The roots of tropical diversity


  • John W. Schroeder, Smithsonian Tropical Research Institute and University of California-Santa Barbara
  • Andrew Dobson, Princeton University, and the Santa Fe Institute
  • Daniel F. Petticord, Smithsonian Tropical Research Institute
  • Edward Allen Herre, Smithsonian Tropical Research Institute
  • Scott A. Mangan, Washington University, St. Louis, MO, USA

Andrew Dobson is a Princeton University professor of ecology and evolutionary biology. Daniel Petticord, Class of 2019, majored in ecology and evolutionary biology and earned a certificate in environmental studies.

How do the tropics maintain tree diversity? That question that has haunted tropical biologists for much of the last two centuries.

A key insight was provided 50 years ago by the work of Dan Janzen and Joe Connell, who suggested that natural enemies — particularly insects that eat the seeds of a particular tree before they are dispersed — would be most abundant at the base of trees, and this would prevent local recruitment of seedlings adjacent to their parent tree. They argued that this would promote the commonly observed pattern in most tropical forests that two adjacent trees are rarely of the same species. Tropical ecologists continue to wrestle with the details of the mechanisms underlying this observation.

A second key insight was provided by Carol Augsburger, Keith Clay and Scott Mangan, who independently showed that fungal pathogens do a much better job than seed-eating insects in minimizing local seedling recruitment. But detailed studies from the network of 50 hectare forest plots around the world to examine the dynamics of tropical forests uncovered a curious pattern. Limitation on local recruitment was much stronger in rare species than common ones, leading some to expect that if negative effects are driven by pathogens they could drive rare species extinct and reduce, rather than maintain, tropical diversity.

A third piece of this puzzle has been brought to light by studies that show local transmission of beneficial fungi from healthy parent trees to nearby offspring.

A team of ecologists tied these threads together in a paper published May 5 in Nature Communications, building on recent empirical studies that illustrate how common trees and rare trees develop different spatial patterns of recruitment. Common tree species can more easily accumulate more of their beneficial mutualists, effectively blunting the local impact of their pathogens. Concomitantly, common species can still be kept in check by their more globally abundant pathogens.

Disk with fungi growing on it
A plate showing fungal colonies found on a single seedling taken from the experimental forest. Seedlings are collected in the field, then surface sterilized. Pieces of stem are then laid in anti-bacterial agar, and the fungi inside are allowed to proliferate and spread, until smaller subsample isolates can be taken. Photo by Daniel Petticord

The authors achieved these insights by running simulations of pathogens, mutualists and trees diffusing across the landscape, then using a machine learning approach to identify which factors are most important in producing the results of the simulations. The work provides important insights into the management of tropical diversity, not least that it will be much easier to maintain these relationships if large areas of forest are left and protected intact, while sharply emphasizing how hard it will be to restore and recreate tropical forests without understanding and including both pathogenic and mutualistic root fungi into any restoration program.

Perhaps the deepest irony of the work at the current time is its emphasis on the importance of the interactions of host trees with microbial pathogens and mutualists in maintaining diversity and abundance in tropical forests. As humans race to find a vaccine for the COVID-19 virus, SARS-CoV-2, we now find that world’s largest vaccination scheme has been operating for millions of years in tropical forests, with trees acting as sources of beneficial mutualistic fungi that effectively vaccinate and protect their offspring against attacks from pathogens.

We suspect that cures for COVID-19 and other emerging pathogens might also be present in some of the compounds that plants and their beneficial fungi use to fight off attacks by insects and other fungal pathogens. As we destroy tropical forests, we simultaneously increase our exposure to pathogens such as SARS-CoV-2, as well as Ebola and Zika, and lose the potential sources for the drugs with which to treat them.

This paper provides vital insights into the mechanisms that maintain patterns of diversity and abundance in tropical forest. There has never been a more important time to ensure we protect as much of the world’s tropics as is possible — not least because they are likely to hold many of the keys for our own survival.

Support for this study was provided by the Smithsonian Tropical Research Institute.

The study, “Mutualist and pathogen traits interact to affect plant community structure in a spatially explicit model,” by John Schroeder, Andrew Dobson, Scott Mangan, Daniel Petticord, and Edward Allen Herre, was published in the journal Nature Communications on May 5, 2020. Nat Commun 11,2204 (2020). DOI: 10.1038/s41467-020-16047-5.