Helen FieldsScience Writer

Ecologists have struggled for years with the question of why tropical forests are so diverse. There are all kinds of hypotheses going around out there – I read papers on many of them in a class in the fall of 1996 and must admit, I’ve forgotten them. Except for one, because I just had to relearn it: Baby trees can’t grow too close to other trees of the same species, because if they do, their enemies – insect herbivores, viruses, whatever – will find them. So if I’m a tree of species X, I won’t do as well near another tree of species X, because special disease Y that only likes tree species X will find me and kill me. If all the species have the same problem, they all have to be spaced out and leave open space for other species to fill. Result: diversity.

Of course, this hypothesis is also proposed for trees in temperate forests, so I still don’t know why they’re less diverse.

But anyway. Today for ScienceNOW I wrote about a related hypothesis, about why some trees are common and some are rare: Maybe the common ones are able to get common because they’re not as susceptible to natural enemies. Maybe they’re more resistant to disease or herbivores or something, so they can survive the experience of being close to another tree of the same species.

For the paper I wrote about today, the researchers took advantage of a giant, long-term forest plot on Barro Colorado Island, a Smithsonian research site in Panama. In the early 1980s, a team laid out a 50-hectare plot and mapped every tree with a diameter of one centimeter or larger. The plot has almost 350 species of trees and shrubs.

In 2001, they took it a step further. A team of field assistants worked for 10 months to mark 20,000 one-meter-squared subplots. In each one, they measured every seedling that was 20 cm or taller and marked it with a bird band from a company that supplies poultry farms – chicken bands are made for mucky, organic environments. “The numbers really hold up in the tropics,” says ecologist Liza Comita, of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California. Every year or so, the seedlings are censused again.

She used those data to look at seedling survival over five years, and found…well, I don’t want to give away the ending. Read my story here. Credit for the funny headline goes to editor Mitch Leslie.

Photo: Liza Comita