Tropical forests are biodiversity hotspots with dozens of tree species per hectare. The mechanisms maintaining such high levels of species richness have been studied for many decades. Previous studies have found that an important process is random changes in environmental conditions over time (temporal environmental stochasticity), and that simple parsimonious models that include this process can explain both static and dynamic patterns of tree diversity. In a previous paper led by Tak, we explored in detail how a changing temporal environment affects species diversity in a large-scale community. In a new paper just published in Journal of Ecology, we extended this work to a small-scale community that, in addition to being subject to a temporally varying environment, receives substantial immigration from a larger scale. This is important because tropical forest dynamics are often studied at small scales (e.g., 50 ha plots) where immigration is important.

Our main finding was that immigration substantially dampens the effect of temporal environmental variation on the species richness of an ecological community. We confirmed this with an application to the 50 ha forest plot on Barro Colorado Island, Panama (see figure below). Thus, while a varying environment may have big effects on population fluctuations of individual species, it does not, somewhat surprisingly, have a big effect on species richness. We expect that our findings will generalise to other tropical forest plots, which also typically receive many immigrants dispersed from surrounding areas, and likely many other ecological communities as well.

We reached our conclusions using novel technical methods for partitioning the effects of temporal environmental variation on species richness into components associated with population-level effects and interspecies-competition effects (see figure below).

The paper was a collaboration with Nao Takashina, who spent a few months in our lab in 2016 and is now an Assistant Professor at the University of Tokyo.

Fung, T., N. Takashina, and R. A. Chisholm. 2023. Mechanistic partitioning of species richness in diverse tropical forest tree communities with immigration and temporal environmental stochasticity. Journal of Ecology