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Abstract:
The example of the Central Amazonian rain forest is used to illustrate an ecological
principle which determines the structure of a tropical ecosystem with a low, but balanced
supply of nutrients. This principle implies that the stability of the biocenosis of such a system increases with the abundance and diversity of its species of biota, that is, the degree
of differentiation and the size of its biomass. Higher abundance and diversity enable the
system to take up the available allochthonous nutrients, to conserve them, and to continuously hold them in shortened nutrient cycles as effìciently as possible. In the Central
Amazonian rain forest, we have an extremely large and diverse plant biomass (fresh
weight), which assimilates the allochthonous nutrients supplied in the rain.
The same principle may also be recognized in the aquatic ecosystems, where the geochemical impoverishment of the Central Amazonian "terra firme" (that land which is not
inundated by the annual floods of the main river system) is expressed even more severely
than in the terrestrial realm. The low levels of electrolytes in the waters of such systems
scarcely permit any primary production, so that here, the food chain is based directly or
indirectly on allochthonous organic material. Formation of a strongly differentiated and
relatively large biomass, which is mostly due to animals in macroscopic size classes, occurs
only where the supply of organic material is relatively great, i.e., in the narrow upper reaches of the streams. A large number of invertebrate animals, especially insect larvae, live
on bacteria and fungi. These decompose the materials such as leaves and wood, which are
poor in nutrients and not directly available to the fauna. The fishes, which make up most
of the biomass act partly as direct filters for the allochthonous materials richer in nutrients,
such as insects, flowers, fruit. etc. which are also contributed to the aquatic systems by the
forest. As the supply of allochthonous organic material decreases relative to surface area
and volume along the courses of the streams, the aquatic biocenosis becomes qualitatively
and quantitatively poorer.
A new ecological situation arises in the lower reaches, which are annually backed up to
form lakes. The main difference is the immigration of brooding fishes from the bordering
waters of the main river system, which have high levels of nutrients and primary production. This introduces allochthonous nutrients and permits the existence of a much larger
biomass. The "filters" for this periodical supply of nutrients are primarily the caimans, in
addition to large piscivorous fishes, turtles, otters and dolphins. In "river-mouth lakes"
which have not been disturbed by man, these predators must constitute a large proportion
of the biomass.
In the Central Amazonian "river lakes", local settlers believe that the fish crops have declined following extermination of the caimans. The ecological interactions which must have
existed between caimans and fishes were presented in a previous study. The present work
is an attempt to determine whether the amounts of nutrients transferred by the caimans
from their largely allochthonous prey, during periods of rapid growth or in longer periods
of starvation, are sufficient to influence the ecosystem. The experiments showed that medium-sized caimans daily eat an amount equal to about 0.6 to 0.8 % of their body weight. The total amounts of N, P, Ca, Mg, Na and K released daily under such feeding conditions
are about 0.20 to o.27 % of their weight; when the animals are starving, this fraction is reduced to 0.08 to 0.10 % of the body weight.
Based on the situation of an hypothetical (but realistic) lake with a surface area of
5 km² and a mean depth of 5 m at medium high water, the following amounts would be
released over a 10-day period (in γ/l):
a) with good nourishment: N-34, P-9.6, Ca-1.5, Mg-0.1, Na-0.1, K 5..0;
b) during starvation: N-17, P-1.5, Ca-0.9, Mg-0.7, Na-0.5, K-1.5.
Over the same period the lake would receive in the rain among other materials, (y/l):
N-14.9, P-0.4, Ca-5.4, Mg-4.5. The groups of nutrients added by caimans and rain complement each other in the biologically important elements, those which are present in limiting amounts in river water. (River water concentrations in y/l : N-280; P-7, Ca-0, Mg- less than 20).
These values show that caimans daily add nutrients (mostly of allochthonous origin) in
quantities that are sufficient to enable an elevation of primary production and thereby an
enlargement of the autochthonous food chain based on it. With their ability to undergo
long periods of starvation, during which accumulated nutrients are continuously released,
the animals are especially well adapted to the ecologically extreme environment of the
river-lake.
