Introduction

The ecology of fish resources is part of a larger picture defined as the fish chain. Diversity is at the base of most ecological processes involving resources and its alteration is viewed as a major source of large ecological and societal changes (Chapin et al. 2000). In addition, diversity is easy to define and conceptualise and is probably the best-studied ecological variable. Differences in the diversity of exploited species are extremely important, for example, the approximately thirty species commonly exploited in the Northeast Atlantic as compared with well over two hundred species in the tropical Western Pacific. Consequences may be numerous at any level of the fish chain, as is illustrated in fig. 3.1.

Diversity has many meanings in marine ecology (Steele 1991). It can apply to a continuum of organisation levels ranging from genetic diversity to ecosystem and landscape diversity. All these levels may be viewed as linked and the factors affecting one level of diversity usually affect the other ones as well (fig. 3.2). In this chapter, the diversity is mainly considered from the genetic to the functional levels.

The ascending arrow indicates that, moving from the genetic to the ecosystem level, changes in diversity depend on larger spatial scales and longer time scales. The descending arrow shows that the total number of components increases as the organisation level decreases.

Diversity can be essentially approached from three perspectives. The first is composition; e.g., of the species or functional groups in an assemblage. The second is the relative abundance of species in an area, as is usually expressed by indices related to the evenness of the distribution of individuals among the total number of species (Legendre and Legendre 1998). The indices make it possible to compare populations or assemblages through time for the same or different areas. The third perspective is the number of items at any organisational level (fig. 3.2), e.g., the number of species in an assemblage. This is often referred to as richness and can be considered at various spatial scales. In this chapter we are interested in the following:

• – Regional diversity: the number of species known in a region. At the taxonomic level, it usually refers to a species checklist, but can also be considered at the functional group, genetic, or other levels.