A new article has just been published in Science entitled “Hyperdominance in the Amazonian Tree Flora” by Hans ter Steege and an army of 100+ authors (including yours truly). Rather than summarizing the article, I am going to share responses by me and two other coauthors (Miles Silman and Paul Fine) to a couple of questions asked of us by a journalist (Deborah Osae-Oppong of the Chicago Field Museum) writing up a press release about the article.
1. In your opinion, what is the most remarkable find in this study?
Ken Feeley: One of the most remarkable and important outcomes of this study is the revelation of how little we actually know about the Amazon, and by extension, other tropical forests worldwide. This study is attempting to answer incredibly basic questions: How many trees are there in the Amazon? How many species of tree are there? What are the most common tree species? How many rare species are there? To even attempt to answer these questions the authors had to work for many years to gather immense amounts of data from across huge expanses of remote and biologically uncharted territories. Even with this unprecedented data set in hand, we are really only able to take what amount to very educated guesses at most of these questions. We now have an idea of how many tree species are in the Amazon and how many of them are common or rare, but we still know next to nothing about those species.
Miles Silman: The most remarkable findings of this study are two-fold, really. And it is that Amazonian forests are spectacularly diverse and monotonous at the same time. There may be up to 16,000 species of trees in the Amazon, yet half of the individual trees one encounters comes from just over 1% of those species.
Paul Fine: For decades, the conventional wisdom was that in Neotropical rainforests it was difficult to find two individuals of the same species in the same forest. The new study shows that this is not the case for the few hundred dominant species but only for the thousands or rarer species.
2. What are the implications of the finding that the so-called “hyper-dominant” species account for half of all the trees in the Amazon?
Ken Feeley: Another hugely important finding of the study is that most of the individual trees in the Amazon actually come from just a relatively small number of hyper-dominant species. As such, it may actually be plausible that we can one day gain a reasonable understanding of how the Amazon works and perhaps more importantly how it will or will not work in the future. This is because we can now focus our research efforts on the hyper-dominant species; once we learn their ecology we will have at least half the pieces needed to put together the puzzle that is the Amazon.
Miles Silman: What it means is that (to a first approximation) half of the ecosystem services that we get from trees comes from just over a percent of the species. At first blush one would think it would make it easier to understand things like carbon cycling in Amazonian forests, and ecosystem responses to global change. I see a danger in focusing too much on the hyperdominants, though. Forests are more than collections of trees. Around 90% of the tree species in Amazonia are obligately tied to animals for pollination and seed dispersal. And, conversely, these animals are tied to the trees. Are the hyper-dominants alone enough to support the vast animal biodiversity of the Amazon, and to keep the ecosystem functioning? Or are the other 98% of tree species, even the very rare species, playing important roles in the web of interactions in the world’s most diverse forest, without which the whole system collapses?
Paul Fine: For me, it suggests exciting new research programs to uncover why the “hyper-dominant” species are so common? To what extent does biogeographical history play a role? Do hyperdominants have some set of successful traits? Defenses against natural enemies?
3. Can you explain, in your own words, the phenomenon of “dark biodiversity”, and the consequences that has for future endeavors into Amazonia?
Ken Feeley: While thanks to this study we now know that the Amazon is dominated by a relatively few super common species, we also now know that there are thousands of extremely rare species hiding out there. Indeed it is these rare species that actually account for most of the biodiversity that the Amazon is so famous for. By their very nature, by the very fact that they are rare, these thousands of species present what may be an intractable problem for ecologists and conservation biologists. Rare species are hard to find, they are hard to recognize, and they offer very small sample sizes making it extremely difficult to ever shed a light on this “dark biodiversity” and learn learn about how rare species work and how they may (or may not) be threatened by climate change, deforestation or any of the other myriad of anthropogenic threats that loom over the Amazon. If we ever hope to understand and protect rare species, and hence the full diversity of the tropical rainforest, we are going to need a lot more than just a 1000+ tree plots
Miles Silman: One of the things that this study really drives home is biodiversity’s dark matter. 99.9% of the individual trees out there hold just a third of the species diversity. A full two-thirds of the tree species in the Amazon are rarer than needles in a haystack, representing just over a tenth of a percent of the total individuals. It is akin to the situation in physics, where we know there is a lot of matter in the universe that we can’t see, and we know it is important, but it is devilishly hard to detect. So, we’re in the situation where we know there is immense biodiversity out there and that we might never be able to detect it. This raises all sorts of questions: Can species really exist at that low a density? What are the limits to small populations? Are the rare species on the way out? Or are they up-and-coming new species? And, as conservationists, do we even try to detect biodiversity’s dark matter?
Paul Fine: We desperately need more investment into the systematics and taxonomy of tropical trees. Most of these rare species are unidentified morphospecies — and these species likely hold the key to understanding the history of Neotropical tree biodiversity and helping to predict how this diversity may change under global climate change.