Chness

quite reasonably, have a diversity of meanings. Yet it is necessary to be specific if the term is to be of any practical use.

In this chapter we restrict our attention to species richness, partly because of its fundamental nature but mainly because so many more data are available for this than for any other aspect of biodiversity. We will address several questions. Why do some communities contain more species than others? Are there patterns or gradients of species richness? If so, what are the reasons for these patterns? There are plausible answers to the questions we ask, but these answers are by no means conclusive. Yet this is not so much a disappointment as a challenge to ecologists of the future. Much of the fascination of ecology lies in the fact that many of the problems are blatant, whereas the solutions are not. We will see that a full understanding of patterns in species richness must draw on our knowledge of all the ecological topics dealt with so far in this book.

As with other areas of ecology, scale the question of scale: is a paramount feature in discussions macroecology of species richness; explanations for patterns usually have both smaller and larger scale components. Thus, the number of species living on a boulder in a river will reflect local influences such as the range of microhabitats provided (on the surface, in crevices and beneath the boulder) and the consequences of species interactions taking place (competition, predation, parasitism). However, larger scale influences of both a spatial and temporal nature will also be important. Thus, species richness may be large on our boulder because the regional pool of species is itself large (in the river as a whole or, at a still larger scale, in the geographic region) or because there has been a long interlude since the boulder was last turned over by a flood (or since the region was last glaciated). Comparatively more emphasis has been placed on local as opposed to regional questions in ecology, prompting Brown and Maurer (1989) to designate a subdiscipline of ecology as macroecology - to deal explicitly with understanding distribution and abundance at large spatial and temporal scales. Geographic patterns in species richness are a principal focus of macroecology (e.g. Gaston & Blackburn, 2000; Blackburn & Gaston, 2003).

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