Marine biodiversity and ecosystem services: an elusive link

Efforts to test the hypothesised positive link between ecosystem services a nd functions and biodiversity are increasing in order to forecast the conse quences of the present erosion of biodiversity on ecosystem functions and t o provide an additional basis for the conservation of biodiversity. These e fforts have been, however, modest in marine ecosystems. An examination of s eagrass communities, which are simple assemblages with a limited membership of about 50 species worldwide and < 12 species in any one community, provi des, however, strong evidence for the existence of such positive link betwe en species richness and ecosystem functions. Ecosystem functions are, howev er, dependent on the particular membership of the community, rather that it s number, for the functions are species-specific properties. Yet evidence, is provided, that an increasing species richness should be, on average, lin ked to an increase in the functional repertoire present in the community, w ill lead to a more efficient use of resources and a greater capacity to ens ure the sustainability of ecosystem functions under disturbance or ecosyste m change. Closer examination indicates that the functional variability of m ixed-species seagrass assemblages is correlated to the variability in speci es size, whereas species of similar size tend to show similar functional ca pacities and, therefore, a greater degree of functional redundancy. In addi tion, the demonstration of positive interactions in seagrass communities, w hich are also dependent on the presence of engineering species in the commu nity that facilitate the growth of other species, provides increasing groun ds to expect an enhanced functional performance of mixed communities over t hat expected from a simple additive contribution of the community members. Multispecific communities also hold, within the functional repertoire they contain, many unrealised functional potentials that may prove instrumental to ensure the sustainability of ecosystem functions in the presence of dist urbance or a changing environment. The arguments offered, illustrated for t he comparatively simple seagrass communities, provide strong reasons to exp ect a strong - if difficult to test experimentally - positive relationship between species diversity and the functions of marine ecosystems and, there by, the services they yield to humanity. (C) 2000 Elsevier Science BN. All rights reserved.