Interdependence and Competition

Organisms need resources to survive and reproduce

A community consists of the populations of different species living and interacting in the same area. Organisms in a community do not live independently: they obtain materials from their surroundings and from other living organisms.

Plants take up water and mineral nutrients from the soil and use light for photosynthesis. Photosynthesis produces glucose, which provides material for growth and can be used in respiration to release energy. Animals obtain food by eating plants or other animals. Food supplies both materials for building their bodies and energy for processes such as movement, growth and reproduction.

These needs create two important relationships: competition, when organisms need the same limited resource, and interdependence, when species rely on one another.

Competition between plants

Competition occurs when a resource is limited: there is not enough for every organism to obtain as much as it needs. It can occur between members of the same species or between different species.

Plants commonly compete for light, space, water and mineral nutrients from the soil. Each resource has a different role:

• Light provides the energy for photosynthesis. A plant shaded by neighbouring plants receives less light, which can limit photosynthesis and growth.
• Space allows leaves to spread out above ground and roots to spread through the soil. Crowded plants have less room to reach the resources they need.
• Water is needed for photosynthesis and is absorbed from the soil by roots. Nearby plants can draw on the same limited water supply.
• Mineral nutrients are needed to make important substances. For example, nitrate ions are used to make proteins, while magnesium ions are needed to make chlorophyll.

Imagine seedlings growing close together. As their leaves and roots spread, they increasingly draw on the same resources. A seedling that gains more light or water may grow more successfully than its neighbours. Competition therefore affects which plants survive and reproduce, and can influence the sizes of their populations.

Competition between animals

Animals commonly compete for food, mates and territory.

Food supplies energy and materials for growth and reproduction. When food is scarce, some animals may obtain too little to survive or produce offspring. Animals competing for the same food do not have to fight: one may simply eat food before another can reach it.

Competition for mates affects opportunities to reproduce and pass on genes. This competition is generally between members of the same species.

A territory is an area occupied and defended by an animal. It can contain food, water, shelter or access to mates. Gaining a territory can therefore secure several resources at once; losing it can reduce an animal's chances of survival or reproduction.

Species also depend on one another

Interdependence means that species depend on other species within their community. Feeding relationships are one example, but the connections extend beyond food:

• Food: herbivores depend on plants, and predators depend on their prey.
• Shelter: plants can provide animals with cover and places to live.
• Pollination: bees transfer pollen between flowers, helping many flowering plants reproduce.
• Seed dispersal: animals can carry seeds away from the parent plant, helping plants spread to new places.

Competition and interdependence can exist together. Two animals may compete for the same plant food while both depend on those plants for survival. Similarly, plants may compete for light while relying on animals for pollination or seed dispersal.

Removing one species can affect the whole community

Because species are connected, removing one can cause changes that spread through a community. The first effect may be on a species directly dependent on it, followed by effects on other species further along the chain of dependence.

For example, consider flowering plants that depend mainly on bees for pollination. If those bees disappear, fewer flowers may be pollinated, so fewer seeds are produced. Over time, the plant population may fall. Herbivores that depend on those plants then have less food, so their populations may decline. Predators feeding on those herbivores may also decline.

This is a possible chain of effects, rather than an inevitable outcome for every plant. Plants pollinated by wind or by other animals would not be affected in the same way. Likewise, a consumer with alternative food sources may be less affected than one that depends on a single food source. To predict a change, follow the actual dependencies in the community rather than assuming that every population changes in the same direction.

What makes a community stable?

A stable community is one in which all the species and environmental factors are in balance, so population sizes remain fairly constant.

Stability does not mean that organisms stop competing, feeding or reproducing. These processes continue, and populations can fluctuate, but there are no large, lasting changes in their sizes. Removing a species or disrupting an important resource can upset this balance because many organisms depend on the same network of relationships.

Competition: shared limited resources

• Plants: light, space, water and mineral nutrients from the soil.
• Animals: food, mates and territory.
• Competition can occur within a species or between species. It affects access to resources and therefore survival, growth and reproduction.

Interdependence: connected species

Species depend on one another for food, shelter, pollination and seed dispersal. Organisms also need materials from their non-living surroundings.

Removing a species can affect the whole community. Trace the dependencies: loss of a pollinator → reduced plant reproduction → less food for herbivores → less prey for predators. Outcomes depend on alternative resources and relationships.