Research on ‘extinction debt’ in the Brazilian Amazon

This month a study in Science was published on the phenomenon of ‘extinction debt’ in the Brazilian Amazon, which was widely quoted in different media like Mongabay, The Guardian and The Huffington Post . This attention was well deserved because in short it told us that the majority of extinctions caused by deforestation in the Brazilian Amazon is yet to come; i.e. this outstanding debt with the Grim Reaper still has to be collected. Though few species are killed off directly in deforested areas, many will disappear gradually as their breeding rates fall and competition increases. But this postponement of extinction makes it still possible to save these species, if we act timely…

Burnt forest in the Amazon. Photo by: Alexander Lees.

Scientists from the Imperial College in London used a modeling approach to foresee how many species are expected to go extinct due to deforestation in the past as well as under  different future deforestation scenarios. They used the famous island theory from ecology, proposed by MacArthur and Wilson, to determine how many species should survive in a habitat island created by deforestation. Because for many animal groups, for instance the reptiles and the insects, information on species richness and occurrence is scarce, they concentrated their efforts on three relatively well known groups: birds, amphibians and mammals. When they compared data on number of species per habitat island with the predicted number of species from the island theory (which is a well established theory, no need to question its validity) they found that around 80-90% of species predicted to go extinct were still holding on. This extinction debt shows us that the majority of species seems able to survive for some time after the deforestation event, but will eventually have to cave in, because the population is not viable anymore. The researchers write: “This time delay offers a window of conservation opportunity, during which it is possible to restore habitat or implement alternative measures to safeguard the persistence of species that are otherwise committed to extinction”.

The extinction debt, measured in number of species, will only increase in the future if deforestation persists. To determine the effects  of future deforestation the researchers ran the model under four different scenarios for the year 2050. The most optimistic scenarios foresee an 80 percent reduction in deforestation by 2020 or deforestation ending for good in the same year while ‘business as usual’ was included as the most plausible scenario, in which no significant reductions in deforestation are taken into account. With the scenario of ‘business as usual’ 5% of species from the groups of mammals, birds and amphibians will have become extinct by 2050 and another 9% make up the extinction debt that will be collected in the decades thereafter. This total of 14% can be reduced to less than 5% when deforestation can be entirely halted or reduced to a minimum.

Map Tree Frog from the Amazon. Credit: Dr. Morley Read

The authors don’t expect such a large reduction in deforestation, partly because of recent changes to the Brazilian Forest Code ( that is viewed by some as a threat to the survival of the Amazonian rainforest. Added to this is a development programme in the Amazon to fuel the economy, including the construction of more than 20 hydroelectric power plants in the Amazon basin. However, recent years have shown a clear decline in deforestation numbers and therefore the authors write that “the declines in deforestation rates over the period 2005 to 2010 have helped widen the window of conservation opportunity for the highly biodiverse Brazilian Amazon”. Their research can help further widen this window, because as one of the researchers, Ewers, said the model reveals hotspots in the Brazilian Amazon where conservation efforts should be focused on the most vulnerable wildlife.

The article in Science was accompanied by a perspective article from Rangel, a Brazilian ecologist of the University of Goiás. He argues that Brazil must “default on its extinction debt,” possibly by creating more conservation areas, particularly by reforesting places that have been abandoned by agriculturalists. He recognisizes the threat from new legislation in brazil that seems to be less environmentally friendly and more in favour of the agricultural businesses who are pushing for more development programmes in the Amazon. But he too points out that a lot of progress has been made in Brazil, especially in reducing deforestation in the last decade and as well Brazil has been expanding its network of protected areas, with currently  more than 50 percent of the Amazon under some form of protection.

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