8 Biogeochemical Interactions and Biodiversity

There has been considerable recent interest in the ecosystem consequences of global and regional shifts in biogeochemical cycling and the loss of species diversity. Since biogeochemical cycling is the result of an interaction of numerous organisms, the link between diversity and biogeochemistry is a natural one. changes in either diversity or biogeochemiical &ling can dramatically influence one another, and the large human impact on both has the capacity to compromise the ecosystem functions on which society relies. A number of significant changes are occurring in the Earth's biogeochemistry (Austin et a]., Chapter 2, this volume; Galloway, Chapter 14, this volume; Chapin et al. 2000). Because of these shifts in global and regional biogeochem-istry, as well as land use changes and the introduction of exotic species, we are in the midst of one of the largest extinction events in the history of life on Earth, with-extinction rates 100-1,000 times greater than prehuman rates. The ecosystem response to these multiple biogeochemical shifts depends largely on the response of the biotic community. For example, shifts in vegetation in response to these human-induced changes can have ecosystem impacts ,that are larger and different in direction than the direct biogeochemical response to these changes (Hobbie 1996; Shaw and Hane 2001). Beyond a shift in composition, loss of diversity can compromise the capacity of a community to perform ecosystem functions, particularly in response to multiple stresses (Grifiths et d. 2000; Degens et d. 2001; Muller et 4.2002). In this chapter we explore the feedbacks between changes in biogeochemical interactions and biodiversity. Plant species richness is the component of biodiversity that is most ficquently studied in relationship to ecosystem function, but we indude other examples where they are available. Shifts in the biotic community in response to a variety of biogeochemical changes ' have been well documented (Table 8.1). Deposition of toxic levels of heavy metals and SO, can decrease the diversity of pl&ts, roil microbes, and several other types of organisms. These decreases in species richness are directional changes through selection for species that can tolerate levels of pollutants that arc potentially toxic to many organisms (Table 8.1). The diirential responses of components of the biotic community to these pollutants can have significant impacts on biogeochemical cycling. For example, heavy metals suppress N 2 0 production more than NO production (Holm-Ha.rn+ig et al. 2002). Plant diversity substantially decreases in response to chronic N inputs across a …