On the Notion of Favorableness in Plant Ecology

For many years plant ecologists have espoused the notion that habitats differ in an intrinsic quality that has been called favorableness. Here I point out that the concept of favorableness is circular and counterproductive. I develop the thesis that species diversity is determined by the balance of several dynamic processes: speciation, competition, immigration, adaptation, and extinction. A simple kinetic model places each of these processes in a definite perspective. My arguments are based on the observation that widespread mesic environments commonly support vegetation containing greater species diversity than environments of more unusual character such as sand dunes, inundated depressions, bogs, mountain tops, saline soils, etc. Habitats such as these, that incorporate uncommon features and are often patchy and of small total area, are called peripheral habitats. Empirical and theoretical considerations both lead to the opinion that the pressure of competition in the species-rich communities of mesic habitats impels an adaptive outflow of species down gradient toward relatively impoverished peripheral habitats. Evolutionary difficulties involved in adapting up a competition gradient argue that the species flow from core to peripheral habitats is virtually unidirectional. Application of the kinetic model to some elementary features of the earth's geography provides a quantitative result that predicts a large and universal tropical-temperate diversity gradient. The mechanism is founded on an area-dependent model of speciation coupled to the estimated rate of environmental change with latitude. It requires no assumptions about stability, food webs, or niche packing. The last section addresses the problem of how the dimensions of plant niche space could vary between different vegetation formations and briefly discusses components of rainforest diversity.