Geoscience Reference
In-Depth Information
Theory of island biogeography
KEY CONCEPT
Islands have held a fascination for biogeographers since the famous studies of Wallace and Darwin in the Pacific
Ocean in the nineteenth century, right up to modern studies by MacArthur and Wilson in the late twentieth century.
Islands can be classed as oceanic, offshore or coral. Another reason for interest has been the studies of colonization
on newly formed islands; the new island of Anak Krakatoa off Sumatra in Indonesia was produced by a violent volcanic
explosion in 1883, and the island of Surtsey on the mid-Atlantic ridge off Iceland was also a product of volcanism in
1963. Studies of biological colonization of these and other islands have shown that islands have special ecological
characteristics. One reason is the rapid rate at which new species are formed when an organism invades an island
('enhanced speciation'). On reaching an island, a plant or animal colonizer can rapidly diverge genetically from the
mainland population. Isolation and genetic drift lead to so-called endemic species, that is, species which evolve within
the confines of a well defined space, whether an island, a mountain range or even a peninsula. Endemismis higher
for relatively stationary organisms such as plants, reptiles, amphibians and non-flying mammals than for mobile
organisms such as bats and birds, though many sub-species can be found even in these mobile groups, as, for
example, the many sub-species of Darwin's finches. The large island of Madagascar illustrates extreme evolutionary
divergence, with some twenty endemic species of shrew and some fifty endemic species of lemurs. This latter
group illustrates another aspect of island ecology, namely the fact that the behaviour of organisms evolves to suit
the environment of the island. In Africa and south-east Asia lemurs are nocturnal animals and very secretive, as a
defence against monkeys and apes. The absence of these competitors on Madagascar means that lemurs can abandon
their nocturnal habit.
Part of the fascination is that islands have very special features and act as ecological laboratories, places where
ecological processes can be studied under more controlled conditions than is usually the case. Islands usually have
far fewer species than the adjacent mainland ('impoverished biota'). Although they gain endemic species, there are
many mainland species unable to colonize. Other things being constant, there is a direct relationship between the
number of species able to colonize islands and the size of the island; the larger the island, the more species will be
found. The species-area relationship can be shown as a mathematical equation:
S = cA
z
where A = area, S = number of species, and c and z are constants. The value of z depends on the groups being
considered, for example whether trees, mammals, or birds, and has values between 0·1 and 0·4. Thus as an
approximate rule of thumb, if an area increases by a factor of ten, the number of species doubles.
Figure 22.5
shows
how the size of bird populations on Mediterranean islands is positively correlated with the size of the island. Two
factors appear to explain this. First, the larger the island the more space there is for larger populations of each species,
and hence there will be lower rates of extinction. Thus there are often very large populations of the fewer species
('density inflation').
Second, larger islands usually have greater habitat diversity, and therefore can support a wider range of species. The
successful in-migrants are usually those species which can successfully colonize a range of habitats ('generalists')
rather than being specialized in one habitat; species on islands often occupy more ecological space than their
equivalents on the mainland ('niche enlargement'). Ecologists have noted that island populations tend to be of different
body size from mainland organisms; the small are larger, and the large are smaller (van Valen's rule). Though there
are many exceptions, famous examples were the Pleistocene mammals of the larger Mediterranean islands such
as Cyprus and Crete, where there were elephant the size of bullocks and hippopotamus the size of pigs. Lack of
predators appears to be the major factor.
Islands are prized because of their unique ecology and rare species. Yet human actions have often decimated their
unique wildlife, whether deliberately or indirectly. About 75 per cent of animal extinctions since
AD
1600 have occurred
on islands where there is 'nowhere to run'. Extinction through hunting is well documented in many islands such as
Madagascar, Mauritius and New Zealand, but the United Nations Environment Programme (UNEP) concludes that
the introduction of alien species on to islands by humans poses the biggest threat. The decimation of island bird
