Experimenting with natural experiments

niche divergence in clover-grass competition

Sometimes, as we have already seen with the gerbils, natural experiments may themselves provide an opportunity for a further - and more informative -experimental manipulation. In one such case, niche divergence was sought in clover, Trifolium repens, as a result of its having to compete with the grass Lolium perenne (Turkington & Mehrhoff, 1990). Clover was examined from two sites: (i) a 'two-species' site, in which clover achieved a ground coverage of 48% and the grass achieved a coverage of 96% (the two added together exceed 100% because their leaves can overlap); and (ii) a site in which clover achieved 40% coverage, but L. perenne covered only 4% (effectively a 'clover-alone' site). A total of three transplant (into the other site) and three re-plant (back into the home site) experiments were carried out (described and numbered in Figure 8.26a). T. repens, from both sites, was planted in: (i) plots at the two-species site cleared of T. repens only; (ii) plots at the two-species site cleared of both T. repens and L. perenne; and (iii) plots at the clover-alone site cleared of T. repens. The extent of competitive suppression or release was assessed from the amount of growth achieved by the different plantings of T. repens. From this the extent of the evolution of niche divergence between 'clover-alone' and 'two-species' T. repens was deduced, as was that between T. repens and L. perenne.

The T. repens population from the two-species site had indeed apparently diverged from the L. perenne population with which it was coexisting (and with which it may otherwise have competed strongly), and had diverged too from the clover-alone

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