Mixed effects of habitat fragmentation on species richness and community structure in a microarthropod microecosystem

1. Theory is unclear about the optimal degree of isolation of habitat fragments where the aim is to maximise species richness. In a field-based micro-ecosystem of Collembola, and predatory and non-predatory mites, moss patches of the same total area were fragmented to varying degrees. The habitat was left for several months to allow the communities to approach a new state of equilibrium. 2. The species richness (in particular of predatory mites) of a given area of habitat was greater when it was part of a large mainland area than part of an island, in agreement with theory. 3. Conversely, species richness and abundance were largely unaffected by fragmentation of a fixed area of island habitat. In this case, it is suggested here that the advantages of several small patches (e.g. reduced impact of environmental stochasticity, wider range of habitats overall) were equally balanced by the advantages of a single large patch (e.g. reduced effect of demographic stochasticity, wider range of habitats within a single patch, reduced edge effect), or that both effects were small. 4. The shapes of rank-abundance curves were similar among the levels of fragmentation of a fixed area of island habitat, implying that fragmentation had little impact on community structure. Conversely, the species composition of non-predatory mites varied weakly, but significantly, by fragmentation.