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.

Factors affecting population growth

A population is a group of organisms of the same species living in an area. Its size increases when births exceed deaths and decreases when deaths exceed births. Factors that affect survival or reproduction therefore affect how quickly the population changes.

Food supply and competition

When food is plentiful, more organisms can survive and reproduce, supporting population growth. If food becomes scarce, fewer organisms may survive or reproduce, so growth slows or the population decreases.

Organisms compete when they need the same limited resources. As a population grows, more individuals may compete for the available food and other resources. Some may not obtain enough to survive or reproduce, reducing population growth.

Predation and disease

Predation occurs when one organism kills and eats another. If more members of a population are killed by predators, deaths increase. This can slow population growth or cause the population to decline.

Disease can also increase deaths in a population. If disease causes deaths to exceed births, the population decreases.

The combined effects of food supply, competition, predation and disease help determine the number of births and deaths, and therefore the rate and direction of change in population size.

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.

Small populations and breeding

When two species compete for a limited resource, such as food, one may obtain more of it than the other. Individuals of the less successful species may then be less likely to survive and reproduce, causing their population to decline.

If the population becomes very small, individuals of a sexually reproducing species may have difficulty finding mates. As a result, fewer offspring may be produced. If too few offspring are produced to replace the individuals that die, the population declines further. It may eventually disappear from the local community.

A small population does not necessarily stop breeding. Its future depends on whether the remaining individuals can find mates and produce enough offspring to replace those that die.