NESTED BIOTAS AND BIOLOGICAL CONSERVATION - METRICS, MECHANISMS, AND MEANING OF NESTEDNESS

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 indi ces exist that allow 'nestedness' to be quantified and its statistical significance assessed. Like the species-area relationship, nested str uctures are observed in many types of habitat and in many taxa, and al so like the species-area relationship, it has been suggested to have i mplications for biological conservation, particularly in connection wi th the 'single large or several small' (SLOSS) debate regarding nature reserve design. In a perfectly nested archipelago of habitats, a larg e, species-rich reserve would necessarily conserve more species than a ny combination of smaller ones because (by definition) these smaller b iotas would contain only those species present in the larger reserve. If the structure deviates from perfect nestedness, however, the situat ion is less straightforward, and depends on the degree of nestedness a nd the processes generating nested structure. Nested structure is gene rally attributed to differential local extinction of species, which wo uld result in smaller biotas converging on the same set of extinction- resistant species. Nested structure can also be produced by differenti al immigration, nested habitat distribution, and passive sampling. Com puter simulations demonstrate that highly nested patterns can be produ ced 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 und erlying causes of nestedness before its full significance for biologic al conservation can be confidently assessed.