Selection experiments

The most direct way of demonstrating the evolutionary effects of competition within a pair of competing species is for the experimenter to induce these direct demonstrations of evolutionary effects of competition have been rare

Figure 8.26 (a) Experimental design to test for the evolution of Trifolium repens (T) in competition with Lolium perenne (L). Indigenous populations of T. repens, and sometimes also L. perenne, were removed. Trifolium repens was removed from the base of the arrow and transplanted, or replanted, at the head of the arrow. Treatment numbers are consistent with the usage of Connell (1980). (b) The results of this experiment are in terms of the total plot dry weight achieved by T. repens in the various treatments. Significance levels for comparisons between pairs of treatments are given in the text. (After Turkington & Mehrhoff, 1990.)

'Clover alone' site

Lolium perenne naturally absent

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-T

-T-L

'Two species' site

Lolium perenne-domlnated site

'Two species' site

Remove Remove grass | Trifolium and Trifolium

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4 2 5 3 Treatment number

Figure 8.27 Apparent evolution of competitive ability in Drosophila serrata.

(a) Of two experimental populations coexisting (and competing) with D. nebulosa, one (I) increased markedly in frequency after around week 20.

(b) Individuals from this population did better in further competition with

D. nebulosa ((-), mean of five populations) than did individuals from population II ((---), mean of five), or individuals from a stock not previously subjected to interspecific competition

effects - to impose the selection pressure (competition) and observe the outcome. Surprisingly perhaps, there have been very few successful experiments of this type. In some cases, a species has responded to the selection pressure applied by a second, competitor species by apparently increasing its 'competitive ability', in the sense of increasing its frequency within a joint population. An example of this with two species of Drosophila is shown in Figure 8.27. Such results, however, tell us nothing about the means by which such apparent increases were achieved (e.g. whether it was as a result of niche differentiation).

To find an example of a demonstrable increase in niche differentiation giving rise to coexistence of competitors in a selection experiment, we must turn away from interspecific competition in the strictest sense to competition between three types of the same bacterial species, Pseudomonas fluorescens, which behave as separate species because they reproduce asexu-ally (Rainey & Trevisano, 1998). The three types are named 'smooth' (SM), 'wrinkly spreader' (WS) and 'fuzzy spreader' (FS) on the basis of the morphology of their colonies plated out on solid medium. In liquid medium, they also occupy quite different parts of the culture vessel (Figure 8.28a). In vessels that were continually shaken, so that no separate niches for the different types could be established, an initially pure culture of SM individuals retained its purity (Figure 8.28b). But in the absence

of shaking, mutant WS and FS types invaded and established (Figure 8.28c). Furthermore, it was possible to determine the competitive abilities of the different types, when rare, to invade pure cultures of the other types (Figure 8.28d). Five of six possible invasions are favored. The exception - WS repels the invasion of FS - is unlikely to lead to the elimination of FS, because FS can invade cultures of SM, and SM can invade cultures of WS. In general, however, the experimental selection of increased niche differentiation amongst competing species appears to be either frustrat-ingly elusive or sadly neglected.

Figure 8.28 (a) Pure cultures of three types of the bacterium, Pseudomonas fluorescens (smooth, SM, wrinkly spreader, WS, and fuzzy spreader, FS) concentrate their growth in different parts of a liquid culture vessel. (b) In shaken culture vessels, pure SM cultures are maintained. Bars represent standard errors. (c) But in unshaken, initially pure SM cultures (•), WS (a) and FS (■) mutants arise, invade and establish. Bars represent standard errors. (d) The competitive abilities (relative rates of increase) when an initially rare type (foot of the arrow) invades a pure colony of another type (head of the arrow). Hence, values >1 indicate an ability to invade (superior competitor when rare) and values <1 an inability. (After Rainey & Trevisano, 1998. Reproduced by permission of Nature.)

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