Rainforest diversity regulators

Dictyophora sp. (credit Bob Thomas)

Tropical forests are important reservoirs of biodiversity, but the processes that maintain this diversity remain poorly understood. The Janzen–Connell hypothesis suggests that specialized natural enemies such as insect herbivores and fungal pathogens maintain high diversity by elevating mortality when plant species occur at high density (negative density dependence; NDD). NDD has been detected widely in tropical forests, but the prediction that NDD caused by insects and pathogens has a community-wide role in maintaining tropical plant diversity remains untested. 

This is the intro to a new study that showed that fungi, often seen as pests, play a crucial role policing biodiversity in rainforests. Researchers found that fungi regulate diversity in rainforests by making dominant species victims of their own success. Fungi spread quickly between closely-packed plants of the same species, preventing them from dominating and enabling a wider range of species to flourish. Seedlings growing near plants of the same species are more likely to die. It has long been suspected that something in the soil is responsible, and the new study shows that fungi play a crucial role. If lots of plants from one species grow in the same place, fungi quickly cut their population down to size, leveling the playing field to give rarer species a chance. 

The researchers sprayed plots in the rainforest of Belize with water, insecticide or fungicide every week for 17 months. They found that fungicides dealt a significant blow to diversity, reducing the effective number of species by 16%. While insecticides did change the composition of surviving species, they did not have an overall impact on diversity. The results suggest that insects disproportionately kill certain plant species, reducing their abundances during the transition from seeds to seedlings. Insects thus strongly influence the structure of plant communities in this forest; however, by doing so relatively independently of plant density, their net effect on plant species diversity is small. 

The initial expectation was that the removal of both fungi and insects would have an effect on the tree species present but it was actually only the removal of the fungi that affected diversity. It was also suspected that the fungus-like oomycetes might play a part in policing rainforest diversity, but treatments with substances that kill oomycetes showed no significant effect on the number of surviving species, suggesting that true fungi and not oomycetes are driving rainforest diversity.

Our experiments highlight that both insect herbivores and pathogens help structure tropical plant communities at the early stages of community assembly and provide support for a pivotal role for natural-enemy-mediated NDD in maintaining species diversity in this tropical forest. Although the magnitude of the NDD we observed was relatively small, this study was conducted over a relatively short timescale (17 months) in a tropical forest of relatively low plant species diversity (approximately 320 tree species have been recorded in the reserve). The effects of NDD will probably accumulate over time, and may be stronger in more species-rich forests. Indeed, similar experiments in other forests are now needed to evaluate the generality of the Janzen–Connell hypothesis as an explanation for variation in species diversity among tropical plant communities.