So many trees

Why is tree biodiversity so large around the equator, moderate at mid-latitudes and monotonous at higher ones? As this is a global phenomenon most possible explanations involve long-term or large-scale mechanisms, such as climate stability (no glaciers in the tropics), rates of speciation (higher in the tropics) or rates of extinction (lower in the tropics according to the fossil record).

In order to explain the high tree species biodiversity in tropical rainforests Daniel Janzen and Joseph Connell independently suggested a very different mechanism that operates at a much smaller scale (conspecific negative density dependence - CNDD). They proposed that host-specific natural enemies which kill seeds and seedlings clumped near parent trees might keep locally common species from dominating a forest and give locally rare species space to flourish. This Janzen-Connell hypothesis is now nearly 50 years old, but it has been hard to evaluate, especially at the global scale. 

Utilizing the Smithsonian Center for Tropical Forest Science-Forest Global Earth Observatory (CTFS-ForestGEO) researchers now analyzed the data from 24 forest plots. Together these plots are home to more than 3,000 tree species and roughly 2.4 million trees. The analysis provided the first evidence that the Janzen-Connell effect contributes to the biodiversity gradient across tropical and temperate latitudes.

The results of the new study show that global patterns in tree species diversity reflect not only stronger CNDD at tropical versus temperate latitudes but also a latitudinal shift in the relationship between CNDD and species abundance. CNDD was stronger for rare species at tropical versus temperate latitudes, potentially causing the persistence of greater numbers of rare species in the tropics.

The key observation on which the Janzen-Connell hypothesis is based is that seedfall is heaviest under a parent tree but the young tend to do better away from their parent. In the tropics, this is stronger for rare species than for common ones. In the temperate zone rare and common species are equally affected, or in some cases it flips and becomes stronger for the common species than the rare ones. The colleagues find this result exciting because it may explain a puzzling characteristic of tropical forests - their diversity is due not to large numbers of species in general but rather to large numbers of rare species.

How can you pack more than a thousand species in a 50-hectare plot in the tropics if the rare species are being negatively impacted by these specialized enemies? You'd think that if these species are rare they'd be more likely to go extinct, so what maintains them in the system? ...Paradoxically, enemies can be beneficial, they kill, but by killing they prevent population booms and busts. If you have no enemies, you're going to have exponential population growth followed by a crash. If you add an enemy that tracks abundance, over time the population stabilizes. It's never going to become large, but the flip side is it's never going to crash. And so these enemies are a stabilizing force.