6.1.2: Competition

Resources are often limited within a habitat and multiple species may compete to obtain them. Competition occurs when organisms use 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 \(\PageIndex{a}\)). Interspecific competition occurs between different species. For example, the invasive vine, kudzu, competes with trees in the southeastern United States for light (figure \(\PageIndex{b}\)). Kudzu is considered invasive in this region because it occurs outside of its historical range (Asia and Australia) and causes ecological harm.

Figure \(\PageIndex{a}\): 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).

Figure \(\PageIndex{b}\): 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).

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 in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. Competition harms one or both competitors because it wastes energy. 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 \(\PageIndex{c}\)). 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.

Figure \(\PageIndex{c}\): 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. 

Attribution

Modified by Melissa Ha from Community Ecology from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)