Interspecific competition early experiments and the competitive exclusion principle

Early in the twentieth century, Tansley (1917) experimentally demonstrated the potential power of interspecific competition in shaping ecological communities. Tansley had observed that closely related plant species living in the same region were often found in different habitats or different soil types. For his experiment he selected two species of an herbaceous perennial, bedstraw, in the genus Galium (Rubiaceae). One species, G. saxatile, is normally found on peaty, acidic soils, while the second species, G. sylvestre, is an inhabitant of limestone soils. Tansley obtained soils from both areas, planted each species singly in each soil type and then placed the two species together in each soil. He found that each species, when planted alone, was able to survive in both soils. Therefore the fundamental niche for both species includes both acidic, peat-rich soil and limestone soil. However, growth and germination were best on the soil where the Galium species was normally found. When grown together on limestone soil, G. sylvestre overgrew and outcompeted G. saxatile. The opposite was true in the acidic peat soil. At this early date, Tansley had established that competitive exclusion could be demonstrated, and that the results differed by environment.

The work by Tansley, however, was not developed further until the publication by Gause (1934) of The Struggle for Existence. Through a series of experiments with yeast (Gause 1932) and protozoans, Gause found that competitive exclusion is observed most often between two closely related species (two species in the same genus, for example), when grown in a simple, constant environment. For example, see Fig. 7.1. Gause prepared organic extracts

Time in days

Figure 7.1 Population dynamics of Paramecium aurelia and P. caudatum: (a) when grown separately; (b) when grown together. Adapted from Gause (1934).

Time in days

Figure 7.1 Population dynamics of Paramecium aurelia and P. caudatum: (a) when grown separately; (b) when grown together. Adapted from Gause (1934).

and introduced bacteria as food. When either Paramecium caudatum or P. aurelia was introduced alone, each flourished and grew logistically, leveling off at a carrying capacity (Fig. 7.1a). When placed together, however, P. caudatum diminished and eventually went extinct, while P. aurelia grew to a steady level (Fig. 7.1b). There are two lessons from this experiment. First, two closely related species were unable to coexist in the simple test-tube environment. Second, even though we declare P. aurelia the "winner," notice that its steady state of approximately 300 per 0.5 ml sample (Fig. 7.1b) is less than the carrying capacity of 500 when this species was grown alone (Fig. 7.1a). Recall the definition of competition as a reciprocally negative interaction, meaning that competition has a negative effect, even on the winners.

Gause's laboratory work inspired many others who worked with yeast, grain beetles, fruit flies, and other organisms easily grown in the laboratory. Crombie (1945, 1946, 1947), Thomas Park (1948) and others did some particularly interesting work with grain beetles. Different species were grown together in vials or other simple environments, usually resulting in competitive exclusion. Crombie, however, showed that the species excluded could change depending on the temperature under which the experiment was run. And when he added glass tubing as a refuge, he found that a grain beetle in the genus Oryzaephilus was able to coexist with a related species in the genus Tribolium. Without the glass tubing, Tribolium drove Oryzaephilus extinct. All of this work suggested that closely related species, whose niches are very similar, are unlikely to coexist in a simple environment. From his research with Paramecium, Gause (1934) proposed what became known as Gause's theorem or principle:

A Two species cannot coexist unless they are doing things differently.

This was eventually rephrased such that competition and the niche concept became integrated.

B No two species can occupy the same ecological niche.

Based on such results, Hardin (1960), three years after the publication of Hutchinson's definition of the niche as an N-dimensional hypervolume, proposed the competitive exclusion principle:

Species which are complete competitors, that is, whose niches overlap completely, cannot coexist indefinitely.

When comparing the fundamental niches of competing species it becomes obvious that their niches overlap on many, if not all, dimensions. On the other hand, if we look at enough niche dimensions, since all species are genetically differentiated from one another, each species will have a unique niche. Therefore, complete niche overlap between two species is virtually impossible. Yet competitive exclusion does occur. A question we might pose is: how closely can niches overlap before competitive exclusion occurs? The other side of that question is: under what conditions do potential competitors coexist?

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