Biological Consequences of Fragmentation

A variety of human land uses, including agriculture and urbanization, fragment intact natural landscapes such as forests into smaller patches of habitat. These patches become isolated “islands,” and depending on the type and degree of fragmentation, this process can have a detrimental effect on wildlife and overall forest health. Linear land uses, including roads, power lines, and even hiking trails, bisect intact habitat and can affect wildlife distribution and ecosystem functioning. Although naturally fragmented systems are found all over the world, human habitat modification is currently the leading cause of fragmentation in the United States.

Fragmentation is problematic because it diminishes both habitat size and connectivity among individuals and populations. Many species have difficulty flourishing or surviving in these modified environments of reduced size, increased isolation, and new ecological boundaries. For example, fragmented habitat islands decrease the area where organisms can find food and eliminate habitat for species that require large unbroken areas of habitat for survival. They also increase risk to predation when organisms have to venture outside habitat patches.

In addition to constraining movement in the landscape, fragmentation creates edge habitat. In the forest context, edge habitat is defined as areas that are adjacent to non-forest land use, such as greenbelts along roadsides or the edges of agricultural fields. Edge habitats have different physical characteristics and species composition than interior habitats. Some native species such as white-tailed deer thrive at the edge but by and large edge forests have lower quality habitat than interior forests. Furthermore, edge forests are disturbed habitats and are thus more susceptible to invasion by invasive species, such as kudzu. Because fragmentation tends to give invasive species a foothold in the landscape, it can have devastating effects even on interior forests.

In general, there is a direct relationship between increased fragmentation of forests and decreased biodiversity. The scale and degree of fragmentation are important determinants of its effects. Overall, habitat islands generally hold fewer species than an area of the same size in continuous habitat. Smaller areas are less likely to hold as many habitat types as larger areas. Furthermore, the farther a habitat island is from another habitat island, the less likely species will be able to migrate to it.