Amazonian forest through the last glacial maximum

Accelerated evolution may occur when a small population of a species – whose genetic variability is therefore limited – becomes isolated from all other members. This is one explanation for the rise of new species, as in the Galapagos archipelago. Creation of such genetic bottlenecks encourages rapid genetic drift away from the main population. It has been suggested to explain sudden behavioural shifts in anatomically modern humans over the last hundred thousand years or so, partly through rapid and long-distance migrations and partly through a variety of environmental catastrophes, such as the huge Toba eruption around 74 ka. Another example has been proposed for the teemingly diverse flora and fauna of the Amazon Basin, particularly among its ~7500 species of butterflies, which has been ascribed to shrinkage of the Amazonian rain forest to isolated patches that became refuges from dry conditions during the last glacial maximum.

Top: Arid ice age climate Middle: Atlantic Per...

Potential forest cover inferred from global climate models for the last glacial maximum (top) the Holocene thermal maximum and at present.. (credit: Wikipedia)

A great deal of evidence suggests that during glacial maxima global climate became considerably drier than that in interglacials, low-latitude deserts and savannah grasslands expanding at the expense of humid forest. Yet the emerging complexity of how climate change proceeds from place to place suggests that evidence such continental drying from one well-documented region, such as tropical Africa, cannot be applied to another without confirming data. Amazonia has been the subject of long-standing controversy about such ecological changes and formation of isolated forest ‘islands’ in the absence of definitive palaeoclimate data from the region itself. A multinational team has now published data on climatic humidity changes over the last 45 ka in what is now an area of dense forest but also receives lower rainfall than most of Amazonia; i.e. where rolling back forest to savannah would have been most likely to occur during the last glacial maximum (Wang, X. et al. 2017. Hydroclimate changes across the Amazon lowlands over the past 45,000 years. Nature, v. 541, p. 204-207; doi:10.1038/nature20787).

Their study area is tropical karst, stalagmites from one of whose caves have yielded detailed oxygen-isotope time series. Using the U/Th dating technique has given the data a time resolution of decades covering the global climatic decline into the last glacial maximum and its recovery to modern times. The relative abundance of oxygen isotopes (expressed by δ18O) in the calcium carbonate layers that make up the stalagmites is proportional to that of the rainwater that carried calcium and carbonate ions dissolved from the limestones. The rainwater δ18O itself depended on the balance between rainfall and evaporation, higher values indicating reduced precipitation. Relative proportions of carbon isotopes in the stalagmites, expressed by δ13C, record the balance of trees and grasses, which have different carbon-isotope signatures. Rainfall in the area did indeed fall during the run-up to the last glacial maximum, to about 60% of that at present, then to rise to ~142% in the mid-Holocene (6 ka). Yet δ13C in the stalagmites remained throughout comparable with those in the Holocene layers, its low values being incompatible with any marked expansion of grasses.

English: View of Amazon basin forest north of ...

Amazonian rain forest north of Manaus, Brazil. (credit: Wikipedia)

One important factor in converting rain forest to grass-dominated savannah is fire induced by climatic drying. Tree mortality and loss of cover accelerates drying out of the forest floor in a vicious circle towards grassland, expressed today by human influences in much of Amazonia. Fires in Amazonia must therefore have been rare during the last ice age; indeed sediment cores from the Amazon delta do not reveal any significant charcoal ‘spike’.

See also: Bush, M.B 2017. The resilience of Amazonian forests. Nature, v. 541, p. 167-168; doi:10.1038/541167a

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2 responses to “Amazonian forest through the last glacial maximum

  1. Brian Roy Rosen

    Dear Steve

    The work you reported was of special interest to me, and I hope you won’t mind if I elaborate on this.

    The phenomenon of regions in which species richness is markedly higher than in surrounding regions is a universal one, both across the globe today, and in the past. As a research question, it has preoccupied me in much of my own research too.

    It seems that a unified theory for this phenomenon has yet to emerge, partly because specialists in terrestrial diversity patterns and origins, and specialists in marine ones, rarely ‘talk’ to each other. Even specialists of different groups of organisms don’t usually talk to each other enough, while biogeographers, ecologists, palaeontologists, evolutionary biologists and taxonomists, often stick to their own scientific sub-community even though all of them are interested one way or another in this same broad question. The result is a fragmentation of effort and ideas. Every research group or individual seems to have their own pet theory about origins of a high diversity pattern which might work for their particular subject but is not necessarily applicable to anyone else. Another problem is that for a very long time, the relevant literature has emphasised ecological conditions, a simple case being the persistent idea that climate warmth simply harbours more species than cold conditions.

    I have argued that for a complete theory of high biodiversity regions, however, we need an integrated model that combines (1) ecological factors, (2) changing distribution patterns through time, and (3) speciations (or ‘originations’) – and conversely, extinctions. Note that such a model in effect combines short-term and long-term processes, the latter on evolutionary and geological time-scales.

    The work you have summarised is refreshing in taking account of processes on both ecological and evolutionary/geological time-scales. But thus far, it only seems to demonstrate how climate and habitat change can cause habitat fragmentation and hence speciations. It does not seem to explain how this in turn might actually generate the species richness we see in their study region. A simple example of this flaw is, if for every species which emerges from fragmentation, an older one goes extinct, then there will be no net gain in species richness within the region. So what other processes are needed which might drive the regional build-up of species?

    The simplest one I have argued (for the marine high diversity region of SE Asia) is for repeated habitat fragmentation combined with repeated partial habitat fusions, i.e. as a ‘dynamic mosaic’ of habitats on a regional and long-term time scale (cf. ‘diversity pump’). Without trying to elaborate further, this implies that if SE Asia has been a dynamic mosaic, its surrounding regions must have been much less dynamic, if at all. The very active tectonics of SE Asia proper, is at least one of the most likely driving factors here, combined probably with glacio-eustasy too.

    Applying this kind of model more generally, relatively frequent tectonic / ecological / climatic / oceanographic / geomorphological [‘TECOG’] events would seem to be the most plausible driving factors for habitat changes. The explanation for Amazonian diversity which you have summarised here shows how such TECOG-driven habitat dynamics might drive speciations, but by implication, how they might also act as a species pump to generate its present regional high diversity pattern. Even though the authors might argue that this is implicit in their study, they don’t appear to have said so.

    Bellwood, D.R., Renema, W., & Rosen, B.R., 2012. Biodiversity hotspots, evolution and coral reef biogeography: a review. Pp. 216-245 in D.J. Gower, K.G. Johnson J.E. Richardson, B.R. Rosen L. Rüber, L. & S.T. Williams (eds). Biotic evolution and environmental change in Southeast Asia. The Systematics Association Special Volume 82, Cambridge University Press, Cambridge.

    Very best

    Brian Roy Rosen

    • Dear Brian

      It rare on Earth-Pages to get such a commentary as yours; and one so clear. Many thanks!

      And other readers, please note – comment on any of the items whenever you feel the urge. all help enliven the site.

      Steve Drury

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