Competition in communities

Organisms need resources to survive and reproduce. Competition occurs when organisms need the same resource but there is not enough for all of them. An organism that obtains less of a resource may be less able to survive or reproduce.

Competition can occur between members of the same species or between members of different species, provided they need the same limited resource.

Resources organisms compete for

Plants often compete for light and space, as well as water and mineral ions from the soil. For example, when plants grow close together, some may receive less light because others shade them. Their roots may also take up water from the same limited supply in the soil.

Animals often compete for food, mates and territory. Territory is an area that an animal or group uses and defends. Competition for mates occurs within a species: members of that species compete for opportunities to reproduce.

Suggesting competition in a habitat

To suggest what organisms may compete for, consider both the organisms present and the resources available. Identify resources that the organisms need in common, then consider which of those resources may be limited.

For example, if several plant species grow close together where little light reaches the ground, they may compete for light. The presence of organisms that need the same resource suggests possible competition, but the resource must also be limited.

Interdependence in communities

A community consists of the populations of different species living in the same place. Species in a community are interdependent: they depend on other species for resources or services.

These dependencies include food and shelter. They also include pollination, when animals transfer pollen between flowers, and seed dispersal, when animals carry seeds away from the parent plant. These services help plants reproduce and spread.

If one species is removed, the populations that depend on it may change. Those changes can then affect other species throughout the community. For example, removing a plant could reduce the food available to insects that feed on it. If those insects become less numerous, birds that eat them may have less food too.

Stability in a community

A stable community is one in which the species and environmental factors are in balance, so population sizes remain fairly constant over time. Stability does not mean that the populations never change: their sizes can fluctuate while the community remains stable.

Population, community and ecosystem organisation

Living organisms can be considered at connected levels of organisation, from one species in an area to all the organisms and their surroundings.

A population is a group of organisms of the same species living in the same area at the same time. For example, all the frogs of one species living in a pond at the same time form a population.

A community consists of all the populations of different species in an ecosystem. In the pond, the frog population is part of a community that also includes populations of plants, other animals and microorganisms.

An ecosystem includes the community and its physical environment as an interacting unit. In the pond, organisms interact with one another and with their surroundings, including the water, light and temperature.

Populations therefore make up a community, while the community and the environment together form an ecosystem.

Interpreting community interaction data

Charts, graphs and tables can show how organisms in a community change or differ in number. To interpret the data, first identify what was measured, then compare the relevant values or patterns and consider what interaction might explain them.

Reading community data

Check the title and labels to identify the organisms and the information shown. In a graph, read both axis labels and their units, and check the scale before reading a value. In a table, use the row and column headings to find the correct entry. A key may be needed to match each line, bar or symbol to an organism.

Read values at the relevant times or under the relevant conditions. Then compare them: for example, identify which population is larger, whether either population increases or decreases, and whether their changes occur at the same time.

Drawing a supported conclusion

Suppose a table shows that, over the same period, population A increases from 20 to 40 individuals while population B decreases from 30 to 15 individuals. The data support the conclusion that A increased while B decreased. If the organisms compete for a resource, competition is one possible explanation for the pattern.

However, the pattern alone does not show that A caused B to decrease, or even establish that they compete. Other factors could have affected either population. A supported interpretation describes what the data show and distinguishes a possible explanation from a conclusion established by the evidence.

Adaptation

An adaptation is an inherited feature that makes an organism better suited to its environment. It can help the organism survive or reproduce, increasing its reproductive success: the chance of producing offspring that themselves survive and reproduce.

An adaptation is useful in relation to a particular environment. For example, a feature that reduces water loss can be advantageous where water is scarce, but may offer little advantage where water is plentiful. If the environment changes, a feature that was once advantageous may become less useful.

Types of adaptation

Adaptations can be grouped according to the kind of feature involved:

• Anatomical adaptations are body structures. For example, a thick, waxy surface on a plant can reduce water loss.
• Physiological adaptations are internal processes. For example, producing concentrated urine allows some animals to conserve water.
• Behavioural adaptations are inherited patterns of action. For example, being active mainly at night can help some desert animals avoid the heat of the day.

Adaptation and natural selection

Individuals in a population can inherit different versions of genes, called alleles. Some of these differences affect features that influence reproductive success in the environment.

If an inherited feature gives its bearers an advantage, they are more likely to reproduce and pass on the alleles associated with it. Over generations, those alleles can become more common in the population. This process is natural selection. As a result, the population can become better adapted to its environment.

Adaptations do not appear because an individual needs them, and individuals do not change their inherited features simply by trying to adapt. Natural selection acts on inherited differences already present in a population.

Changes in inherited characteristics across generations are part of evolution. Because different environments favour different features, adaptation through natural selection contributes to the diversity of living organisms.