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8.3: Competition

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    Competition

    There is a finite amount of most resources on earth which drives species into competitive interactions. Competition occurs when organisms use the the same resources, and one or both organisms is harmed. Competing species are often occupy the same trophic level and compete for food, but species on different trophic levels could still compete for space, water, etc. Intraspecific competition occurs within a species. For example, most penguins defend territories from other individuals of the same species because they compete for suitable habitat and other resources (figure \(\PageIndex{1}\)). Interspecific competition occurs between different species. For example, the invasive vine, kudzu, competes with trees in the southeastern United States for light (figure \(\PageIndex{2}\)). Kudzu is considered invasive in this region because it occurs outside of its historical range (Asia and Australia) and causes ecological harm.

    Definition: Competition

    Competition occurs when members of the same species (intraspecific) or members of different species (interspecific) complete for access to a limited resource such as food, water, or habitat.

    Adelie penguins
    Figure \(\PageIndex{1}\): Adélie penguins at Cape Adare in Ross Sea, Antarctica. These penguins defend small territories in which they nest. Image by Brocken Inaglory (CC-BY-SA).
    The vine kudzu grows over trees and other structures along a slope, totally covering them.
    Figure \(\PageIndex{2}\): Kudzu is a vine that covers trees, shrubs, and other structures. It can fully block its competitors' access to light, eventually killing them. Image by Katie Ashdown (CC-BY).

    Interspecific Competition and Extinction

    When populations of different species in a community depend on the same resources, there may not be enough resources to go around. If one species has a disadvantage, such as more predators, it may get fewer of the necessary resources. As a result, members of that species are less likely to survive, and the species will have a higher death rate than the other species. Fewer offspring will be produced and the species may eventually die out in the area.

    In nature, interspecific competition has often led to the extinction of species. Many other extinctions have occurred when humans introduced new species into areas where they had no predators. For example, rabbits were introduced into Australia in the mid-1800s for sport hunting. Rabbits had no predators in Australia and quickly spread throughout the continent. Many species of Australian mammals could not successfully compete with rabbits and went extinct.

    Interspecific Competition and Specialization

    Another possible outcome of interspecific competition is the evolution of traits that create distinct differences among the competing species. Through natural selection, competing species can become more specialized. This allows them to live together without competing for the same resources. An example is the anolis lizard (Figure \(\PageIndex{4}\)). Many species of anolis live and prey on insects in tropical rainforests. Competition among the different species led to the evolution of specializations. Some anolis evolved specializations to prey on insects in leaf litter on the forest floor. Others evolved specializations to prey on insects on the branches of trees. This allowed the different species of anolis to co-exist without competing.

    The Competitive Exclusion Principle

    Ecologists have come to understand that all species have an ecological niche: the unique set of resources used by a species, which includes its interactions with other species. The competitive exclusion principle states that two species cannot occupy the exact same niche for long. In other words, different species cannot coexist in a community if they are competing for all the same resources. Competition reduces the fitness of one or both competitors because it wastes energy. However, competition is usually asymmetric with one species dominating the competition and the weaker competitor suffers a stronger fitness cost. In an experimental test of competitive exclusion two species of the freshwater microbe Paramecium were cultured separately and together. When cultured separately, both species reproduced, and the number of cells (individuals) increased. However, when the two species were grown together, one species (P. aurelia) grew, and the the other species (P. caudatum) was eliminated. Because both species occupied the same ecological niche, they could not coexist. Paramecium aurelia was the superior competitor in this case (figure \(\PageIndex{3}\)). Competitive exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day. This is called resource partitioning. The two species are then said to occupy different microniches. These species coexist by minimizing direct competition.

    paramecium cultures alone and together
    Figure \(\PageIndex{3}\): Graphs a, b, and c all plot number of cells versus time in days. When Paramecium aurelia and P. caudatum were grown individually in the laboratory (a and b), the two species both reproduced and reached a relatively high cell density. When the two species were placed together in the same test tube (habitat, c), P. aurelia (top, green) reached nearly the same cell density as it exhibited when grown alone, but P. caudatum (bottom, purple) hardly grew at all, and eventually its population drops to zero. Paramecium aurelia outcompeted P. caudatum for food, leading to the latter’s eventual extinction.

    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. Alternatively, species can coexist on the same resources if each species is limited by different resources, or differently able to capture resources. Different types of phytoplankton can coexist when different species are differently limited by nitrogen, phosphorus, silicon, and light. In the Galapagos Islands, finches with small beaks are more able to consume small seeds, and finches with large beaks are more able to consume large seeds. If a species' density declines, then the food it most depends on will become more abundant (since there are so few individuals to consume it). As a result, the remaining individuals will experience less competition for food. Although "resource" generally refers to food, species can partition other non-consumable objects, such as parts of the habitat. For example, warblers are thought to coexist because they nest in different parts of trees . Species can also partition habitat in a way that gives them access to different types of resources. As previously stated, anole lizards appear to coexist because each uses different parts of the forests as perch locations. This likely gives them access to different species of insects.

    resource partitioning
    Figure \(\PageIndex{4}\): Resource partitioning among anole lizards. (CC-BY via Libretexts)

    Character Displacement

    Character displacement occurs when similar species that live in the same geographical region and occupy similar niches differentiate in order to minimize niche overlap and avoid competitive exclusion. Several species of Galapagos finches display character displacement. Each closely related species differs in beak size and beak depth, allowing them to coexist in the same region since each species eats a different type of seed: the seed best fit for its unique beak. The finches with the deeper, stronger beaks consume large, tough seeds, while the finches with smaller beaks consume the smaller, softer seeds.

    Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not overlap. This pattern results from evolutionary change driven by biological competition among species for a limited resource (e.g. food). The rationale for character displacement stems from the competitive exclusion principle, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.

    Galapagos finches
    Figure \(\PageIndex{5}\): Several species of Galapagos finches, shown above, display character displacement. Each closely-related species differs in beak size and beak depth, allowing them to coexist in the same region since each species eats a different type of seed: the seed best fit for its unique beak. The finches with the deeper, stronger beaks consume large, tough seeds, while the finches with smaller beaks consume the smaller, softer seeds. (CC VT-SA 4.0; Awakening Conscience via Wikipedia)

    For example, Darwin's finches can be found alone or together on the Galapagos Islands. Both species' populations actually have more individuals with intermediate-sized beaks when they live on islands without the other species present. However, when both species are present on the same island, competition is intense between individuals that have intermediate-sized beaks of both species because they all require intermediate sized seeds. Consequently, individuals with small and large beaks have greater survival and reproduction on these islands than individuals with intermediate-sized beaks. Different finch species can coexist if they have traits—for instance, beak size—that allow them to specialize on particular resources. When Geospiza fortis and Geospiza fuliginosa are present on the same island, G. fuliginosa tends to evolve a small beak and G. fortis a large beak. The observation that competing species' traits are more different when they live in the same area than when competing species live in different areas is called character displacement. For the two finch species, beak size was displaced: beaks became smaller in one species and larger in the other species.

    Attribution

    This page is a modified derivative of:

    This page titled 8.3: Competition is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Sara Kappus (Open Educational Resource Initiative at Evergreen Valley College) .