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Biology library

Course: Biology library > Unit 28

Lesson 4: Intro to community ecology
Science
Biology library
Ecology
Intro to community ecology
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Niches & competition

AP.BIO:
ENE‑4 (EU)
Google Classroom
What an ecological niche is. How species with overlapping niches compete for resources. Resource partitioning to reduce competition.

Key points:

  • In interspecies competition, two species use the same limited resource. Competition has a negative effect on both of the species (-/- interaction).
  • A species' niche is basically its ecological role, which is defined by the set of conditions, resources, and interactions it needs (or can make use of).
  • The competitive exclusion principle says that two species can't coexist if they occupy exactly the same niche (competing for identical resources).
  • Two species whose niches overlap may evolve by natural selection to have more distinct niches, resulting in resource partitioning.

Introduction

Humans compete with other humans all the time – for jobs, athletic prizes, dates, you name it. But do we compete with other species? If you've ever gone camping and had your food stolen by an enterprising raccoon, bear, or other critter, you've had a little taste of interspecific competition – competition between members of different species that use overlapping, limited resources.
Resources are often limited in a habitat, and many species may compete to get ahold of them. For instance, plants in a garden may compete with each other for soil nutrients, water, and light. The overall effect of interspecific competition is negative for both species that participate (a -/- interaction). That is, each species would do better if the other species weren't there.
In this article, we'll look at the concept of an ecological niche and see how species having similar niches can lead to competition. We'll also see how species can evolve by natural selection to occupy more different niches, thus divvying up resources and minimizing competition.

The niche concept

A species' niche is its ecological role or "way of life," which is defined by the full set of conditions, resources, and interactions it needs (or can make use of)start superscript, 1, end superscript. Each species fits into an ecological community in its own special way and has its own tolerable ranges for many environmental factors. For example, a fish species' niche might be defined partly by ranges of salinity (saltiness), pH (acidity), and temperature it can tolerate, as well as the types of food it can eat.
As we'll see, two organisms with exactly the same niche can't survive in the same habitat (because they compete for exactly the same resources, so one will drive the other to extinction). However, species whose niches only partly overlap may be able to coexist. Also, over long periods of time, they may evolve to make use of more different, or less overlapping, sets of resources.

Competitive exclusion principle

The competitive exclusion principle tells us that two species can't have exactly the same niche in a habitat and stably coexist. That's because species with identical niches also have identical needs, which means they would compete for precisely the same resources.
A famous example of the competitive exclusion principle is shown in the figure below, which features two types of single-celled microorganisms, Paramecium aurelia and Paramecium caudatum. When grown individually in the lab, both species thrive. But when they are grown in the same test tube (habitat) with a fixed amount of nutrients, both grow more poorly and P. aurelia eventually outcompetes P. caudatum for food, leading to P. caudatum's extinction.
Graphs a, b, and c all plot number of cells versus time in days. In Graph (a), P. aurelia is grown alone. In graph (b), P. caudatum is grown alone. In graph (c), both species are grown together. When grown separately, the two species both exhibit logistic growth and grow to a relatively high cell density. When the two species are grown together, P. aurelia shows logistic growth to nearly the same cell density as it exhibited when grown alone, but P. caudatum hardly grows at all, and eventually its population drops to zero.
Image modified from "Community ecology: Figure 7," by OpenStax College, Concepts of Biology, CC BY 4.0.
In nature, it's rarely the case that two species occupy exactly identical niches. However, the greater the extent to which two species' niches overlap, the stronger the competition between them will tend to besquared.

Resource partitioning

Competitive exclusion may be avoided if one or both of the competing species evolves to use a different resource, occupy a different area of the habitat, or feed during a different time of day. The result of this kind of evolution is that two similar species use largely non-overlapping resources and thus have different niches. This is called resource partitioning, and it helps the species coexist because there is less direct competition between them.
The anole lizards found on the island of Puerto Rico are a good example of resource partitioning. In this group, natural selection has led to the evolution of different species that make use of different resources. The figure below shows resource partitioning among 11 species of anole lizards. Each species lives in its own preferred habitat, which is defined by type and height of vegetation (trees, shrubs, cactus, etc.), sunlight, and moisture, among other factors.
Diagram representing resource partitioning among species of anole lizards. Some live high in a tree, others in the middle, others on the trunk. Other anole species live in bushes or cactuses. Also, some species live in a sunnier, drier environment, while others live in a shadier, moister environment. There are 11 species pictured in all, each with a slightly different type of environment it occupies.
Image credit: "Community ecology: Figure 9, by Eva Horne, modified from Williams et al.cubed, source article is CC BY 4.0.

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