Urban Natural Areas #2 – Habitat Fragmentation
Habitat fragmentation occurs when large, contiguous areas of habitat are reduced in area and divided into two or more smaller units (Wilcove et al. 1986). The fragmented units are isolated from one another and embedded in a modified or degraded landscape. Fragmentation within metropolitan regions occurs at a coarse scale when large habitats are cleared for human use. As urbanization intensifies fragmentation occurs at a finer scale through the division of habitat by roads, utility rights-of-way, railroads, and other barriers which impede movement of wildlife.
Habitat fragments are impacted in two ways: 1. the fragments have a proportion of edge greater than the original area, and 2. the center of the habitat fragment is closer to the edge than the original. These effects reduce habitat values for particular species and may lead to gradual loss of species diversity within the fragments.
The degree of isolation and the strength of the barriers between the fragmented habitat patches have a direct influence on the ability of plant, and animals to disperse and colonize. In contiguous environments, seeds, spores, and animals can move passively or actively through the landscape. Populations shift to meet changing environmental factors to increase the suitability of their local and seasonal ranges. This continual shifting of species and populations leads to a pattern of local extinctions and recolonizations.
Many forest bird, mammal, and insect species will not cross even very narrow unprotected open areas (Laurance and Bierregaard 1997). Such open areas present a dangerous landscape to them, harboring numerous predators. Harper (1977) found that dispersal of herbaceous and shrub plant species within older forest patches was restricted and could take up to several decades, even without human infrastructure barriers such as roads and towns. Fleshy fruits and sticky seeds that are dispersed by mammals are also affected (Santos and Telleria 1994). Natural local extinctions within the isolated fragments lead to species impoverishment, as new species are no longer able to bridge the barriers and recolonize the site. As a consequence only those species that are able to move through the barriers and tolerate the fragmented habitat will increase in abundance and distribution.
Reduced size of habitat leads to a constriction of foraging area. Access to scattered foraging resources and to specialized sites for mating and rearing may no longer be available. The reduced size of the habitat also inhibits the ability to find suitable mates and to form social groups. Plants may lose their reproductive ability as butterflies, bees, and other symbiotic associates can no longer reach the fragment to pollinate the flowers. Uncommon or widespread populations can decline and go locally extinct. Reduction in total numbers of local populations of some species will lead to a loss of genetic vigor.
Wilcove D.S. McLellan C.H. and Dobson A.P. 1986. Habitat fragmentation in the temperate zone. pp. 237- 256 in Conservation biology: the science of scarcity and diversity (M. E. Soule, Ed.). Sinauer Associates Inc.
Laurance W. F. and Bierregaard R.O.Jr., (eds.). 1997. Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities . The University of Chicago Press, Chicago, Illinois, 616 pp.
Harper, J.L. 1997. Population Biology of Plants. Academic Press, London, 842 pp.
Santos, T. and Telleria J.L. 1994. Influence of forest fragmentation on seed consumption and dispersal of Spanish juniper. Biological Conservation 70:129-134.