PHILIP HALL / NOAAThe foothills of Aconcagua, in Chile: the Andes had housed vast forestsPHILIP HALL / NOAA
During the peak of the last ice age lived through by the planet, between 18,000 and 14,000 years ago, the Cerrados, savannahs, and Caatingas (semi-arid Brazilian shrubland , the most open and typical vegetation of these types of dry climate, dominated South America. Thanks to natural adversaries, the imposing tropical forests and their gigantic trees dropped back and were obliged to occupy extremely reduced areas. Known as sanctuaries, these limited spaces represented a survival alternative for plants and animals accustomed to humid weather, which made use of this protection and reproduced more intensely. When the Earth warmed up again. The forests once again expanded and the species of the different sanctuaries ended up finding each other. The biological richness of the Amazon would be one of the consequences of this mixture.
Initially proposed by the zoologist Paulo Vanzolini, but conceptually formulated by the German Jürgen Haffer in 1969 and applied to the Brazilian reality by the geographer Aziz Ab’Sáber, this scenario – known as the Model of Forest Refugia – represented during at least three decades the most accepted vision of the southern portion of the American continent, including Brazil, during the most recent frozen times. The consensus, nevertheless, makes up part of the past. Sustained by the analysis of the pollen of the plants that existed over the last 20,000 years, today preserved in sediments, and by the dynamics of the birth of rivers and of the country’s hydrographic basins, Brazilian and international studies are highly critical – and even deny – the idea of sanctuaries.
“It’s not possible to defend a monolithic climatic version for an era that was so unstable and globally complex, even more so in a country the size of Brazil”, says Paulo Eduardo de Oliveira, an agronomy engineer and with his doctorate degree in botany and ecology, who is a professor at the University of Guarulhos. There are, as yet, no other theories to explain the diversity of animals and plants in Brazil after the last ice age, only discoveries that do not fit into the old model. “We have records from that glacial period that reveal diverse regions of humid and cold climate and forests much greater than those that would correspond to sanctuaries.” Oliveira and the American Mark Bush, from the Technology Institute of Florida, published in the January to April edition of the magazine entitled, Biota Neotropica, in an article that examines a series of studies that test the validity of the hypothesis of sanctuaries.
The research shows, for example, the existence of dense forests at the foot of the mountain chain of the Andes, where the temperature would have gone down by five Celsius degrees during the last ice age. During the same era, similar vegetation would be found in the oriental portion of the Brazilian Amazon, whilst in the the Amazon central region the climate in fact must have been a little drier, but not sufficient to eliminate the forestry formations. “There was always someone who was quick to say that we’d simply found a sanctuary area when we’d pointed towards the existence of forests related to humidity and the cold”, says Oliveira. Under these circumstances, the pollen could carry valuable information: as it represents the reproductive organ of flowering plants, it can be thought of as a structure of species identification. The morphological analysis of its structure indicates the plant family to which it belongs.
The way out of this impasse was conceived by the Australian Simon Haberle, from the National University of Australia, in 1997. Haberle collected sediments deposited in the delta of the Amazon River, a strategic area, exactly as it brings together pollen originating from distinct areas of the river basin. He made use of this potential of the pollen to provide information about plants, which had already been demonstrated in 1979 by the Brazilian botanist Maria Lucia Absy in her doctorate thesis at the University of Amsterdam, Holland, and confirmed: to a large extent they were from forests, and not from grasslands, which defined the configurations of the Amazon landscape during the most recent ice age. Haberle analyzed data relating to a huge area and, in the opinion of Oliveira, it would be a contradiction to sustain the idea that one was dealing with a sanctuary.
Oliveira, Bush and Paul Colinvaux, from the Woods Hole Marine Biological Laboratory, of Massachusetts, the United States, were the first to contest the idea of sanctuaries in a study published in 1996 in the magazine Science. By way of the analysis of pollen grains collected in lakes in the region of the upper Negro river, in the Pico da Neblina National Park, they rebuffed the possibility, taken up by the Model of Forest Refugia, of the existence of savannah or of whatever other type of vegetation of a dry climate in that area, over the last 40,000 years. Years later, in 2003, professor Peter Wilf, from Pennsylvania State University in the United States, based on the analysis of fossilized leaves of 102 different species of plants from the Argentinean Patagonia with at least 52 million years of age, confirmed that before the last glaciation there were already an enormous variety of species of plants and animals.
According to the Model of Forest Refugia, this biodiversity would have sprung up at the end of the glaciation period, when the few green areas would have returned to expand. The biologist Fátima Praxedes Leite, after having analyzed the Amazon sediments aged between 23 million to 6 million years, had already detected, during 1997, this diversity, revealing what professor Oliveira classifies as “huge ecological stability”.
Tectonics
As well as the pollen and the sediments, the formation of the Brazilian rivers basins and the origin of rivers, offer other arguments for contesting the Model of Forest Refugia. The biologist Alexandre Cunha Ribeiro, currently at the University of São Paulo (USP) Ribeirão Preto campus, showed in the April to June 2006 edition of magazine entitled, Neotropical Ichthyology that the evolutionary history and the geographical distribution of an elevated diversity of species of the fish that inhabit the country’s rivers is directly associated to tectonic movements and to the dislocation of blocks of superficial rocks. In the case of Brazil, the extensive zones of rock fractures that make up the continental foundation – more susceptible to ruptures – coincide with the limits of separation between the basins. When the continental movements intensified, they re-opened up old fractures, making the blocks of rock move themselves, so that they could rise or fall.
The final result of these movements is slight or brusque changes in the courses of rivers, literally launched in other directions, different from their original trajectories. In another study, published in June in the magazine Ichthyological Exploration of Freshwaters, the researchers Flávio Lima, Claudio Riccomini, Naércio Menezes and Ribeiro demonstrated how the high course of the Guaratuba river, an ancient tributary of the Tietê river, managed to conquer its own life and transformed itself into one of the coastal rivers independent from the region between Santos and Ubatuba. “Ancient geological faults, reactivated between 60,000 and 10,000 years ago in the upper part of the Sierra do Mar, captured the river, which changed its direction away from the Tietê river”, he comments.
Carried away by these changes of route, the fish species either spread out or isolated themselves and populations of the same species can be found in rivers of distinct hydrographic basins. “The general architecture of the major Brazilian hydrographic basins, such as those of the rivers Paraná, São Francisco and Uruguai, should already have been formed a little more than 100 million years ago, when Brazil separated from Africa”, points out Ribeiro. Afterwards came the fine adjustments, given impulse by the most recent tectonic processes – river basins and consequently its fauna of fish do not respect the limits of any sanctuary and invade Cerrados, Caatingas and forests. Nevertheless, this dispersion also depends on the capacity of adaptation of each species to the natural conditions of each environment. The Piabina argentea, for example, a little fish of up to 5 centimeters in length, can be found in turbid, clear and rocky waters, in tributaries of the rivers Paraná, São Francisco, Paraíba do Sul and some other coastal rivers of the Brazilian Southeast, although comparison studies suggest that the common ancestor to these populations had originated in the basin of the river Paraná.
On the other hand, the fish Glandulocauda melanogenys, also only a few centimeter in length, is a fish endemic to cold and clean waters, only found in the tributaries of the upper head of the river Tietê, as well as the upper part of the river Guaratuba, which was a tributary of the Tietê before the tectonic movements that led to its capture. “Different species react in a distinctive manner to the same tectonic movements”, says Ribeiro.
“The fact that a determined species of fish possesses a wide distribution or is restricted to a small area of a river basin depends on its biological characteristics. Species with low capacity towards displacement or very demanding from the environmental point of view can be restricted to a small area where such conditions exist. Indeed, in spite of the river basins repeatedly mixing their faunas via the process of river and stream capture through tectonic activity, not all of the species will have an equivalent geographical distribution. As the mixing of the fauna by the tectonic process in continuing with time, the result is a very complex history involving ancestral populations and their descendants in the large South American river basins.”
Fossil registers of fish also indicate that the diversity of species with characteristics of modern fauna was already great during the Tertiary Period, around 50 million years ago, a moment well before that contemplated for the sanctuaries.
The fossils of the fish Corydoras revelatus are from the Tertiary Period , which is of the same genre that includes the “bottom cleaner” or tamboatá, currently common throughout Brazil. The fossil fauna of the Taubaté river basin, aged between 23 million and 35 million years, is able to count upon genres still today found in Brazilian rivers and streams, such as the large catfish Steindachneridion, the piraputanga Brycon, the curimbatazinho Cyphocharax and the lambari Lignobrycon.
“Science is made up of getting it right and getting it wrong and it’s important that paradigms pass through a critical evaluation, especially when new technologies come forward”, says Oliveira. For him, the era in which it was conceived and in the face of technical limitations, the Model of Forest Refugia based itself on what is called indirect evidence – associations between similar geographical landscapes and analogues between similar rocks, as well as the analysis of plants, amphibians and butterflies of close species found in distant locations.
Currently, as well as the analysis of pollen, it is possible to work with sophisticated DNA tests and with uranium and carbon dating and isotopes of other chemical elements. Attentive to the contradictions of the Concept, Oliveira also questions how the plants and animals could have diversified themselves if they had been confined to restricted and extremely competitive environments. “It doesn’t make sense. In reality, because of the fight for survival, the sanctuaries would have to have brought about a reduction in the number of species”, he evaluates.
Another problem unresolved by the sanctuaries is with respect to temperatures. Caatingas and savannahs are environments of a warm climate; they would be incompatible with an era of glaciation, even if Brazil has only suffered indirectly the effects of global cooling, more intense in the northern hemisphere. “The theory took into consideration only humidity and rainfall”, says Oliveira. “Either the idea of the sanctuaries is re-formulated or it will be abandoned”, he says.
Vanzolini rebuffs the critics and recalls that in the Amazon the fauna is not uniform. “The sanctuaries continue to be valid as a consistent explanation for this animal diversity. Up until now, nobody has managed to present another sustainable proposal to substitute this theory.” Professor Ab’Sáber also rebuffs the doubts. For him, the information that led the critics to think that the theory does not work is related to more recent happenings, which do not correspond to the time of glaciation: “The theory’s fundamentals of redoubts continue absolutely untouchable.”
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