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B iomes are compri sed of the major, regionally distinct biotic communities. They are the largest ecosystem units, delineated at a global scale. Biomes are not distinguished by the taxonomic identities of the organisms they contain, but rather on the basis of the life forms of these organisms, their structure, life history, and responses to environmental change. Although specific plant and animal species differ among continents, the same biomes with similar structure, seasonality, productivity, niches and uses by humans exist in different regions. For example, northern conifer forests exist in North America and Asia, and tropical rain forests are found in Africa, Central and South America, and south and southeast Asia. Biomes are usually associated with their climax community vegetation; however, they encompass successional and subclimax community species and animal species, as well as soils. Generally, they are not divided by sharp boundaries; rather, adjacent biomes grade into each other, interact, and function as interdependent parts of the biosphere as a whole.
Classifying the earth into major biome types is a useful approach that allows the development of a common framework and mapping system for these large-scale systems. This facilitates communication among scientists, comparative analyses, and the development of resource and environmental management strategies. However, it is important to note that maps of biomes are human constructions and not usually drawn to reflect a current reality. Rather, they tend to depict an imagined world devoid of human impacts and influence, and one in which processes of succession have reached a climax end-state.
Biomes are delineated by a combination of ecological gradients, including temperature, precipitation, altitude/depth, latitude, longitude, proximity to various features such as oceans and mountains, soil type, salinity, and range of tidal activity. These factors determine the assemblage of animals and plants that live in the biome, and their biological productivity.
Human Impact on Biome Classes
Various classification systems have been developed to organize biomes. Some scientists apply the biome concept exclusively to terrestrial systems because their structure and connections to other aquatic environments differ from terrestrial systems, and they are perceived to be less responsive to climatic cues. Other scientists, however, include freshwater and marine systems in their biome classifications. Whittaker provides a classification system that is more detailed than some, with 36 discrete biome types. Cox and Moore identify ten terrestrial and four aquatic biomes. These encompass the arctic tundra, northern coniferous forest, temperate forest, tropical rain forest, tropical seasonal forest, temperate grassland, tropical savanna grassland and scrub, desert, chaparral, mountains, freshwater, oceans, rocky shores, and muddy or sandy shores.
Desert biomes are arid, with low and often irregular precipitation coupled with high evaporation. They have relatively low productivity and are one of the harshest environments on earth. Human activities, such as animal grazing, have actually extended the range of deserts in the world through the process of desertification. Tundra is identified with low temperatures and permafrost, and is predominantly found in northern polar regions with less occurring in the southern hemisphere. Many animal populations in the tundra are migratory and/or have large cyclic changes in abundance. Significant human impacts on the tundra include those associated with fossil fuel and mineral extraction, military operations, and the exploitation of both marine and terrestrial animal resources.
The northern coniferous forest (or taiga) occurs adjacent to the tundra region, encircling the northern latitudes of continents, and is also found in high-altitude regions in lower latitudes. It is comprised primarily of evergreen conifers and represents one of the world’s largest, most intact biomes. Logging coupled with large-scale mining, however, are degrading this biome. This degradation is exacerbated by the impacts of acid rain (caused by emissions of air pollutants primarily from midlatitudes) on terrestrial and freshwater aquatic systems. In the mid-latitudes, temperate forests have a seasonal climate distinguished by warm summers, cold winters, and deciduous treed land cover. A large extent of these biomes has been converted to human settlements and boasts large agricultural, urban, and industrial areas with concomitant deforestation, habitat fragmentation, and production of waste and air pollution.
Tropical rainforests occupy the equatorial region between the Tropics of Cancer and Capricorn, and are characterized by high solar radiation, temperature precipitation, species diversity, and nutrientpoor soils. Intense logging, especially in the Amazon region of South America, coupled with land conversion for agricultural operations particularly animal grazing are of special concern. Cox and Moore estimate that at the current projected rates of destruction, the tropical rainforest biome could be eradicated completely within this century.
Temperate grasslands have precipitation levels greater than those found in deserts, but too low to support forest vegetation. They are characterized by grasses, large herds of grazing mammals, and soil rich in organic matter. These biomes have been altered by development of animal and plant agricultural operations and by the introduction, both purposeful and accidental, of new plant species. This has changed the ecological balance and made soils vulnerable to moisture loss and erosion, and as a result, intact examples of this biome are quite rare. Tropical savannas have warm climates with significant dry seasons and generally poor soils. They are comprised of grasslands, shrubs, and woodlands with significant grasses as well as a diverse fauna, including large herds of grazing mammals. Fire is an important abiotic aspect of this biome. Chaparral or sclerophyll ecosystems are characterized by a Mediterranean climate of wet, mild winters and dry, hot summers. The short trees and shrubs found in these areas are adapted to withstand summer droughts. These areas support large human settlements and, as a result, have been degraded by urbanization, pollution, and introduced species.
Overarching all of these human impacts is the impact of greenhouse gas pollution on the earth’s atmosphere and its climate system. All of the world’s biomes are affected by climate change, with the arctic tundra and ice-covered regions experiencing the most serious and visible impacts. Future impacts of climate change pose arguably the greatest threat to the stability of the world’s biomes.
- Barry Cox and Peter D. Moore, Biogeography: An Ecological and Evolutionary Approach, 5th ed. (Blackwell Scientific Publications, 1993);
- Mitchell B. Rambler, Lynn Margulis, Rene Fester, , Global Ecology: Towards a Science of the Biosphere (Academic Press, 1989);
- Ian Simmons, Biogeographical Processes (George Allen & Unwin, 1982);
- Robert H. Whittaker, Communities and Ecosystems, 2nd (Macmillan, 1975).