Fragmentation of habitat and patchiness may affect various individual, population and community level processes. Endeavors of ecologists and conservationists to protect wildlife populations in such habitats have resulted in various conservation theories and management options.
One early theory—the theory of island biogeography—was proposed by McArthur and Wilson (1967).
The theory of island biogeography states that in a fragmented community, the species number remains constant, but that local turnover among islands changes the species composition of island communities.
Levins (1970) furthered this concept for populations in fragmented habitats through the rates of meta-population extinction theory.
A meta-population is a dynamic demographic unit composed of several smaller sub-populations of a species where extinction in one patch is balanced by re-colonization in another. Levin’s theory states that if re-colonization rates match the extinction rates of sub-populations in a meta-population, then the persistence of a meta-population is possible.
However, the ability of a sub-population to recolonize is dependent on how well the patches are connected. However, an important precondition is that the constituent sub-populations should be “isolated enough to constitute separate populations; and yet be interconnected enough to permit re-colonization to balance extinction of this isolated sub-populations” (Harrison 1992).
It is worthwhile to mention in support of the above theories that as islands are isolated, and in many cases the species found on them are endemic, extinction has been particularly common on islands. About 93 per cent of the bird species whose extinction has been recorded since 1600 have been island species.
The Percolation theory deals with modelling irregular animal behaviour in a hostile matrix. The theory is important with respect to animal movements and use of resources. One other theory—the Source-Sink theory evolved from the demographic model suggested by Pulliam (1988, 1996).
The theory defines ‘Source’ as a population in which births exceed deaths and immigration rates. On the other hand, a ‘Sink’ is defined as a population showing negative balance between offspring and deaths and in which juvenile population does not have the capability to replace adult mortality.
Merriam (1984) defines ‘connectivity’ as a parameter to measure the processes by which the sub-populations of a landscape are interconnected into a demographic functional unit. Connectivity can be seen as the inverse correlate of hostility of the inter-patch habitat.
It is a functional parameter that varies according to specific organisms and is frequently not related to landscape structure (Farina 1998). However, connectivity itself stems from the complex interactions between species-specific movement patterns and landscape features. For example, woodland connectedness plays a fundamental role for species that need tree cover for their movements (ibid.).
It is important to note that that ‘connectedness’ and ‘connectivity’ between habitats are two different terms. Thus, while connectedness—the structural connection between habitat patches—enhances the possibility of connectivity, connectivity itself may not be guaranteed by connectedness. Whatever the dynamics of a population in a fragmented landscape, Beier and Noss (1998) argue that connectivity “enhances population viability for many species”.