Habitat Fragmentation
The habitat of a living organism, plant, animal, or microbe, is a place, or a set of environmental conditions, where the organism lives. Net loss of habitat obviously has serious implications for the survival and well-being of dependent organisms, but the nature of remaining habitat is also very important. One factor affecting the quality of surviving habitat is the size of remaining pieces. Larger areas tend to be more desirable for most species . Various influences, often a result of human activity, cause habitat to be divided into smaller and smaller, widely separated pieces. This process of habitat fragmentation has profound implications for species living there.
Each patch created when larger habitat areas are fragmented results in more edge area where patches interface with the surrounding environment . These smaller patches with a relatively large ratio of edge to interior area have some unique characteristics. They are often distinguished by increased predation when predators are able to hunt or forage along this edge more easily. The decline of songbirds throughout the United States is due in part to the increase of the brown-headed cowbird competing with other birds along habitat edges. The cowbird acts as a parasite by laying its eggs in other birds' nests and leaving them for other birds to hatch and raise. After hatching, the young cowbirds compete with the smaller birds of the nest, almost always killing them.
In the smaller patches formed from fragmentation, habitat is less protected from adverse environmental events, and a single storm may destroy the entire area. A disease outbreak may eliminate the entire population. When the number of breeding adults becomes very low, some species can no longer reproduce successfully.
Some songbirds found in the United States are declining in number as their habitat shrinks or disappears. When they migrate south in the winter they find that habitat is more scarce and fragmented. When they return from the tropics in the spring, they discover that the nesting territory that they used the previous year has disappeared.
Species dispersal is decreased as organisms must travel farther to go from one habitat area to another, increasing their exposure to predation and possibly harmful environmental conditions. Populations become increasingly insular as they become separated from related populations, losing the genetic benefits of a larger interbreeding population.
Road building often divides habitat areas, seriously disrupting migration of some mammals and herptiles (frogs , snakes, and turtles). Large swaths of land used by modern freeways are particularly effective in this regard. In earlier times, railroads built across the Great Plains to connect the west coast of the United States with states east of the Mississippi, divided bison habitat and hampered their migration from one grazing area to another. This was one of the factors that led to their near extinction .
Habitat fragmentation, usually a result of human activity, is found in all major habitat types around the world. Rain forests, wetlands , grasslands , and hardwood and conifer forests are all subject to various degrees of fragmentation. Globally, rain forests are currently by far the most seriously impacted ecosystem . Because they contain 50% or more of the world's species, the resulting number of species extinctions is particularly disturbing. It is estimated that 25% of the world's rainforests disappeared during the twentieth century, and another 25% were seriously fragmented and degraded. In the last two centuries, nearly all of the prairie grassland once found in the United States has disappeared. Remaining remnants occur in small, scattered, isolated patches. This has resulted in the extinction or near extinction of many plant and animal species.
Ability to survive habitat fragmentation and other environmental changes varies greatly among species. Most find the stress overwhelming and simply disappear. A few of the common species that have been very successful in adapting to changing conditions include animals such as the opossum, raccoon, gray squirrel, and European starling. Plant examples include dandelions, crab grass, creeping Charlie, and many other weed species. The wetland invader, purple loosestrife , originally imported from Europe to the United States, is rapidly spreading into disrupted habitat previously occupied by native emergent aquatic vegetation such as reeds and cattails. Animal species that once found a comfortable home in cattail stands must move on.
[Douglas C. Pratt Ph.D. ]
Cunningham, W. P., and Cunningham, M. A. Principals of Environmental Science: Inquiry and Applications. McGraw-Hill, 2002.
Environmental Encyclopedia
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