Early arrivals may facilitate the appearance of later species by changing the environment. For example, early herbaceous species may increase soil fertility. species may inhibit establishment of later species. Early species may tolerate later species but neither hinder nor help their colonization. Concept 53.4 Biogeographic factors affect community biodiversity • Two key factors correlated with a community’s biodiversity (species diversity) are its geographic location and its size. • In the 1850s, both Charles Darwin and Alfred Wallace pointed out that plant and animal life were more abundant and varied in the tropics. They also that small or remote islands have fewer species than large islands or those near continents. • Such observations suggest that biogeographic patterns in biodiversity conform to a set of basic principles. Species richness generally declines an equatorial-polar gradient. • Tropical habitats support much larger numbers of species of organisms than do temperate and polar regions. • What causes these gradients? The two key factors are probably evolutionary history and climate. • Over the course of evolutionary time, species diversity may increase in a community as more speciation events ° Tropical communities are generally older than temperate or polar communities. ° The growing season in the tropics is five times longer than that in a tundra community. Biological time thus runs five times faster in the tropics. ° Repeated glaciation events have eliminated many temperate and polar communities. • Climate is likely the primary cause of latitudinal gradients in biodiversity. ° Solar energy input and water availability can be combined as a measure of evapotranspiration, the evaporation of water from soil plus the transpiration of water from plans. Actual evapotranspiration, determined by the amount of radiation, temperature, and water availability, is much higher in hot areas with abundant rainfall than in areas with low temperatures or precipitation. Potential evapotranspiration, a measure of energy availability, is determined by the amount of solar radiation and temperature. The species richness of plants and animals correlates with both measures of evapotranspiration. Species richness is related to a community’s geographic size. • The species-area curve quantifies what may seem obvious: the larger the geographic area of a community, the greater the number of species. Larger areas offer a greater diversity of habitats and microhabitats than smaller areas.
This study demonstrates that the amount of nutrients leaving an intact forest ecosystem is controlled by the plants. Results of the Hubbard Brook studies assess natural ecosystem dynamics and provide insight into the mechanisms by which human activities affect ecosystem processes. Concept 54.5 The human population is disrupting chemical cycles throughout the biosphere activities and technologies have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems worldwide. The human population moves nutrients from one part of the biosphere to another. • Human activity intrudes in nutrient cycles. Nutrients from farm soil may run off into streams and lakes, depleting nutrients in one area, causing excesses in another, and disrupting chemical cycles in both places. Humans also add entirely new materials—many toxic—to ecosystems. • In agricultural ecosystems, a large amount of nutrients are removed from the area as crop biomass. After a while, the natural store of nutrients can become exhausted. The soil cannot be used to grow crops without nutrient supplementation. • Nitrogen is the main nutrient lost through agriculture. Plowing and mixing the soil increase the decomposition rate of organic matter, releasing usable nitrogen that is then removed from the ecosystem when crops are harvested. • Recent studies indicate that human activities have approximately doubled the worldwide supply of fixed nitrogen, due to the use of fertilizers, cultivation of legumes, and burning. This may increase the amount of nitrogen oxides in the atmosphere and contribute to atmospheric warming, depletion of ozone, and possibly acid precipitation. • The key problem with excess nitrogen seems to be critical load, the amount of added nitrogen that can be absorbed by plants without damaging the ecosystem. Nitrogenous minerals in the soil that exceed the critical load eventually into groundwater or run off into freshwater and marine ecosystems, contaminating water supplies, choking waterways, and killing fish. • Lakes are classified by nutrient availability as oligotrophic or eutrophic. In an oligotrophic lake, primary productivity is relatively low because the mineral nutrients required by phytoplankton are scarce. Overall productivity is higher in eutrophic lakes. • Human intrusion has disrupted freshwater ecosystems by cultural eutrophication. Sewage and factory wastes and runoff of animal wastes from pastures and stockyards have overloaded many freshwater streams and lakes with nitrogen. This results in an explosive increase in the density of photosynthetic organisms, released from nutrient limitation.