Tropical Blackwater Rivers, Animals, and Mast Fruiting by the Dipterocarpaceae

  • Janzen D
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It is proposed that tropical nutrient-poor white sand soils produce blackwater rivers, rivers that are rich in humic acids and poor in nutrients, because the vegetation growing on these soils is exceptionally rich in secondary compounds. The humic acids (= tannins and other phenolics) may even be only the more conspicuous of the secondary compounds that leach out of the living vegetation and the litter. While the water and the soil (including litter) may be expected to have a low productivity and animal biomass solely on the basis of its low nutrient content, it is quite possible that large amounts of secondary compounds are also debilitating to the animal community. An exceptionally high concentration of secondary compounds is expected in the vegetation growing on white sand soils for two reasons. First, this is an expected outcome in habitats where the loss of a leaf to an herbivore or through deciduous behavior is relatively a much greater loss than on nutrient-rich soils. Second, the plants growing there belong for the most part to families exceptionally rich in secondary compounds, a characteristic which is in turn selected for by the chemical defense requirements of plants growing in low diversity stands. The small amount of data that is available from Sarawak white sand habitats shows that the carrying capacity for animals is very greatly reduced. The postulated cause is reduced primary productivity and/or much of the productivity being used by the plant for secondary compounds (unharvestable productivity), or stored for seed crops at very long intervals (unavailable productivity). It is proposed that mast fruiting at the community level, as displayed by trees in the Dipterocarpaceae, is a mechanism of escape from seed predators that is unique to this part of the tropics (S.E. Asia) because this area has reduced animal communities (both on white sand soil sites and in general), and because the climate is sufficiently uniform for such an intra- and inter-population cueing system to evolve. Without experimentation, it is impossible to know, however, if the animal community is reduced solely due to overall lowered primary and harvestable productivity, or as well to the inevitable reduction in animal numbers when many of the trees in a habitat wait more than a few years for their highly synchronized seed crops. The occurrence of numerous tropical habitats with a very low diversity of trees inviolates the currently popular dogma that diversity is mandatory for stability in tropical habitats. I propose that the trees in such monotonous habitats are exceptionally well-protected chemically with respect to foliage, and have either very toxic seeds or well-developed mast cycles.

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  • Daniel Janzen

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