Why trees have to leave home
Tropical forests are a challenge for ecologists, because there are so many species. It’s not just a practical challenge to identify them, but a theoretical question of why they’re all there. Why don’t the better adapted ones win out, while others go extinct? Does every species really have its own ‘niche’—some set of conditions where it outcompetes everything else? Or are the different species much the same? (I’ve discussed neutral theory before)
A third idea is that trees can’t grow too close to their parents (or others of the same species), perhaps because pests and diseases spread between them. This is called the Janzen-Connell hypothesis, after the scientists who, independently, thought of it. If each species has to space itself out to escape pests, other species can grow in the gaps, without outcompeting each other. Some new evidence backs this up.
One study analysed where thousands of seedlings were growing, on an intensely studied plot on Barro Colorado Island, Panama. After five years, saplings with others of the same species nearby were less likely to have survived. Interestingly, the effect was apparently greater for rare species than for common ones; perhaps that helps to explain why they are rare.
Another experiment looked at seedlings grown in pots, with soil taken from under the same tree species, or another one. Seedlings grown in ‘home’ soil once again fared worse, suggesting that a disease or a pest in the soil could be the key.
Owen T. Lewis (2010) Ecology: Close relatives are bad news. Nature (News & Views) 466, 698–699