Pathogens and insect herbivores drive rainforest plant diversity and composition

• Published in: Nature (London) 2014-02, Vol.506 (7486), p.85-88
• Main Authors: Bagchi, Robert, Gallery, Rachel E., Gripenberg, Sofia, Gurr, Sarah J., Narayan, Lakshmi, Addis, Claire E., Freckleton, Robert P., Lewis, Owen T.
• Format: Article
• Language: English
• Subjects: 631/158/2450; 631/158/2454; 631/158/670; 631/158/853; Animals; Belize; Biodiversity; Confidence intervals; Density dependence; Flowers & plants; Fungi - drug effects; Fungi - physiology; Fungicides; Fungicides, Industrial - pharmacology; Herbivores; Herbivory; Humanities and Social Sciences; Hypotheses; Insecta - drug effects; Insecta - physiology; Insecticides; Insecticides - pharmacology; Insects; letter; Methacrylates - pharmacology; Models, Biological; multidisciplinary; Pathogens; Pesticides; Plant communities; Plant diversity; Plant species; Pyrimidines - pharmacology; Rainforests; Science; Seedlings; Seedlings - drug effects; Seedlings - microbiology; Seedlings - parasitology; Seedlings - physiology; Seeds; Seeds - drug effects; Seeds - physiology; Species composition; Species diversity; Species richness; Strobilurins; Studies; Trees - drug effects; Trees - microbiology; Trees - parasitology; Trees - physiology; Tropical Climate; Tropical forests; Tropical plants
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12911

Suppressing fungi in a tropical forest plant community lowers diversity by reducing the negative effects of density on seedling recruitment, and removing insects increases seedling survival and alters plant community composition; this demonstrates the crucial role of pathogens and insects in maintaining and structuring tropical forest plant diversity. Plants' foes promote biodiversity The exceptional species richness of tropical rainforests takes some explanation, and one explanation that is well favoured, the Janzen–Connell effect, sounds counterintuitive. It proposes that community diversity is promoted by natural enemies such as fungal pathogens and insect herbivores on the grounds that they prevent any individual host species from becoming too common. This paper reports on experiments in the Chiquibul Forest Reserve, Belize, in which fungi and insects were removed from forest plots in controlled conditions. The results confirm that the fungal pathogens can promote high plant diversity, and that insect herbivores alter the composition of these plant communities. Tropical forests are important reservoirs of biodiversity 1 , but the processes that maintain this diversity remain poorly understood 2 . The Janzen–Connell hypothesis 3 , 4 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 5 , 6 , 7 , 8 , 9 , but the prediction that NDD caused by insects and pathogens has a community-wide role in maintaining tropical plant diversity remains untested. We show experimentally that changes in plant diversity and species composition are caused by fungal pathogens and insect herbivores. Effective plant species richness increased across the seed-to-seedling transition, corresponding to large changes in species composition 5 . Treating seeds and young seedlings with fungicides significantly reduced the diversity of the seedling assemblage, consistent with the Janzen–Connell hypothesis. Although suppressing insect herbivores using insecticides did not alter species diversity, it greatly increased seedling recruitment and caused a marked shift in seedling species composition. Overall, seedling recruitment was significantly reduced at high conspecific seed densities and this NDD was greatest for the species that were most abundant as seeds. Suppressing fungi reduc