Laboratory studies of competition
The strength of competition should be easiest to measure in a comprehensive way using species that are small in size, have short generation times, and can be maintained under controlled conditions.
These requirements led Gause (1936) to study competition between two protozoans in laboratory culture vessels, and Park (1948) to study competition between pairs of flour beetle species in small jars. Both authors observed large effects of interspecific competition on abundance, including exclusion. Several early laboratory studies of interspecific competition during the MacArthur era featured work on Drosophila species. The seminal work by Ayala (1969) has already been mentioned in Chapter 1. By quantifying relative short-term changes in responses from a wide range of starting densities, his results provided clear evidence of strong nonlinearity in the interaction. However, Ayala’s initial interpretation of the results was that they invalidated the competitive exclusion principle. Gilpin (Gilpin and Ayala 1973) reinterpreted these experiments, and Gilpin led a group that initiated an expanded set of experiments involving almost thirty Drosophila species. These culminated in a study by Pomerantz et al. (1980) on intraspecific competition in close to 30 species and a related study by Gilpin et al. (1986) on a related interspecific system involving up to ten species in a single treatment. The intraspecific results demonstrated strong nonlinearity. Although Pomerantz (1981) argued otherwise, the intraspecific findings strongly suggested similar nonlinearity in interspecific competition (Abrams 1983a).The interspecific competition experiments described in Gilpin et al. (1986) revealed that laboratory studies often do not escape many of the problems they attributed to studies in natural environments (see the quotation in Section 7.1). The experiments often could not be run for long enough to distinguish outcomes of highly unequal coexistence from exclusion.
They also did not allow a large enough number of treatments to examine the full spectrum of competitive effects over a wide range of abundances for any given pair of species. Nevertheless, the results confirmed that alternative multi-species outcomes were possible in some cases, and that pairwise interactions were not independent of the presence of additional species. Coexistence of more than three species did not occur in the 30 treatments initialized with ten species present (Gilpin et al. 1986), and survival of only two species was the most common outcome. This probably reflected the relatively (but not completely) homogeneous resource conditions in the laboratory environment.Other studies of laboratory microcosms from the 1970s confirmed that nonlinear effects were common. Neill (1974, 1975) constructed a series of replicated aquatic microcosms involving four microcrustacean consumers of algae. He studied the impacts of removal of different subsets of the species on the abundances of the remaining species. He also examined the effects of adding one or two predatory fish; each of these treatments had different per capita impacts on the four invertebrate species. The results ofboth types of manipulation were inconsistent with the Lotka- Volterra model in that effects of consumers on each other were non-additive, and the effects of different magnitudes of perturbation were not linearly related. An earlier study of competition between four protozoan species by Vandermeer (1969) had concluded that the Lotka-Volterra model provided a good description of their dynamics. This may have been true, but the experiments constituted a relatively weak test of this claim.
The laboratory systems described above all revealed high levels of competition between many pairs of species. However, these systems lacked the spatial variation that is found in almost all natural systems, and they were also characterized by a relatively small or unknown number of resource types. The earlier laboratory studies of competition (Gause 1936; Park 1948) also involved one or a very small number of resources.
More recent years have seen many laboratory studies of competition, and I will not attempt to review all of them here. A number of the studies have examined competition among groups of species where the small spatial scale in the lab is not so different from natural conditions. Bacterial communities have proven to be useful for such multi-species experiments (e.g., Friedman et al. 2017; Abreu et al. 2020). While not all results are consistent with Lotka-Volterra models, some studies have had a high degree of consistency with those simple models (Friedman et al. 2017). However, the resource utilization patterns of the bacteria are usually poorly known, and precisely controlled changes in the mortality of a single species/strain are usually not possible.
7.3
More on the topic Laboratory studies of competition:
- Serum Laboratory Studies
- Field studies of competition
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- Theory regarding the strength of competition
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- Historiographical studies
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