Fragmentation
Fragmentation is often defined as a process by which a natural landscape, in this case forest, is broken up into small patches, isolated from one another in a matrix of lands dominated by human activities (Forman, 1995; Malcolm et al., 2007). The process of habitat fragmentation involves three main factors, each of which have an important effect on plant and animal species: 1) Fragmentation leads to the breaking up of large patches into numerous smaller patches (e.g. see the Figure), resulting in a net habitat loss and change in spatial structure (Forman, 1995). This results in a decrease in the amount of resources and area for shelter available to plant and animal species dependent on this habitat and therefore leads to a general reduction in the number of individuals that can be hosted (Lindenmayer and Fischer, 2006). The change in spatial structure and the loss of habitat due to fragmentation could also alter the population structure and size of individual plant species and therefore community diversity, richness and composition. Fragmentation also has an impact on forest shape. Changes of forest shape due to fragmentation can play a major role in patch species composition. Irregular and rectangular patch shapes are believed to have less core area which reduces the abundance of species dependent on forest interior habitat compared with circular shapes (McGarigal and Marks, 1994). Empirical studies indicate that community composition can also be altered as a result of change in forest shape, e.g. Hill and Curran (2005) who found that irregular-shaped fragments had a higher proportion of regenerating, light-demanding pioneers and animal-dispersed tree species than regular-shaped forest patches. 2) By opening core areas to the effects of exterior habitats, fragmentation of continuous habitat patches may lead to a dramatic increase in edge effect (Sih et al. 2000; Turner et al. 2001; McGarigal et al., 2002; Harper et al., 2005). In highly modified landscapes, many edges can be created by humans. Edge effect refers to changes in biological and physical conditions that occur at boundaries and within adjacent areas of affected forest (Laurance et al., 2002; Lindenmayer and Fischer, 2006). 3) Habitat fragmentation results in the geographic isolation of patches among a matrix of, e.g., agricultural crop and grazing land (Bender et al., 2003) - see Figure 1.1B. This might greatly restrict the mobility of plant propagules and less mobile animals (Andreassen et al. 1996), and thereby isolate some populations, especially of species with specialist habitat requirements that are obligately dependent on the habitat type restricted to the isolated patch islands. Small isolated populations can be threatened by inbreeding, which represents a serious problem for their survival (Schmitt and Seitz, 2002). Moreover, small populations are more sensitive to stochastic events, such as epidemic outbreaks, that could drive local populations to extinction (Sutherland, 2000). As the isolation of habitat patches increases, the probability of species recolonizing individual patches, and forming a viable metapopulation across the landscape may decrease (Parker and Nally, 2002). |
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