The species-area rule
... [T]he "species-area curve" ... is based on the simple
observation that every community of species needs a habitat.
The larger the habitat, the more species it can support.
In the 1960s, [E. O.] Wilson and the late Robert H. MacArthur
tallied the number of species on islands of various sizes,
eventually constructing what is now known as the theory of island
The theory is usually summed up by the rule that N,
of species, is proportional to A.27
where A is the area.
Extinction curves are calculated by inverting the relationship:
treating habitats as "islands" and asking what happens to
species as the island shrinks.
Clearly, if a habitat drops below a minimum size, the community as
a whole will cease to exist.
But how fast does this take place?
How much room is there for recovery?
"The rule that is followed for teaching purposes," Wilson says,
"is that for every 90% loss in area, the number of species that
can live indefinitely there is cut by one-half."
from C. C. Mann, Extinction: Are Ecologists Crying Wolf?,
Science 253, 736--738 (1991)
(C) copyright American Association for the Advancement of Science;
reproduced here for a private educational purpose
Why does this rule work?