Species abundance and the distribution of specialization in host-parasite interaction networks

1. 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.