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Habitat Fragmentation

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California’s Natural Community Conservation Planning Act has fostered the development of regional preserve systems with the aim of maintaining natural ecosystems while balancing growth. In San Diego County, this preserve system falls under the regulatory umbrella of the Multiple Species Conservation Plan (MSCP) and Multiple Habitat Conservation Plan (MHCP). Yet, little is understood about what areas are serving or under-serving the species for which they were intended. It is well-established that habitat fragmentation decreases the ability of the remaining land to support the full complement of native species originally found there, and that the resulting risks to species are not randomly or equally distributed, but are affected by fragment and species characteristics. Studies of birds, carnivores, and small mammals in southern California have found: that habitat fragments of 10 to 100 hectares are subsequently unable to support the full complement of native species originally found there for more than a few decades; that only the most abundant species survive; and that most are doomed to local extinction within 100 years (Soule et al. 1992). Yet, relatively little is understood regarding how these fragmentation effects currently operate within the preserve system.

Some habitat fragments are more likely to continue supporting more species than others. Fragment size, age and isolation are known to be the strongest landscape predictors of species distribution and abundance in a fragmented landscape (Crooks 2002, Bolger et al. 1997, Soule et al. 1988). Although smaller and older fragments and are known to support less species, those with quality corridors to other natural areas are known to better promote continued species persistence. In some cases, distance to the nearest habitat remnant was also strongly predictive of the number of species continuing to persist in habitat fragments (Soule et al. 1992). Studies where this effect was not seen have generally been interpreted as indicating that the species in question are unable to cross the matrix between habitat remnants, increasing their risk of extinction (Bolger et al. 1997). Thus, Soule et al. (1998) noted that the provision of corridors appears to be the most effective design and planning feature for preventing elimination of species in a fragmented habitat, although this study focused on birds.

Some species are more susceptible to extinction risks in fragmented landscapes than others. Species with higher densities in unfragmented habitats generally have a higher chance of surviving in fragmented landscapes (Bolger et al. 1991, and Soule et al. 1998). Species with less mobility appear to be more susceptible due to their decreased ability to cross the matrix between habitat fragments, while species with larger body and range sizes require larger habitats to survive, increasing their extinction risks in fragmented landscapes (Crooks 2002, Soule et al. 1988).

Certain species are also known to provide indications of various levels of habitat connectivity and success of remnant habitat fragment functionality (Crooks 2002). Mountain lions, with a large body weight and range size, are highly sensitive to habitat fragmentation, making them a good species to monitor in large, intact habitat areas. Bobcats, with a strictly carnivorous diet but smaller body size, are intermediately affected by disturbance, making them a valuable indicator of landscape connectivity in fragmented landscapes. Coyotes provide a good indicator of functional connectivity in highly fragmented areas, particularly if bobcats and mountain lions have already disappeared from the areas in question.

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