Laboratory Experiments Support the Lotka–Volterra Model.
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.
13.6 Competition Is Influenced by Nonresource Factors
Interspecific competition involves individuals of two or more species vying for the same limited resource. However, features of the environment other than resources also directly influence the growth and reproduction of species (see Chapters 6 and 7) and therefore can influence the outcome of competitive interactions. For example, environmental factors such as temperature, soil or water pH, relative humidity, and salinity directly influence physiological processes related to growth and reproduction, but they are not consumable resources that species compete over.
For example, in a series of field and laboratory experiments, Yoshinori Taniguchi and colleagues at the University of Wyoming examined the influence of water temperature on the relative competitive ability of three fish species that show longitudinal replacement in Rocky Mountain streams. Brook trout (Salvelinus fontinalis) are most abundant at high elevations, brown trout (Salmo trutta) at middle elevations, and creek chub (Semotilus atromaculatus) at lower elevations. Previous studies have shown that interference competition for foraging sites is an important factor influencing the relative success of individuals at sites where the species co-occur. Based on the distribution of these three species along elevation gradients in the Rocky Mountain streams and differences in physiological performance with respect to temperature, the researchers hypothesized that the brook trout would be competitively superior at cold water temperatures, brown trout at moderate water temperatures, and creek chub would be competitively superior at warmer water temperatures. To test this hypothesis, Taniguchi and his colleagues used experimental streams to examine competitive interactions at seven different water temperatures: 3, 6, 10, 22, 22, 24, and 26°C.
Prior to each test, fish were thermally acclimated by increasing or decreasing the temperature by 1°C per day until the test temperature was reached (see Section 7.9 for discussion of thermal acclimation). For each test, individuals of each species were matched for size (<10%) and placed in the experimental stream together. Aggressive interactions and food intake were monitored. Competitive superiority was based on which species consumed the most food items because food intake is considered a limiting factor for these drift-feeding, stream fishes.
Patterns of food consumption clearly show changes in the relative competitive abilities of the three fish species across the gradient of water temperatures (Figure 13.5). At 3°C, brook trout exhibited the highest rate of food consumption, although differences between the two trout species were minimal below 20°C, and both trout species consumed significantly more food than creek chub. However, as temperature increased, food consumption by creek chub increased. At 24°C, food intake by brook trout dropped to zero, whereas intake rate of brown trout still exceeded that of creek chub. At 26°C, the rate of food intake reversed for the two species and food intake by creek chub exceeded that of brown trout. In an additional series of experiments, the researchers were able to establish that the observed patterns of food intake during the competition trials were a result of differences in competitive ability and no changes in appetite because of water temperature.
The transition in competitive ability from 24 to 26°C in the laboratory experiments are in agreement with the transition in dominance from trout species to creek chub at a similar temperature range in the field. The results of Taniguchi and his colleagues provide a clear example of temperature mediation of competitive interactions. The relative competitive abilities of the three fish species for limiting food resources are directly influenced by abiotic conditions, that is, water temperature.
A similar case of competitive ability being influenced by nonresource factors is illustrated in the work of Susan Warner of Pennsylvania State University. Warner and her colleagues examined the effect of water pH (acidity) on interspecific competition between two species of tadpoles (Hyla gratiosa and Hyla femoralis). The two species overlap broadly in their geographic distribution, yet differ in their responses to water acidity. The researchers conducted experiments using two levels of water pH (4.5 and 6.0) and varying levels of population densities to examine the interactions of pH and population density on both intra- and interspecific competition. The results of the experiments indicated that interspecific interactions were minimal at low water pH (4.5); however, at higher water pH (6.0), interspecific competition from H. fermoralis caused decreased survival and an increased larval period for H. gratiosa. The latter resulted in decreased size at metamorphosis for H. gratiosa individuals.