speciesarea

The species-area relationship (SAR) describes how the number of observed species tends to increase with the area sampled. It is a widely observed pattern in ecology and biogeography, and while the exact form varies, larger areas typically harbor more species, though the rate of increase slows at larger sizes.

The most common mathematical form is a power law: S = cA^z, where S is species richness, A is area, c is a constant, and z is the slope on a log-log plot. Taking logs gives log S = log c + z log A. Typical z values range from about 0.2 to 0.4 for islands and 0.1 to 0.3 for continental terrains, though estimates depend on taxon and region.

Mechanisms underlying SAR include increased habitat diversity and resource availability in larger areas, larger population sizes that reduce extinction risk, and higher immigration rates. On islands, the theory of island biogeography links area to colonization and extinction dynamics and predicts predictable differences in SAR with isolation. The relationship also holds across broad spatial scales, from local plots to continental extents, where arrangement of habitats and edge effects can influence the pattern.

Scale, variation, and methods: Researchers distinguish between sample-based and contiguous-area SAR and use tools such as rarefaction or extrapolation to compare different sample sizes. Habitat diversity and spatial configuration influence the shape of the SAR and the estimated z value.

Applications and caveats: SAR informs biodiversity impact assessments, reserve design, and extinction risk estimation under habitat loss. Limitations include taxon-specific variation, non-equilibrium dynamics, habitat quality, edge effects, and sampling biases that can affect parameter estimates.

Summary: As a foundational concept in macroecology and island biogeography, the species-area relationship helps quantify how geography shapes biodiversity but must be interpreted within its ecological and methodological context.