Friday, May 12, 2006
Bunker and Naeem have written a letter highlighting the importance of the recent study by Wills reported earlier by this blog. The Wills study noted that nonrandom processes play a key role in maintaining diversity in tropical forests -- forest tree diversity increases as individuals age because of preferential survival by individuals of locally rare species.
Bunker and Naeem argue that all three mechanisms supported by those results imply that species diversity increases ecosystem functioning:
First, the Janzen-Connell model (1, 2) predicts that species escape their specialist herbivores, predators, and pathogens when they are locally rare, whereas common species are more readily attacked. Losses of carbon and nutrients to natural enemies result in lower growth rates and thus lower primary productivity (3). Second, niche complementarity occurs when species exploit resources in different ways and results in more complete resource utilization and thus higher productivity (4) and has been shown to contribute to increased functioning with diversity (5, 6). In tropical forests, tree species may differ in their ability to acquire soil resources, resulting in more complete resource capture and thus higher productivity. Third, facilitation occurs when one species directly benefits another but experiences no harm (7) and has been shown to contribute to increases in ecosystem functioning with diversity (8, 9). In tropical forests, a tree species might fix nitrogen that becomes available to its neighbors. In this case, the tree's neighbors will experience increased growth rates if nitrogen is limiting.
When any of these three nonrandom mechanisms are operating, species extinctions will result in a decrease in productivity due to increased losses to natural enemies, failure to fully utilize essential resources, or the loss of direct benefits of facilitation. In contrast, neutral models of species coexistence (10-12) assume species are functionally equivalent, and therefore ecosystem functioning will not be dependent on species diversity. Wills and colleagues provide strong circumstantial evidence that even in highly diverse tropical forests, biodiversity enhances ecosystem functioning.