Why the Amazon Rainforest is species rich

By Razib Khan | May 5, 2011 4:08 pm

A monkey frog

The Pith: The Amazon Rainforest has a lot of species because it’s been around for a very long time.

I really don’t know much about ecology, alas. So my understanding of evolution framed in its proper ecological context is a touch on the coarse side. When I say I don’t know much about ecology, I mean that I lack a thick network of descriptive detail. So that means that I have some rather simple models in my head, which upon closer inspection turn out to be false in many specific instances. That’s what you get for relying on theory. Today I ran into a paper which presented me with some mildly surprising results.

The question: why is the Amazon Rainforest characterized by such a diversity of species? If you’d asked me that question 1 hour ago I would have said that it was a matter of physics. That is, the physical parameters of a high but consistent rainfall and temperature regime. This means the basic energetic inputs into the biome is high, and its consistency allows the organisms to plan their life schedule efficiently, maximizing the inputs. All that naturally produces a lot of diversification in the “climax” ecosystem. To some extent I would acknowledge this was pretty much a “Just-So,” but I’d have thought it was a good shot, and probably representative of the internal logic of many people.

ResearchBlogging.orgBut no, a new paper in Ecology Letters seems to imply that that the answer we must look to is history and not physics. From the perspective of someone who is rooted in are reductionist conception of evolutionary biology this isn’t the answer I was “rooting” for, but if it is, it is. What’s their logic?

First, the abstract, Phylogenetic origins of local-scale diversity patterns and the causes of Amazonian megadiversity:

What explains the striking variation in local species richness across the globe and the remarkable diversity of rainforest sites in Amazonia? Here, we apply a novel phylogenetic approach to these questions, using treefrogs (Hylidae) as a model system. Hylids show dramatic variation in local richness globally and incredible local diversity in Amazonia. We find that variation in local richness is not explained primarily by climatic factors, rates of diversification (speciation and extinction) nor morphological variation. Instead, local richness patterns are explained predominantly by the timing of colonization of each region, and Amazonian megadiversity is linked to the long-term sympatry of multiple clades in that region. Our results also suggest intriguing interactions between clade diversification, trait evolution and the accumulation of local richness. Specifically, sympatry between clades seems to slow diversification and trait evolution, but prevents neither the accumulation of local richness over time nor the co-occurrence of similar species

Thankfully species richness is pretty easy to understand. It’s a count of the number of species in a given area. In this case they limited their count to a specific clade, the tree frogs. This clade seems to have a common ancestor ~60-80 million years before the present from which it descends.

Below is a phylogenetic tree (scaled to time on the horizontal) representing the relationships of contemporary tree frog species, as well as a distribution of the species across the world:

Visual inspection tells you immediately that Amazonia is overloaded with tree frog species. But to get at the question of what explains the variation in species richness the authors used standard statistical techniques relating predictors such as temperature and precipitation values to the outcome, species richness. The authors did find a relationship between precipitation and temperature and species richness. But once they controlled for phylogeny in their regression, that is, take into account history, the relationship went away. In other words the correlations may have been an artifact of the fact that the Amazon is warm and wet and rich with species. Controlling for the phylogeny of the clade, which is a record of contingent history, the expected picture relating physical parameters to diversification changes. The two panels above and to the left show the relationships between species richness (y-axis) and first colonization event. The left panel is pegged from the first colonization of any tree frog lineage, while the second sums up distinct colonization events by different clades (so the x-axis has a larger magnitude). The r-squared, the proportion of the variance of y explained by variance in x, is nearly 0.50 in the left and 0.70 in the right. That’s pretty good.

There’s some interesting material in the paper sympatry vs. allopatry in regards to the tree frogs. Basically, how they vary in size and diversity as a function of whether they co-occur in the same ecosystem or whether they’re physically separated (so allopatric speciation is when two lineages are separated while sympatric is when they are geographically overlapping but diverge anyhow, perhaps through occupation of differing niches).

But that’s not my primary concern or interest. How generalizable are these results form tree frogs? I don’t know this literature well. Surely someone has done a phylogenetic least squares with a lot of different clades and checked for this? If the results here are generalizable then the diversity of the Amazon ecosystem is in large part a function of its longer term stability and persistence. I have posited that at the “end of history” natural selection will have shaped an exceeding simply and energetically optimized biosphere, dominated by a few species. But in Amazon is a case in the opposite direction, as clade diversification increases as a function of the time of ecosystem integrity. Is this monotonic? In other words, is there going to be a time when a rare evolutionary event may given rise to a species which sweeps away all the accumulated variation?

Those are questions for the future I suppose.

Citation: Wiens JJ, Pyron RA, & Moen DS (2011). Phylogenetic origins of local-scale diversity patterns and the causes of Amazonian megadiversity. Ecology letters PMID: 21535341

Image credit: Colin Burnett

  • http://washparkprophet.blogspot.com ohwilleke

    The European and North American examples suggest that habitat destruction is a powerful factor in reducing species diversity and that at least in the Holocene, living somewhere that humans haven’t yet made it a priority to occupy and remake according to their own desires is critical.

  • Ian

    Mittelbach et al (2007). provide an overview of current thinking on the question, and access to the article happens to be free, at least for the time being.

  • bob sykes

    I was under the impression that the modern Amazonian rain forest (and others) is fairly recent. That during the last ice age much of the Amazon basin was savanna, and the rain forest ecology existed as relict islands scattered throughout the basin.

    If that is true (and I am not yet again confused), then the modern species richness might be a result of Gould-Eldredge punctuated equilibrium.

  • Markk

    I’m not sure of the specifics of the article and I agree with #3 above, but I think you are not taking a big enough view when saying it is history, not physics. W.A. guessing coming. It sounds like underneath their simple non-correlations lies some kind of stochastic model of species formation to produce the tree of species and and time is one input. I can’t believe energy and space and some kind of diversity measure of habitat aren’t others. Diversity measure also including what species of trees are around, etc. Some of which likely are correlated with energy input also.

    I guess I am saying something like this: you would not get any number of tree frogs without that energy and water input, but given that input other events will determine rate of speciation, and thus make time an important factor. I get that history is important but history is important in pretty much every complex (i.e. realistic, phenomenological) model, the question to me would be something like: Is the space of high probability possible outcomes with zillions of tree frog species constrained by energy and water input? I think that what they showed is that -within- the space of high probability outcomes number of species looks like some Markov chain model where the longer you wait the more you get till the reaper comes.

    Hope that made some kind of sense.

  • dave chamberlin

    The link provided by Ian really helped me to grasp the complexity of why there is such biodiversity in the rainforest. I’ll quote another point of view coming from page 185 ‘The story of Life” by T.R.E. Southwood. “It used to be considered that the great variety of life in rain forests was a reflection of their stability, that they remained unchanged over countless years. This is now known to be incorrect. On the shortest scale, both of time and size, these forests are always changing because the gigantic trees are both shallow rooted and relatively short lived, the heart often rotting; they fall and when they do so, the gap is recolonized by a succession of plants. These gaps open at random across the forest and what we see is actually a mosaic of vegetation all in different stages of succession; this plays a major role in the maintenance of diversity.”
    We have the time and area hypothesis, the evolutionary and biogeographical hypothesis and we even have the evolutionary speed hypothesis which links biodiversity to temperature through it’s effects on mutation rates. Once again the closer we look at life processes the more complex they become.

  • Ian

    There’s a lot to think about in that article. What makes this dataset unusual, I think, is the number of species in their phylogeny (350/850 spp., and all the genera). What’s more, the phylogeny is apparently calibrated against fossils.

    What really makes this dataset unusual, I think, is its depth. If you read the supplemental material, you’ll see that the clade is at least 144 million years old – early Cretaceous, before any major radiation of the angiosperms. This is important because hylid diversity isn’t going to be a primary response to climate.

    Hylid diversity is likely to be related to the abundance and diversity of food species, and to the physical diversity of the habitat. The more closely-related taxa that coexist, the more finely they’re likely to partition resources (as MacArthur demonstrated for birds in Panama and Puerto Rico back in the 60s). Plant diversity (trees, epiphytes) is likely to structure habitat diversity for treefrogs. It also drives insect diversity (cf. Terry Erwin’s fogging experiments in the early 80s, which found high species-specificity between insects and the trees they lived on), which I’m guessing is the main food source for treefrogs. So some of the pattern observed probably reflects diversification driven by the rise of the angiosperms. Since this diversification also crosses the K-T boundary, it seems likely that this also is part of the overall signal of diversification.

    The whole “time” issue is pretty interesting really. If the clade continues to diversify at an apparently constant rate, both in their core area and in areas that they have more recently colonised, it raises some interesting questions about limiting similarity and species “saturation”. Conventional wisdom suggests that for two species to coexist there must be some minimum (ecological) difference between them. If that’s the case, then the ability for hylids to continue diversifying at similar rates in all habitats suggests that even in Amazonia there are still “empty” niches available for new species to occupy. An alternative, of course, would be to invoke Stephen Hubbell’s neutral theory…that treefrog species are competitively equivalent.


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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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