Effects of Diversity on Productivity: Quantitative Distributions of Traits

Abstract

Biodiversity relates to ecosystem function on islands in two distinct ways. First, the isolation of some islands from larger land masses has led to unique patterns of biodiversity, caused by limited numbers of species introductions, absence of certain “functional groups” of species, limited refugia during past climate changes, and development of endemism and adaptive radiation within taxa. The importance of these biodiversity patterns to ecosystem function is often revealed by biological invasions having direct effects on ecosystem function, such as the bird-dispersed nitrogen-fixing shrub Myrica faya altering the N cycle in montane Hawaiian forests (Vitousek and Walker 1989). Island patterns of biodiversity also lead to what might be considered indirect consequences of invasions, such as the vulnerability of island plant species to relatively recent changes such as introduction of goats (Baker and Reeser 1972; Spatz and Mueller-Dombois 1973), and consequent ecosystem degradation from overgrazing. Second, although the effects of biodiversity per se on the function of a given ecosystem are not always unique to islands, islands may present unique opportunities for their investigation. For example, the dominant role of a single tree species, Metrosideros polymorpha, in Hawaiian forests has permitted much clearer identification of controls of ecosystem carbon and nitrogen accumulation than would be possible in more diverse plant communities (Vitousek 1995).