Environmental Engineering Reference
In-Depth Information
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.
Endemism
is 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:
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 approxi
