Terrestrial ecosystems and biomes

An ecosystem is a system of interactions of a community of organisms with its surroundings. The largest fundamental region of an ecosystem is the biome. A biome consists of similar plants and animals that can be distinguished from each other by vegetation and climate. Some examples of biomes are grasslands, tundra, forests and tropical rainforests (Bryant, 1997). A biome includes both aquatic and terrestrial ecosystems. Two laws control the boundaries of climatically determined biomes. These are the law of the minimum and the law of the maximum (Bryant, 1997). The law of the minimum states that plants have a minimum requirement for growth and reproduction controlled by a range of factors, like nutrients, availability of water, sunlight and temperature. The factor that is least available has the greatest effect on plant growth, so that small changes in that parameter can have a profound influence on a plant's survival. In Fig. 4.5, the distribution of biome types depending on latitude, temperature and precipitation is shown. For instance, the disappearance of tropical forest in favour of savannas as the result of a decrease in precipitation is an example of change through not meeting the minimum requirement limit for the rain forest (Zheng and Lei, 1999). The law of the maximum states that too much of a certain factor also limits the area where a plant can survive; for instance, the replacement of wetland species by aquatic plant species when the water level rises in an area, or the replacement of one mangrove species by another, when the concentration of salt increases and exceeds a certain threshold (Ellison et al., 2000). As a result of these laws and other environmental factors, biomes show up spatially as distinct zones of vegetation, mainly spaced parallel to the temperature gradient that exists either latitudi-nally between the equator and the poles, or with elevation on mountains.

Climate change affects the distribution of biomes through both of these laws. An increase in temperature exceeds the maximum requirements for some species, which will disappear from that place. For other species the rise in temperature could make

Mean annual precipitation [mm] Fig. 4.5. Classification of biome types based upon climatic classification of Holdridge (1947).

certain places more suitable for habitation and they will migrate to these places. The same effects can be expected for changes in precipitation (Bryant, 1997).

Important drivers for changes in terrestrial ecosystems are migration, changes in fire regime and changes in water availability. How these changes affect the terrestrial biomes situated in river basins, forests, grasslands, plains and mountains is now described.

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