“Ant mutualism increases long-term growth and survival of a common Amazonian tree”

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Selene Báez, David A. Donoso, Simon A. Queenborough, Liliana Jaramillo, Renato Valencia, and Olivier Dangles

Interactions with mutualist ants determine the establishment and decay of Amazonian ‘devil’s gardens’

A devil’s garden or supay chakra in the Yasuni Forest Dynamics Plot in Ecuador.
(Credit: Esteban Baus C., biologist and photographer, Pontificia Universidad Católica del Ecuador)

So-called “devil’s gardens” are a striking feature of Amazonian rain forests. These clearings in otherwise crowded and high-diversity forest are centered around a single species of tree and maintained by a single species of ant. The ants live in hollow swellings in the tree branches and keep the clearings (supay chakras in the native Quichua) free of other tree seedlings. The tree (Duroia hirsuta, of the coffee family Rubiaceae) and the ant (Myrmelachista schumanni) live in a mutualistic relationship, presumably of equal benefit to both partners. However, the origin and dynamics—and the specific benefits—of these gardens have remained a mystery.

A team of researchers based at institutions in Ecuador (Universidad Técnica Particular de Loja, Pontificia Universidad Católica del Ecuador, and Escuela Politécnica Nacional), France (Institut de Recherche pour le Développement (IRD-CNRS-Paris Saclay)), and the US (Yale School of Forestry and Environmental Studies) took advantage of a long-term project located in the Yasuni National Park in Ecuador and led by the Universidad Católica del Ecuador in Quito, to explore how the mutualist ants influence their tree hosts—information that could only be uncovered over many years and across large areas of forest. By monitoring the growth of Duroia trees and the ants they hosted over 18 years in a 25-ha forest plot, they found that trees with the mutualist ant grew twice as fast and had higher survival than trees without the ant and that this benefit was greater in smaller trees with many neighbors. Conversely, larger trees grew better without the mutualist ant and were less affected by neighboring trees.

These findings indicate that the establishment and decline of devil’s gardens is tightly linked to the trees’ relationship with their ant partners. The role of the mutualist ant is extremely important during the establishment of the devil’s garden, as it promotes Duroia tree growth and reduces mortality and competition among young trees. As the devil’s garden ages, the trees tend to host other types of mutualist ants (possibly less costly or better defenders) that do not benefit young trees. Ultimately, the gardens may disappear once all the trees within it contain other species of ants that do not make the clearings and provide little benefit to new trees. Long-term monitoring of plant and ant populations is therefore essential in order to understand how changes in the relationship between ants and their hosts may affect the number and size of devil’s gardens, and the structure and function of tropical forests more broadly. Read the Article