Only bystanders? The involvement of plants in their interactions with their environment

It is easy to think of plants as static organisms in the ecosystem. As you walk through the forest and look for plants, you do not have to carefully monitor the movement of the organisms or listen for a particular song. The plants are there, waiting for you to observe them. While they sit still, it does not been that millions of things are not happening inside the organism and that they do not react their environment as any bird, insect or mammal do.

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A view of the forest (Photo by Anne-Sophie Caron)

While at BCI, we started our immersion in tropical biology with a series of lectures on tropical plant ecology, with a particular interest in the interaction of plants with their environment, especially insect herbivores, micro organisms and soil composition.

Tom Kursar and Lissy Coley have shown with their studies the importance of secondary metabolites in the ecology and evolution of tropical trees. Their work on the genus Inga has shown that herbivore could be selecting for certain defenses and that this could help explain why so many species of this same genus can be coexisting. In fact, the chemical defenses present in Inga are not what we could expect at first. Species that are closely related do not necessarily have similar defenses. It seems these trees have some key metabolic switchpoints where they can turn on and off the production of certain compounds. Therefore, they can respond faster and more drastically to the evolutionary pressure insects put on them. This process results in the different species having different defensive profiles and occupying a different “herbivore niche”, even though they otherwise play similar roles in the environment.

Scott Mangan and Andy Jones’ lectures highlighted the importance of the interaction between plants, the soil they grow on and, especially, the soil microbial community that is at the center of this interaction. Whereas aboveground plant composition can determine the soil microbial community, it seems to be more important in determining the fungi community. On the other side, the same microbial community can be detrimental to some tree species, as some species react more strongly as saplings. This stage in the tree’s life is especially important and the survival at this stage can determine which species while make up the forest and their abundance.

Finally, Brian Sedio, who studies diversity of the genus Psychotria, used a novel approach to address the evolution of plant defenses. Using Mass Spectroscopy, he creates molecular network of the different compounds found in the different Psychotria species. Using this method means you don’t have to isolate every compound since you can show the link between those using the molecular building blocks they are made of. By overlaying the plant and herbivore data on top of this network, you can investigate the interaction that result from the chemistry. In this particular context, there was an influence of compound on herbivore host use (even though the data is made up of a high number of singletons) and on co-occurrence of closely related plant species (as in Inga).

After hearing about such research, it is hard to continue ignoring the interconnectedness of plants with their environment and how much they can influence each other as well as the species that depend on them.

References:

Kursar, T. A., Dexter, K. G., Lokvam, J., Pennington, R. T., Richardson, J. E., Weber, M. G., … & Coley, P. D. (2009). The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga.Proceedings of the National Academy of Sciences, 106(43), 18073-18078.

Coley, P. D., & Kursar, T. A. (2014). On tropical forests and their pests.Science, 343(6166), 35-36.

Barberán, A., K.L. McGuire, J.A. Wolf, F.A. Jones, S.J. Wright, B.L. Turner, A. Essene, S.P. Hubbell, B.C. Faircloth, N. Fierer. 2015. Relating belowground microbial composition to the taxonomic, phylogenetic, and functional trait distributions of trees in a tropical forest. Ecology Letters. 18: 1397-1405.

Mangan, S. A., Schnitzer, S. A., Herre, E. A., Mack, K. M., Valencia, M. C., Sanchez, E. I., & Bever, J. D. (2010). Negative plant-soil feedback predicts tree-species relative abundance in a tropical forest. Nature, 466(7307), 752-755.

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