Demography of Annual Plants: The Role of Habitat Heterogeneity and Competition
There is abundant evidence that local heterogeneity in habitat conditions may influence the demography of plant populations (Fowler and Antonovics 1981; Mack and Pyke 1983; van Tienderen 1992; Tielbörger and Kadmon 1995). It is also well evident that competition for resources may be important in determining the survival, growth and reproduction of individual plants (Weiner 1988; Keddy 1989a; Pantastico-Caldas and Venable 1993; Tremmel and Bazzaz 1993; Kadmon 1995; Grace and Platt 1995). Much less is known about the degree to which, and the manner by which, competition interacts with local variation in habitat conditions in determining the demography of plant populations. Interactions between the effects of habitat conditions and competition are expected to be common in natural plant communities because factors affecting the intensity of competition, such as population density (Keddy 1982; Condit et al. 1994), standing crop (Wilson and Keddy 1986a; Reader et al. 1994), productivity (Turkington et al. 1993; Kadmon 1995), abiotic stress (Keddy 1981; Stadt et al. 1994) and herbivory (Swank and Oechel 1991; Burger and Louda 1994; Shabel and Peart 1994), may vary considerably between one habitat and another, even at very small spatial scales. Under such circumstances, any attempt to interpret observed patterns of demographic variation must take into account the interactive effects of spatial heterogeneity in habitat conditions and competition. Moreover, since plant species differ from each other in their tolerance to both abiotic factors (Chapin et al. 1993; Aerts and de Caluwe 1994) and competition (Goldberg and Fleetwood 1987; Keddy 1989b; Silvertown and Dale 1991), coexisting species may respond differentially to the same changes in habitat conditions and competition, depending on their life-history traits. Such species-specific responses may be important in determining opportunities for the coexistence of competing species, as well as community-level patterns of species composition and diversity.