Human-induced changes in diversity include species introductions, local reduction of species richness, alteration of the relative abundance of species, changes in the spatial distribution of species and species extinctions. While marine habitats have apparently experienced relatively low rates of contemporary extinctions compared with terrestrial and freshwater habitats (e.g. Glynn and de Weerdt 1991; Glynn and Feingold 1992; Vermeij 1993; but see Carlton 1993; Lubchenco et al. 1995), other aspects of marine diversity have been profoundly altered by humans (Norse 1993; Lubchenco et al. 1995; National Research Council 1995). Overfishing, species introductions, habitat deterioration and destruction, eutrophication and pollution all impact diversity (Chandler et ai. 1995; Done el al. 1995; Lubchenco et al. 1995; Twilley et al. 1995).

This chapter considers the likely community- and ecosystem-wide consequences of these changes in diversity for nearshore coastal systems. We begin by reviewing ways in which human activities are changing diversity. For the most part, the consequences of these changes are complex and not well documented or understood. Many of the changcs, however, are the same kinds of changes that have been created experimentally on local scales by investigators seeking to understand the causes of community structure and differences among communities. We summarize the relevant results from experiments performed on rocky shores around the world. We focus on

Functional Roles of Biodiversity: A Global Perspective ^

Edited by H.A. Mooney, J.H. Cushman, E. Medina. O.E. Sa!a and E.-D. Sehulze ftcftfj © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd m uni r experiments with high trophic level species because there is a wealth of information on them which relates directly to an understanding of community dynamics (e.g. Menge and Farreli 1989; Paine 1994; Underwood and Chapman 1995). Our results reinforce emerging findings from other assessments (Chapin et al. 1995; Mooney et at 1995a, b) that the identity of the species being added or removed and its similarity to other species are often of critical importance in determining overall effects. Hence discussions of diversity need to be concerned not only with numbers of species, but also with kinds of specics and their functional similarity to other species in the system. We conclude by synthesizing results from the experimental studies into a conceptual framework for considering the general relationship between certain changes in diversity and community or ecosystem consequences.

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