Fragmented systems tend to exhibit distinctive patterns of species richness and species composition.
Because local extinctions on fragments are no longer balanced by immigration from surrounding areas, their species-area relationships characteristically exhibit high slopes. Small fragments often support many fewer species than an equivalent area of contiguous habitat. In addition, fragments often support nested subsets of species, where the species comprising smaller local assemblages constitute a proper or included subset of the species in richer ones. Nestedness appears most prevalent and most strongly developed in fragmented systems, where species composition has been sculpted by local extinction. However, this structure also characterizes other kinds of ecological systems.