SUBJECT: Aggregation in Marine Ecosystems
AUTHOR: William Silvert
DATE: 16th April 2003

I think it is useful to have this catalogue of differences between marine and terrestrial ecosystems, but I would add an observation that has significant implications for understanding marine biodiversity. Highly aggregated models have proved very useful in modelling marine ecosystems, and I think that this has implications for the way in which these ecosystems function.

In particular, size structured models in which organisms are characterised just by size work very well, and marine ecosystems have very similar size spectra with approximately equal biomasses in each size range (on a logarithmic scale - i.e., the biomass of organisms between 1 and 10 cm is
roughly the same as that between 0.1 and 1.0 mm). This suggests that the role of individual species in determining the structure of marine ecosystems is relatively minor, and that the ecosystem determines the species in it rather than the species shaping the ecosystem. In other words, an ecosystem is like a play - the individual actors are important, but it is the structure of the play that determines their roles.

This has major implications for studies of biodiversity. Whereas in terrestrial systems (or even semi-terrestrial, the polar bear is a recurring example) a single species plays a dominant role, there are usually several
top predators in a marine ecosystem. Marine ecosystems are probably more robust in terms of their response to the loss of a few species, although this may not appear to be the case for humans who see commercially desirable species replaced by inedible "trash fish" and invertebrates.

There are also major differences associated with the links between primary production and grazers. We know a lot about forest and crop biodiversity, but algal biodiversity is quite another matter, further complicated by the existence of many kinds of harmful algae.

In short, our understanding of terrestrial biodiversity is not a great deal of help in dealing with marine biodiversity, and the political aspects of defending marine biodiversity are bound to be very different. And of course when we start talking about the political aspects we have to remember that whereas organisms that grow on land are a national resource, those that live in the sea are common resources and must be managed globally.

A contribution by:

William Silvert
Instituto de Investigacao das Pescas e do Mar
Postal address Av. de Brasilia, s/n
1449-006 Lisboa
Portugal