for phylogenetic influences, differences in host body surface area, and sampling effort. In most regions, we found significant positive relationships between flea abundance and either the number of host species they exploited or the average taxonomic distance among those host species. This was true whether we used mean flea abundance or the maximum abundance they achieved on their optimal host. Although fleas tended to exploit more host species in regions with either larger number of available hosts or more taxonomically diverse host faunas, differences in host faunas between regions had no clear effect on the abundance-host specificity relationship. Overall, the results support the resource breadth hypothesis: fleas exploiting many host species or taxonomically unrelated hosts achieve higher abundance than specialist fleas. We conclude that generalist parasites achieve higher abundance because of a combination of resource availability and stability.     

Deconstructing spatial patterns in species composition of ectoparasite communities: the relative contribution of host composition,environmental variables and geography

Aim We determined whether dissimilarity in species composition between parasite communities depends on geographic distance, environmental dissimilarity or host faunal dissimilarity, for different subsets of parasite species with different levels of host specificity. Location Communities of fleas parasitic on small mammals from 28 different regions of the Palaearctic. Method Dissimilarities in both parasite and host species composition were computed between each pair of regions using the Bray–Curtis index. Geographic distances between regions were also calculated, as were measures of environmental dissimilarity consisting of the pairwise Euclidean distances between regions derived from elevation, vegetation and climatic variables. The 136 flea species included in the dataset were divided into highly host‐specific species (using 1–2 host species per region, on average), moderately host‐specific species (2.2–4 hosts per region) and generalist species (>4 hosts per region). The relative influence of geographic distance, host faunal dissimilarity and environmental dissimilarity on dissimilarity of flea species composition among all regions was analysed for the entire set of flea species as well as for the three above subsets using multiple regressions on distance matrices. Results When including all flea species, dissimilarity in flea species composition was affected by all three independent variables, although the pure effect of dissimilarity in host species composition was the strongest. Results were different when the subsets of fleas differing in host specificity were treated separately. In particular, dissimilarity in species composition of highly host‐specific fleas increased solely with environmental dissimilarity, whereas dissimilarity for both moderately specific and non‐specific fleas increased with both geographic distance and dissimilarity in host species composition. Main conclusions Host specificity seems to dictate which of the three factors considered is most likely to affect the dissimilarity between flea communities. Counter‐intuitively, environmental dissimilarity played a key role in determining dissimilarity in species composition of highly host‐specific fleas, possibly because, although their presence in a region relies on the occurrence of particular host species, their abundance is itself mostly determined by climatic conditions. Our results show that deconstructing communities into subsets of species with different traits can make it easier to uncover the mechanisms shaping geographic patterns of diversity.     

Similarity in ectoparasite faunas of Palaearctic rodents as a function of host phylogenetic, geographic or environmental distances: Which matters the most?

Different host species harbour parasite faunas that are anywhere from very similar to very different in species composition. A priori, the similarity in the parasite faunas of any two host species should decrease with increases in either the phylogenetic distance, the distinctness of the environments occupied or the geographical distance between these hosts. We tested these predictions using extensive data on the faunas of fleas (Insecta: Siphonaptera) and gamasid mites (Acari: Parasitiformes) parasitic on rodents across the Palaearctic. For each pair of host species, we computed the similarity in parasite faunas based on both species composition as well as the phylogenetic and/or taxonomic distinctness of parasite species. Phylogenetic distances between hosts were based on patristic distances through a rodent phylogeny, geographic distances were computed from geographic range data, and environmental dissimilarity was measured from the average climatic and vegetation scores of each host range. Using multiple regressions on distance matrices to assess the separate explanatory power of each of the three dependent variables, environmental dissimilarity between the ranges of host species emerged as the best predictor of dissimilarity between parasite faunas, especially for fleas; in the case of mites, phylogenetic distance between host species was also important. A closer look at the data indicates that the flea and mite faunas of two hosts inhabiting different environments are always different, whilst hosts living in similar environments can have either very similar or dissimilar parasite faunas. Additional tests showed that dissimilarity in flea or mite faunas between host geographic ranges was best explained by dissimilarity in vegetation, followed by dissimilarity in climatic conditions. Thus, external environmental factors may play greater roles than commonly thought in the evolution of host-parasite associations.     

Nested pattern in flea assemblages across the host's geographic range

Understanding non-random patterns in the taxonomic composition of communities occurring in insular or fragmented habitats remains a major goal of ecology. Nested subset patterns are one possible departure from random community assembly that has been reported for communities of both free-living and parasitic animals. Here, we investigate the effects of extrinsic factors on the occurrence of nestedness among the assemblages of fleas found in different populations of the same host species, using data on 25 mammalian host species. The patterns of flea species composition among host populations spanned the entire spectrum from significantly nested to significantly anti-nested. After controlling for host phylogeny, we found that across host species, the tendency for flea assemblages to approach nestedness increased with increasing host geographic range size and with decreasing latitude of the host's geographic range. This tendency also decreased with an increase in a composite variable combining data on mean January and July temperature. The number of closely-related mammalian species living in sympatry with a given host species had no influence on whether or not the structure of flea assemblages among its populations departed from randomness. We propose explanations for these results that include: the possible gradual loss of flea species as a host expands its range from its initial area of origin, the lack of specific flea faunas in narrowly-distributed host species, interspecific differences in the dispersal abilities of flea species becoming amplified in hosts with broad geographical ranges, and the effect of latitude, climate and environment on the probabilities of host-switching and extinction in fleas. Overall, our results suggest that the structure of flea assemblages in mammalian hosts may be driven by features of host biology.     

Coevolutionary arms races: increased host immune defense promotes specialization by avian fleas

We investigated the relationship between host defense and specialization by parasites in comparative analyses of bird fleas and T-cell mediated immune response of their avian hosts, showing that fleas with few main host species exploited hosts with weak or strong immune defenses, whereas flea species that parasitized a large number of host species only exploited hosts with weak immune responses. Hosts with strong immune responses were exploited by a larger number of flea species than hosts with weak responses. A path analysis model with an effect of T-cell response on the number of host species, or a model with host coloniality directly affecting host T-cell response, which in turn affected the number of host species used by fleas, best explained the data. Therefore, parasite specialization may have evolved in response to strong host defenses.     

Geographical range size and host specificity in ectoparasites: a case study with Amphipsylla fleas and rodent hosts

Aim  To identify the factors that determine the geographical range sizes of ectoparasites with different degrees of host specificity.
Location  The study used data on the distributions of fleas of the genus Amphipsylla and their rodent hosts across the Holarctic.
Methods  All known points of occurrence of 32 flea species and 51 species of their rodent hosts were mapped. The shape and size of the geographical range of each species were estimated using a combination of the minimal convex polygon technique and modelling with the garp algorithm. Factors determining the geographical range sizes of the fleas were identified using stepwise multiple regression analysis.
Results  The geographical range size of fleas that are strongly host-specific across their entire ranges correlated positively with the geographical range size of the fleas' principal hosts, and negatively with the geographical range size of the fleas' potential competitors. The geographical range sizes of both (1) fleas that are locally host-specific but that shift their host preferences geographically, and (2) host-opportunistic fleas were positively correlated only with the area of the geographical ranges of their principal hosts. Strongly host-specific fleas occupied 0.2–80.0% of the geographical range of their principal hosts, whereas this figure was 0.9–83.7% in locally host-specific fleas and 16.6–63.7% in host-opportunistic fleas.
Main conclusions  The main determinant of the geographical range size of a flea species is the size of the geographical range of its hosts. The role of potential competitors in determining the geographical range size is stronger in host-specific than in host-opportunistic fleas. Cases in which the geographical range of a parasite is smaller than the geographical range(s) of its host(s) owing to narrower parasite environmental tolerances are much more frequent in host-opportunistic than in host-specific fleas.     

Combining phylogenetic and ecological information into a new index of host specificity

Host specificity has 2 independent facets: the extent to which different host species are used by a parasite, and the phylogenetic distances among these hosts. Although the number of host species exploited by a parasite commonly is used as a measure of host specificity, it fails to capture ecological and phylogenetic differences among hosts. Here, a new index of host specificity, S(TD)*, is developed and illustrated. This index measures the average taxonomic distinctness among the host species used by a parasite, weighted for the parasite's prevalence in the different hosts. For a given number of host species, the index approaches its minimum value when a parasite achieves high prevalence in a few closely related host species, and the index approaches its highest value when a parasite reaches its highest prevalence values in distantly related host species. Simple hypothetical examples are used to demonstrate the index's computation and some of its properties. The new index is influenced independently both by the taxonomic (or phylogenetic) affinities of a set of host species and by the distribution of prevalence values among these hosts. A single value cannot truly capture all the nuances of a phenomenon as complex as host specificity; nevertheless, the proposed index incorporates the features of specificity that are most relevant to parasitologists and will be a useful tool for comparative studies.     

Host specificity and geographic range in haematophagous ectoparasites

A negative interspecific correlation between the degree of habitat specialization and the size of a species' geographic range has been documented for several free living groups of organisms, providing support for the niche breadth hypothesis. In contrast, practically nothing is known about the geographic range sizes of parasitic organisms and their determinants. In the context of the niche breadth hypothesis, parasites represent ideal study systems, because of the well documented variation in host specificity among parasite species. Here, we investigated the relationship between host specificity (a measure of niche breadth) and geographic range size among flea species parasitic on small mammals, using data from seven distinct geographical regions. Two measures of host specificity were used: the number of host species used by a flea species, and a measure of the average taxonomic distance between the host species used by a flea; the latter index provides an evolutionary perspective on host specificity. After correcting for phylogenetic influences, and using either of our two measures of host specificity, the degree of host specificity of fleas was negatively correlated with the size of their geographic range in all seven regions studied here, with only one minor exception. Overall, these results provide strong support for the niche breadth hypothesis, although other explanations cannot be ruled out.     

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.     

Ecological characteristics of flea species relate to their suitability as plague vectors

The ability of vector-borne diseases to persist and spread is closely linked to the ecological characteristics of the vector species they use. Yet there have been no investigations of how species used as vectors by pathogens such as the plague bacterium differ from closely related species that are not used as vectors. The plague bacterium uses mammals as reservoir hosts and fleas as vectors. The ability of different fleas to serve as vectors is assumed to depend on how likely they are to experience gut blockage following bacterial multiplication; the blockage causes fleas to regurgitate blood into a wound and thus inject bacteria into new hosts. Beyond these physiological differences, it is unclear whether there exist fundamental ecological differences between fleas that are effective vectors and those that are not. Here, using a comparative analysis, we identify clear associations between the ability of flea species to transmit plague and their ecological characteristics. First, there is a positive relationship between the abundance of flea species on their hosts and their potential as vectors. Second, although the number of host species exploited by a flea is not associated with its potential as a vector, there is a negative relationship between the ability of fleas to transmit plague and the taxonomic diversity of their host spectrum. This suggests a correlation between some ecological characteristics of fleas and their ability to develop the plague blockage. The plague pathogen thus uses mainly abundant fleas specialized on a narrow taxonomic range of mammals, features that should maximize the persistence of the disease in the face of high flea mortality, and its transmission to suitable hosts only. This previously unrecognized pattern of vector use is of importance for the persistence and transmission of the disease.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

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.     

Feeding performance of fleas on different host species: is phylogenetic distance between hosts important?

We asked if and how feeding performance of fleas on an auxiliary host is affected by the phylogenetic distance between this host and the principal host of a flea. We investigated the feeding of 2 flea species, Parapulex chephrenis and Xenopsylla ramesis, on a principal (Acomys cahirinus and Meriones crassus, respectively) and 8 auxiliary host species. We predicted that fleas would perform better (higher proportion of fleas would feed and take larger bloodmeals) on (a) a principal rather than an auxiliary host and (b) auxiliary hosts phylogenetically closer to a principal host. Although feeding performance of fleas differed among different hosts, we found that: (1) fleas did not always perform better on a principal host than on an auxiliary host; and (2) flea performance on an auxiliary host was not negatively correlated with phylogenetic distance of this host from the principal host. In some cases, fleas fed better on hosts that were phylogenetically distant from their principal host. We concluded that variation in flea feeding performance among host species results from interplay between (a) inherent species-specific host defence abilities, (b) inherent species-specific flea abilities to withstand host defences and (c) evolutionary tightness of association between a particular host species and a particular flea species.     

Sampling fleas: the reliability of host infestation data

The use of measures of host infestation as a reliable indicator of a flea population size to be used in interspecific comparisons was considered. The abundance of fleas collected from host bodies and collected from host burrows was compared among 55 flea species, controlling for the effect of flea phylogeny. The mean number of fleas on host bodies correlated positively with the mean number of fleas in host burrows/nests both when the entire data pool was analysed and for separate subsets of data on 'fur' fleas and 'nest' fleas. This was also true for a within-host (Microtus californicus) between-flea comparison. The results of this study demonstrate that, in general, the index of host body infestation by fleas can be used reliably as an indicator of the entire population size.     

Relationships between parasite abundance and the taxonomic distance among a parasite's host species: an example with fleas parasitic on small mammals

Opportunistic parasite species, capable of exploiting several different host species, do not achieve the same abundance on all these hosts. Parasites achieve maximum abundance on their principal host species, and lower abundances on their auxiliary host species. Taxonomic relatedness between the principal and auxiliary host species may determine what abundance a parasite can achieve on its auxiliary hosts, as relatedness should reflect similarities among host species in ecological, physiological and/or immunological characters. We tested this hypothesis with fleas (Siphonaptera) parasitic on small Holarctic mammals. We determined whether the abundance of a flea in its auxiliary hosts decreases with increasing taxonomic distance of these hosts from the principal host. Using data on 106 flea species from 23 regions, for a total of 194 flea-locality combinations, we found consistent support for this relationship, both within and across regions, and even after controlling for the potentially confounding effect of flea phylogeny. These results are most likely explained by a decrease in the efficiency of the parasite's evasive mechanisms against the host's behavioural and immune defences with increasing taxonomic distance from the principal host. Our findings suggest that host switching over evolutionary time may be severely constrained by the coupling of parasite success with the relatedness between new hosts and the original host.     

Trait‐based and phylogenetic associations between parasites and their hosts: a case study with small mammals and fleas in the Palearctic

We investigated the associations between ecological (density, shelter structure), morphological (body mass, hair morphology) and physiological traits (basal metabolic rate) of small mammals and ecological (seasonality of reproduction, microhabitat preferences, abundance, host specificity) and morphological (presence and number of combs) traits of their flea parasites that shape host selection processes by fleas. We adapted the extended version of the three‐table ordination and linked species composition of flea assemblages of host species with traits and phylogenies of both hosts and fleas. Fleas with similar trait values, independent of phylogenetic affinities, were clustered on the same host species. Fleas possessing certain traits selected hosts possessing certain traits. Fleas belonging to the same phylogenetic lineage were found on the same host more often than expected by chance. Certain phylogenetic lineages of hosts harbored certain phylogenetic lineages of fleas. The process of host selection by fleas appeared to be determined by reciprocal relationships between host and flea traits, as well as between host and flea phylogenies. We concluded that the connection between host and flea phylogenies, coupled with the connection between host and flea traits, suggests that the species compositions of the host spectra of fleas were driven by the interaction between historical processes and traits.     

Abundance patterns of two Oropsylla (Ceratophyllidae: Siphonaptera) species on black-tailed prairie dog (Cynomys ludovicianus) hosts.

Behavioral, genetic, and immune variation within a host population may lead to aggregation of parasites whereby a small proportion of hosts harbor a majority of parasites. In situations where two or more parasite species infect the same host population there is the potential for interaction among parasites that could potentially influence patterns of aggregation through either competition or facilitation. We studied the occurrence and abundance patterns of two congeneric flea species on black-tailed prairie dog (Cynomys ludovicianus) hosts to test for interactions among parasite species. We live-trapped prairie dogs on ten sites in Boulder County, CO and collected their fleas. We found a non-random, positive association between the two flea species, Oropsylla hirsuta and O. tuberculata cynomuris; hosts with high loads of one flea species had high loads of the second species. This result suggests that there is no interspecific competition among fleas on prairie dog hosts. Host weight had a weak negative relationship to flea load and host sex did not influence flea load, though there were slight differences in flea prevalence and abundance between male and female C. ludovicianus. While genetic and behavioral variation among hosts may predispose certain individuals to infection, our results indicate apparent facilitation among flea species that may result from immune suppression or other flea-mediated factors.     

Stability in abundance and niche breadth of gamasid mites across environmental conditions, parasite identity and host pools

There is substantial variability among populations of the same species in basic features such as abundance or niche breadth, and it is unclear to what extent these are true species traits as opposed to the product of local environmental factors. In parasites, abundance and niche breadth, i.e. host specificity, show repeatability among different populations of the same species, but may also be influenced by external forces, depending on the parasite taxa studied. We tested whether the abundance and host specificity of gamasid mites parasitic on small mammals from 26 different geographic regions of the Palaearctic, are species-specific or instead determined by host identity and/or parameters of the biotic and abiotic environment. Values of abundance and host specificity (measured as the number of host species used) were significantly more similar among populations of the same mite species than among different mite species; despite also showing consistency within particular host species or regions independently of mite species identity, both abundance and the number of host species used appear to be true mite species traits. In contrast, the taxonomic distinctness of host species used by a mite showed little repeatability among populations of the same mite species, and appears mostly determined by the local pool of available host species. Within given mite species, all three variables (abundance, number of host species used, and their taxonomic distinctness) covaried to some extent with one or more environmental factors (e.g., nature of the local host assemblage, temperature, precipitation) across geographical regions, but there was no universal pattern among results from different mite species. These results are similar to those obtained earlier on other taxa, e.g. fleas, and suggest that there are general laws acting on spatial patterns of parasite abundance and host specificity.