A key driver of the current trend in biodiversity loss is the conversion of land for agriculture, settlement, or other human uses. When land is transformed from one use to another, basic elements of the ecosystem upon which a species relies for shelter, food, and reproduction may be altered or may no longer be available. Some species are able to adapt more readily than others. For example, in areas of great biodiversity, species tend to be highly specialized in terms of the food they eat, where they live and reproduce, as well as the niche services they provide. When there is a loss of habitat, these species are at a greater risk of extinction than those which have larger habitat ranges.
Conversely, a few species may thrive under the new conditions since predators may no longer be a key factor or there is increased availability of food sources. However, any shift in species dominance can affect both the total number of species in an altered habitat as well as the genetic diversity of the species that remain in the habitat. An additional threat to endemic plants and animals can occur from the introduction of non-native species that migrate or are transported from other areas, gaining a foothold in a changing habitat. Marine species are particularly vulnerable to these types of changes.
Many factors can contribute to habitat loss. There are natural circumstances, such as wind, fire, and flooding, as well as factors of population growth, economic fluctuation, and sociopolitical changes that affect how land is used by humans. As agricultural economist Gerald Nelson has noted, global trends in such drivers can influence changes in local ecosystem management. For example, in Brazil's Cerrado region, government policies promoted the expansion of large-scale commercial agriculture for export from the 1960s to the 1980s, leading to a dramatic loss of biodiversity in the Hotspot.
Human preference for certain lands and certain species is also reducing the number of habitats, creating more uniform landscapes across the globe. Approximately 25% of all land on Earth has been transformed into cultivated systems, many of which are producing the same monoculture crops for sale on the commodities market, which has been shown to decrease both species and habitat diversity. Existing habitats can also become fragmented when bisected by roadways, dams, and other physical alterations to the landscape. These habitat fragments generally support fewer species; tigers and great apes are two species especially susceptible to habitat fragmentation.
Biomes with concentrated populations have undergone the most conversion, but the rate of change is now highest in developing countries within Southeast Asia and South America . Historically, the areas with the greatest percentage of their original footprint converted for human use have been the temperate grassland, savannah, and shrubland biomes and the Mediterranean forest, woodland, and scrub biomes. And, over the last 50 years, both terrestrial and marine habitats have experienced conversion greater than any other time in history.
Currently, grasslands and tropical dry forests are being converted faster than any other biome. Under the scenarios predicted by the Millennium Ecosystem Assessment, an additional 10% to 20% of grass and forestland is projected to be converted ? primarily to agriculture ? by 2050. Growing coastal communities are also seeing an increase in habitat loss and degradation due to dredging, port expansion, and shoreline stabilization efforts. And, mangroves are being degraded or destroyed at nearly twice the rate of tropical forests.
Not all biomes or nations are experiencing rapid habitat loss. North America, Europe, and China are characterized by declining or stabilizing trends, and in the Mediterranean and temperate mixed forest, the rate of loss has leveled since 1950 because most of the arable land had already been converted. There is concern, however, that as the human population continues to expand, increased demand for natural resources may reverse this trend. New technologies are now enabling humans to explore (and transform) previously inaccessible or unusable ecosystems, including the polar seafloor.
Management interventions, such as protection in reserves and tighter regulations on drivers of change like pollution or land conversion, are generally attributed to shielding both land and aquatic species from major changes in habitat. The majority of international and national policies aiming to preserve biodiversity also now include recommendations or incentives for curbing habitat loss. Since habitat loss is often caused by the confluence of several drivers, methods to stabilize or reduce problems can take on a variety of forms, including instituting local policies, skills training, economic penalties, and the establishment of protected areas.
Ecosystems and Human Well-being: Current States and Trends Section 4.3.1 Habitat Change, Loss, and Degradation of the Millennium Ecosystem Assessment examines how the causes of species extinction are changing from historical trends affecting island species to present-day habitat loss and degradation affecting continental species.
Scientific Facts on Ecosystem Change Greenfacts.org summarizes the essential points of the Millennium Ecosystem Assessment, illustrating what types of ecosystems have changed over time and the observed effects on biodiversity.
Living Planet Report Released annually since 1998, WWF's Living Planet Reports show the state of the natural world and the impact of human activity through tracking and analysis of the human ecological ?footprint? and species diversity in a sampling of regions around the world.
LandScope America LandScope draws from and links to a variety of sources in order to provide a clear, up-to-date perspective on the changing landscape. The site also includes a useful image gallery, with profiles of the nation's iconic habitats.
Share the Adventure! Tracking Habitat Change Educator's Guide In this electronic field trip created by the Bureau of Land Management, students "travel" to Nevada and New Mexico to join scientists as they examine factors contributing to changes in the habitat of the sage-grouse and the lesser prairie chicken. [Grades 4-9]
Ellis, Erle and Robert Pontius, 2007. "Land-use and land-cover change," Encyclopedia of Earth. Ed. Cutler J. Cleveland (Washington , D.C.: Environmental Information Coalition, National Council for Science and the Environment).
Foley, J.A., R. DeFries, G.P. Asner, C. Barford, G. Bonan, S.R. Carpenter, F.S. Chapin, et al. 2005. Global Consequences of land use. Science 22 July: 570-574.