Module 13: Habitat Fragmentation, Deforestation, and Desertification

Habitat Fragmentation and Corridors

By Dr. James A. Danoff-Burg, Columbia University

Habitat Fragmentation

Undisturbed natural areas are home to a great many species that are found only there. However, intact natural areas frequently become fragmented and degraded for a variety of large-scale industrial purposes, chiefly farming, logging, grazing, mining, or oil. In addition, conversion of natural areas for residential use is rapidly increasing with the rapidly increasing human population (Module 4).

A characteristic pattern of human encroachment accompanies most of these land conversions. The first step in the process is construction of a road traversing the area, usually a single lane dirt road that receives only occasional use. Subsequently the road is widened, additional branches are added, and the roads are frequently paved to ease human passage. Human traffic subsequently increases, bringing further human ingression and impacts, including increased land use, pollution, litter, and resource depletion. The final step is when humans completely convert an area into artificial (e.g. a parking lot) or heavily managed land. The disturbed land (or matrix) between fragments is maintained in the same arrested state of succession by human activities.

Land clearing and fragmentation is presently the most significant threat to biodiversity. Matrix clearings that are long lasting usually reduce the movement of many species of animals from fragment to fragment, isolating gene pools and reducing local genetic diversity. As we encountered earlier in the semester, reduced genetic diversity diminishes the ability of species to respond from population-reducing perturbation events (Module 4).

Edge Effects

Edge effects combine with population isolation to further endanger endemic species by reducing total suitable habitat area within fragments. Edge effects include abiotic microenvironmental changes such as increased light penetration, windiness, aridity, exposure, heat, and soil compaction. These edge-related changes in a forest can penetrate 200 m into otherwise pristine natural areas for some animal species.

With increasing fragmentation, intrusion by both introduced exotics and disturbance specialist species increases (Module 14), further reducing the habitat available to endemic forest species. Avian nest predators and brood parasites have also been shown to use linear clearings to gain access to nests, indicating that enhanced predation and parasitism also accompany these habitat changes. All of these factors combine to contribute to an increased risk of local extirpation and even global extinction of many species. Increasing fragmentation will almost certainly lead to a homogenization and reduction in global biodiversity.

Corridors

A possible way to help mitigate the effects of fragmentation is to create corridors, relatively narrow connections between fragments. These connections are portions of the previously disturbed matrix that are modified and enhanced to be made more habitable. Corridors are often created either by introducing key endemic plants or by not disturbing a linear area and allowing it to undergo natural succession.

Corridors cannot serve as permanent habitats for most forest-dwelling endemics because of their narrow width and enhanced edge effects relative to an intact forest, but are instead a dispersal pathway between two fragments. The matrix surrounding the connective corridor is usually less desirable than either the corridor or the fragment. Therefore, dispersing animals from a fragment will stay within the corridor, avoid entering the less desirable matrix, and not permanently remain within the corridor. Eventually, most dispersing organisms will end up in one of the fragments that are connected by these corridors. If this model of animal movement works well, then corridors would enlarge the effective metapopulation size of the animal species that can use them. In so doing, corridors would help to reduce the effects of some of the main deleterious effects that fragmentation has on local biodiversity, including local population genetic diversity.

With continued exponential human population growth around the world certain, a better understanding of the effects of historical and future habitat fragmentation is essential. Knowledge of these effects will help us to design creative solutions to the biodiversity threats posed by fragmentation.

Refer to the Wilson and Perlman CD for more information on habitat fragmentation (Main Menu/Threats to Biodiversity/Habitat Fragmentation) and corridors (Main Menu/Conservation Practice/Reserve Design/Corridors). Explore the Interactive Model, Explorations and Further Thought exercises on your own.

Additional Relevant Online Resources

A chapter titled Landscape Heterogeneity and Change is available from "Principles of Ecosystem Ecology", a draft textbook available online from Chapin, Matson, and Mooney (authors).

A Land Cover Change Monitoring Program: A Federal Agency Initiative (A Report to the Committee on Environment and Natural Resources), is available from D.L. Skole and C. O. Justice.

A Land-Use and Land-Cover Change Science/Research Plan is available from B.L. Turner II, David Skole, Steven Sanderson, Gunther Fischer, Louise Fresco and Rik Leemans

A report entitled "What Drives Deforestation In The Brazilian Amazon? Evidence from Satellite and Socioeconomic Data" is available from Alexander S.P. Pfaff of the Massachusetts Institute of Technology.

A report discussing the connection between Landscape Ecology and Ecosystems Management is available from Thomas G. Barnes of the University of Kentucky. It is also available for downloading as an Adobe pdf file and discusses matrixes among many other related subjects.

Itaipu Binacional discusses many of the features of its ambitious reforestation and corridor construction plan that it has developed and implemented as part of its massive hydroelectric dam on the Paraná River in Brazil.

Ecosystem Fragmentation and its implications for Canadian Parks is discussed by Canadian Heritage and Parks Canada.

An excellent discussion of the impacts of edge effects in urban areas is available from the California State Parks.

A whimsical, but nonetheless interesting, discussion of avian nest parasites entitled "Brood Parasites or Where Did I Put That Egg?" is available from Christine Tarski at About.com.

The Green Corridors Management Strategy for the Upper Parramatta River Catchment Trust located near Sydney, Australia is an interesting application of the use of corridors for mitigating biodiversity loss from fragmentation.

Habitat fragmentation, corridor construction in Florida, and its impact on the Florida Panther is available from Florida Panther Net.

An impressive bibliography of the Effects of Highways and Other Linear Developments on Wildlife Populations is available from Anthony P. Clevenger of Parks Canada.

A page from Dr. Steve Archer of Texas A & M discusses how multiple factors lead to tree encroachment in grasslands, including land use practices (grazing, fire suppression), climate, and chemical atmospheric changes (such as elevated carbon dioxide and nitrogen deposition). It also has an extensive bibliography discussing these factors.

All Materials Copyright © 2000 by James Danoff-Burg

All Rights Reserved.