Abstract

This chapter looks at the factors controlling large-scale biogeographical patterns of pelagic taxa, the present biogeography of pelagic communities, and the origins of pelagic biodiversity. Oceanic versus neritic biodiversity is considered before the existing patterns of distribution of bathyetric communities are discussed.

Introduction

Biodiversity, according to the Biodiversity Convention, is defined as ‘the variability among living organisms from all sources including inter alia, terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are a part; this includes diversity within species, between species and of ecosystems’. Biodiversity therefore includes all variability in the natural world, at all scales in time and space, and at all levels of organisation from the genetic variability within the organisms to the structure of ecosystems. In trying to understand the causal factors determining the variability, the general approach has been to look for correlations between the patterns in the biological characteristics and other abiotic and biotic factors, the inference being that positive and negative correlations indicate causal relationships.

The end of the nineteenth and beginning of the twentieth centuries saw the great transoceanic expeditions that laid the foundations for modern biogeographical studies of pelagic faunas and floras (e.g. Ekman, 1953). But as more detailed studies were conducted, incompatibilities emerged between the interpretations based on studies of large-scale distributions and those of variability over finer space and time scales.