Unit 9: Biodiversity Decline // Section 7: Habitat Loss: Causes and Consequences
Most endangered species are threatened by multiple factors, but habitat loss is generally viewed as the largest single cause of biodiversity loss worldwide. When humans convert wild areas for agriculture, forestry, urban development, or water projects (including dams, hydropower, and irrigation), they reduce or eliminate its usefulness as a habitat for the other species that live there.
As discussed in Unit 4, "Ecosystems," a species' ecological niche is the sum of all of the ranges of tolerance under which it can survive in a specific ecosystem, including temperatures, climate, type of shelter, food sources it needs, and many other factors. Generalist species can adapt to many different types of living conditions, but more specialized organisms may not be able to adjust when their habitat is changed or disappears as a result of land development.
All forms of development alter natural ecosystems. Commercial forestry involves road-cutting through forests and the harvesting of trees that are important as shelter or food for some species. Dams change river flow patterns, dissolved oxygen levels, and water temperatures and may prevent fish from swimming upstream to spawn. Farmers clear land, withdraw large quantities of water from local sources, and introduce pesticides and chemical fertilizers to the environment. Ranching impacts land physically through grazing and generates air and water emissions from animal wastes. Urban development clears land and paves it, which changes local water cycles by increasing surface runoff and reducing groundwater supplies. It also generates air and water pollution from industrial activities and transportation. (For more on these impacts, see Unit 7, "Agriculture," Unit 8, "Water Resources," and Unit 11, "Atmospheric Pollution.")
According to the Millennium Ecosystem Assessment (MA), a four-year, multinational analysis of the health of global ecosystems, cultivated land (including land used for livestock production and aquaculture) now covers one-quarter of Earth's terrestrial area. Mediterranean and temperate forests have been most heavily impacted by land conversion, but substantial conversion of tropical forests is also projected to occur by 2050 (Fig. 8). In contrast, boreal forests and tundra have experienced almost no conversion, although they are threatened by other forces, such as global climate change.
Figure 8. Terrestrial habitat transformation
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Source: © 2005. World Resources Institute. Millennium Ecosystem Assessment. Ecosystems and Human Well-Being: Synthesis, p.4 (Washington, DC: Island Press).
Land can become less suitable as habitat even if it is not directly converted to other uses. When actions such as suburban development and road-building carve large sectors of land into fragments, the undeveloped parcels may be too small or isolated to support viable populations of species that thrived in the larger ecosystems. This process, which is called habitat fragmentation, reduces biodiversity by:
- Splitting populations into smaller groups, which may be less viable because it is harder for the isolated individuals within the groups to defend themselves or find mates
- Increasing crowding and competition within the fragments
- Reducing species' foraging ranges and access to prey and water sources
- Increasing friction between animals and humans as animals range into developed areas
Fragmentation of natural ecosystems intensifies edge effects, impacts that stem from the juxtaposition of two different ecosystems—for example, a meadow and a paved street. The edges of natural ecosystems are more susceptible to light, wind, and weather than interior areas, so they are less suitable habitat for species that live in sheltered areas. Edges also are vulnerable to invasive species.
"We're seeing mortality rates [of old-growth rainforest tree species] go through the ceiling as a consequence of edge effects. It's obvious that the ecology of the rain forest is being altered in a profound way by fragmentation," says Bill Laurence of the Smithsonian Tropical Research Institute, who studies edge effects in Panamanian rain forests.
The theory of island biogeography, developed by ecologists Robert MacArthur and E.O. Wilson to explain the uneven distribution of species among various islands, offers some insights into how habitat fragmentation affects local species. According to the theory, the number of species on an island is a balance between the rate of colonization by new species and the rate of extinction of existing species. An island's population will approach an equilibrium level where the two trends are balanced and the number of species remains stable. Large islands typically have more resources, so they can be expected to support larger equilibrium numbers of species. Accordingly, extinction rates should increase with habitat fragmentation because smaller habitat fragments support fewer species.
Island biogeography also suggests that habitat fragmentation will reduce the rate at which species colonize new areas because they have trouble crossing gaps in between the smaller sections of remaining habitat. For example, as illustrated in Figure 9, many animals are killed crossing highways that divide their ranges (and are at risk of losing genetic diversity if they cannot reach other local populations to breed).
Figure 9. Habitat fragmentation and species mobility
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Source: © United States Department of Transportation, Federal Highway Administration.
One solution that is attracting increasing interest is to create corridors of land linking separate habitat zones, making it easier for wildlife to move from one sector to another without injury or interference. Research has shown that these corridors increase wildlife movement between habitat zones, although it can be difficult to maintain access for animals when the corridors cross human structure such as highways and railroad tracks (footnote 17).