Earth's Biosphere and Climates

The distribution of organisms around the planet is dependent upon several factors. Scientists classify factors as biotic or abiotic. A biotic factor is a living thing that influences an ecosystem, such as food resources, predators, and pathogens. An abiotic factor is a nonliving thing, such as water, temperature, and minerals, that affects an ecosystem. An ecosystem includes all organisms living in one geographic region and all the environmental factors that affect those organisms.

Both types of factors determine whether an organism is likely to reside in a particular location. Biotic factors include the presence or absence of other organisms. The presence of natural predators may drive a population to either leave an area or become extinct. An absence of natural prey or other nutrient-providing species may have a similar effect. For example, either the presence of a large population of wolves or the absence of plentiful grasses may cause a population of rabbits to leave an area or die out. Competition can also play a role in where a species is found. Competition exists when species fight each other directly or when they use the same resource. A population may be physically capable of thriving in a given environment but be pushed out of an area by another species that is a better at finding resources. For example, if there are no wolves and plenty of grasses, but a large population of grasshoppers consumes the grasses before the rabbits can find them, the rabbit population may leave the area or die out.

In addition to biotic factors, the global distribution of species is also dependent on abiotic, or nonliving, factors. Organisms tend to be found in places well suited for their physiological needs and physical constraints. Water is among the most important abiotic factors because water is critical for life. Access to some amount of freshwater, which accounts for only a small percentage of all water on Earth, is important for terrestrial organisms. Temperature is another factor that determines where life is found on Earth. Many organisms survive within a narrow body-temperature range. Depending on its metabolism, an organism has a greater or lesser tolerance for extreme environmental temperature fluctuations. In addition, some organisms are better adapted to cold temperatures, while others thrive at warm temperatures. As a result, more organisms are found in locations with moderate or stable temperatures than in locations with extreme or highly variable temperatures. Sunlight is another important abiotic factor affecting the geographic distribution of life. Photosynthetic organisms require access to sunlight, and these organisms are often the source of energy for ecosystems. Subterranean caves and deep-ocean environments contain organisms that use alternative energy sources, such as feeding on leftovers that sink from shallow depths. Pressure is yet another abiotic factor that can affect living things. In the deep ocean, where pressure is high, only organisms specifically adapted to those conditions can thrive. Similarly, at high mountain altitudes, where air pressure is low and oxygen is scarce, specially adapted organisms are found. Oxygen is important for respiring organisms, but the presence of other critical elements in air, soil, and water can be equally important. For example, organisms also require carbon, nitrogen, and phosphorus to carry out life's processes. Regional variability in each of these factors helps explain the variety of organisms found across different environments around the globe.

Additional factors affect where populations thrive. Some geographic patterns of living things are global. There tends to be greater biodiversity (number and variety of species in a given area) at low and mid-range elevations than at higher elevations. It is suggested this is because there is less habitable area at higher elevations. There tends to be greater biodiversity closer to the ocean's surface than in the deepest depths of the oceans, because of the availability of light. In addition, more biodiversity exists near the equator than near Earth's poles. As in all aspects of biology, there are exceptions to these general trends. For example, a hydrothermal vent is a deep-sea fissure releasing extremely hot fluid that contains hydrogen sulfide. Areas around hydrothermal vents have much greater biodiversity than the surrounding sea floor.

Earth's biomes (areas defined by climatic and other characteristics) and ecosystems (living things and their environment) are diverse because climate varies geographically. Climate is the long-term, wide-range weather patterns that an area experiences, such as the amount of precipitation, the average temperature, and the average humidity. Climate patterns are the result of Earth's movement around the Sun and Earth's daily rotation on its tilted axis. Areas around the equator experience direct sun exposure for most of the year. However, areas away from the equator experience seasonal variation in sun exposure, depending on which part of Earth is tilting toward or away from the Sun. One hemisphere experiences summer when it's tilted toward the Sun and catches its greatest annual portion of direct rays. Six months later, the other hemisphere will be in a similar position and experience its summer. In addition to predictable annual variation in sun exposure, there are major global patterns in air movement. Air rises at the equator and falls at 30 degrees latitude. Air rises again at 60 degrees and falls at the poles. This pattern exists in both hemispheres. These large wind patterns affect climate because they determine global patterns in rainfall. Near the equator where air is rising, there is high rainfall and warm temperatures. At 30 degrees (north and south), cool, dry air sinks and is reheated, resulting in some of the world's great deserts. At 60 degrees, air rises again and drops rainfall, forming the cool, temperate forests. At the poles, cold, dry air falls again, leading to low-rainfall polar climates. In addition to rainfall patterns based on latitude, there is a local effect that occurs when wind blows toward a mountain range. A rain shadow is an effect of air movement over a mountain range that causes wet conditions on one side of the mountain range and dry conditions on the other side. As the air moves up the mountain range, it loses water, causing rain on the windward side. When the air reaches the peak of the range, the dry air begins to descend and warm, causing a desert on the leeward side of the mountain range. Individual mountains also show an ecosystem gradient; high elevations lead to cool, dry conditions. The highest elevations are similar in climate to polar regions.