Talk about not being able to stand your relatives. Cinnamomum triplinerve, a member of the avocado family known as the sigua blanca tree, is less likely to survive if it grows too close to members of the same species, according to a new study. Another avocado relative, Beilschmiedia pendula, meanwhile, doesn't have this problem. The finding helps explain why some species are much more common than others in tropical forests.
Liza Comita, an ecologist at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California, wanted to test a hypothesis that's been knocking around ecology for decades: seedlings do better when they grow farther from members of their own species. Experiments have supported this. If you plant seedlings at varying distances from an adult tree, the more distant ones are more likely to survive. But Comita wanted to find out whether the finding held for lots of species simultaneously, in a tropical forest.
She and colleagues looked at about 30,000 seedlings of 180 tree species in a 50-hectare, long-term plot on Barro Colorado Island in Panama. In 2001, 20,000 one-meter-square subplots were laid out in the forest, and every seedling in them was measured and labeled. Every year or so, a team of field assistants goes out and resurveys the seedlings. Comita looked to see how many of the seedlings from 2001 were still alive in 2006, whether they had other seedlings of the same or different species in the plot with them, and what adult trees were within 30 meters. The researchers found that seedlings were more likely to survive if they were farther away from adults or other seedlings of the same species.
But based on some earlier analysis, Comita suspected there would also be a difference between rare and common tree species. And she found it: Seedlings of rarer species were less likely to survive having a neighbor of the same species than were seedlings of common species, her team reports online this week in Science. Seedlings of common species seemed more comfortable being packed in, which could help explain how they're able to become more common.
Nobody knows what keeps trees of the same species from growing close together. The most popular hypothesis is that the farther a seedling is from members of its own species, the better its chance of avoiding enemies—viruses, leaf-eating insects, and so on—that only attack its species. "When one kid in a preschool gets a cold, it spreads to all the other kids because they're at high density and they're all susceptible," Comita says. "Seedlings of different species are not equally susceptible to that pathogen."
Timothy Paine, a community ecologist at the University of Zürich in Switzerland, says he was surprised to see that rarer species were more sensitive to neighbors, because common species would seem like such an appealing target for pathogens and insects. "I would have expected that the more common species would be more hard hit. They're the ones that ought to have more transmissible diseases," he says.
Further experiments could show whether common species are more resistant to diseases and insects. Another new study published today in Nature, suggests that this is the case. When researchers grew seedlings in the presence of soil taken from under a tree of the same species and from a tree of a different species, they found that the rare species didn't grow as well in the presence of soil from a tree of the same species, while common species had less of a problem. Those researchers think soil pathogens are to blame.