Species–Area Relationship

Abstract

Abstract The species–area relationship (SAR) describes the increase in species numbers with increasing area and is often referred to as one of ecology's few genuine laws. Different (mathematical) regression models can be fitted to species–area data to generate a species–area curve. The power model, S = cA z , is the most commonly used, but more than 20 models have been proposed. Community ecologists of the early nineteenth century were the first to propose a mathematical model. Subsequently, the SAR has become an important part of biogeography, macroecology and conservation. Two main types of SARs are described: those generated from sample areas (mainlands) and those generated from isolates (islands), more or less isolated from each other. There are many applications of the SAR: to identify, explain and compare patterns in nature, to extrapolate (upscale) species numbers and to forecast changes in species numbers – for example extinctions from habitat loss. Key Concepts Species‐area curves result from graphing the model, typically either the power model or the logarithmic model, fitted to species‐area data. Isolate species–area relationships (iSARs) result from a comparison of the number of species on oceanic islands or other types of isolates, for example mountain tops (‘sky islands’) or forest remnants. Sample‐area species–area relationships (saSARs) result by accumulating the number of species when adding new, typically continuous, sample areas (or increase the surveyed area) on mainlands (also referred to as ‘mainland SARs’). Nested sampling areas are spatially organised so that each smaller area is completely contained within the next area, larger than the previous. The z ‐value is the exponent of the power‐law SAR and the most preferred SAR measurement. It is often described as the ‘slope’, because it becomes the slope in log–log space, although in reality, z is the rate at which the species–area curve decelerates. Self‐similarity (or scale invariance), resulting if the species–area relationship is power law, causes the same proportional (or percentage) increase in species number for each doubling of area size. The z ‐value controls this proportion. Minimum‐area effects (MAEs) result from resource restrictions, when the isolate (or island) becomes too small to sustain viable populations of some species. The equilibrium theory of island biogeography was proposed by MacArthur and Wilson in 1967 to explain species richness of oceanic islands, and also applies to other isolates.