The Species-Area Curve

Species Area Curve

What is the Species-Area Relationship Curve?

The Species-Area curve is a graphical representation of species richness or number of species found in a habitat, plotted against area of habitat. It shows that species richness increases with increase in habitable area.

The graph is a rectangular hyperbola, when plotted in non-logarithmic scale, whereas a straight ascending line, when plotted in logarithmic scale.

Equation on Non-Logarithmic Scale

    \[S=c A^{z}\]

where, S is species richness (number of species), A is habitable area, z is regression coefficient and c is constant of proportionality. The value of c depends on the unit used for area measurement, and equals the number of species that would exist if the habitat area was confined to one square unit.

Value of z (Regression Coefficient)

Ecologists have discovered that the value of z lies in the range of 0.1 to 0.2, regardless of the taxonomic group or the region (whether it is the plants in Britain, birds in California or molluscs in New York state, the slopes of the regression line are amazingly similar). But, if you analyse the species-area relationships among very large areas like the entire continents, you will find that the slope of the line to be much steeper (z values in the range of 0.6 to 1.2).

For example, for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is found to be 1.15.

Equation on Logarithmic Scale

On a logarithmic scale, the relationship is a straight line described by the equation:

    \[log (S)=\log \left(c A^{z}\right)=\log (c)+z \log (A)\]

Famous American ecology Henry A. Gleason gave the semi-log equation in which species number is non-logarithmic, whereas, other factors in the equation are taken as logarithmic, to obtain a straight line curve. Such a curve is called a semi-log axes curve.

    \[(S)=\log \left(c A^{z}\right)=\log (c)+z \log (A)\]

Common Uses of this Model

Species–area curves are often graphed for islands (or habitats that are otherwise isolated from one another, i.e. not dynamically interacting, such as woodlots in an agricultural landscape) of different sizes. Although larger islands obviously tend to have more species, it is possible that a smaller island might have more species than a larger one.

Limitations

The S-A curves works only with major assumptions. In this model, we assume that larger areas tend to contain larger numbers of species, and empirically, the relative numbers seem to obey systematic mathematical relationships. The species–area curve is usually plotted for a single type of organism, such as all vascular plants or all species of a specific trophic level within a particular site.

It is rarely, if ever, constructed for all types of organisms (if simply) because of the prodigious data requirements and therefore increased chances of errors.