Biotic and Abiotic Factors

There are many abiotic (physical) factors which affect the distribution of plants and animal in their habitats.

Light is a major abiotic factor, as it is the source of energy for photosynthesis, which in turn means it influences all producers and consumers (directly or not). The amount of light available is a major factor in deciding where plants can survive. For instance terrestrial plants which are shade-tolerant can be found underneath trees, whereas those who are not only grow where there is a plentiful supply of sunlight. Similarly in aquatic environments some plants must have leaves on the surface of the water in order to photosynthesise, while others can survive under the water, where less light can penetrate.

The temperature of a habitat will provide further limitations to which organisms can survive there. Nearly all living organisms have an optimum temperature range to live, so changes to the temperature will affect the rate they grow, and will affect their evolution. This is less significant with aquatic plants and animals as water has a high specific heat capacity, which means it takes lots of energy to heat water, so its temperature does not fluctuate greatly. However the heat does affect the distribution of organisms in aquatic environment in one way at least, this is that higher temperatures cause a reduced amount of dissolved oxygen in the water which leads to a reduced amount of aerobic organisms. It is mainly the sun which dictates the temperature, and the latitude will be the main factor in how hot the habitat will be. So some organisms will be suited to the temperatures on the equator while others will be suited to the poles.

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Through interaction with other physical features and organisms, the wind will affect the distribution of the organisms. The growth of trees and shrubs can be affected by strong prevailing winds. Wind speeds also affect the rate of transpiration, and the likelihood of the exposed side of the tree freezing. These three affects could mean that a certain plant could no longer survive in the area, so they would not be as widely distributed. It would follow that the lack of the plant could adversely affect the distribution of other plants and animals (this would be a biotic factor - which we will look at in greater detail later). The wind can also affect the migration of birds and seed and spore dispersion in plants, as such the wind can affect the distribution of plants and animals in this way.

The availability of water will be a factor in determining which organisms can live where. Plants can be divided into three categories with respect to their ability to live with water. Hydrophytes are those which are adapted to living in waterlogged or submerged conditions, xerophytes are those who are adapted to living in conditions with little water and finally mesophytes are those who are adapted to living in conditions where there is an adequate supply of water. So depending on the amount of water available different species of plants will be distributed accordingly. Terrestrial animals are also heavily affected by the availability of water, meaning they are also affected by the availability of water. Animals which are evolved to conserve water and to lose as little through evaporation will be found in dry hotter environments, while those who are not as good at that will be found in the places where water is less scarce.

Salinity can affect the distribution of organisms, especially plants because it is harder for water to pass into the roots by osmosis when outside the roots there is a high water potential. Halophytes are plants which can tolerate high levels of salt, they are usually found in estuaries and salt marshes, where other plants cannot be found.

In aquatic habitats the flow of water can effect the distribution of organisms. The flow of water can move organisms from one place to another; very fast flowing water can often make it difficult for any organisms to survive in one place, as they are washed away. The fast flowing water is often more oxygenated, as more of its surface area is exposed to the air. Where the water is more oxygenated there will be more animals and photosynthetic plants as the supply of oxygen will be greater compared to that where the water is less oxygenated, as such oxygen concentration is itself an abiotic factor. Some animals are specifically adapted for living in water with low oxygen concentrations while other will simply die without sufficient oxygen.

Soil type affects habitats because it determines the type of vegetation which can grow there, which also determines which animals will live there. One type of soil profile is podzol, and this is associated with coniferous woodland and heath land, at higher altitudes. Brown earth (another soil profile) is associated with deciduous woodland at lower altitudes. The pH of the surroundings, be it soil or water, will affect the distribution of plants and animals. Some plants, such as heather grow well in acidic conditions, while others such as dog's mercury thrive in alkaline conditions, so you can see that the pH will determine the distribution of terrestrial plants. Because of their affect on plants, they have a knock on effect on consumers, as they rely on plants for their food. Aquatic animals are also affected by pH, while some can survive in a wide range of pH, such as the freshwater shrimp, others are only found in water with specific pH's.

Along with these abiotic factors there are several biotic factors which also affect which affect the distribution of plants and animals.

Competition between organisms is a major biotic factor, as organisms compete for food, water, light, minerals, shelter and a mate. Organisms don't just have to compete with others from within their species (intraspecific competition) but also with organisms from other species (interspecific competition). This competition can lead to the displacement of one species - i.e. their distribution. While organisms have to deal with competition they also have predation to cope with. The distribution of a species is also determined by the presence or absence of prey and/or predators.

Some organisms produce chemicals which repel other organisms (which may be of the same species - or not). This mostly applies to terrestrial animals that use chemicals to mark their territories, in order to deter other members of the species. This can reduce the distribution of some organisms in an area. Some ants produce pheromones when in danger, these pheromones warn other members of the species of the danger.

The dispersion of some plants rely upon animals to disperse their seeds. Similarly many plants rely on insects for pollination, and without the insect they would be unable to reproduce effectively.

Probably the most significant biotic factor which affects organisms is the influence of humans, we often dictate which organisms grow where, while being hunters, fishers, farmers, developers and polluters to name a few activities which affect other organisms.

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