Phytoplankton diversity: discontinuous assembly responses to environmental forcing

High diversity is a feature of many biological systems and it is generally considered to be important for their healthy functioning. Whether this is always or only sometimes true (see LAwroN 1997), high species richness and genetic represenration in ecosystems are seen to be threatened and, therefore, they are often objectives for conservation. It follows that 'protecting biodiversity' must be a good thing, even if we are nor quite sure how to do i t. Biodiversity (originally BioDiversity, coined as a shorthand for 'biological diversity': WILSON & PETER 1986), has no precise definition and is some way away from tangible quantification. 'Species richness' is biased by the search efficiency and taxonomic skill of the user. Gene pools can be guessed at but are scarcely measurable. The functions fulfilled by organisms in communities provide another focus for understanding the diversity o f the assemblage (MARTINEZ 1996), as account is taken of the physiological and morphological adaptive specialisms of rhe organisms present. Nevertheless, competition is still expected to select in favour of the functional specialists and to exclude the less well adapted contenders. It is nor difficult to echo the deduction ofTILMAN (1999) that the mechanistic explanation for the maintenance of the generally high biodiversity within given systems remains puzzling.