The Species-Pool Hypothesis and Plant Community Diversity

Abstract

One of central problems in ecology is to explain species diversity in communities, and a considerable number of hypotheses with this purpose have been formulated during last three decades. One very influential group of hypotheses, largely emanating from Hutchinson (1959, 1961), seek explanations in terms of mechanisms of coexistence. The question addressed by these hypotheses is: why are there so many coexisting species? (implicitly assuming that most potential inhabitants of a certain community are present, or have at least had opportunity to be present, but may have been subsequently outcompeted). These hypotheses usually predict that species, in order to coexist, in some way must avoid negative effects of competition. A common theme for wide array of mechanisms suggested to have this, or alike, effects (e.g. Hutchinson 1961, MacArthur 1972, Grubb 1977, Grime 1979, Huston 1979, Tilman 1982, Fagerstrom 1988) is that they are confined to, in a strict sense, ecological phenomena, such as spatial or temporal separation of resource use, herbivore-mediated suppression of dominants, or chance effects during recruitment. An alternative (or complementary) view of diversity in communities stresses importance of historic, or phylogenetic, aspects of species diversification. Present-day diversity is a result of speciation and extinction processes that have occurred over long time. In his overview of diversity in land communities, Whittaker (1977) concluded that there is not much evidence, either theoretical or empirical, for that species diversity is in equilibrium. Hence, in Whittakers view, to understand patterns of diversity, large scale aspects of speciation, extinction and biogeography must be accounted for. This line of reasoning can be exemplified by some studies of diversity in tropical rain forests. These communities are well known for their exceptional plant species richness (Gentry 1982), and one general explanation for their high diversity is that tropical forests have accumulated species under stable conditions experienced during long time (Stebbins 1974). In a study of tropical tree species diversity, Hubbell and Foster (1986) suggested that tree communities are in a state of non-equilibrium; new species' (either immigrants or newly evolved ones) are capable of invading communites due to a permanent occurrence of empty sites. Since risk of species exclusion from a plant community was found to be very small, diversity would be determined by regional species richness and immigration rates. Thus, variation in species diversity among communities, or among guilds within communities, reflect rate-determining mechanisms behind speciation and extinction. High species diversity is expected when existing speciespool contains many species, and comparatively low species diversity will be found when species-pool is small. Similar suggestions have been made by several authors, in various contexts, (e.g. Grime 1973, 1979, Cracraft 1985, Hodgson 1986, 1987, Connell and Lowman 1989, Hart 1990, Brooks and McLennan 1991, Cornell and Lawton 1992, Zobel 1992), and Taylor et al. (1990) coined term the species-pool hypothesis for explanations of local diversity by reference to size of regional or global pool of species: All else being equal, larger local and/or global area of a habitat type and older its geological age, greater past opportunity for speciation and hence, greater number of available species adapted to a particular habitat type. However, if different phylogenetic lineages do not diversify at same average rate, age (or size) of a habitat type may be only weakly related to richness of species-pool. Furthermore, Zobel (1992) stressed importance of evolutionary factors (i.e. speciation rate) in explanations of species diversity in plant communities, but did not specify what might determine speciation rate per se. Cracraft (1985) approached diversity problem from a different angle and suggested that community diversity is explicable by basic processes speciation and extinction; speciation rates are mainly determined by large scale changes in lithospheric complexity, and extinction rates are dependent on envi-