The Resource Ratio Hypothesis and the Meaning of Competition

Thompson, K

Tilman's (1980, 1985, 1986) resource ratio hypothesis is an interesting contribution to the current debate about whether the structure and dynamics of vegetation are accounted for by equilibrium or non-equilibrium models. Recently, the weight of evidence seems to have been in favour of non-equilibrium hypotheses (e.g. Grubb, 1977; Connell, 1979; Huston, 1979) and Tilman's ideas can be seen as an attempt to revive the concept of niche differentiation as an important force in structuring plant communities. The resource ratio hypothesis is not described in detail here (a useful summary is in Begon, Harper & Townsend, 1986) but in essence it explains coexistence of competing species by their differing abilities to compete for pairs of resources where the ratio of abundance of the resources varies. The hypothesis is an ingenious attempt to overcome the frequently voiced objection (e.g. Grubb, 1977) that there are not enough different resources for niche differentiation to make a major contribution to the diversity of plant communities. Tilman has pointed out that even a few resources can generate a wide range of resource ratios and that such ratios, together with a plausible amount of environmental heterogeneity, may account for the structure of plant communities. An attractive feature of the hypothesis is that it accounts for both diversity of stable communities (via spatial heterogeneity of resource ratios) and for species replacement during succession (via change in ratios over time). Unfortunately some of the assumptions and predictions of Tilman's hypothesis, particularly as applied to succession, seem to me to be question- able. Below I list five statements which represent some essential elements of the hypothesis. Some of these statements are assumptions on which the hypothesis is based, others are predictions derived from the hypothesis. Following each statement I examine the evidence and finally I consider to what extent my skepticism about the hypothesis may be ascribed to a fundamental disagreement about the nature of competition.

The Resource Ratio Hypothesis and the Meaning of Competition

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