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Species-Area Relations

S = cA^z

or

log S = log c + z log A

where S is the number of species (richness), A the area, z the exponent, and c the constant of
proportionality or the expected number of species occurring on one unit of area (one related to
the units of measure of area, e.g., sq. mile, km², etc.).

In studies of birds:

S = 2.53 A ^0.165 (Harris)
S = 1.16 A^0.176 (British breeding birds)
S = 1.17 A^0.24 (wintering British birds)

and mammals

S = 1.19 A^0.326

where A is in square miles.

z typically ranges from 0.25 to 0.35. (It may be very close to 0.33 or a cube-root.)

when z = 0.30, a 10-fold increase in area is necessary to double the number of species. A 90% reduction in area can lead to a 50% decrease in species in the area.

Rafe et al. (1985: 332), faced with taking the logarithm of a zero area, assigned all habitats not present an arbitrary value of 0.1 ha partially on the grounds that such an area was "... too small to hold a major assemblage of species."

A typical strategy is to use the conventional transformation of species (y) and area (x) as log (y +1) and log (x + 1).

An optional view is to study mean density of species per occupied plot, an old ecological trick.

One study found a species-elevation relationship

S = 101.1 - 4.4 x 10-3X - 8 x 10-7x2

Rafe et al. (1985: 331) found, to no one's surprise, that a greater amount of variation in richness is explained by species-habitat relations than by species-area relationships. Increasing the area has an effect over and above that due to increase in habitat diversity with area (Rafe et al. 1985: 332). For sites of equal area, greatest richness will occur on the sites with the most habitat types.

Species-area curves are used for:

• estimating species present, richness or its synonym diversity
• predicting number of extinctions due to habitat destruction
• estimating the minimum size of a refuge needed to preserve a given species or number of species

Beta or gamma species are found within more than one habitat type.

Species tend to be more numerous where

• islands are close to "shore" or a source
• islands are large
• islands are diverse heterogenous
• islands are complex (mountainous, ect.) climatic
• islands have foilage height diversity
• islands are more mature (time since volcanoes etc.)

Future developments are to be found in treating regions (counties, provinces, watersheds, etc.) as "islands."

Literature Cited

Harris, L.D. 1984. The fragmented forest. Univ. Chicago Press. Chicago, IL. 211 pp.

Rafe, R.W., M.B. Usher, and R.G. Jefferson. 1985. Birds on reserves: the influence of area and habitat on species richness. J. Appl. Ecol. 22: 327-335.

This Web site is maintained by R. H. Giles, Jr.
Last revision January 17, 2000.