Georgy I. Shenbrot

Host–habitat relations as an important determinant of spatial distribution of flea assemblages (Siphonaptera) on rodents in the Negev Desert

We studied flea assemblages on rodents in different habitats of the Ramon erosion cirque in the Negev Desert to examine whether host-habitat relations influence flea spatial distribution. Eleven flea species parasitizing 12 rodent species were recorded. There was significant positive relationship between flea species richness and body mass of the host species; no relationships were found between relative richness of flea assemblage and either the number of habitats occupied by the host species or the size of host geographical range. The differences in pattern of flea parasitism among habitat types within host species were determined by both environmental features of a habitat and the specific pattern of habitat use by rodents. There was replacement of Xenopsylla conformis by Xenopsylla ramesis on Meriones crassus and Gerbillus dasyurus among different habitats. The results of ordination of the flea collections from each individual host demonstrated that the flea assemblages were segregated mainly along 4 axes, which explained 86% of total variance. Each of the ordination axes corresponded with a change in flea species composition. The directions of these changes were (1) among-hosts within a habitat and (2) among-habitats within a host.

Community Structure of Desert Small Mammals: Comparisons Across Four Continents

Presence/absence data for the small-mammal species at sites in seven deserts were analyzed for evidence of similarity in community structure. The deserts studied were located in North and South America (268 and 118 sites, respectively), Australia (245 sites), Israel (54 sites), and greater Eurasia (Thar, 15 sites; Turkestan, 36 sites; Gobi, 98 sites). Patterns observed in all deserts included: (1) low a diversity (2-4 species per site); (2) high P diversity (species turnover between sites); and (3) local coexistence of 20-30% of the species in the regional pool. Additionally, the number of species with which a species co-occurred increased with the number of sites at which that species occurred. Although these results suggested that some features of community structure were similar across deserts, other aspects, especially trophic structure, differed widely. Deserts in the northern hemisphere possessed more granivores, and the Turkestan Desert more folivores, than other deserts. Carnivorous small mammals were most strongly represented in Australia, and omnivores in South America, Australia, and the Thar. The structure of desert small-mammal communities is strongly influenced by historical factors; different taxonomic groups with distinctive trophic adaptations proliferate in different desert regions where they are subject to some common structuring processes of community assembly.

Sex-biased parasitism, seasonality and sexual size dimorphism in desert rodents

We investigated seasonality of gender differences in the patterns of flea infestation in nine rodent species to test if sex-biased parasitism in terms of mean abundance, species richness, prevalence and the level of aggregation (a) varies among hosts and between seasons, and (b) is linked to sexual size dimorphism. Sexual size differences were significant in both summer and winter in Acomys cahirinus, Gerbillus pyramidum and Meriones crassus, and in winter only in Acomys russatus, Gerbillus dasyurus, Gerbillus nanus and Sekeetamys calurus. Sexual size dimorphism was male biased except for A. russatus in which it was female biased. Manifestation of sexual differences in flea infestation was different among hosts between seasons. A significant effect of sex on mean flea abundance was found in six hosts, on mean flea species richness in five hosts and on prevalence in two hosts. Male-biased parasitism was found in summer in one host only and in winter in five hosts. Female-biased parasitism occurred in winter in A. russatus. Gender differences in the slopes of the regressions of log-transformed variances against log-transformed mean abundances occurred in three hosts. No relationship was found between sexual size dimorphism and any parasitological parameter in any season using both conventional regressions and the method of independent contrasts. Our results suggest that sex-biased parasitism is a complicated phenomenon that involves several different mechanisms.

THE EFFECT OF HOST DENSITY ON ECTOPARASITE DISTRIBUTION: AN EXAMPLE OF A RODENT PARASITIZED BY FLEAS

The pattern of parasitism of the flea species Xenopsylla dipodilli and Nosopsyllus iranus theodori on the desert rodent species Gerbillus dasyurus was studied to test the hypothesis that the relationships between flea abundance and host density conform to pre-existing models of R. M. Anderson and R. M. May, with the correction that the density of those host individuals that possess permanent burrows (residents) is substituted for the overall host density. It was predicted that: (1) the intensity of flea infestation would increase in curvilinear fashion with increase of host density to a plateau that would be attained at a lower level of host density than would be expected from the basic model, and (2) the prevalence of flea infestation plotted against host density would be hump-shaped. The results indicated that intensity of flea infestation increased in either curvilinear fashion to an asymptote (for X. dipodilli) or linearly (for N. i. theodori) with increase of host density. As host density increased, the prevalence of infestation changed either unimodally (X. dipodilli) or logarithmically (N. i. theodori). In addition, there was a positive relationship between the mean number of fleas per host and the percentage of hosts infested. Both basic and corrected models describing the relationships between flea burden and host density fit the observational data well. However, simulations of the fraction of resident hosts demonstrated that this parameter influences the relationship between host density and flea burden only when residents comprise ≤50% of all host individuals.

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.

Phylogenetic signal in module composition and species connectivity in compartmentalized host-parasite networks.

Across different taxa, networks of mutualistic or antagonistic interactions show consistent architecture. Most networks are modular, with modules being distinct species subsets connected mainly with each other and having few connections to other modules. We investigate the phylogenetic relatedness of species within modules and whether a phylogenetic signal is detectable in the within- and among-module connectivity of species using 27 mammal-flea networks from the Palaearctic. In the 24 networks that were modular, closely related hosts co-occurred in the same module more often than expected by chance; in contrast, this was rarely the case for parasites. The within- and among-module connectivity of the same host or parasite species varied geographically. However, among-module but not within-module connectivity of host and parasites was somewhat phylogenetically constrained. These findings suggest that the establishment of host-parasite networks results from the interplay between phylogenetic influences acting mostly on hosts and local factors acting on parasites, to create an asymmetrically constrained pattern of geographic variation in modular structure. Modularity in host-parasite networks seems to result from the shared evolutionary history of hosts and by trait convergence among unrelated parasites. This suggests profound differences between hosts and parasites in the establishment and functioning of bipartite antagonistic networks.

Annual cycles of four flea species in the central Negev desert.

Abstract Bionomics of fleas (Siphonaptera) parasitizing rodent hosts, mostly the gerbil Gerbillus dasyurus (Wagner) and the jird Meriones crassus Sundevall (Gerbillidae), were investigated in the central Negev desert of Israel. Populations were sampled weekly (by Sherman trapping of hosts) from August 2000 to July 2001. Among 1055 fleas of nine species captured, four species predominated (94%). Two species of Pulicidae, Xenopsylla dipodilli Smit and X. ramesis (Rothschild), reproduced perennially, whereas adults of Nosopsyllus iranus theodori Smit (Ceratophyllidae) and Stenoponia tripectinata medialis Jordan (Hystrichopsyllidae) occurred only in cool months (October–March). During their main activity season on the most infested host species (estimated from > 300 trap‐nights/month), prevalence of these four flea species reached 40–70%, 20–30%, 100% and 50–70%, respectively, with infestation intensities of 2–2.7, 7–12, 2–3.5 and 2.5–7 fleas per infested rodent, respectively.

Density-dependent host selection in ectoparasites: An application of isodar theory to fleas parasitizing rodents

Parasites should make the same decisions that every animal makes regarding fitness reward. They can maximize reproductive success by selection of those habitats that guarantee the greatest fitness output. We consider the host population as a habitat of a parasite population. Consequently, hosts (=habitats) that differ quantitatively or qualitatively will support different numbers of parasites. The nature of habitat selection can be detected by isodars, lines along which habitat selection yields equivalent fitness reward. We applied this approach to study host selection of five fleas, each infesting two desert rodents. Xenopsylla conformis, Xenopsylla ramesis, Nosopsyllus iranus theodori and Stenoponia tripectinata medialis parasitize Gerbillus dasyurus and Meriones crassus. Synosternus cleopatrae pyramidis parasitizes Gerbillus andersoni allenbyi and Gerbillus pyramidum. Three fleas (X. conformis, X. ramesis and S. c. pyramidis) were able to perceive quantitative (amount of the resource; e.g. organic matter in the nest for flea larvae) and/or qualitative (pattern of resource acquisition; e.g. host defensiveness) differences between hosts. Two other fleas did not perceive between-host differences. X. conformis was a density-dependent host selector that showed sharp selectivity at low density. X. ramesis and S. c. pyramidis were density-independent host selectors with a direct correspondence of density with habitat quality. N. i. theodori and S. t. medialis were non-selectors with no relationship at all between density and host quality. The results of the application of the isodar theory suggest that ectoparasites, like other animals, behave as if they are able to make choices and decisions that favour environments in which their reproductive benefit is maximized.

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.

ADDITIONAL RECORDS OF FLEAS (SIPHONAPTERA) ON WILD RODENTS IN THE SOUTHERN PART OF ISRAEL

ABSTRACT Results, of a survey of fleas on small rodents of southern Israel conducted from 1992 to 1998 are presented. From 18 localities, 14 rodent species were sampled, yielding 14 species of fleas. Nosopsyllus iranus theodori is reported from Gerbillus gerbillus, Gerbillus henleyi, and Mus musculus in Israel for the first time. Additional records of Coptopsylla africana in Israel substantiate the presence of this species in Israel. Other distributional records and host preferences are reported and discussed.