Role of Caimans in the Nutrient Regime of Mouth-Lakes of Amazon Affluents (An Hypothesis)

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The formerly abundant caimans of the central Amazon region have nearly beconme extinct because of the demand for their hides. Inhabitants of the region have discovered, to their surprise, that the fish populations in certain mouth-lakes have diminished following the disappearance of the caimans; even piranhas have decreased in number. Close interdependence exists, or seems to have existed, between caimans and the fish in the limnologically very extreme, central Amazonian water bodies, in the manner outlined below. Those tributaries of the Amazon which do not come from the peripheral regions, but arise in the tertiary central Amazonian plains, are so extremely poor in essential electrolytes that primary production is scarcely possible in them. The food chain is based on organic material which enters the water from the surrounding forests. In addition, swarms of fish originating in the electrolyte- and nutrient-rich waters of the Amazon River and its bordering flood basins migrate into the usually broadened, lake-like lower reaches of these rivers to breed. Many of the fish serve as prey here for the predatory fish, turtles, caimans, and aquatic mammals. Part of this allochthonous food is immediately transformed into nutrients by rapid metabolism. These nutrients support a limited primary production, and serve as the basis for building up a food chain which, in turn, benefits the rising generation of fish. The greater and more differentiated is the biomass in these electrolyte-poor water bodies, the more stable seems to be the biocenosis. For the more numerous and varied are the single components of the biotic communities, the more efficient must be the biological filters for the allochthonous sources of nutrition which they form. THE AMAZON REGION differs from the humid tropics of the Old World, among other ways, in that man has not yet succeeded in visibly altering the natural landscape. Rain forest, so far undisturbed, extends over millions of square kilometers and encloses uncounted smaller and larger flowing waters within this gigantic drainage system. We cannot yet find human wastes which influence the biocenoses in streams and rivers. However, the few people who do live there (ca. 0.3 per kiM2) have been able to effect great changes in other ways in the natural biological communities of the flowing waters. They have, in the last decades, almost exterminated a number of formerly very abundant species of aquatic animals, and thereby, it appears, removed important components of the ecosystem. In the first place among nearly extinct animals stands the caiman, whose ecological significance must have been very great. Other seriously threatened animals include turtles, otters, and manatees. In several areas, the populations of the economically very important fish, Api'apaima gigas Cuv., have noticeably decreased.