Environmental Engineering Reference
In-Depth Information
Programme (UNEP) concludes that the introduction of alien species on to islands by
humans poses the biggest threat. The decimation of island bird populations by introduced
cats and rats has been a major cause of extinctions on islands such as Ascension Island,
whilst the escape and spread of alien 'garden' species ('invasive plants') can cause
problems for endemic species. Thus on Lundy Island in the Bristol Channel, UK, the
endemic Lundy cabbage is threatened by the spread of rhododendron, which not only
outcompetes the Lundy cabbage physically but also secretes a chemical into the soil
which attacks the cabbage ('allelopathy'). Conservation bodies are trying to limit the
damage to island wildlife. Physical methods are erecting fences or clearing vegetation,
herbicides can be sprayed, and biological control can be attempted by introducing
chemical contraceptives and viruses (e.g. myxomytosis in rabbits) against alien species.
Reintroducing lost species can be implemented; Scottish Natural Heritage has had a
successful programme for reintroducing sea eagles on the island of Rhum since 1975, and
the Jersey Wildlife Trust has a breeding programme for reintroducing the pink pigeon
and the Mauritius kestrel on to the island of Mauritius. Such attempts are expensive, and
will not have guaranteed success unless the underlying cause is dealt with.
greater diversity to arise in the tropics. The reasons why this greater diversity is able to
survive are a more complex question but are clearly related to a combination of two
important factors - environmental stability (which allows more niche specialization and
niche overlap) and a larger energy flux (which speeds up mechanisms of speciation and
divergent evolution).
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Biological diversity at any particular moment in time for any particular 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 in order to conserve and sustain plant
and animal species in a particular habitat. What size of oak woodland is necessary to
preserve its plant and animal population? One argument from
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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
warblers, 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
