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Some differences between land and marine diversity(11) It appears that fewer species live in
the ocean than on the land, based on the number of species currently known. Consideration
solely of the number of species, however, can be a misleading measure of diversity since at
higher taxonomic levels such as class or phylum, marine ecosystems have a significantly
higher degree of genetic diversity. Almost every major division of life has at least some
representation in the sea, and many divisions are principally or wholly marine. In contrast,
only one third of these large categories occur on land. Of the 33 animal phyla listed by
Grassle et al.,(12) 28 occur in the sea. Thirteen, including comb jellies, lamp shells and
echinoderms, are exclusively marine; and several more, including sponges, coelenterates and
bryozoans, are nearly so (more than 95 per cent of species). In contrast, 11 animal phyla are
terrestrial, and only one exclusively so. Thus the seas host nearly the entire extant
diversity of basic animal body plans, while land and freshwater animals comprise myriad
variations on relatively few themes, notably insects, possibly nematodes, and a smattering of
other groups. Genetic diversity too tends to be higher in marine organisms, with individuals
in many taxa exhibiting much greater levels of gene locus heterozygosity than, for example,
birds or mammals.(13)

While some genetic characteristics are in both land and sea organisms, in many ways the
marine realm is a world unto itself, and differences in media, dimensionality and scale have
momentous implications for marine conservation. The most obvious difference between the
marine and terrestrial realms is the seawater above the geological substratum. The bouyancy,
viscosity, currents and surface tension of seawater allow very different kinds of organisms
to exist in the sea. In contrast to terrestrial species, these organisms are not mostly
restricted to a thin, relatively two-dimensional zone within a hundred metres of the soil
surface as on land, but can be found through numerous zones from the greatest depths (more
than 11 700 metres) to a few metres above sea level.

In addition, marine and terrestrial ecosystems differ significantly in their functional
responses to environmental change. Three principal functional relations exemplify this
diversity:


Size, life span, and trophic status. In the ocean and on land, the larger organisms are
usually more long-lived, but they differ in trophic status and their place in the food web.
In the sea, the longest living organisms are the big carnivores at the top of the food chain,
whereas the longest living terrestrial organisms are trees, which are primary producers (at
the bottom of the food chain).

Relationship of organisms with physical processes.  Marine reproductive cycles, particularly
in the larval and juvenile phases, are closely linked to water movements. Terrestrial animals
tend to evolve reproductive patterns that are not affected by short (approx. 1 year) to
medium (approx. 10 years) term variability in the physical environment.


Differences in processes on the land and sea, observed on the same time scale. Some of this
contrast can be eliminated by using different time scales; the same responses can occur in
different systems at different time scales. The rates, rather than the magnitudes, of
environmental change may have more effect on the disturbance of community structure.


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Last modified: 28 November 1995