Toucans and other large fruit-eating birds have almost disappeared from many tropical forests in Brazil after losing their habitat to deforestation. Their dispersal has rippled through the forest ecosystem in far-reaching and unexpected ways, affecting the character of a palm tree critical to the survival of other species, according to a study published today in the journal Science.
Agriculture and other human activities have had a profound effect on the global ecosystem; some biologists have even gone so far as to say that humans are the cause of a sixth mass extinction in animal and plant life, akin to the one that took out the dinosaurs. Earlier research from 2001, 2008 and 2011 has demonstrated that human activity is responsible for rapid evolutionary changes in insects, microorganisms and commercially exploited species of fish. The current study, lead by researchers from the Universidade Estadual Paulista in Brazil, provides further evidence that humans are able to set off a “cascading effect” through large natural ecosystems.
In the 1800s, humans began cutting down the Brazilian rainforest and using the land for farming, growing mainly coffee and sugar cane. Toucans and other large fruit eating birds like toucanets and large cotingas need to live in expansive tracts of forest, and are unable to thrive in the smaller forest fragments broken up by agriculture. The birds are essential players in the reproduction of the palm tree species Euterpe edulis — their wide throat openings are the only ones big enough to swallow the palm’s larger seeds, about the size of a golf ball, which they then disperse by excretion or regurgitation. Scientists believe that seed size is related to the plant’s survival prospects, with larger seeds producing trees more likely to sprout, grow, and produce more seeds themselves.
The researchers compared the seed size of 22 palm populations in areas of the Brazilian Atlantic forest where large fruit-eating birds still reside and areas where they have fled. In the absence of toucans and other large birds, small-throated thrushes become the main dispersers of the palm seeds. Normally, the thrushes consume 33 percent of the fruits in a given area, but when the large fruit-eating birds are driven out, the hardier thrushes consume up to 98 percent of the fruits. Their small throat openings can’t handle the palm’s larger seeds, however, and so the smaller seeds become more likely to be passed on. Indeed, the researchers said that the seeds were significantly smaller at sites depopulated of large birds.
Since seed size can be associated with many environmental factors, the scientists also examined other ecological variables, such as climate, soil fertility, and forest cover, but only the population of fruit-eating birds had a large enough effect to explain the discrepancy.
Smaller seeds produce weaker trees, and are more vulnerable to drying out in hot weather before they can germinate. After confirming the reduction in larger seeds at the thrush dominated forest sites, the researchers performed a genetic analysis to determine the number of generations needed to account for the change. They found that the selection for smaller seeds could have occurred less than 100 years after an initial disturbance — a timeline that concords with the advent of large-scale farming and deforestation in the area.
At the same time, there is evidence that humans have accelerated global warming, leading to an upturn in extreme weather events and drought in South America. If Euterpe edulis must increasingly rely on smaller seeds for reproductive success, its future will become even more precarious. As a keystone species, the palm supplies food for birds and small mammals year-round, and its loss could set off a chain reaction that would reverberate up the food chain. The study highlights the awesome power humans have to drive evolutionary changes in whole ecosystems, and suggests we should be more careful with how we wield it.