In a world with rapidly rising population space is usually a sparse commodity. Man needs space for agriculture, industries, housing, military establishments, railways and roads etc. and at times it becomes difficult to set aside large areas of biological significance exclusively for wild life conservation. The question asked in this connection is what should be the minimum critical area which is just sufficient for the conservation of a given set of flora or fauna?

When a large forest is fragmented into small patches its capacity to sustain viable populations of different species declines drastically. A number of species are either forced to flee away or are pushed to extinction. The number of species which a small isolated patch of forest can support is a function of its productivity, habitat requirement of the species concerned and the conditions of environment – both abiotic and biotic.

According to the bio-geographic principles, a tenfold decline in the area of the patch could reduce the species to about half of their original number (Mc Arthur and Wilson 1967, Wilson 1985). It has been observed that size of the habitat has much influence on the number of viable population of many species inhabiting the system.

In Illinois Graber and Graber (1976) found that the number of bird species in small patches of grassland decline at a much faster rate than the simple reduction in the total area of the patch would suggest. Sampson (1980) has computed the probability of occurrence of breading populations of selected grassland bird species in small habitat patches.

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It was found that eastern meadow larks (Sturnella magna) can occur in a patch of one hectare only. The horned lark (Eremophile alpestris) and grass-hopper sparrow (Ammodramus sarannarum) require a habitat patch more than one hectare.

Henslow’s sparrow (Ammodramus henslowii) and the upland piper (Bartramia longicanda) need a patch of habitat more than ten hectares. Thus each species appears to have a different size requirement as far as the extent of protected habitat is concerned. Habitats too small than the required dimensions are unable to support populations which need larger patches, resulting ultimately in their disappearance.

Thus a protected area should be large enough to support the population of the species which we intend to conserve. This population should be a viable population – populations which are capable of growing and multiplying actively.

If the population in the system is unable to multiply or multiplication occurs at a rate slower than the mortality, the entire conservation effort will be wasted as at some point of time in future the species shall be eliminated. Moreover, in order to strengthen chances of survival, the populations should be large enough to encompass as much genetic diversity as possible.

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A population consisting of individuals with a homogenous genetic makeup is usually more susceptible to chance variations in environmental conditions and attacks by pathogens as compared with populations having heterogenous genetic makeup. Thus, in order to contain, a greater number of species with populations, large enough to have a secure future, the habitat should be large enough to feed all individuals of all viable-populations of all species and also to meet the habitat size requirements of the species to be preserved.

For sanctuaries and national parks which are intended for the preservation of one, two or few species we have simply to ensure that the habitat should be large enough for existence and multiplication at a rate higher than the mortality of the species or group of species concerned.

In Nature reserves or biosphere reserves which are usually meant for the conservation of the total biotic spectrum of the locality or the climatic region, the overall area should be large enough to support all the life forms in viable state. Of course, we can protect a number of species in viable state in a relatively small area if larger ones are not available.

But this shall require intensive management and in the process many important life forms may be omitted because of our limitations. The practice of intensive management could have some undesirable effects on the organisms being protected as human care may eliminate factors which cause wild life to adopt and adjust to an ever changing environment.