Species-Area Relationship

The species-area relationship is the positive relationship between the number of species and the size of the area in which they live. As the area increases, the number of species that are found within the area of interest also increases. The number of species found in an area is referred to as species richness. The species-area relationship is a general and widely documented pattern of community ecology and biogeography. It has become so well accepted that it is sometimes referred to as a rule or a law. Due to its fundamental importance in describing ecological relationships, the species-area relationship has been prominently incorporated into models of island biogeography and has been used in conservation decisions.

Fundamentally, the species-area relationship is a very simple conceptual relationship between species richness and area. When a small area of a landscape is observed, only a few of the species within the landscape will be included within the boundaries of the area. These species are likely the more common species within the landscape. As the observed area is expanded, it is more likely to include some of the rarer species in the landscape, thus increasing the observed species richness. Once a starting point is defined and the study area is expanded around that point, species richness will never decline. The number of species either increases as the study area increases or [Page 2672]remains the same. Even if the loss of species occurs within an area through time, the relationship between species richness and area within each time snapshot will not be negative.

While the species-area relationship is a positive relationship between species richness and area, the relationship is not linear. As the observed area is expanded and more rare species are observed, the number of unrecorded species in the landscape declines, and fewer additions to the species richness value occur. This relationship was modeled in 1920 by Arrhenius as

S = cAz,

where S is the species richness, c is a constant, A is the area, and z is a fitted value that represents the slope of the linear relationship for the log of the equation. This model is better known as the power model. Another common model used to describe the relationship between species richness and area is the Gleason model, which was developed in 1922. The equation of the Gleason model is

S = d + k log(A),

where S is the species richness, d is the y-intercept, k is the slope of the line, and A is the area. Both equations have been used extensively to describe how species richness increases as area increases, but neither is considered to be applicable in all cases. A major criticism of both equations is that they do not approach a maximum species richness value as area increases. In theory, there are a limited number of species within a landscape, but both the Arrhenius and Gleason models allow species richness to increase without limit as area increases. While in a practical sense, there will be a limit to the area that is being studied, thus making the Arrhenius and Gleason models not completely applicable in most studies, it is important to recognize that the models are just representations of the observed relationship between species richness and area. The inherent flexibility of the equations allows them to be fitted to maximize their representation of a data set. They are tools to quantitatively describe how species richness increases as area increases.

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