Originalité et diversité des écosystèmes tropicaux

In the tropical area climatic factors play a major role in determining the ecosystem structure and function. Temperatures are constantly high but the rainfall regime is responsible for the differences between seasons. In addition to many local variations, the climate varies gradually from one of abundant rainfall, spread out over the year near the Equator, to one with a predominant dry season, becoming longer and more severe towards the Tropics of Cancer or Capricorn. The vegetation does not follow this gradual progression and three main types may be distinguished: 1. rain forest, either green or semideciduous ; 2, humid preforestal savanna, generally poor in wood species ; 3. dry savanna, with a more or less large number of wood species but a less developed herbaceous layer. The differentiation between these various types of vegetation is closely linked to fires, which do not occur in the forests but in the humid savannas have an important impact on the wood species due to the high quantity of combustible grasses. High temperatures and abundant rainfall have a direct favourable effect on the vegetation. They also have an indirect and fundamental influence through the soil which is easily leached and desaturated. The biomass of forests in the intertropical zone is nevertheless very high (400-500 t/ha). Their productivity is also high -from 20 to 30 t/ ha/yr- but reduced by a high respiration rate and, in contrast to the biomass, depends on the nature of the sub-soil. Animals eat only a tiny fraction of the living biomass of the plant matter produced. The majority is consumed by decomposers (more than 90 %), the only organisms which can cope with the deficiency in nitrogen. The same remark is true for humid savannas; the limiting factor is the lack of nutrients in the soil, which means that grass becomes useless as fodder for animals after a few weeks’ growth, due to the lack of protein. Grass can be consumed only as dead matter by the joint action of detritus-eaters and microorganisms, bacteria and fungi. As this process is not immediate, a large part of the above-ground plant matter is destroyed by fire. In the drier savannas, where water is the limiting factor to plant growth, the protein content of grass is high enough to allow almost all the above-ground biomass to be eaten as fodder. Consequently fire has less effect, large herbivorous ungulates can thrive and it is economically possible to raise cattle, in spite of a lower global productivity.