Geoscience Reference
In-Depth Information
Can the theory of island biography help
conservation?
APPLICATIONS
Biological diversity at any moment in time for any geographical location is the balance of immigration and extinction.
In successions on new land surfaces (volcanic islands, salt marshes), new species arrive and colonize; initially the
rate of immigration exceeds the rate of extinction, but as more species compete for space the rate of extinction
increases. If it equals the rate of immigration a state of dynamic equilibrium will exist. New species arrive, old species
disappear and the composition is always changing, but the number of species at any particular moment is constant.
An important application of the theory of island biogeography is to determine the minimum critical size of populations
and habitat to conserve and sustain plant and animal species in the habitat. What size of oak woodland is necessary
to preserve its plant and animal population? One argument from S = cA
z
may be that for 100 ha of woodland it is
better to have ten patches of 10 ha, each with two species, than 100 ha with its four species. However, if the two
species are the same in each case, there would clearly be no increase in diversity. The discussion is complicated by
the fact that different species have different ranges. Sparrow hawks require larger territories than blue tits, and thus
the conservation of sparrow hawks requires large nature reserves. Unfortunately too little is known about the territories
of animal species, especially in relation to emigration and immigration. For example, if one is keen to conserve
hedgehogs, it is important to know whether the population in a particular habitat is an isolated entity or whether it
reflects immigration of individuals from outside which is balanced by emigration to areas outside. This is where the
DNA fingerprinting of individuals and the use of radio tracking can establish the minimal critical size needed to conserve
the population.
Studies of birds in Britain have shown that the number of species found in woodland does indeed reflect area.
However, area seems to be an indicator of species diversity rather than a cause; a larger area is usually associated
with greater habitat diversity in the form of floristic diversity and canopy height. Smaller woodlands possess fewer
kinds of species than larger areas, and also contain smaller populations of particular species. This in turn leads to
genetic drift, inbreeding and loss of genetic diversity, especially where habitat islands are physically isolated from
each other. The net result is that population numbers may fall below a critical threshold, become vulnerable to a
physical disturbance and may become locally extinct.
On a global scale the tropical rain forests are being destroyed at a rate of 2 per cent per year, and it is estimated that
the present area of 8 M km
2
is about half that in immediate postglacial times. The rate of loss is increasing and will
reduce the cover to 4 M km
2
by
AD
2020. The question arises: what proportion of species will disappear? The answer
will lie between 10 per cent (zvalue 0·15) and 23 per cent (zvalue 0·35). This elimination of 10-23 per cent represents
5-10 per cent of all species on Earth, at a conservative estimate. The species-area equation accounts for most,
though not all, of this loss. Hence many tropical countries try to preserve 'islands' of forest, as in Brazil, where a
government law requires landowners to leave at least 50 per cent of their land under forest. Studies of such 'islands'
show that diversity decreases more rapidly the smaller is the island. Winds and desiccation reduce shade-loving
insects like ants and butterflies in plots less than 10 ha in size, as well as amphibians, mammals and birds which
depend on them. Large ground-dwelling mammals migrate quickly but some species of birds and monkeys flourish
around the forest edges.
of Dutch Elm Disease in 1975-85, which involved a long-
term recovery. Perturbations need to be defined in terms
of area of impact and time of impact. For that reason
it is difficult to compare widely different ecosystems,
although comparisons should be possible for similar
ecosystems.
human-made perturbations include deforestation, over-
grazing, agrochemicals, acid precipitation and pollution.
Some perturbations cause very large changes in the
abundance of species, like the severe British winter of
1962-63, which decimated bird populations. Others may
involve the removal of some species, as in the outbreak
