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.

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.