Competition and Niche Partitioning

Essential Question

What happens when two species need the same resources?

Food, water, light, and space are limited, so organisms often compete for them. In this lesson, you’ll learn what a niche is, how competition between species can change populations, and how species can share a habitat by using resources in different ways.

Niches and Competition

• Niche: a species’ role in its community
• Competition: using the same limited resource
• Intraspecific: within one species
• Interspecific: between different species

A niche is the role a species plays in its community: the resources it uses, where and when it lives and feeds, and how it interacts with other species. Competition happens when organisms need the same resource and the supply is limited. Competition among members of the same species is intraspecific, and competition between different species is interspecific. The more two niches overlap, the stronger the competition is likely to be.

Competitive Exclusion

• Species with very similar niches compete strongly
• A small advantage can matter over time
• The weaker competitor may decline in that habitat
• Seen in lab experiments such as Gause’s Paramecium

In the 1930s, G. F. Gause grew two species of Paramecium in the same container with a limited food supply. Each species grew well alone, but when grown together one species declined while the other kept growing. The graph shows illustrative data modeled on that kind of result. This outcome is called competitive exclusion: when two species compete for the same limiting resource, a small advantage can lead to one species declining in that habitat. It is easiest to see in simple lab conditions. In nature, many factors affect the result, and it often takes a long time.

Resource Partitioning

• Species may use different parts of a resource
• Different place, food, or time of activity
• Less niche overlap means less competition
• Lets similar species live together

Similar species can often live in the same area if they use resources differently. This is called resource partitioning. In the 1950s, Robert MacArthur studied several warbler species in spruce trees and found that they spent most of their feeding time in different parts of the tree. Because each species mostly used its own zone, competition between them was likely reduced. Species can also divide a resource by feeding at different times of day or on different kinds of food.

Outcomes of Competition

• Competition can lower growth of one or both species
• One species may decline locally
• Or species can come to use resources differently
• Outcomes depend on the conditions

Competition does not always end the same way. If two species use the same resource in the same way, one may decline in that habitat. If their niches differ, even slightly, they may be able to share the habitat. Over long periods, competition can also favor individuals that use slightly different resources, which can make the niches of competing species more different. Environmental conditions matter too, because a change in temperature or food supply can change which species has the advantage.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Two Species in One Container

Use the graph. When the two species were grown together, which one was outcompeted? Write a claim, evidence, and reasoning.

Show the solution

1. Read the graph: grown together, species B peaks at 38 cells/mL on day 6 and then falls to 0 by day 16. Species A is at 164 cells/mL on day 14.
2. Claim: species B was outcompeted by species A in this container.
3. Evidence: each species grew well alone (A to about 180, B to about 122 cells/mL), but together B fell from 38 to 0 cells/mL while A kept rising.
4. Reasoning: the species had the same limited food supply. Species A used it better, so B could not keep growing. This fits competitive exclusion in simple lab conditions.
5. Answer: Species B was outcompeted: it fell from 38 to 0 cells/mL while species A kept growing.

Example 2: A New Warbler Arrives

Use the spruce tree model. A new warbler species arrives that feeds mostly on the middle branches near the trunk, the zone species 2 uses. Predict what might happen and explain with the idea of niche overlap.

Show the solution

1. Overlap: the new species and species 2 would use the same zone, so their niches overlap strongly.
2. Competition: the same insects would now be shared between two species, so competition is likely to increase in that zone.
3. Possible outcomes: one species might decline there, or one or both might shift to different parts of the tree or different foods over time.
4. Species 1 and 3: they mostly use other zones, so they are likely to be affected less.
5. Answer: Competition is likely to increase in the middle zone, and species 2 or the newcomer may decline or shift to different resources.

Example 3: How Much Did Species B Decline?

In the graph where both species are grown together, species B peaks at 38 cells/mL on day 6 and is at 5 cells/mL on day 12. What percent decrease is this? Round to the nearest tenth of a percent.

Show the solution

1. Change: 38 − 5 = 33 cells/mL.
2. Percent decrease: change ÷ starting value × 100 = 33 ÷ 38 × 100 = 86.8%.
3. Answer: 86.8% decrease (33 cells/mL fewer).

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Practice

Tell competition within a species from competition between species, read population data as evidence, use a model of resource partitioning, and calculate a percent change. Try it on your own first, then check your work with the answer key.

• Competition and Niche Partitioning Practice Worksheet (PDF)
• Competition and Niche Partitioning Practice Answer Key (PDF)

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Lesson Slides

• 3.4 Competition and Niche Partitioning Slides (PDF)

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Big Ideas

• A niche is a species’ role in its community
• Competition happens when organisms use the same limited resource
• Strong niche overlap can lead to one species declining locally
• Resource partitioning lets similar species share a habitat
• Outcomes depend on conditions and often take time

Back to the essential question: when two species need the same resources, they compete. If their niches overlap strongly, one species may decline in that habitat. If they use resources in different ways, they can often live together. Next, you’ll look at predators, prey, and keystone species.

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