The study was co-authored by Liza Comita, an assistant professor of tropical forest ecology at F&ES.



The Janzen-Connell hypothesis, a widely recognized theory introduced in the early 1970s, proposes that a diversity of trees coexist because specialized “natural enemies” — whether they’re herbivorous insects or fungal pathogens — reduce the survival rates of seeds and seedlings. This outcome is especially prevalent when seeds are dispersed near their parents, or near members of the same species, where these natural enemies are more likely to be found. The phenomenon is called conspecific negative density dependence (CNDD).



According to the hypothesis, rare species benefit more from this phenomenon — since there tends to be more patches of earth “free” of their enemies compared with common plant species — giving them a better survival chance than common seedlings. This, theoretically, prevents any species from becoming too common or too rare.



In recent years, however, scientists have found that the relationship tends not to be the same for all species, Stump said. “Increasingly people have found that some species are largely immune to these natural enemies while some other species are much more susceptible.”



To test the theory, Stump and Comita developed a theoretical model that simulates competition between trees. It predicts how a species’ vulnerability to its “natural enemies” affects whether it goes extinct, and how patterns of seed dispersal affect these outcomes.



According to their findings, communities are less stable if rare plant species are more susceptible to natural enemies, particularly when dispersal is low. Furthermore, they found that variation in the sensitivity to enemies is potentially more harmful to diversity than many other competitive factors.