Course 29.3Lesson 2 of 3
In competition, both species are harmed, because both need the same limited resources. Species that are not even related can compete. This lesson gives the competitive exclusion principle, the barnacles of the Scottish coast, competitive release, and the way in which species share resources to live together.
Try the code and the simulations beside the lesson.
Two birds sit on one tree. Each wants the same fruit, and there are not enough for both. This is competition. In competition, the fitness of one species is clearly lower when another species is present. Fitness here is measured by , the rate of increase of the species.
Competition is not only about food. It can be for space, or for a place to breed. Closely related species compete most, because their needs are alike. But species that are not related can also compete.
Competition can also be by interference. One species reduces the feeding efficiency of the other by its presence, even if food and space are plentiful.
The Russian biologist G. F. Gause stated this principle. Two closely related species that compete for the same resources cannot live together for ever. The weaker competitor is eliminated in the end. It holds when the resource is limited. If the resource is plentiful, the two can stay.
The figure shows a model of two species in one place. When one presses on the other more than on itself, the weaker one is eliminated. When both press on each other gently, they stay.
Joseph Connell studied two barnacles on the rocky coast of Scotland. The larger barnacle, Balanus, is the better competitor. It dominates the intertidal area and keeps out the smaller barnacle, Chthamalus, from that zone.
When a species is held to a small range by a better competitor, and the better competitor is removed, the species spreads over a much larger range. This is competitive release.
If two similar species are to live together, they must divide the resources. This is resource partitioning. The ecologist Robert MacArthur studied five closely related species of warblers on one tree. They did not compete to the end, because each of them fed in a different part of the tree. They differed in how and where they hunted for insects.
You should find that the use of the shared middle height falls fast as the distance between the best heights grows. Species that feed at different places meet less.
Words for A-Level and IB. The role of a species in its habitat, with all that it uses, is its niche. Competition between members of the same species is intraspecific, and competition between different species is interspecific. The overlap of the niches is a number that falls as the species partition the resource. The program below gives it with an exact formula.
The program uses the model of the figure to give the numbers of two species after 200 years, for three strengths of competition.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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