Today, the pressure of human activity, population and development are fragmenting wild biological communities into small patches surrounded by urban or agricultural land. These patches virtually become small islands in the sea of man dominated land-scope and the bio-geographic principles of species-area relationship can be applied to them.
When a habitat is fragmented into small patches its capacity to support or sustain viable populations of different species declines drastically and as a consequence the excess numbers of species are removed. Small forest patches suffer from rapid changes in micro climate which normally affects only the margins of a large forest. Winds and enchroachment by various agencies, damage trees and progressively reduce the size of forest patch or fragment.
The immediate consequence of habitat fragmentation is movement of most of the mobile forms from margins of the patch to deep interior. The margins which are frequently subjected to encroachments lose much of their species. Deeper zones of the patch are subjected to crowding where competition for space, food and wan causes the weaker organisms to perish.
Crowding often results in stepped up predation or overgrazing which destroys vegetation and the organisms are forced to live in a stressful environment. The rate of reproduction slows down or at times organisms fail to reproduce altogether while life under stressful conditions shortens their life span.
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According to bio-geographic principles, under ideal conditions a ten-fold increase in area of the isolated patch is required to enable the system to support a doubling of the number of species. In other words, a decline in the area of the patch by one tenth could most likely, reduce the number of species in the patch by half of its original number (Mc Arthur and Wilson 1967, Wilson 1985).
Using area species relationship the ultimate reduction in biodiversity due to fragmentations of rain forests of New World Tropics has been calculated. If the present rate of deforestation continues there will be inevitable loss of about 12% bird species and about 15% of the plant species in South and Central America (Simberloff 1984). This shall be so when the river Amazon and Orinoca basins will still have the largest continuous tracts of rain forests of the world.
At other places the prospects are more dismal because less extensive tracts are far more threatened. For example, the Western forests of Ecuador, which were largely undisturbed till 1960, where newly constructed network of roads led to rapid human settlements and clearance of much of the forest area, have been fragmented into small patches of one to few square kilometres. Such a patch, about 0.8 square kilometres in area at Rio Palenque Biological station now contains only about 1033 plant species many of which are represented by a single specimen only and are endemic to the locality. Before 1960 the intact forest had thousands of species as found in any other tropical regions of the world.
It is not only that fragmentation of habitat causes decline in the number of species but it also makes the species which survive more vulnerable to future extinction. Few individuals of a species do not form a viable population. The drastic reduction in genetic diversity of species concerned enfeables the chances of the species’ survival in future.