‘Species Repulsion’ Enables High Biodiversity in Tropical Trees

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.

Social Distancing for Trees

This idea of repulsion — formally known as conspecific negative density dependence, or CNDD — goes back to the 1970s when the ecologists Daniel Janzen and Joseph Connell independently suggested that insects, herbivores and pathogens that prey selectively on one species could make the area around an adult tree dangerous for its seeds. Other species would not be prevented as effectively from growing in the area, though they would still be limited by nonspecific problems like a lack of sunlight under an adult tree’s canopy. The result would be that adult trees of a species would tend to maintain a kind of minimum “social distance” from each other.

Tests in recent decades, primarily on seedlings, provide support for CNDD, said Michael Kalyuzhny, the lead author of the new paper, whose work as a postdoctoral researcher at the University of Texas, Austin, has led to his now starting a lab at the Hebrew University of Jerusalem. Seeds often don’t grow as well in soil taken from spots near their parent trees as they do in soil from spots near unrelated trees.

Yet even a cursory look at the BCI data shows that adult trees in that forest do not seem to be repelling one another. Instead, the trees cluster, forming loose groups of the same species throughout the plot.

Kalyuzhny and Annette Ostling, the community ecologist who leads the lab where Kalyuzhny works, set out to model different scenarios that might explain the observed tree distributions, to learn whether something might be obscuring the role of CNDD in the forest.

Chance alone wasn’t the answer: When they ran a null model in which trees were just spaced randomly, the adult trees in a species would get too spread out.

So the researchers introduced the effect of seed dispersal into their models, which could be by wind and by birds and other creatures. Helene Muller-Landau and her collaborators at BCI have made estimates of how far a seed can travel from its source within the 50-hectare plot, using nets that capture seeds as they drift to the forest floor. The seeds are regularly collected and the distance from their parents is calculated. Kalyuzhny and his colleagues drew on that data to model tree arrangements shaped only by dispersal limitation.

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