Hyperdominance in the Amazonian tree flora

Hyperdominance in the Amazonian Tree Flora. ter Steeg et al. Science 342 (6156). DOI:10.1126/science.1243092. Hyperdominance in the Amazonian tree flora

I can't see the Amazon for all these tree plots - taken from ter Steege et al.

I can’t see the Amazon for all these tree plots – taken from ter Steege et al.

Jun Lim

Jun Lim

Among other things, the authors set out to try to estimate how many tree species are in the Amazon, how they are distributed in space and among habitat types. They did this in part by extrapolating the total Amazonian tree diversity by fitting the mean rank-abundance data for over half a million trees within 1,000 well-studied plots in the Amazon to Fisher’s log-series. They found that there were “hyper-dominant” tree species, which represented about 1.4% of Amazonian tree diversity while representing over half of all trees in the Amazon. On the other hand, the rarest 11,000 species accounted for a paltry 0.1% of all trees! Furthermore, as it turns out, this inequity in abundance among tree species in the Amazon had an interesting spatial pattern. These so-called “hyper-dominants”, had on average larger geographic ranges, but the majority of them were found to be dominant in only one or two out of several distinct forest types, suggesting that dominant species were habitat-specialists.

This paper, however, leaves me with an intense wanting, although these findings are clearly the first of many awesome papers to come. Firstly, it would be interesting to see how these patterns of dominance generalize to other tropical tree systems, such as the ever-wet forests of South-east Asia (where I grew up, albeit in the virtually deforested Singapore). Anybody who knows even a little of the rainforests of Malaysia and Indonesia thinks immediately of the dipterocarp trees (Dipterocarpaceae) that form much if not most of the canopy.

Another thing that really got me thinking was the idea of scalar-dependency in niche specialism. Sure, if you are a species that does well in one particular soil type, all the small-scale heterogeneity in habitat does not matter much to your distribution (kinda like generalism). But in the larger scheme of things, you start to appear much more constrained (specialism). What’s interesting is that this plays out in a consistent and similar way across many species and across many different habitats. From a community assembly standpoint, this brings up the age old question of the relative importance of dispersal limitation (high numbers of species can effectively coexist even if a habitat was homogeneous) and niche-based explanations for high species diversity, especially considering the strong role of habitat heterogeneity across many systems (which filters for different communities in different parts of the landscape). I feel it is still unlikely that the relative role of these two processes will be disentangled any time soon, although this paper was a big step towards that goal.

Will Pearse

Will Pearse

It’s hard to think of an image in ecology that represents much more blood, sweat, and tears than the one at the top of this article. 567 plots would be a lot anywhere in the world, but dense Amazonian rainforest does not make for easy fieldwork. That you can go online, register, and download much of this data right now makes this even more wonderful; thank you everyone involved in RAINFOR!

Like Jun, I sense this is the first of many papers working with this dataset (and is an excellent start at that!), but I sense it may still take us some time yet to get a handle on the Amazon! I’m having some trouble getting my head around the spatial scale of this dataset, and so I’m unsure how I feel about the most common (hyperdominant) species in the Amazon being habitat specialists. In many ways I’d be shocked if species were truly generalists in the sense that they were everywhere across the Amazon, although I suppose I’d need a dataset like this to be sure! I’m particularly interested by how more diverse genera are less likely to contain hyperdominants. I’m tempted to infer that because hyper-diverse genera are more similar to one-another and have similar ranges, they are competing too strongly with one-another to become dominant. Given there’s a taxonomic effect to hyperdominance, perhaps a phylogenetic analysis would help get at these issues (…although I’d rather not be the one making a phylogeny of the Amazon…!)

I think it’s legitimate to ask how many species there are in the Amazon using this dataset, and I’m frankly amazed by how flat the middle of the rank-abundance plot in figure 2 is for the Amazon. While I agree with the authors’ general conclusions here, I am slightly concerned about extrapolating out into such low population sizes. It’s probably fair to say that no one single curve can describe both the hyperdominant and extremely rare species in the Amazon, and the extrapolation is based on assuming that a straight line that follows the medium-richness species will cover everything. I’m sure the authors would agree that this is a simplification; naively I would expect expect this estimate to be too high, yet their estimate of ~15000 species in the Amazon actually struck me as quite low when I first saw it. I guess much of this might fall back to what we’d be happy considering a species; trees are not known for playing by phylogeneticists’ rules, and maybe very rare species can survive for quite a while in the Amazon thanks to outcrossing and hybridisation.

Lynsey McInnes

Lynsey McInnes

I always open a Science paper with a slight sense of foreboding that if I want to understand even a little of what the authors have done, I’ll have to trawl through endless supplementary files. So, first things first, I really appreciate the slightly longer format of this article so that by the end of it, you have a sense of what was done, alongside the pitfalls and the potential implications. A real, whole paper; result!

And what a paper. My mind is still a bit fluffy on the spatial scale and connectedness of each plot (I was surprised there was no map figure of the interpolated richness per 1 degree cell), but that is a minor grumble. This is a massive, impressive collaborative effort to probe the distribution of trees in Amazonia. Naively, I was taken aback by a number of findings: that there are potentially >16,000 tree species (seems like a lot to me), that only 227 of them seem to dominant, that each hyper-dominant was kinda habitat-restricted. Conversely, I wasn’t surprised that species in different families were distributed differently (one or a few hyper-dominants vs. tons of restricted-range endemics) or that two well-chosen traits didn’t predict hyper-dominance.

It’s clear that such a big effort is going to generate tons of follow-on papers, many of which have been primed in this first article. There seemed to be some confusion whether to focus on the hyper-dominants and what made them so vs. the thousands of species with tiny or unknown distributions many of which the authors suggest are close to extinction. An interesting route by which the authors will follow up this paper will likely be to try to find out how important the non-dominant species are for ecosystem functioning. They seem to suggest that perhaps they are not so important. I wonder what the cut-off for usefulness vs. exciting rarity is? Does it vary among plant families? How much could we lose without having any impact at all? How much complementarity is there? How easy (and valid) will be to model ecosystem functioning or resource cycling concentrating only or mostly on the hyper-dominants?

I also found it funny that the authors did not wonder why their Maxent models predicted populations of many species in places where extensive surveys suggest they are not found. What is found in their place? Populations of phylogenetically or functionally related species? Or was some environmental or topographic variable missing? An easy (but perhaps quite dull) follow-on paper perhaps…

A final follow-on that I could think of would be to compare spatial patterns of some of the species’ patterns (a mix of hyper and non-hyper dominants) among regions and forest types (kinda like figure 4) to see if one can tease out any patterns of dispersal limitation. I think the authors conclude that many species are restricted to one or two forest types: are these generally within a region or across all or multiple regions (this might have been covered, but I missed it). If you are happy with the high levels of extrapolation (interpolation?) involved, this dataset is a treasure chest for dispersal ecologists…

So many options! And more or less freely-available online already (see link above). Let’s get going!


About will.pearse
Ecology / evolutionary biologist

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