A typical example of the species-area relationship, showing the shape of the curve.
For the purposes of thinking about extinction, the species-area
relationship is key. One (admittedly simplified) way of conceiving of what
humans are doing to the world is that we are everywhere changing the
value of A. Consider, for example, a grassland that once covered a
thousand square miles. Let’s say the grassland was home to a hundred
species of birds (or beetles or snakes). If half of the grassland were
eliminated—converted into farmland or shopping malls—it should be
possible to calculate, using the species-area relationship, the proportion
of bird species (or beetles or snakes) that would be lost. Very roughly
speaking, the answer is ten percent. (Here again, it’s important to
remember that the relationship is not linear.) Since it takes a long time for
the system to reach a new equilibrium, you wouldn’t expect the species to
disappear right away, but you would expect them to be headed in that
direction.
In 2004, a group of scientists decided to use the species-area
relationship to generate a “first-pass” estimate of the extinction risk
posed by global warming. First, the members of the team gathered data on
the current ranges of more than a thousand plant and animal species.
Then they correlated these ranges with present-day climate conditions.
Finally, they imagined two extreme scenarios. In one, all of the species
tuis.
(Tuis.)
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