Exploitation of more than two resources

We have seen how two diatom species may coexist in the laboratory on two shared limiting resources. In fact, Tilman's resource competition theory predicts that the diversity of coexisting species should be proportional to the total number of resources in a system that are at physiological limiting levels: the more limiting resources, the more coexisting competitors. Interlandi and Kilham (2001) tested this hypothesis directly in three lakes in the Yellowstone region of Wyoming, USA using an index (Simpson's index) of the species diversity of phytoplankton there (diatoms and other species). If one species exists on its own, the index equals 1; in a group of species where biomass is strongly dominated by a single species, the index will be close to 1; when two species exist at equal biomass, the index is 2; and so on. According to resource competition theory, this index should therefore increase in direct proportion to the number of resources limiting growth. The spatial and temporal patterns in phytoplankton diversity in the three lakes for 1996 and 1997 are shown in Figure 8.37. The principal limiting resources for phytoplankton growth are nitrogen, phosphorus, silicon and light. These parameters were measured at the same depths and times that the phytoplankton were sampled, and it was noted where and when any of the potential limiting factors actually occurred at levels below threshold limits for growth. Consistent with resource competition theory, species diversity increased as the number of resources at physiologically limiting levels increased (Figure 8.38).

These results suggest that even in the highly dynamic environments of lakes where equilibrium conditions are rare, resource competition plays a role in continuously structuring the phyto-plankton community. It is heartening that the results of experiments performed in the artificial world of the laboratory are echoed here in the much more complex natural environment.

Our survey of interspecific competition has concluded with a realization that we need to understand much more about the mechanisms underlying the interactions between consumers and their resources. If these resources are alive, then we normally refer to such interactions as predation; and if they were alive once, but are now dead, we refer to them as detritivory. It would seem, therefore, that the distinction normally made between competition and predation is, in a very real sense, an artificial one (Tilman, 1990). None the less, having dealt with competition here, we turn next, in a separate series of chapters, to predation and detritivory.

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