The species-area relationship: new challenges for an old pattern

The species-area relationship (i.e., the relationship between area and the number of species found in that area) is one of longest and most frequently studied patterns in nature. Yet there remain some important and interestin g questions on the nature of this relationship, its causality, quantificati on and application for both ecologists and conservation biologists. Traditionally, the species-area relationship describes the very general ten dency for species number to increase with island area; a relationship whose slope declines (but remains positive) as area increases. The true relation ship, however, may be much more complicated than this, and may in many case s approximate a sigmoidal relationship. On small islands, species number ma y vary independently of island area. Species richness then increases as we consider larger islands, but the curve eventually slows and asymptotes or l evels off when richness equals that of the the source or mainland pool. The relationship may also include a secondary phase of increase in richness if island area becomes large enough to allow in situ speciation. Causal explanations for this relationship may, therefore, need to be multif actorial and include a range of processes from disturbance and stochastic v ariation in habitat quality on the very small islands, to ecological intera ctions, immigration, extinction and, finally, evolution on the larger islan ds.