Ecosystem Organisation and Interdependence

An ecosystem includes this living community, the non-living environment and the interactions between them. Light intensity, water availability and mineral ions are non-living components; plants and animals are living components. An ecosystem is therefore more than a collection of organisms.

These levels describe what is included, rather than a fixed physical size. An ecosystem can be as small as a garden pond or as extensive as Antarctica. A habitat, by contrast, is the place where an organism lives: woodland is a habitat for the organisms described above.

Environmental changes affect populations differently

An abiotic factor is a non-living environmental factor. Examples include temperature, light intensity, water availability and the presence or concentration of pollutants. Biotic factors involve the living components of the environment, including competition and predation.

Environmental changes can affect both population size and distribution. Population size means the number of organisms; distribution means where they occur. A change can increase the population of one species while decreasing that of another, so a community does not necessarily respond as a single unit.

How temperature, light, water and pollutants affect communities

Temperature affects enzyme activity and therefore processes such as respiration and growth. Organisms have a range of temperatures in which they can survive and reproduce successfully. Conditions outside this range can reduce their population size or restrict where they live. Different species tolerate different temperatures, so a temperature change can alter the mixture of species in a community.

Light intensity affects photosynthesis. Where light limits photosynthesis, plants make less glucose, which can reduce their growth. Some plants grow well in shade, whereas others require brighter conditions. Light therefore influences which plants grow in an area and how much food and shelter they provide for animals.

Water availability also affects plant growth: plants need water for photosynthesis and other cell processes. A shortage of water can reduce plant growth, leaving less food for plant-eating animals. Too much water can also be harmful. In waterlogged soil, roots may receive too little oxygen for aerobic respiration. Species differ in their tolerance of dry or waterlogged conditions, affecting their distribution.

Pollutants can harm sensitive organisms. For example, lichens sensitive to sulphur dioxide cannot grow where the concentration of this air pollutant becomes too high. Such pollution can restrict where these lichens occur and reduce their population size.

These effects can spread through feeding relationships. If dry conditions reduce grass growth, rabbits may have less food; a decline in rabbits can then leave less food for foxes. An abiotic change can therefore affect animals indirectly through its effect on plants.

Competition for limited resources

Competition occurs when organisms need the same resources and there is not enough for all of them. It can occur between individuals of the same species or between different species.

Plants compete for light, water, mineral ions and space to grow. Animals compete for food, water and territory; animals of the same species can also compete for mates. Organisms that obtain fewer resources may grow less well, be less likely to survive or produce fewer offspring. Their population can consequently decrease.

Red and grey squirrels provide a real example. Grey squirrels were introduced to Britain from America in the nineteenth century. They share habitats and food sources with native red squirrels and can outcompete them for food. Competition has contributed to reductions in red squirrel numbers and distribution. Grey squirrels can also carry squirrelpox virus, which can be deadly to red squirrels: more than one biotic factor can affect the same population.

Predation connects population sizes

Predation is the killing and eating of one organism by another. The organism doing the eating is the predator, and the organism eaten is the prey. For example, foxes are predators of rabbits.

More predation can reduce the number of prey. This may happen when a new predator enters a community. The effects can extend to other species: if prey become scarce, existing predators may have less food and their populations may also decline. Conversely, fewer predators can allow prey numbers to increase, potentially increasing the amount of vegetation those prey consume.

Why species depend on one another

Organisms in a community do not live independently of all the other species around them. They can depend on other species for food, shelter, pollination and seed dispersal. This dependence is called interdependence.

Each relationship connects populations. If a species that provides food or shelter is removed, organisms that depend on it may also be affected. Likewise, losing organisms that carry pollen or disperse seeds can affect the plants that depend on those services. Removing one species can therefore have consequences beyond that species, changing the wider community.

Parasitism: one species benefits at another's expense

In parasitism, a parasite lives in or on another organism, called its host, and obtains resources from it. These resources may include food, shelter or a suitable place to reproduce. The parasite benefits, but the host is harmed rather than benefiting.

For example, fleas live on mammals such as dogs and feed on their blood. The flea gains nutrients and depends on the host as its food source. The dog loses blood and may suffer irritation. This is a form of dependence, but not one in which both participants benefit.

Mutualism: both species benefit

In mutualism, both species benefit from their relationship. Bees and many flowering plants are an example.

Bees obtain nectar from flowers as food, providing energy. As a bee visits a flower, pollen can attach to its body. When it visits another flower of the same species, it can transfer that pollen, helping to pollinate the plant. The bee gains food, while the plant gains help with reproduction.

If these pollinators decline, plants that depend on them may reproduce less successfully. If the flowering plants decline, bees may have less food. The relationship shows how the continued survival of organisms can depend on the presence of other species.

Abiotic factors:

• Temperature affects enzyme activity, growth and survival.
• Light affects photosynthesis and plant growth.
• Water shortage limits plant growth; waterlogging can limit oxygen for root respiration.
• Pollutants can harm sensitive species: high sulphur dioxide concentrations prevent sensitive lichens from growing.

Biotic factors:

• Competition for limited resources can reduce survival and reproduction. Grey squirrels compete with red squirrels for food.
• Predation can reduce prey numbers; fewer prey can leave predators with less food.

Interdependence

Species depend on others for food, shelter, pollination and seed dispersal. Changes to one population can affect the wider community.

Parasitism and mutualism

• Parasitism: parasite benefits; host is harmed. Fleas obtain blood from dogs.
• Mutualism: both species benefit. Bees obtain nectar; flowering plants gain pollination that helps reproduction.