The strength of interactions among species in a network tends to be highly asymmetric. We evaluate the hypothesis that this asymmetry results from the distribution of abundance among species, so that species interactions occur randomly among individuals. We used a database on mutualistic and antagonistic bipartite quantitative interaction networks. We show that across all types of networks asymmetry was correlated with abundance, so that rare species were asymmetrically affected by their abundant partners, while pairs of interacting abundant species tended to exhibit more symmetric, reciprocally strong effects. A null model shows that abundance provides a sufficient explanation of the asymmetry structure in some networks, but suggests the role of additional factors in others. Although not universal, our hypothesis holds for a substantial fraction of networks analyzed here, and should be considered as a null model in all studies aimed at evaluating the ecological and evolutionary consequences of species interactions.

Summary1. Understanding the structure of ecological networks is a crucial task for interpreting community and ecosystem responses to global change. 2. Despite the recent interest in this subject, almost all studies have focused exclusively on one specific network property. The question remains as to what extent different network properties are related and how understanding this relationship can advance our comprehension of the mechanisms behind these patterns. 3. Here, we analysed the relationship between nestedness and modularity, two frequently studied network properties, for a large data set of 95 ecological communities including both plant-animal mutualistic and host-parasite networks. 4. We found that the correlation between nestedness and modularity for a population of random matrices generated from the real communities decreases significantly in magnitude and sign with increasing connectance independent of the network type. At low connectivities, networks that are highly nested also tend to be highly modular; the reverse happens at high connectivities. 5. The above result is qualitatively robust when different null models are used to infer network structure, but, at a finer scale, quantitative differences exist. We observed an important interaction between the network structure pattern and the null model used to detect it. 6. A better understanding of the relationship between nestedness and modularity is important given their potential implications on the dynamics and stability of ecological communities.

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Summary1. Recent studies have evaluated the distribution of specialization in species interaction networks. Species abundance patterns have been hypothesized to determine observed topological patterns. We evaluate this hypothesis in the context of host-parasite interaction networks. 2. We used two independent series of data sets, one consisting of data for seven sites describing interactions between freshwater fish and their metazoan parasites and another consisting of data for 25 localities describing interactions between fleas and their mammalian hosts. We evaluated the influence of species abundance patterns on the distribution of specialization in these host-parasite interaction networks with the aid of null models. 3. In parallel with recent studies of plant-animal mutualistic networks, our analyses suggest that host-parasite interactions in these systems are highly asymmetric: specialist parasites tend to interact with hosts with high parasite richness, whereas hosts with low parasite richness tend to interact mainly with generalist parasites. 4. The observed distribution of specialization was predicted by a null model that assumed that species-specific probabilities of being assigned a link during the randomization process were roughly proportional to their relative abundance. Thus, abundant hosts tend to harbour richer parasite faunas, with a high proportion of rare specialists.

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.

The strength of interactions among species in a network tends to be highly asymmetric. We evaluate the hypothesis that this asymmetry results from the distribution of abundance among species, so that species interactions occur randomly among individuals. We used a database on mutualistic and antagonistic bipartite quantitative interaction networks. We show that across all types of networks asymmetry was correlated with abundance, so that rare species were asymmetrically affected by their abundant partners, while pairs of interacting abundant species tended to exhibit more symmetric, reciprocally strong effects. A null model shows that abundance provides a sufficient explanation of the asymmetry structure in some networks, but suggests the role of additional factors in others. Although not universal, our hypothesis holds for a substantial fraction of networks analyzed here, and should be considered as a null model in all studies aimed at evaluating the ecological and evolutionary consequences of species interactions.

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.

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.