The Struggle of Existence

The 'struggle for existence' was coined by Darwin in regard to the problem that species in nature interact strongly as competitors and predators; therefore, what is the nature of species interactions that enables some species to be dominant by driving other species to extinction, and similarly, how do species persist with the plethora of strong interactions? Early in the twentieth century, Lotka and Volterra separately developed mathematical equations that addressed the population dynamics of

200

P. aurelia

• * *

' . *

150 100

Separately

tySV

In mixed population

50

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160

P. caudatum

120 80

Separately

In mixed population

40

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1 1 1 1 1

Figure 1 Population growth for Paramecium aurelia and Paramecium caudatum when grown alone ('separately') and in competition ('in mixed population'). This is an example of competitive exclusion between two similar species. From Gause GF (1934) The Struggle for Existence. Baltimore: Williams and Wilkens.

competing species and the conditions when extinction or coexistence will occur in these populations. For example, on a single resource, only one species can persist (Figure 1), but if intraspecific competition is greater than interspecific competition in the two competing populations, then similar species may stably coexist. In 1934, Gause published The Struggle for Existence which consisted of the first experimental tests of Lotka-Volterra equations. Specifically, Gause used unicellular organisms (protozoa and yeast) in a series of laboratory experiments to quantify species interactions in competition to determine the conditions necessary for coexistence. One of Gause's competition experiments showed that two Paramecium species exhibited logistic growth when grown alone, but when grown together, one was always driven to extinction (Figure 1). This pattern, of competitive species excluding others that share the same resources, had been mentioned earlier by naturalists and theoreticians; Gause united the niche and competitive interactions conceptually and empirically.

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