For ecologists, tropical rainforests hold many enigmas. A single hectare can contain hundreds of tree species, far more than in forests closer to the poles. Somehow these species coexist in such dizzying abundance that, as naturalists and ecologists have sometimes noted, tropical forests can feel like botanical gardens, where every plant is something new.
For such throngs of species to be packed so densely, they must coexist in a very particular balance. Evolution seems not to favor situations where any single species thrives too aggressively, instead favoring ones where organisms are surrounded by species other than their own. Squaring those facts with what’s understood about how species distribute themselves, compete and influence one another is a challenge.
To study this extraordinary diversity, years ago scientists began to set up plots of forest where they could record and track the location and condition of every single tree over decades. One of the earliest such plots, at Barro Colorado Island (BCI) in Panama, is 500 meters wide by 1,000 meters long (the area of about 70 soccer fields) and contains more than 300 species. Since 1980, researchers around the world have pored over the detailed records of its inhabitants.
In a paper recently published in Science, researchers at the University of Texas, Austin modeled several distribution scenarios and compared them to BCI data. They found that patterns in the dispersal of seeds on the wind or by birds and other wildlife, as well as more random processes, were not sufficient to explain the distribution of adult trees in the forest. They suggest that this is evidence for “species-specific repulsion,” a long-standing theory that trees of the same species get naturally spaced out because the environment immediately around a parent tree is specifically hostile to the tree’s own offspring. [Continue reading…]