Ecology: Biology of Interaction. IV-12. Competition and Ecological Niches
In interspecific competition, two populations can negatively affect one another in two different ways. The first (indirect, or exploitative competition) is related to the use of the same resource by both populations. If the abundance of one population increases, its consumption of the shared resource rises, and less remains for the other.
IV-12. Competition and Ecological Niches As we have established, interspecies competition is an interaction between populations of two different species, where an increase in each of these populations causes a decrease in the population size of the other. However, in addition to interspecies competition, there is also intraspecific competition, which occurs within a single population. In interspecies competition, two populations can negatively affect each other in two different ways. The first (indirect, or exploitative competition) is related to the fact that two populations use the same resource. If the population size of one of these populations increases, its consumption of the resource common to both competitors will intensify, and the other population will receive less of it. The second (direct, or interference competition) is related to the expenditure of energy by individuals to cause harm to each other. An example of direct competition is allelopathy – the release of substances by plants that inhibit other species. But why do organisms expend energy during interference competition? This phenomenon would not be observed if such expenditures did not bring some benefit to the competing populations. This benefit is a reduction in losses from exploitative competition. Therefore, although direct competition, which can be accompanied by conflicts between individuals of different populations, seems more obvious and spectacular, it is only a consequence of hidden, indirect competition. Which species compete more strongly with each other: similar or different ones? The more similar the species are, the more their resource needs overlap, and the sharper the competition between them will be. The concept of an ecological niche proves very useful for describing competitive relationships. We have already mentioned that many key concepts in ecology are used in different senses. Perhaps the concept of an ecological niche is a "record holder" in this regard. The term "ecological niche" was first used by J. Grinnell in 1917. He used it to denote the characteristic habitat of a species, the set of conditions in which a particular species is found in nature. C. Elton in 1927 defined the ecological niche as the place of a species in a community, its position in the food web. J. Hutchinson in 1957 presented the ecological niche as the set of all values of ecological factors that permit the existence of a species (see section V-09). Finally, E. Odum explained that a niche is a characteristic of a species' requirements for its environment given its way of life. For example, we can assess how a particular species uses environmental resources and use this assessment as a slice of its ecological niche. Figure IV-12.1 shows a two-dimensional slice of the ecological niche of one of the insectivorous birds of American oak forests – the blue-gray gnatcatcher. As you can see, its niche is characterized here by two parameters: what prey the birds catch and where (at what height) they do it. It can be assumed that for any other bird species living in the same habitats as the blue-gray gnatcatchers, a similar picture would look different. Fig. IV-12.1. Two-dimensional ecological niche of the blue-gray gnatcatcher; the frequency of catching insects of different sizes at different heights from the ground is shown. "The ecological niche of a particular organism depends not only on where it lives, but also on what it does (how it transforms energy, its behavior, how it reacts to and changes the physical and biological environment) and how it is limited by other species. The following analogy can be drawn: a habitat is the "address" of an organism, and an ecological niche is, biologically speaking, its "profession" (Odum, 1975). Combining such diverse approaches, we can say that an ecological niche is a complex characteristic of a species' way of life, including the resources it consumes and the conditions under which it can exist. The value of the concept of an ecological niche lies in the fact that it allows us to compare the ways of life of different species. For example, if two species can inhabit similar conditions but differ in the resources they consume (e.g., in the size of their prey), we can compare the niches of these species based on this parameter. Now it will be clear to you why we started talking about the concept of a niche when discussing competition. For a resource consumed by only one species, interspecies competition is impossible. However, for that part of the resource diversity that can be used by two species (i.e., the part where the niches of these species overlap), competition will arise between these species (Fig. IV-12.2). Fig. IV-12.2. Comparison of ecological niches of two species based on the parameter by which they differ (e.g., by the size of prey consumed by predators)