Functional groups, determinism and the dynamics of a tropical forest

  1. Unravelling the drivers of forests dynamics is one of the main challenges in tree community ecology. These drivers include niche differentiation, dispersal limitation and stochasticity. Previous work has demonstrated that these mechanisms likely interact such that no one process is responsible for forest dynamics.
  2. One possibility is that the functional composition in a forest changes in a deterministic fashion, but the abundances of individual species that are functionally similar and have similar life-history strategies drift in a neutral fashion. This framework aligns with the functional group-based version of the neutral theory proposed more than 30 years ago, but it has remained poorly understood and is not well-integrated in tree community ecology.
  3. To investigate the possibility that determinism and neutrality may operate on the functional and species levels, respectively, we studied the long-term dynamics of trees on Barro Colorado Island, Panama. Specifically, we sorted tree species into functional groups. We defined them as groups of species that cluster together based upon continuous functional trait measurements that are believed to reflect key life-history trade-offs. This information was then used to quantify the observed species and functional group dynamics in the forest and to compare them to that expected from neutral simulations.
  4. We found that forest dynamics are likely governed by deterministic processes at the between-functional group level where species relative abundances change or drift through time within group. Species rank distributions for each functional group remained relatively stable suggesting that these groups may act as broad adaptive zones for both common and rare species that may promote species coexistence. Moreover, we found that these functional groups associate with different habitats in the forest.
  5. Synthesis. The results indicate that deterministic processes control the relative proportion of each functional group in the tropical forest studied and that individual species within groups may have apparently neutral dynamics. Furthermore, the results demonstrate that observed stable functional group relative proportions can be produced by deterministic or neutral dynamics, which underscores the importance of comparing the observed functional dynamics to that expected under neutral dynamics.
Vanessa Rubio & Nathan G. Swenson
Journal of Ecology