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American Association for the Advancement of Science

Human activity echoes through the Brazilian rainforest

A toucanet (Selenidera maculirostris) eating palm fruits in the Atlantic forest. These large birds play a fundamental role in the forest as seed dispersers of large seeds.
[Image courtesy of Edson Endrigo]

The disappearance of large, fruit-eating birds from tropical rainforests in Brazil has caused the forests' palm trees to produce smaller, weaker seeds over the past century, researchers say. The finding makes it clear that human activity can trigger fast-paced evolutionary changes in natural ecosystems.

Mauro Galetti and researchers from Brazil, Mexico and Spain collected more than 9,000 seeds from 22 different palm populations in patches of the Brazilian rainforest—some of which had been disrupted by coffee and sugar cane development during the 1800's and others that remained undisturbed. The researchers then used a combination of statistics, genetics and models of evolution to show that the loss of large-beaked birds was to blame for the observed decrease in palm seed size.

The researchers considered many environmental factors, like climate, soil fertility and forest cover, but in the end, only the absence of large, seed-carrying birds could account for the smaller seeds in the area. Genetic studies revealed that this decrease in seed size among forest palms could have taken place within the past 100 years.

In light of their findings, Galetti and the other researchers suggest that the conversion of tropical forest for agriculture—a change that displaced many large bird populations—triggered a rapid evolution of forest palms that resulted in smaller, less successful seeds. Long periods of drought and warmer climate, which is predicted by climate models of South America, could be particularly harmful to tropical plants that rely on animals to spread their seeds, the researchers warn.