The theoretical Lotka–Volterra equations.
The theoretical Lotka–Volterra equations stimulated studies of competition in the laboratory, where under controlled conditions an outcome is more easily determined than in the field. One of the first to study the Lotka–Volterra competition model experimentally was the Russian biologist G. F. Gause. In a series of experiments published in the mid-1930s, he examined competition between two species of Paramecium, Paramecium aurelia and Paramecium caudatum. P. aurelia has a higher rate of population growth than P. caudatum and can tolerate a higher population density. When Gause introduced both species to one tube containing a fixed amount of bacterial food, P. caudatum died out (Figure 13.3). In another experiment, Gause reared the species that was competitively displaced in the previous experiment, P. caudatum, with another species, Paramecium bursaria. These two species coexisted because P. caudatum fed on bacteria suspended in solution, whereas P. bursaria confined its feeding to bacteria at the bottom of the tubes. Each species used food unavailable to the other.
In the 1940s and 1950s, Thomas Park at the University of Chicago conducted several classic competition experiments with laboratory populations of flour beetles. He found that the outcome of competition between Tribolium castaneum and Tribolium confusum depended on environmental temperature, humidity, and fluctuations in the total number of eggs, larvae, pupae, and adults. Often, the outcome of competition was not determined until many generations had passed.
In a much later study, ecologist David Tilman of the University of Minnesota grew laboratory populations of two species of diatoms, Asterionella formosa and Synedra ulna. Both species require silica for the formation of cell walls. The researchers monitored population growth and decline as well as the level of silica in the water. When grown alone in a liquid medium to which silica was continually added, both species kept silica at a low level because they used it to form cell walls. However, when grown together, the use of silica by S. ulna reduced the concentration to a level below that necessary for A. formosa to survive and reproduce (Figure 13.4). By reducing resource availability, S. ulna drove A. formosa to extinction.
13.5 Studies Support the Competitive Exclusion Principle
In three of the four situations predicted by the Lotka–Volterra equations, one species drives the other to extinction. The results of the laboratory studies just presented tend to support the mathematical models. These and other observations have led to the concept called the competitive exclusion principle , which states that “complete competitors” cannot coexist. Complete competitors are two species (non-interbreeding populations) that live in the same place and have exactly the same ecological requirements (see concept of fundamental niche in Chapter 12, Section 12.6). Under this set of conditions, if population A increases the least bit faster than population B, then A will eventually outcompete B, leading to its local extinction.
Competitive exclusion, then, invokes more than competition for a limited resource. The competitive exclusion principle involves assumptions about the species involved as well as the environment in which they exist. First, this principle assumes that the competitors have exactly the same resource requirements. Second, it assumes that environmental conditions remain constant. Such conditions rarely exist. The idea of competitive exclusion, however, has stimulated a more critical look at competitive relationships in natural situations. How similar can two species be and still coexist? What ecological conditions are necessary for coexistence of species that share a common resource base? The resulting research has identified a wide variety of factors affecting the outcome of interspecific competition, including environmental factors that directly influence a species’ survival, growth, and reproduction but are not consumable resources (such as temperature or pH), spatial and temporal variations in resource availability, competition for multiple limiting resources, and resource partitioning. In the following sections, we examine each topic and consider how it functions to influence the nature of competition.
