[Coral-List] Reefs the most diverse
martin.pecheux at free.fr
Wed Jan 4 21:31:35 EST 2012
Here is a submitted article, in the glory of reefs.
Coral reefs are the most diverse Earth ecosystem.
Les récifs de coraux forment l'écosystème de la Terre le plus divers.
Abstract: Comparisons are made of biodiversity between coral reefs and tropical rain forests. It is clear that coral reefs are the most diverse ecosystem of the Earth with its spectrum of variability and when ecological processes are considered, not just species numbers.
Key-words: reef, coral, biodiversity, tropic, forest, symbiosis, CO2
Résumé: Je fais la comparaison de la biodiversité des récifs de coraux et des forêts tropicales humides. Il est clair que les récifs de coraux forment l'écosystème terrestre le plus divers, avec son large spectre de variabilité, et quand les processus écologiques sont considérés, non pas seulement le nombre des espèces.
Mots-clef: récif, coraux, biodiversité, tropique, forêt, symbiose, CO2
Reefs are often quoted as one of the most diverse marine ecosystems. No. Reefs are the most diverse of all Earth ecosystems.
We do not have to count biodiversity just in term of number of species, but in term of processes/patterns, of original enzymatic pathways, of fit to and evolving in a particular ecological niche, with both abiotic ("Court Jester", ) and biotic ("Red Queen") factors, at different time and spatial scales, i.e. of life invention. Of course this is not quantifiable, but a good measure is given by DNA divergence (but with my Y chromosome – 1.61% length -, I am nearest to male chimpanzees – 1.23% difference - than to womans). An approximation of this divergence is the number of phyla. For animals, reefs are the richest Earth ecosystem with 30 represented phyla whereas there is only 19 terrestrial ones .
The only animal phylum found on land and not in reefs is the anecdotal Onychophora (cousins of arthropods and tardigrades, marine at first in the Precambrian, actually 11 genera). Three other animal phyla are marine but not found in reefs: Orthonectida, 24 species of parasitic worms of marin invertebrates, Cycliophora, 3 microscopic species, one undescribed, cousin of rotifers, living in Atlantic and Mediteranean crustacean buccal pieces, and the famous Pogonophores, the big red annelids living from H2S near rift events (of which half of taxonomists make it a class or put it in an other family) (for all, source: Wikipedia). So, almost nothing.
Of course, the oldest, the more time for radiation and DNA divergence. Reefs are the oldest biotope on the Earth since 3.4 billions years ago with reef stromatoliths , and the proeminent place of life till planktonic appraisal (500 My at least, from isotope, ) and land conquest (in Silurian, 425-475 My by Rhynia, Cooksonia from green algae).
I will not consider the case of the bentic deep sea ecosystem, said to be surprisingly rich, with a so wide areal surface extent. It is often quoted that reefs are the rain forests of the sea. No: rain forests are the reefs of the land. Tropical rain forests are the most diverse land ecosystem, said to be the one with 40%-70% of all biotic species, etc…
In fact, tropical rain forests are said to be especially diverse primarily by the number of insect species. Erwin (1982, in ) and his colleagues gassed 19 trees in Panama, collecting about 1000 unique beetle species. So they extrapolate to 30 millions species. Hamilton et al.  estimate more reasonnably that there is 2.5-3.7 millions tropical arthropod species. It is very difficult to know exactly, but insects would represent about 80% of all forest species (T. Bourgoin, specialist of tropical insects, Collection Director at National Museum of Natural History, Paris, com. pers.). Why a so great number of species? They don't seem to play exceptional ecological roles: predation on plants, and between them, decomposition, recently pollinisation, even if each one is at its own place. It appears that there is so much beetles in part because they are more or less specialized feeders on few tree species (cf. ).
For the main primary trophic level and equivalently morphological shaping groups of the ecosystem, in reefs, there are about 108 genera and 800 species of hermatypic corals, Hubbell et al.  estimate the number of Amazon tree species to be 12 500, and Hamilton et al. , for whole tropics, to be 50 000. But introgression by commun pollinisators, the presence of interspecific hybrids maintening a flux of genetic pools between related species, might be more important than currently evaluated . Such phenomen is not excluded in corals, where the morphological definition of species is not perfect. Curiously, the number of hermatypic coral species is similar to the number of genera of tropical trees: Hubbell et al.  reporte that 514 tree genera have been found in Amazon over 300 000 samples. Symbioses of corals with zooxanthellae is though less evolved but less intricated, than endosymbioses of trees with chloroplasts. Diversity of zooxanthellae clades appears usual for dinoflagellates, but a priori greater than chloroplasts, with their own short DNA, and with a nuclear part. The accessory groups of the primary trophic level in forests (small plants, grasses, epiphytes, lichens, bryophytes and particularly vines) are somewhat like trees, whereas in reefs there is a huge diversity: green, brown, red fleshy and crustose algae, big Tridacna mollusks with zooxanthellae, seven order of sponges associated with zooxanthellae or cyanobacteria, large foraminifers symbiotic with zooxanthellae, diatoms, chlorophytes, rhodophytes or free undigested chloroplasts, and the chordate ascidians with the particular pro-eu-karyotic Prochloron sp. (synthesis in ). Rare species contribute surely over all equally to complex ecosystem stability (cf. ), the best measure of fit biodiversity.
Main groups of the upper trophic level seem roughly at equality, echinoderms, crustaceans, big mollusks and above, numerous fishes and few marine upper vertebrates (sea serpents, seven marine turtles of two separated lineages, few iguana and crocodiles, sea birds, cetaceans), compared to amphibians, reptiles, birds, and above, mammals.
Groups of the intermediate levels seems of small size, which could hypothetically explain their species number, as for example of micro-mollusks, foraminifers, worms, compared to insects, arachnids, worms. Small, perhaps because mollusks and arthropods are protostomians. Nematods and plathyhelminths too, but the annelids ("worms", roughly 15 000 species) appeared to me, aside the large evolved deuterostomia, as a third superphyllum, with mouth and anus derived from the blastopore (which can be called "schizostomia"), but this is not the purpoise of this article.
And for micro-organisms, most remains to be known. Reefs have the "help" of been bathed by the whole planktonic ecosystem, of course, important in their economy.
Regionalism is similar, with two main provinces in reefs, Caribbean and Indo-Pacific, and three in tropical forests, Amazon, Congo basin and South-East Asia, and many minor ones.
There is a great difference between coral reefs and tropical forests: the superficially. Tropical forest occupy in the past about 10%-15%, and now 5%-7% of Earth surface (convergent sources), whereas reefs, with 284 803 km2 (, but deeper water not evaluated) on Earth mean radius 6 371 km, occupy only 0.551%. How an ecosystem with twenty times less spatial distribution has more variability ? It is quite amazing. Moreover, many reefs like those scattered across the Pacific Ocean are isolated .
Why are tropics more diverse ? An old question. Perhaps because with temperature, energy activation of enzymes is lower, allowing a greater facility to create new pathways. Why are tropical animals so coloured ? That, I really don't know.
So, to conclude, it appears to me clearly that reefs are the most diverse Earth ecosystem. This is a supplementary reason to worry of strong reef decay for a global cause, CO2, whereas we "know" how to stop rapidly deforestation.
I am redevant to Charles Birkeland, who helps me for data and correction of the English. This work was supported by French AAH n°2504010.
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