The organism in the environment

A habitat is the place where an organism lives. It includes the places where the organism finds food, mates, sleeps and reproduces. A community is the total number of all the organisms of different species living together in one habitat. An ecosystem is made up of a community of organisms which interact with the non-living (abiotic) elements of their environment. Biodiversity is used as a measure of the variety of all the different species of living organisms present in an ecosystem. A population is the number of organisms of one species which are living within one habitat.

The population of any organism in a habitat can be determined using quadrats.

A quadrat is a square frame of known dimensions. The quadrat is placed at random points within the habitat and the number of organisms of that species that are within that area are counted.

The quadrat is then placed into several new positions and the number of organisms in each position are counted. The population size of the organism in that habitat is determined as the mean number of organisms per quadrat. This is calculated as the total number of organisms divided by the number of quadrats used.

The population of various organisms in the same habitat can be determined using the same method and this allows scientists to investigate the distribution of organisms in the same habitat and therefore measure the biodiversity in that habitat.

The population size and distribution of organisms can vary widely and is affected by abiotic and biotic factors.

Abiotic factors

Abiotic factors are non-living factors which interact with each other to create environments in which the animals and plants making up a community can live.

The following are examples of abiotic factors which can affect living organisms and therefore the population size and distributions:

• Temperature – as temperature is a limiting factor for photosynthesis, the growth of green plants in the community may be limited in cold climates. The limited number of green plants limits the number of herbivores in the community as they do not have enough food to survive, and consequently limits the number of carnivores which feed on the herbivores
• Light intensity – light is also a limiting factor for photosynthesis so will also affect the distribution of plants and animals in a community. In low light levels green plants may be replaced by fungi or mosses as they are more suited to this environment
• Amount of water available – plants and animals rely on water for survival. For example, after it rains in a desert many plants will grow and release seeds where water supplies are adequate. These plants are eaten by animals which may move into the community to feed on them
• Levels of pollutants – if the levels of pollutants in an area are excessive, organisms may not survive. For example, lichens can’t survive in high sulphur dioxide concentration
• Wind intensity and direction – in areas where the winds are very strong, the shape of trees and landscapes can be changed. Strong winds can also cause the rate of transpiration in plants to increase but reduce the amount of wind pollination which occurs as the pollen is blown away from the other plants
• Soil pH and mineral content – the level of mineral ions and pH of the soil determines the types of plants which can grow and survive in an environment. For example, if the level of nitrate ions in the soil is high carnivorous plants can survive, but other types of plants may not
• Availability of oxygen – the survival of organisms living in watery habitats depends on the availability of oxygen. Some invertebrates can survive in very low oxygen levels, whereas most fish require high oxygen levels for survival
• Availability of carbon dioxide – the amount of carbon dioxide is also a limiting factor for photosynthesis so in low levels of carbon dioxide plants are unable to photosynthesise and grow

Biotic factors

Biotic factors are the living factors which affect the distribution of organisms in communities, and examples include:

• Competition – organisms compete with members of their own and other species for the same resources such as food and shelter. For example, red and grey squirrels may live in the same habitat and therefore need to eat the same food. There is a limit to the amount of food available so if the grey squirrels eat most of the food there will be very little left for the red squirrels and their numbers will decrease as they cannot survive to reproduce
• Predation – the number of predators is determined by the amount of prey available. If the number of prey animals decreases, there will not be enough food for the predators. The predators will die, and their numbers will decrease whilst the prey animals become more likely to survive and reproduce. When new predators are introduced to a community, the prey animals are not familiar with them and have no defences against them. The prey animal numbers may decrease rapidly as they cannot survive
• Disease caused by pathogens or parasites – if a new pathogen or parasite is introduced to a community, the organisms in that community may have no resistance and their populations may reduce so much that they die out