Nested biotas and biological conservation: metrics, mechanisms, and meaning of nestedness

Abstract The biotas of archipelagos and fragmented habitats frequently show a ‘nested’ structure. That is, the species composition of a small island or fragment tends to be a subset of the next larger island or fragment, and the set of biotas as a whole forms a nested series. Several indices exist that allow ‘nestedness’ to be quantified and its statistical significance assessed. Like the species-area relationship, nested structures are observed in many types of habitat and in many taxa, and also like the species-area relationship, it has been suggested to have implications for biological conservation, particularly in connection with the ‘single large or several small’ (SLOSS) debate regarding nature reserve design. In a perpectly nested archipelago of habitats, a large, species-rich reserve would necessarily conserve more species than any combination of smaller ones because (by definition) these smaller biotas would contain only those species present in the larger reserve. If the structure deviates from perfect nestedness, however, the situation is less straightforward, and depends on the degree of nestedness and the processes generating nested structure. Nested structure is generally attributed to differential local extinction of species, which would result in smaller biotas converging on the same set of extinction-resistant species. Nested structure can also be produced by differential immigration, nested habitat distribution, and passive sampling. Computer simulations demonstrate that highly nested patterns can be produced by passive sampling, and that these tend to be ‘outlier-rich’. In this case, large reserves have no advantage over aggregates of smaller reserves with equal total area. More work needs to be done on the underlying causes of nestedness before its full significance for biological conservation can be confidently assessed.