Geography Reference
In-Depth Information
Figure 15.1
A hierarchy of processes determining
species diversity.
Figure 15.2
Although the small local populations (A)
frequently go extinct they combine to produce a
metapopulation (B), which is relatively stable over time.
reference to regional processes of immigration and
extinction. One of the very few ecological models
to take such processes into account was the
equilibrium theory of island biogeography
developed by MacArthur and Wilson (1967). Their
model attempted to account for the number of
organisms found on offshore islands in terms of
the size and degree of isolation of the island. These
two variables determined the equilibrium
between the rate at which new species arrived and
at which established species became extinct
(Figure 15.3). These ideas were adopted, rather
uncritically, by conservation biology. Rapid rates
of habitat fragmentation are a major cause of
decline in many species populations, and it was
believed that residual habitat fragments might
behave in a similar way to islands (Plate 15.1)
(Simberloff and Abele 1976). This triggered a
prolonged debate over what has become known
as the SLOSS (single large or several small)
problem; do a few large reserves maximise the
chances of long-term survival for more species
than a larger number of small reserves?
Regrettably, this debate ignored the important
contribution of metapopulation dynamics to the
understanding of the behaviour of fragmented
habitats. Although specific extinction and dispersal
proposed that since many of these local
populations were small they would commonly go
extinct, only to be re-established sometime later
by the arrival of new individuals from elsewhere.
They viewed the 'metapopulation' as rather like
the lights on a Christmas tree. The tree is lit all the
time, but individual lights flash on and off (Figure
15.2). This gives an important spatial structure to
populations that ecology has failed to recognise.
Many of the changes that occur in a local
population cannot be understood without
