Coconut palms block nutrient flow in tropical ecosystems
Journal Watch Online 25 Jan 10;
The sight of coconut palms on tropical islands is so common that it’s hard to believe these trees are invasive transplants from Asia. And while their presence might be good for tourism, it turns out that the palms also prevent valuable nutrients from reaching the soil.
As described in the Proceedings of the National Academy of Sciences, researchers studied an ecosystem with coconut palms (Cocos nucifera) on an atoll in the Pacific. Birds shied away from landing on the palms, instead nesting and roosting eight times more densely in forests with mainly native trees. The birds might avoid coconut palms because these trees don’t offer “platforms” to nest on and can house predatory rats, the team says.
Since birds typically shower the soil in nutrient-rich guano, their absence has deprived the ground of nitrate and phosphate, the researchers report. That depletion also caused some plant leaves to contain fewer nutrients, making them less appetizing to herbivorous animals. In lab experiments, katydids ate 12 times more leaf material from a forest dominated by native trees than from coconut palm forests, and crabs ate 23 times as much.
The case of the coconut palm counters the usual notion that introduced plants add nutrients to the soil, the researchers say. Since this ubiquitous species can have such widespread ecological effects, they write, “the implications of the global expansion of the coconut should be more closely monitored and considered.” – Roberta Kwok
Source: Young, H., McCauley, D., Dunbar, R., & Dirzo, R. (2010). Plants cause ecosystem nutrient depletion via the interruption of bird-derived spatial subsidies Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0914169107