Structure of compound and component communities of fleas parasitic on small mammals in six different regions as revealed by environmental-based co-occurrence geometry analyses.

We inferred the patterns of co-occurrence of flea species in compound (across all host species) and component (across conspecific hosts) communities from six regions of the world (Mongolia, Northwest Argentina, Argentinian Patagonia, West Siberia, Slovakia, and South Africa) using the novel eigenvector ellipsoid method. This method allows us to infer structural community patterns by comparing species' environmental requirements with the pattern of their co-occurrences. We asked whether: (a) communities are characterized by species segregation, nestedness, or modularity; (b) patterns detected by the novel method conform to the patterns identified by traditional methods that search for non-randomness in community structure; and (c) the pattern of flea species co-occurrences in component communities is associated with host species traits. The results of the application of the eigenvector ellipsoid method suggested that the co-occurrence of flea species was random in all compound communities except in South Africa, where this community demonstrated a tendency to be nested. Flea species co-occurrences were random in many component communities. Species segregation was detected in the flea community of one host, whereas the flea communities of 14 hosts from different regions appeared to be nested. No indication of a modular structure in any community was found. The nestedness of flea component communities was mainly characteristic of hosts with a low relative brain mass. We concluded that the application of this novel method that combines data on species distribution and their environmental requirements allows better identification of the community structural patterns and produces more reliable results as compared with traditional methods.

Dissimilarity in flea and host assemblages and their interaction networks along a spatial distance gradient: different patterns revealed by different network dissimilarity metrics.

We investigated the distance-decay pattern (an increase in dissimilarity with increasing geographic distance) in regional assemblages of fleas and their small mammalian hosts, as well as their interaction networks, in four biogeographic realms. Dissimilarity of assemblages (ßtotal) was partitioned into species richness differences (ßrich) and species replacement (ßrepl) components. Dissimilarity of networks was assessed using two metrics: (a) whole network dissimilarity (ßWN) partitioned into species replacement (ßST) and interaction rewiring (ßOS) components and (b) D statistics, measuring dissimilarity in the pure structure of the networks, without using information on species identities and calculated for hosts-shared-by-fleas networks (Dh) and fleas-shared-by-hosts networks (Df). We asked whether the distance-decay pattern (a) occurs among interactor assemblages or their interaction networks; (b) depends on the network dissimilarity metric used; and (c) differs between realms. The ßtotal and ßrepl of flea and host assemblages increased with distance in all realms except for host assemblages in the Afrotropics. ßrich for flea and host assemblages increased with distance in the Nearctic only. In networks, ßWN and ßST demonstrated a distance-decay pattern, whereas ßOS was mainly spatially invariant except in the Neotropics. Correlations of Dh or Df and geographic distance were mostly non-significant. We conclude that investigations of dissimilarity in interaction networks should include both types of dissimilarity metrics (those that consider partner identities and those that consider the pure structure of networks). This will allow elucidating the predictability of some facets of network dissimilarity and the unpredictability of other facets.

Functional similarity affects similarity in partner composition in flea-mammal networks.

Functional signal in an interaction network is a phenomenon in which species resembling each other in their traits interact with similar partners. We tested the functional signal concept in realm-specific and regional flea-host networks from four biogeographic realms and asked whether the species composition of (a) host spectra and (b) flea assemblages is similar between functionally similar flea and host species, respectively. Analogously to testing for phylogenetic signal, we applied Mantel tests to investigate the correlation between flea or host functional distances calculated from functional dendrograms and dissimilarities in sets of interacting partners. In all realm-specific networks, functionally similar fleas tended to exploit similar hosts often belonging to the same genus, whereas functionally similar hosts tended to harbour similar fleas, again often belonging to the same genus. The strength of realm-specific functional signals and the frequency of detecting a significant functional signal in the regional networks differed between realms. The frequency of detecting a significant functional signal in the regional networks correlated positively with the network size for fleas and with the number of hosts in a network for hosts. A functional signal in the regional networks was more frequently found for hosts than for fleas. We discuss the mechanisms behind the functional signal in both fleas and their hosts, relate geographic functional signal patterns to the historic biogeography of fleas and conclude that functional signals in the species composition of host spectra for fleas and of flea assemblages for hosts result from the interplay of evolutionary and ecological processes.

Searching for common patterns in parasite ecology: species and host contributions to beta-diversity in helminths of South African ungulates and fleas of South American rodents.

We searched for common patterns in parasite ecology by investigating species and host contributions to the beta-diversity of infracommunities (=assemblages of parasites harboured by a host individual) in helminths of three species of South African ungulates and fleas of 11 species of South American rodents, assuming that a comparison of patterns in distinctly different parasites and hosts would allow us to judge the generality or, at least, commonness of these patterns. We used data on species' composition and numbers of parasites and asked whether (i) parasite species' attributes (life cycle, transmission mode, and host specificity in helminths; possession of sclerotized combs, microhabitat preference, and host specificity in fleas) or their population structure (mean abundance and/or prevalence) and (ii) host characteristics (sex and age) affect parasite and host species' contributions to parasite beta-diversity (SCBD and HCBD, respectively). We found that parasite species' morphological and ecological attributes were mostly not associated with their SCBD. In contrast, parasite SCBD, in both ungulates and rodents, significantly increased with either parasite mean abundance or prevalence or both. The effect of host characteristics on HCBD was detected in a few hosts only. In general, parasite infracommunities' beta-diversity appeared to be driven by variation in parasite species rather than the uniqueness of the assemblages harboured by individual hosts. We conclude that some ecological patterns (such as the relationships between SCBD and parasite abundance/prevalence) appear to be common and do not differ between different host-parasite associations in different geographic regions, whereas other patterns (the relationships between SCBD and parasite species' attributes) are contingent and depend on parasite and host identities.

Relationships between functional alpha and beta diversities of flea parasites and their small mammalian hosts.

We studied the relationships between functional alpha and beta diversities of fleas and their small mammalian hosts in 4 biogeographic realms (the Afrotropics, the Nearctic, the Neotropics and the Palearctic), considering 3 components of alpha diversity (functional richness, divergence and regularity). We asked whether (a) flea alpha and beta diversities are driven by host alpha and beta diversities; (b) the variation in the off-host environment affects variation in flea alpha and beta diversities; and (c) the pattern of the relationship between flea and host alpha or beta diversities differs between geographic realms. We analysed alpha diversity using modified phylogenetic generalized least squares and beta diversity using modified phylogenetic generalized dissimilarity modelling. In all realms, flea functional richness and regularity increased with an increase in host functional richness and regularity, respectively, whereas flea functional divergence correlated positively with host functional divergence in the Nearctic only. Environmental effects on the components of flea alpha diversity were found only in the Holarctic realms. Host functional beta diversity was invariantly the best predictor of flea functional beta diversity in all realms, whereas the effects of environmental variables on flea functional beta diversity were much weaker and differed between realms. We conclude that flea functional diversity is mostly driven by host functional diversity, whereas the environmental effects on flea functional diversity vary (a) geographically and (b) between components of functional alpha diversity.

Nestedness of flea assemblages harboured by small mammalian hosts revisited: phylogenetic and functional nestedness do not follow compositional nestedness.

We studied compositional, phylogenetic, and functional nestedness in the flea assemblages of 14 host species across regions. Our main questions were (a) are a host's flea assemblages compositionally, phylogenetically, or functionally nested? (b) Do similar processes drive these nestedness facets? (d) Are a host's biological traits associated with nestedness of its flea assemblages? Rows of host matrices were ordered by decreasing species richness/the sum of the branch lengths of a phylogenetic tree/functional dendrogram or by decreasing region area or by increasing distance from the centre of a host's geographic range. None of the matrices sorted by species richness/sum of branch lengths were nested from a compositional perspective, but they were significantly nested from phylogenetic and functional perspectives. Compositional, phylogenetic, and functional nestedness of matrices sorted by region area or by distance from the host's geographic range centre varied between hosts. In some hosts, flea assemblages were nested from all three perspectives independently of how matrix rows were sorted, whereas in other hosts, the occurrence of significant nestedness depended on the order of the matrix rows. The degree of phylogenetic and functional nestedness for matrices sorted by the sum of branch lengths was associated with a host species' morphoecological traits and the latitude of its geographic range. We conclude that consideration of nestedness based solely on species composition does not allow a comprehensive understanding of the patterns of parasite community structure. Nestedness should also be considered from phylogenetic and functional perspectives.

Environment and traits affect parasite and host species positions but not roles in flea-mammal networks.

We studied spatial variation in the effects of environment and network size on species positions and roles in multiple flea-mammal networks from four biogeographic realms. We asked whether species positions (measured as species strength [SS], the degree of interaction specialization [d'], and the eigenvector centrality [C]) or the roles of fleas and their hosts in the interaction networks: (a) are repeatable/conserved within a flea or a host species; (b) vary in dependence on environmental variables and/or network size; and (c) the effects of environment and network size on species positions or roles in the networks depend on species traits. The repeatability analysis of species position indices for 441 flea and 429 host species, occurring in at least two networks, demonstrated that the repeatability of SS, d', and C within a species was significant, although not especially high, suggesting that the indices' values were affected by local factors. The majority of flea and host species in the majority of networks demonstrated a peripheral role. A value of at least one index of species position was significantly affected by environmental variables or network size in 41 and 36, respectively, of the 52 flea and 52 host species that occurred in multiple networks. In both fleas and hosts, the occurrence of the significant effect of environment or network size on at least one index of species position, but not on a species' role in a network, was associated with some species traits.

Metabolic rate and ecological traits of ectoparasites: a case study with seven flea species from the Negev Desert.

We studied the relationship between fleas' metabolic rate and their ecological traits, using data on standard metabolic rate (SMR), mean abundance, host specificity, and geographic range size in males and females of seven desert flea species. SMR was measured via mass-specific CO2 emission, whereas host specificity was measured as (a) the mean number of host species used by a flea per region in regions where this flea was recorded; (b) the total number of host species a flea exploited across its geographic range; and (c) the phylogenetic diversity of the flea's hosts. To control for confounding effects of phylogeny when analysing data on multiple species, we applied the Phylogenetic Generalised Least Squares (PGLS) model. We found that the only ecological trait significantly correlating with flea SMR was the phylogenetic diversity of hosts utilized by a flea across its geographic range. The strength of the association between SMR and host phylogenetic diversity was higher in male than in female fleas. We explain the relationship between flea SMR and their host specificity by the necessity of host-opportunistic species to compensate for the high energetic cost of neutralizing multiple defences from multiple hosts by increased SMR.

Compositional and phylogenetic nestedness of host assemblages exploited by generalist ectoparasites across their geographic ranges: drivers and associations with ectoparasite traits.

We investigated compositional and phylogenetic nestedness in the host assemblages of 26 host-generalist fleas across regions within the Palearctic. We asked the following questions: (i) are host assemblages exploited by a flea species compositionally or phylogenetically nested (=C-nested and P-nested, respectively) across regions?; (ii) if yes, what are the processes that generate nestedness, and does phylogenetic nestedness follow the same processes as compositional nestedness?; and (iii) are the biological traits of a flea species associated with its host assemblages' degree of nestedness? Nestedness was calculated for matrices with rows ordered either by decreasing region area (=a-matrices) or increasing distance from the centre of a flea's geographic range (d-matrices). Significant C-nestedness was found in either a- (three fleas) or d-matrices (three fleas) or both (10 fleas). Significant P-nestedness was detected in either a- (three fleas) or d-matrices (four fleas) or both (two fleas). In some but not other species, P-nestedness followed C-nestedness. The probability of C-nestedness to be significant, as well as its degree for d-matrices, was associated with a flea's morphoecological traits, whereas this was not the case for either a-matrices or the P-nestedness for either type of ordered matrices. We conclude that compositional, but not phylogenetic, nestedness is (i) generated by similar mechanisms in many flea species and (ii) may be simultaneously driven by different mechanisms in the same flea. In contrast, mechanisms promoting phylogenetic nestedness differ between flea species and seem to act separately.

Free-living gerbils with higher testosterone take fewer risks.

Among the physiological differences between the sexes are circulating androgen levels. Testosterone (T) is an androgen that has been linked to aggression and risk-taking in male vertebrates, so that males with higher T are generally more aggressive and take more risks. In females, T is not often measured, and its relationship with behaviour has been less studied. The costs of elevated T are assumed to be higher for reproductive females, while the benefits higher for males. Here, we tested the association between endogenous T and risk-taking behaviours in both males and females under well-studied experimental settings in free-living Baluchistan gerbils (Gerbillus nanus; Gn). In addition, we experimentally elevated Gn T levels using implants and measured risk-taking behaviour. Surprisingly, we found that there were no differences in the association between T and risk-taking behaviours between males and females, and that in both sexes, Gn with higher T levels took fewer risks. We also found that Gn spent equal time foraging between risky (open habitat) and safe (under a bush) experimental food patches. We expected Gn, which are nocturnal, to take fewer risks during full moon nights, but found that Gn were more active during moon lit nights than during dark (new moon) nights. This study demonstrates that T has many functions, and that its effects are complex and often unpredictable. It also shows that hypotheses regarding the propensity to take risks under specific coverage and light regimes are not universal, and likely include variables such as species, environment, context, and predator-specific behavioural strategies.

Phylogenetic patterns in regional flea assemblages from 6 biogeographic realms: strong links between flea and host phylogenetic turnovers and weak effects of phylogenetic originality on host specificity.

We investigated phylogenetic patterns in flea assemblages from 80 regions in 6 biogeographic realms and asked whether (a) flea phylogenetic turnover is driven by host phylogenetic turnover, environmental dissimilarity or geographic distance; (b) the relative importance of these drivers differs between realms; and (c) the environmental drivers of flea phylogenetic turnover are similar to those of host phylogenetic turnover. We also asked whether the phylogenetic originality of a flea species correlates with the degree of its host specificity and whether the phylogenetic originality of a host species correlates with the diversity of its flea assemblages. We found that host phylogenetic turnover was the best predictor of flea phylogenetic turnover in all realms, whereas the effect of the environment was weaker. Environmental predictors of flea phylogenetic turnover differed between realms. The importance of spatial distances as a predictor of the phylogenetic dissimilarity between regional assemblages varied between realms. The responses of host turnover differed from those of fleas. In 4 of the 6 realms, geographic distances were substantially better predictors of host phylogenetic turnover than environmental gradients. We also found no general relationship between flea phylogenetic originality and its host specificity in terms of either host species richness or host phylogenetic diversity. We conclude that flea phylogenetic turnover is determined mainly by the phylogenetic turnover of their hosts rather than by environmental gradients. Phylogenetic patterns in fleas are manifested at the level of regional assemblages rather than at the level of individual species.

Latitudinal gradients in body size and sexual size dimorphism in fleas: males drive Bergmann's pattern.

We tested for the effects of latitude and geographic range size (GRS) on body size, leg length, and sexual size dimorphism (SSD) across 103 species of fleas, taking into account phylogenetic between-species relationships. When the data on body size were combined for males and females, the positive correlation between body size and latitude, but not GRS, was revealed. When the analysis was restricted to one sex only, the effect of latitude appeared to be non-significant for females, whereas male body size increased with an increase in latitude. Intraspecific body size variation was not associated with either the latitude or the latitudinal span of the geographic range, independently of which data subset was analyzed. No evidence of association between size-independent tibia length and latitude was found for either females, males, or both sexes combined. The degree of SSD decreased with a decrease in latitude but was not affected by GRS. We conclude that macroecological patterns might be manifested differently in males and females. This should be kept in mind while searching for these patterns.

The species composition of local flea assemblages at a small scale in two South American regions is predominantly driven by niche-based mechanisms.

We applied a step-down factor analysis (SDFA) and multi-site generalised dissimilarity modelling (MS-GDM) to local flea communities harboured by small mammals (i.e., collected at small sampling sites over a short time period) in two South American regions (Patagonia and the Northwestern Argentina) with the aim of understanding whether these communities were assembled via niche-based or dispersal-based processes. The SDFA allows us to determine whether clusters of flea assemblages across different types of climates, vegetation and soils can be distinguished (suggesting niche-based assembly). MS-GDM allows us to determine whether a substantial proportion of the variation in flea species turnover is explained by specific climate-associated, vegetation-associated and soil-associated variables (indicating niche-based assembly) or host turnover (indicating dispersal-based assembly). Mapping of assemblages on climate, vegetation and soil maps, according to their loadings on axis 1 or axis 2 of the SDFA, did not provide clear-cut results. Clusters of similar loadings could be recognized within some, but not other, climate, vegetation and soil types. However, MS-GDM demonstrated that the effect of environmental variables (especially air temperature) on flea compositional turnover was much stronger than that of host turnover, indicating the predominance of niche-based processes in local community assembly. A comparison of our results with those on the mechanisms that drive species assembly in regional communities allows us to conclude that local and regional communities result from the joint action of niche-based and dispersal-based processes, with the former more important at a smaller spatial scale and the latter at a larger spatial scale.

Regional flea and host assemblages form biogeographic, but not ecological, clusters: evidence for a dispersal-based mechanism as a driver of species composition.

We used data on the species composition of regional assemblages of fleas and their small mammalian hosts from 6 biogeographic realms and applied a novel method of step-down factor analyses (SDFA) and cluster analyses to identify biogeographic (across the entire globe) and ecological (within a realm across the main terrestrial biomes) clusters of these assemblages. We found that, at the global scale, the clusters of regional assemblage loadings on SDFA axes reflected well the assemblage distribution, according to the biogeographic realms to which they belong. At the global scale, the cluster topology, corresponding to the biogeographic realms, was similar between flea and host assemblages, but the topology of subtrees within realm-specific clusters substantially differed between fleas and hosts. At the scale of biogeographic realms, the distribution of regional flea and host assemblages did not correspond to the predominant biome types. Assemblages with similar loadings on SDFA axes were often situated in different biomes and vice versa. The across-biome, within-realm distributions of flea vs host assemblages suggested weak congruence between these distributions. Our results indicate that dispersal is a predominant mechanism of flea and host community assembly across large regions.

Phylogenetic signals in flea-host interaction networks from four biogeographic realms: differences between interactors and the effects of environmental factors.

The structure of ecological interaction networks is associated with evolutionary histories of the interacting species. This is reflected by the phylogenetic signals (PS) in these networks when closely related species interact with similar partners because some traits inherited from the ancestors may determine ecological interactions. We investigated PS for small mammalian hosts and fleas in 80 regional interaction networks from four biogeographic realms (the Palearctic, the Nearctic, the Afrotropics, and the Neotropics). We asked (i) whether the relative strength of PS in host-flea networks is similar between hosts and fleas and/or between realms; (ii) how environmental variation affects the PS of hosts and fleas in their interaction networks; and (iii) whether the PS for hosts or fleas is affected by the phylogenetic diversity of either hosts or fleas, respectively. We found that the PS for hosts was stronger than that for fleas in all realms. An environmental effect on the PS for hosts, but not for fleas, was found in three of the four realms (except the Neotropics). In the Palearctic and the Nearctic, a stronger PS was characteristic for cooler and/or drier regions, whereas the opposite was the case for the Afrotropics in regard to precipitation. The phylogenetic diversity of regional host and flea assemblages was not associated with the values of the respective PS in any realm. We conclude that the pattern of the relative strength of the PS for hosts and fleas in their interaction networks is similar in different biogeographic realms with vastly different host and flea faunas. However, the environmental effects on the PS are geographically variable and might be associated with the history of host-flea associations, as well as the spatial pattern of environmental variation, within a realm.

Fitness consequences of host colonization in two generalist fleas: Context-dependency and the effect of spatial co-occurrence.

We studied the fitness consequences of colonizing a novel host by experimental lines of fleas (Synosternus cleopatrae and Xenopsylla ramesis) maintained for 18-22 generations on the principal or novel (sympatric or allopatric) hosts via number, developmental success and size of the offspring of the fleas exploiting these hosts. We asked whether (a) fitness on non-principal hosts increases after prolonged maintenance; (b) the colonization success depends on the spatial co-occurrence of a flea and a host and (c) colonization of a novel host is accompanied by a decreased ability to exploit an original host. The ability of fleas to colonize novel hosts differed between species, with S. cleopatrae, but not X. ramesis, increasing its offspring production on novel hosts. Spatial co-occurrence did not affect colonization success. Maintenance on an alternative host was not accompanied by decreased adaptation to the original host. When fleas returned to the original host, their reproductive output was higher than that of their ancestors. We conclude that the success of colonizing a novel host is (a) context-dependent and varies between flea and host species and (b) not accompanied by the loss of ability to exploit an ancestral host but may lead to an increase in this ability.

Host phylogeny and ecology, but not host physiology, are the main drivers of (dis)similarity between the host spectra of fleas: application of a novel ordination approach to regional assemblages from four continents.

We investigated the patterns of phylogenetic and functional (dis)similarity in the species composition of host spectra between co-habitating generalist flea species in regional assemblages from four continents (Europe, Asia, North America and Africa) using a recently developed ordination approach (Double Similarity Principal Component Analysis). From the functional perspective, we considered physiological [body mass and basal metabolic rate (BMR)] and ecological (shelter depth and complexity) host traits. We asked (a) whether host phylogeny, physiology or ecology is the main driver of (dis)similarities between flea host spectra and (b) whether the patterns of phylogenetic and functional (dis)similarity in host spectra vary between flea assemblages from different continents. Phylogenetic similarity between the host spectra was highest in Africa, lowest in North America and moderate in Europe and Asia. In each assemblage, phylogenetic clusters of hosts dominating in the host spectra could be distinguished. The functional similarity between the host spectra of co-occurring fleas was low for shelter structure in all assemblages and much higher for body mass and BMR in three of the four assemblages (except North America). We conclude that host phylogeny and shelter structure are the main drivers of (dis)similarity between the host spectra of co-habitating fleas. However, the effects of these factors on the patterns of (dis)similarity varied across continents.

Dark host specificity in two ectoparasite taxa: repeatability, parasite traits, and environmental effects.

We applied the concept of dark diversity (species that may potentially inhabit a locality but are absent) to the host spectrum of a parasite and defined it as dark host specificity (DHS). We studied the trait-associated and geographic patterns of dark host specificity in fleas and gamasid mites parasitic on small mammals, asking the following questions: (a) Is dark host specificity repeatable across populations of the same species? (b) Is it associated with morphological and/or ecological species traits? (c) What are the factors associated with geographical variation in the DHS among populations of the same species? The DHS was repeatable within species with a large proportion of variance among samples, accounted for by differences between species. The average DHS of fleas, but not mites, was affected by parasite traits, with the DHS being higher in fleas with larger geographic ranges, higher characteristic abundance levels, and summer reproduction peaks. In the majority of ectoparasites, the regional DHS decreased with an increase in either structural or phylogenetic host specificity. The associations between the DHS and the environmental or host-associated characteristics of a region were revealed in a few species (eight of 22 fleas and three of 12 mites). The DHS decreased with (a) an increase in air temperature in two fleas, (b) a decrease in precipitation in two fleas, and (c) an increase in regional host species richness (in three fleas and three mites). Overall, our results suggest that dark host specificity in arthropod ectoparasites is a species-specific character associated, to a large extent, with the breadth of their host-related niches, while the influences of parasite traits and local environmental conditions are minor.

Temporal variation of metacommunity structure in arthropod ectoparasites harboured by small mammals: the effects of scale and climatic fluctuations.

We applied the elements of metacommunity structure (EMS) approach and studied the temporal dynamics of metacommunity structure in arthropod ectoparasites (fleas, gamasid mites and ixodid ticks) harboured by six small mammalian hosts sampled for three decades in the same locality in Western Siberia at three hierarchical scales (inframetacommunities, component metacommunities and a compound metacommunity). All metacommunities were positively coherent. Inframetacommunity structures varied across sampling periods in all host species. The main structural pattern in an inframetacommunity of the same host varied across sampling times but was mostly characterized by clumped species distributions (Clementsian, Gleasonian and their quasi-versions). Component metacommunities in five of the six host species were characterized by either a Clementsian or a quasi-Clementsian distribution. In four of the six host species, this pattern was driven by mite distribution. The temporal structure of compound metacommunity was characterized by a Clementsian pattern. In contrast to the majority of component metacommunities, this pattern was driven by fleas, whereas the temporal structure of gamasid mite compound metacommunities demonstrated a Gleasonian distribution. The temporal gradient in infracommunity composition was not associated with temporal changes in either air temperature or precipitation, whereas the precipitation gradient was positively correlated with the structure of component (in five host species) and compound metacommunities. In conclusion, the best-fit metacommunity structure of ectoparasites varies temporally due to temporal changes in distribution patterns that can be associated with year-to-year climatic variation, affecting both hosts and parasites.

Dark diversity of flea assemblages of small mammalian hosts: effects of environment, host traits and host phylogeny.

An assemblage of species in a locality comprises two components, namely (i) species that are present (realised diversity) and (ii) species from the regional pool that may potentially inhabit this locality due to suitable ecological conditions, but that are absent (dark diversity). We investigated factors affecting the dark diversity of component communities of fleas parasitic on small mammals in the northern Palearctic at two scales. First, we considered the dark diversity of flea assemblages of the same host (for 13 host species) across regions and tested for the effects of environmental factors and the number of available host species on the dark diversity of within-region flea assemblages. Second, we considered the dark diversity of fleas across host species within a region (for 20 regions) and asked whether within-host dark diversity is associated with host phylogeny and/or traits. We found that the dark diversity of flea assemblages harboured by small mammals varied substantially (i) within the same host species across space (in 12 of 13 host species) and (ii) between host species within a region (in eight of 20 regions). The size of the dark diversity of flea assemblages of the same host across regions was generally affected by environmental factors (mainly by the amount of green vegetation), whereas the size of the dark diversity of flea assemblages of a host species within a region was affected by host traits (mainly by the degree of host sociality and the structure of its shelter and, to a lesser degree, by its geographic range size) but was not associated with host phylogenetic affinities. We conclude that application of the dark diversity concept to parasite communities across space or hosts allows a better understanding of the factors affecting the species richness and composition of these communities.