Some Thoughts on Resource Competition and Diversity in Plant Communities

Abstract

The most diverse terrestrial plant communities in the world, i.e. those that are richest in species, occur on some of the most nutrient-poor soils. The fynbos of the Cape Province of South Africa and the heaths of Australia are two such cases, and both occur on soils that are very poor in nutrients (Goldblatt 1978; Taylor 1978; Kruger 1979; Specht 1979; George et al. 1979; Naveh and Whittaker 1980; Chapter 19, this volume). Although possibly not as diverse as the fynbos (Kruger and Taylor 1979), tropical forests are also relatively rich in species and also occur on relatively poor soils (Holdridge et al. 1971; Huston 1979). This association of high plant diversity with nutrient-poor habitats is not limited to mediterranean and tropical climates, nor to terrestrial plant communities. In a broad variety of plant communities — in temperate and tropical regions, in grasslands, forests, lakes, rivers, estuaries, and the oceans — plant species diversity within a geographical area is locally higher in habitats relatively poor in nutrients, and lower in sites very poor, or richer in nutrients (Huston 1979). For instance, the nutrient-poor, unproductive waters of the Sargasso Sea support a very diverse phytoplankton assemblage, whereas the more productive and nutrient-rich temperate oceans support a much less diverse plant community (Fischer 1960; Guillard and Kilham 1977). Similarly, nutrient-rich marine upwelling regions tend to be much less diverse than nearby regions that do not experience nutrient upwelling (Blasco 1971; Dugdale 1972; Smayda 1975). The addition of nutrients (cultural eutrophication) to coastal marine areas, estuaries, rivers and lakes has consistently led to decreased species diversity (Patrick 1963, 1967; Williams 1964; Schleske and Stoermer 1971).