Big-Mouthed Toucans Key To Forest Evolution

By Christopher Joyce

Published May 31, 2013 at 5:00 AM EDT

Channel-billed toucans are important seed dispersers in rain forests.

Brazil is a paradise for birds; the country has more than 1,700 species. Among them is the colorful toucan, a bird with an almost comically giant bill that can be half as long as its body. There are lots of different types of toucan — red-breasted, channel-billed, keel-billed, saffron toucanet — each with its own color-scheme and distinctive call.

The jucara is the dominant palm tree in the Brazilian Atlantic forest. Its seeds are dispersed mostly by birds.

Unfortunately, as more humans have moved into Brazil's Atlantic coastal forests, the increase in hunting, logging and farming has taken a toll on the number of toucans. Now scientists have discovered that the drop-off in birds is reshaping the forest as well.

According to research published this week in the journal Science, here's what happened: The forest's dominant palm tree, called the jucara ( Euterpe edulis),depends on birds to eat its fruit and then defecate the seed in a place where it can germinate and grow to become a new tree. According to Mauro Galetti, a biologist at the Estadual Paulista University in Brazil who studies the process, the toucans give jucara palms with large seeds a reproductive edge.

"These large birds, they have an extremely important role in the forest," Galetti says, "not only because they disperse seeds, but because they disperse large seeds."

The largest palm seeds — some the size of marbles inside a thin-skinned fruit — are too big for most birds, except for those big-mouthed toucans. So as the toucans disappeared, the palms that made big seeds were out of luck — no seed dispersers. Meanwhile, jucara trees that were genetically predisposed to make small seeds did just fine. In fact, they thrived and started to dominate the forest.

"The extinction of large birds changed the evolution of this palm," Galetti says. His team checked other possible causes of evolutionary change — shifts in soil fertility or rainfall, or in other seed dispersers. But the genetic change in the palm populations occurred most often where the toucans were scarcest.

A yellow-legged thrush eats the fruit of the jucara tree. These birds eat only small seeds and spread them throughout the Atlantic rain forests.

Now, humans have always been messing with nature. When we cut down forests or prairie or when we overhunt, we change a region's mix of plants and animals. That's an "ecological" change, Galetti explains. But in this case, humans actually altered the genetic makeup of a wild palm tree population — in just a century, indirectly, and by accident.

So what happens if these small-seed palms eventually take over? Well, it turns out that smaller seeds aren't so good. They dry up and die faster than big ones in hot, dry weather. And scientists predict that climate change will make parts of Brazil hotter and drier, so much so that the jucara may not survive. Says Galetti: "The impacts on the forest could be quite dramatic because several animal species ... rely on this palm for food." In fact, about 60 species of animals depend on this palm for food. So the take-home message is: As the toucan goes, so goes the forest.