Habitat Destruction and Degradation



Habitat Destruction and Degradation

W. Daniel Edge, Department of Fisheries and Wildlife, Oregon State University

Habitat destruction and loss is the principle cause for the decline in biological diversity. Habitat destruction and loss outranks over exploitation, introduction of exotic species, predation and competition, natural disasters, and inadequate laws as causes of species extinctions and those threatened with extinction. Furthermore, habitat loss and destruction is a direct result of increasing human populations. The greatest destruction of habitats and largest pulse of species extinctions have occurred over the past 150 years. In 1850, the global population was approximately 1 billion people and increased to 2 billion by 1930 (80 years); in 1990 (60 years), it was 5.3 billion and is projected to reach 6.5 billion by the year 2000. This alarming growth in human populations has been possible because of significant progress in medicine that lowered infant mortality rates and increased life expectancy, and advances in modern agriculture that made food plentiful. Although population growth has slowed in industrialized countries, it still remains high in the tropical countries of Asia, Africa, and South and Central America. Increasing rates of human population growth in tropical countries presents a special concern for biological diversity because these countries contain the greatest number of species.

In many countries of the world, substantial portions of natural habitats have already been destroyed; on island countries where human densities are particularly high, most of the original habitat is gone. In 49 out of 61 countries in Africa and Asia, more than 50% of the habitat has been destroyed.

Threatened Habitats

Deforestation of rain forests is a particular concern because of the high biodiversity in these habitats. Tropical rain forests make up approximately 7% of the earth's surface area, yet they contain over 50% of the plant and animal species. For example, about 30% of the world's bird species occur in tropical forests of Asia, Africa, and South and Central America (2,600 species). Approximately 180,000 km2 of rainforest are lost each year, an area larger than the state of Florida. At the current rate of destruction, rainforests will be virtually eliminated by the year 2040. Conversion to agriculture, commercial logging, and fuel-wood production are the principle causes of deforestation.

Wetlands and aquatic areas are also species-rich habitats that are being destroyed. Wetlands are critically important habitats for a large number of vertebrate species. Wetlands make up approximately 5% of the land area in the United States and contain approximately 15% of the threatened and endangered species. Wetlands are converted and drained worldwide for development projects or for agriculture. Wetland and aquatic habitats are also altered or transformed by channelization of waterways and dam projects. In the United States, over 50% of the wetlands present 200 years ago are now gone.

Mangrove forests are unique and important habitats that are being destroyed for wood production and agriculture and aquaculture. Mangroves are special forest habitats that occur on shallow, intertidal coastal areas in the tropics. These habitats are critically important as a breeding area of many aquatic organisms and are important for preventing erosion damage from storms. Between 50 and 60% of the commercial species of fish and shrimp caught in the tropics are dependent on mangrove areas. These areas are also important breeding habitats for many birds species. Mangroves are cleared for the production of rice and aquaculture, and for fuel wood and construction materials. In some countries in Asia, over 30% of these habitats have been destroyed or degraded.

Temperate grasslands are biologically important habitats that have been destroyed or degraded primarily through agricultural practices. Because grasslands typically occur on well-developed soils and gentle terrain, they are prime spots for crop production. Sites that are not converted for crop production are highly desired as grazing land for livestock production. Crop production destroys grassland habitats while overgrazing degrades these habitats, allowing the invasion of exotic and early successional stage species. The net result is habitats that no longer support the diversity of vertebrates that they once did.

Habitat Fragmentation

Habitat fragmentation is a less obvious form of habitat degradation but is a principle cause for the decline in biodiversity. Habitat fragmentation is a process by which historically contiguous habitats are reduced in size and the remaining habitats are divided into patches (Fig. 1). The patches that remain are often isolated from each other within a modified or degraded landscape. Roads, agriculture, urbanization, and other human-induced changes on the landscape cause habitat fragmentation. Habitat fragmentation has been identified as a principle cause in the decline of many species native to North America.

| [pic] |

| Fig. 1. Habitat fragmentation occurs when contiguous habitats (left) are modified by human or natural disturbances. Disturbances create patches of habitat, and |

|as fragmentation continues the disturbed habitats dominate the landscape and the original habitat occurs in isolated patches (right). |

Fragmented habitats differ from the original contiguous habitats in several important ways. Not only are the original habitats reduced in total area, but the remaining patches of habitat have a greater amount of edge relative to their area. These habitat patches also have smaller core areas, which might influence the survival and reproduction of species living there. Furthermore, these patches might be too small to be used by some wildlife species. Finally, as the patches become more and more isolated, animal movements among patches may be reduced or entirely eliminated.

Ecological processes may accelerate at habitat edges, which in turn may influence the survival and reproduction of species living there. Although many wildlife species (perhaps as much as 60% of all species) are attracted to edges because of the various foraging and cover opportunities provided there, some species are particularly vulnerable in edge habitats. Brood parasitism by brown-headed cowbirds and increased predation on nests by generalist predators are two factors that have been identified as causing a decline in some North American songbird species.

Brood parasitism is a unique reproductive strategy used by some birds in which one species (the brood parasite) lays its eggs in the nest of another species (the host), which raises the brood parasite's young. Often, differences in size or behavior of the young of the two species are pronounced and the host species only successfully raises the young of the brood parasite. The brown-headed cowbird is a classic example. The brown-headed cowbird parasitizes the nests of many songbirds in North America and is a species that only nests in habitat edges. As habitats such as the eastern deciduous forests become more and more fragmented and habitat edges become more abundant relative to interior habitats, cowbird populations increase and songbird populations decline.

Predation on bird nests also increases at the edge of habitats because many generalist predators, such as crows, jays, raccoons, skunks, foxes, and domestic cats preferentially hunt these areas, again because of the various foraging and cover opportunities there. Several studies have documented very high rates of predation on bird nests near habitat edges (75-100%), and the rate of predation increase the closer birds nest to the edge (Fig. 2). Thus, predation may increase with the amount of edge. Again, the songbirds nesting in deciduous forest of the eastern U.S. provide us with an alarming example.

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| Fig. 2. Predation rates on artificial nests decline with distance from a forest edge. Edge-related predation extends 300_600 m into the |

|forest (from Wilcove 1985). |

Some habitat patches in a fragmented landscape may be too small to be attractive to some species, while others may be too small to provide the basic needs for survival and reproduction. The black-footed ferret, one of North America's most endangered mammals, feeds almost exclusively on prairie dogs and only occurs in these unique habitats. A single ferret may require up to 200 ha of prairie dog town to survive. Prairie dogs were (and still are in some areas) poisoned because they were perceived as competitors for livestock forage. Consequently, prairie dog towns were either totally eliminated or greatly reduced in size. Many of the prairie dog towns that remain are much smaller than 200 ha and do not provide adequate habitat for the ferrets. Spotted owls in the old-growth forests of the Pacific Northwest provide another example, and many larger animals such as elephants and tigers face similar problems as native habitats are reduced to small, isolated parks and reserves.

Some species, for reasons that are not clear, simply prefer areas that are much larger than they need to survive or reproduce. Some songbirds in the eastern U.S. are not detected in small woodlots, even though their territory requirements are smaller than the size of the woodlot (Fig. 3). For example, the probability of sighting a wood thrush in mature deciduous forest in Maryland was found to increase from 20% for 0.1 ha woodlots to 80% in woodlots greater than 100 ha.

Small patches of habitat are more vulnerable to catastrophes and other environmental factors. Some environmental factors such as wind, temperature, and humidity differ between core and edge habitats. Old-growth forests of the Pacific Northwest provide a familiar example. When adjacent to clearcuts, old-growth patches have higher temperatures and lower humidity for up to two tree-lengths (300 feet) into the old-growth patch. These differences in microclimate result in differences in plant community composition and structure. In addition, blowdown from wind is much more likely at the edge of these patches. This build-up in dead wood may increase the likelihood that the entire patch is destroyed by fire.

As habitat patches become smaller and more isolated, movement of animals among patches is likely to be disrupted. The long-term survival of a species may depend upon the ability of individuals to disperse (move from the site where it was born to another site to reproduce as an adult) and colonize other habitat patches. The likelihood of a particular patch being discovered or colonized decreases as the patches get smaller and/or farther apart. A major cause of mortality to young northern spotted owls is thought to occur when they disperse as juveniles in order to find additional suitable habitat. The black-footed ferret provides another example. Prairie dog towns, which were once large and abundant in grasslands of the western U.S. are now small and isolated. Sites where the U.S. Fish and Wildlife Service is trying to reintroduce the ferret contain many large dog towns in close proximity to each other.

Recommended Readings

Harris, L. D. 1984. The fragmented forest. University of Chicago Press, Chicago, IL.

Noss, R. F., and A. Y. Cooperrider. 1994. Saving nature's legacy: protecting and restoring biodiversity. Island Press, Washington, DC.

Robbins, C. S., D. K. Dawson, and B. A. Dowell. 1989. Habitat area requirements of breeding forest birds of the Middle Atlantic States. Wildlife Monographs 103:1_34.

Wilcove, D. S. 1985. Nest predation in forest tracts and the decline of migratory songbirds. Ecology 66:1211_1214.

Wilcox, B. A., and D. D. Murphy. 1985. Conservation strategy: the effects of fragmentation on extinction. American Naturalist 125:879887.

Wilson, E.O. 1992. The diversity of life. Belknap Press of Harvard University Press, Cambridge, MA.

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