SUBJECT: Assumptions
AUTHOR: Ian Davies
DATE: 16th April 2003

Christos and Tasso propose a series of assumptions to summarise our forum:
1. Diversity at higher taxonomic levels is much greater in the sea (fourteen “endemic” marine phyla, compared to one terrestrial)
The endemic terrestrial phylum is the Onycophora and does not occur in Europe.
2. A greater variety of species at a higher trophic level are commercially exploited in the sea.
There is more than this. Man exploits natural populations only in the marine environment. There are no terrestrial animals and plants, in Europe, that are harvested from wild populations, whereas the majority of the resources we get from the sea derive from natural populations.
3. The marine physical environment is totally different from the terrestrial physical environment. OK, no further comment.
4. Dispersal of species may occur over much broader ranges than on land. There are many facets of this problem. Remember that many terrestrial organisms migrate (think of the birds). This apparently logical statement might lead to many counterintuitive facts, like the paradox of Rockall.
5. The main primary producers are very small sized and often mobile in the sea. This is true, but the turn-over rates are very fast. and this leads to point 6.
6. The standing stock of grazers is higher than that of primary producers in the sea. This is true in a static situation (standing) but turnover rates contribute to justify how comes that those who eat are more abundant than those who are eaten.
7. Ocean productivity is on average lower than land productivity. If you take Valiela's textbook, marine Ecological Processes and go at page 29 there is a table related to production (not productivity) in various environments it appears that the greatest production is that of freshwater phytoplankton in nutrient rich waters, then tere are corals, kelp & rockweeds, then benthic microalgae, then rain forests and so on. The picture shows that, even taking the surface of available space for that kind of production, the sea is incomparably more productive than the land.
8. High level carnivores often play key roles in structuring marine biodiversity. This is true, but remember that these carnivores are not necessarily large animals. Mnemiopsis is a more efficient predator than all cetaceans, birds (and men) that insist on the Black sea.And this is probably true for many other apparently inconspicuous top predators. We did not investigate this aspect enough, especially in the mediterranean.
9. Marine biodiversity is the most exposed to pollution. Yes, everything goes to the sea sooner or later. But, in the sea, there are no ultraendemics like on land. There are little molluscs or little flowers that occur only on the tip of a given mountain: If you affect that mountain the species might become extinct. There is nothing like that in the sea.

A contribution by:

Ferdinando Boero
Universita de Lecce
Via Prov. Le Lecce-Monteroni
73100 Lecce
Italy