We have just published a review paper, led by Lisa Hülsmann of the University of Regensburg, about conspecific negative density dependence and its ability to explain tree diversity. The predominant pattern in global tree diversity is increased species richness towards the tropics. One proposed explanation for this is the greater climatic stability of tropical forests, which allows greater prevalence of pests (e.g., herbivorous insects and fungi), which in turn keep the abundances of their host tree species in check, thus maintaining overall tree diversity. For this mechanism to work, a pest must have greater per-tree impacts when the host tree is at high population density. This is an example of a more general phenomenon called conspecific negative density dependence.
The idea that pests maintain the tree diversity of tropical forests was proposed 50 years ago by Daniel Janzen and Joseph Connell and eventually became known as the Janzen–Connell hypothesis. In the years since, many empirical studies have reported that tree species do suffer more when surrounded by individuals of their own species, consistent with the hypothesis. These observations have provoked optimism among forest ecologists that the Janzen–Connell hypothesis is close to proven.
In our review, we present a more cautious appraisal. Our summary of the current state of knowledge reveals two important unresolved questions. Firstly, it is not clear whether the effect of neighbouring conspecific trees is strong enough to have a substantial influence on the overall tree diversity in a forest. Secondly, it is not yet possible to say whether the regulatory effect is indeed stronger or more frequent in the tropics.
We conclude that the explanation of Janzen and Connell remains a hypothesis yet to be proven. More precisely, although the existence of the mechanism is relatively well established, its importance in comparison to many other alternative explanations for tropical tree diversity remains unclear. To weigh these hypotheses against each other and to test the Janzen–Connell hypothesis in its entirety, new data and collaborations between experimental and theoretical ecologists will be necessary.