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The "chicken or the egg" of the ecologists

New York Times

This is the ecologists' version of the chicken-and-egg question: Is it the presence of the carnivores that is responsible for creating a diverse and healthy ecosystem, by preventing the plant-eating population from multiplying and allowing the plants to thrive, or is it the vegetation that supports this diversity, by providing food in an amount that keeps the plant-eaters and their predators in the environment ? In other words, is the regulation of the ecosystem done from the top of the food pyramid (by the predators) or from the base (by the plants)?

Scientists from Duke University and eight other research institutions tried to shed light on this issue. Their research laboratory – a group of small islands in a lake in Venezuela – was ideal. The lake was created by a hydroelectric dam, and the islands were created when the lake filled; Each island is actually a miniature and isolated ecosystem. Because the islands are so small (their size ranges from about 2 dunams to about 100 dunams), large predators are absent from them - those that feed on other vertebrates. There are only insect, seed and plant eaters, including birds, howler monkeys and leaf-cutter ants.

The researchers, who described their work in the journal "Science" conducted a survey of the population of plants and animals on the islands for several years, and found that in the absence of predators the animals that eat the plants and seeds thrived, including the ants: the density of the animal populations on the islands was 10 to 100 times greater than that on the mainland . It was also found that the density of the saplings and young trees on these islands was considerably less than that on the mainland.
These findings, the scientists say, provide evidence that the lack of control from the top of the pyramid creates anarchy in the lower ecological layers.

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