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.

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.

Effects of Biotic Factors on Community Populations

A community contains populations of different species living in the same area. Its biotic factors are the living components of the environment, such as food organisms, predators, pathogens and competitors.

A change in a biotic factor can affect how many organisms survive or reproduce. As a result, the size of a population may increase or decrease.

Food availability and populations

When food limits a population, a reduction in its availability means that fewer individuals can obtain enough to survive or reproduce. The population may then decrease. An increase in food availability may allow more individuals to survive and reproduce, supporting a larger population.

Predators and pathogens

A newly arriving predator may reduce the population of its prey by eating individuals. A new pathogen, a disease-causing microorganism, may spread among susceptible organisms. Disease can reduce their survival or reproduction, causing their population to fall.

Competition and loss from a community

Organisms compete when they need the same limited resources. Competition may reduce a species’ access to a resource so much that fewer individuals survive or reproduce and its population falls. If too few individuals remain to breed successfully, the species may be lost from that community.

Effects on other populations

The effect of a change depends on the organisms and conditions involved. A change in one population can also affect other species that depend on it. For example, if a prey population falls, predators that depend on it have less food available.