Published online by Cambridge University Press: 05 July 2014
Humans have dramatically altered biotic communities around the world. During the process of converting forests, grasslands, and wetlands for agriculture, urban development, and transportation, the remnants of natural habitat have become increasingly fragmented. This fragmentation of habitat has many biological consequences. Foremost is the reduction of available habitat resulting in a reduced size of populations of species dependent on natural land covers (Andren 1994; Fahrig 1997, 2003). Fragmentation, though, is more than just habitat loss; it is also the division of remaining habitat into patches that experience at least partial isolation from other such fragments (Fahrig 2003). Fragmentation increases habitat edges and, consequently, the exposure of populations to the resulting alterations in microclimate and biota associated with edge environments (Suarez et al. 1998; Fagan et al. 1999). As fragments become more isolated, the frequency of movements between fragments is reduced. Furthermore, when localized extinctions in isolated fragments occur, immigration from neighboring fragments can be insufficient to allow recolonization (Tilman et al. 1994; Fahrig 2003). Extinctions associated with demographic stochasticity are expected to be more common in small fragments than in more continuous habitat tracts (Griffen & Drake 2008).
The consequences of fragmentation have the potential to influence genetic diversity in species requiring continuous tracts of habitat (see Chapter 8 by J. Hamrick). Localized population declines cause allele frequencies to drift; moreover, widespread population declines due to fragmentation across the range of a species may reduce its effective population size (Pannell & Charlesworth 2000; Alo & Turner 2005). With declines of census and effective population size, inhabitants of isolated fragments begin to resemble independent populations, become more vulnerable to stochastic processes, lose genetic diversity, and increase in relatedness (Barrowclough 1980; Leberg 2005).