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Citation. Tilman, D. 1985. The resource-ratio hypothesis of succession. The American Naturalist 125:827-852. [1160 LTER]
Abstract. In this paper, I present an alternative, simple theory of succession, the resource-ratio hypothesis. Ecologists have long been intrigued by succession because its repeatability in an area and the similarities among areas worldwide suggest that much of the pattern of vegetation dynamics following a disturbance may result from a few underlying processes (e.g., Cowles 1899; Cooper 1913; Clements 1916; Gleason 1917, 1927; Lawrence 1958; MacMahon 1981). As shown by Connell and Slatyer (1977), Peet and Christensen (1980), McIntosh (1980, 1981), Horn (1981), Tilman (1982), and others, however, there is, as yet, no consensus in ecology as to which mechanisms are most important in controlling succession. Different processes have been considered important in primary succession, in secondary succession, and in maintaining the composition of vegetation once change ceases (e.g., Drury and Nisbet 1973; Connell and Slatyer 1977; McIntosh 1981). Clearly, the uniqueness of the species involved in successions in different areas, the uniqueness of each particular habitat, and various historical factors all limit the potential predictive ability of any model of vegetation dynamics and structure. In the interest of parsimony, however, it may be useful to explore the possibility that the same theory can explain the general patterns observed in primary successions, in secondary successions, and in mature vegetation. The resource-ratio hypothesis of succession is an attempt to explain what MacMahon (1981, p. 277) called "the surprising degree of pattern to successional processes in various parts of the world." It is an extension of a graphical theory of plant competition for resources in spatially heterogeneous habitats and along spatial gradients (Tilman 1982). The resource-ratio hypothesis of succession, which is meant to apply to species that are dominant sometime during succession, has two main elements; interspecific competition for resources and the long-term pattern of supply of limiting resources, which I call the resource-supply trajectory. According to this hypothesis, succession results from a gradient through time in the relative availabilities of limiting resources. In addition, succession should be a directional or repeatable process only to the extent that the resource-supply trajectory is repeatable or directional. In this paper, I suggest that a major axis for the evolution and differentiation of terrestrial plants has been the gradient from habitats with resource-poor soils but high availability of light at the soil surface to habitats with resource-rich soils but low availability of light. This gradient and the evolution of plant life histories in response to it may be the cause of the otherwise difficult-to-explain similarity of primary and secondary succession. The graphical theory of consumer- resource interactions used in this paper was developed in Tilman (1980, 1982) and is an extension of work by MacArthur (1972), Maguire (1973), Leon and Tumpson (1975), and Taylor and Williams (1975). So that its critical assumptions may be readily apparent, the resource-ratio hypothesis has been kept as simple as possible. Other potentially important processes, such as species-specific herbivory, differential colonization abilities, and temperature-dependent growth, have not been included in the model. If this simple formulation of the model proves useful, it would be possible to add other elements to the model to determine whether they could explain a significantly larger portion of the observed variance.
Keywords. resource ratio hypothesis, succession, limiting resources, consumer-resource interactions