Environment Factors

l Dispersal and Distribution

The movement of individuals away from their area of origin or from centers of high population density, called dispersal, contributers to the global distribution of organisms. The dispersal of organisms is critical to understanding both geographic isolation in evolution and the broad patterns of current geographic distribution of species.

- Natural Range Expansions

The importance of dispersal is most evident when organisms reach an area where they did not exist previously.

- Species Transplants

To determine if dispersal is a key factor limiting the distribution of a species, ecologists may observe the results of intentional or accidental transplants of the species to areas where it was previously absent. For a transplant to be considered successful, some of the organisms must not only survive in the new area but also reproduce there. If a transplant is successful, then we can conclude that the potential range of the species is larger than its actual range; in other words, the species could live in certain areas where it currently does not.

l Behavior and Habitat Selection

Some of organisms do not occupy all of their potential range, even though they may be physically able to disperse into the unoccupied areas. When individuals seem to avoid certain habitats, even where the habitats are suitable, the organism’s distribution may be limited by habitat selection behavior.

l Biotic Factors

In many cases, a species cannot complete its full life cycle if transplated to a new area. This inability to survive and and reproduce may be due to negative interactions with other organisms in the form of predation, parasitsm or competition. Alternatively, survival and reproduction may be limited by the absence of other species on which the transplanted species depends, such as pollinators for many flowering plants. Predator (organisms that kill their prey) and herbivores (organisms that eat plants or algae) are common examples of biotic factors that limit the distribution of species. Simply put, organisms that eat can llimit the distribution of organisms that get eaten. In addition to predation and herbivory, the presence of absence of food resources, parasites, pathogens and competing organisms can act as biotic limitations on species distribution. Some of the most striking cases of limitation occur when humans accidentally or intentionally introduce exotic predators or pathogens into new areas and wipe out native species.

l Abiotic Factors

Abiotic factors, such as temperature, water, salinity, sunlight, or soil, might be limiting a species distribution. If the physical conditions at a site do not allow a species to survive and reproduce, then the species will not be found there. Although two regions of Earth may experience different conditions at any given time, daily and annual fluctuations of abiotic factors may either blur or accentuate regional distinctions. Furthermore, organisms can avoid some stressful conditions temporarily through behaviors such as dormancy or hibernation.

- Temperature

Environment temperature is an important factor in the distribution of organisms because of its effect on biological processes. Cells may rupture if the water they contain frezzes, and proteins of most organisms denature at temperature above 45℃. Few organisms can maintain an active metabolism at very low or very high temperatures, though extraordinary adaptations enable some organisms, such as thermophilic prokaryotes, to live outside the temperature range habitable by other life.

-Water

The dramatic variation in water availability among habitats is another important factor in species distribution. Species living at the seashore or in tidal wetlands can desiccate (dry out) as the tide recedes. Terrestrial organisms face a nearly constant threat of desiccation and the distribution of terrestrial species reflects their ability to obtain and conserve water.

- Salinity

The salt concentration of water in the environment affects the water balance of organisms through osmosis. Most aquatic organisms are restricted to either freshwater or saltwater habitats by their limited ability to osmoregulate. Although many terrestrial organisms can excrete excess salts from specialized glands or in feces, salt flats and other high-salinity habitats typically have few species of plants or animal.

- Sunlight

Sunlight absorbed by photosynthetic organisms provides the energy that drive most ecosystems, and too little sunlight can limit the distribution of photosynthetic species. In forests, shading by leaves in the treetops make competition for light especially intense, particularly for seedlings growing on the forest floor. In aquatic environments, every meter of water depth selectively absorbs about 45% of the red light and about 2% of the blue light passing through it. As a result, most photosynthesis in aquatic environments occurs relatively near the surface. Too much light can also limit the survival of organisms. The atmosphere is thinner at hugher elevation, absorbing less ultraviolet radiation, so the sun’s rays are more likely to damage DNA and protein in alpine environment. In other ecosystems, such as deserts, high light levels can increase temperature stress if animals are unable to avoid the light or to cool themselves through evaporation.

- Rocks and Soil

The pH, mineral composition, and physical structure of rocks and soil limit the distribution of plants and thus of the animals that feed on them, contributing to the patchiness of terrestrial ecosystems. The pH of soil and water can limit the distribution of organisms directly, through extreme acidic or basic conditions, or indirectly, through the solubility of nutrients and toxins. In stream and rivers, the composition of the substrate (bottom surface) can affect water chemistry, which in turn influences the resident organisms. In freshwater and marine environment, the structure of the substrate determines the organisms that can attach too it or burrow into it.

- Climate

Climatic factors, particulary temperature and water availability, have a major influence on the distribution of terrestrial organisms. Climate patterns can describe on two scales: macroclimate, patterns on the global, regional and local level; and microclimate, very fine patterns, such as those encountered by the communit of organisms that live beneath a fallen log.