Laboratory experiments and competition

The classic works of Thomas Park and his colleagues (Park 1954, Neyman et al. 1956) illustrate cases 1-3, as well as the fact that competitive outcomes are not necessarily deterministic (see Table 7.1). In Park's most often cited study, he raised two species of flour

Table 7.1 Results of competition between two species of flour beetles {Tribolium confusum and T. castaneum) in six different temperature and humidity combinations. Based on Park {1954).

Temperature

Percent relative

"Climate"

Percent of replicates in

(°C)

humidity

which one species or

the other wins

T. confusum

T. castaneum

34

70

Hot-moist

0

100

34

30

Hot-dry

90

10

29

70

Warm-moist

14

86

29

30

Warm-dry

87

13

24

70

Cold-moist

71

29

24

30

Cold-dry

100

0

beetles, Tribolium castaneum and T. confusum, in vials of sifted flour under different temperature and humidity regimes (Table 7.1).

In these experiments the initial numbers of individuals were equal. That is, N1 = N2 at t = 0. In the top and bottom rows of the chart we have typical deterministic results, illustrating the first two graphical analyses (Figs 7.4 and 7.5) in which competitive exclusion occurs. The other four results are stochastic in that the result of any single experiment is unpredictable. Neyman et al. (1956) conducted further experiments using the cold-moist regime. They found that one species or the other was a deterministic winner when given a large initial numerical edge (Fig. 7.6). But there still existed an "indeterminate" zone (in the region of N1 ~ N2) where either species could still win.

A natural extension of these two-species competitive interactions was the work of Vandermeer (1969). Building on the work of Gause (1934), Vandermeer raised four species of protozoans in monocultures, thereby determining their rm and carrying capacities. Next he grew each of the species in pair-wise combinations. From these experiments he estimated the pair-wise competition coefficients. The general results were as follows: Paramecium aurelia depressed the growth of P. caudatum, drove P. bursaria extinct, and depressed the growth of Blepharisma sp. P. caudatum also drove P. bursaria extinct and drove Blepharisma sp. to a very low population level. Blepharisma and P. bursaria had little effect on each other and coexisted.

Based on the pair-wise competition coefficients and K-values, Vandermeer predicted the outcome of placing all four species together in a community, including predicted growth rates and carrying capacities. His predictions and the actual outcomes were surprisingly similar, leading to the conclusion that higher-order interactions (the combined effects of two species on a third) and nonlinear relationships were not significant in this community.

Other multi-species studies, however, did not confirm Vandermeer's findings. For example, Neill (1974) conducted a series of replicated removal experiments in a laboratory microcosm containing four species of microcrustaceans and associated algae and bacteria. Each species of crustacean was grown in pairs and estimates of population density were made under each regime. Computation of competition coefficients showed that the a-values depended on the community composition. The joint effects of two species on a third (in a three-species community) were not as predicted from the separate interactions in the two-species systems. Such results helped push ecologists to look for a different theoretical approach to competition.

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