Sunday, 16 December 2012

Island Extinctions and the Case of Madagascar

Island Extinctions
Islands which were only inhabited by humans after the late Pleistocene megafauna extinction event offer interesting ‘control experiments’. A compelling argument for human factors driving extinction is the time lag between continent extinctions and those of nearby islands (Martin and Steadman 1999). For example, New Zealand’s moas lasted 30,000 years longer than Australia’s extinct giant bird, the mihirung, which went extinct during a Australia’s wave of rapid megafauna extinction (before the late Pleistocene).

The argument for prey naiveté on early contact with human hunters finds support also in the remarkable tameness of wild birds in remote islands which were undiscovered by prehistoric explorers. These include the Galapagos, Christmas Islands, etc. Galapagos’ avifauna were unafraid of humans, as depicted in historic accounts from 17th century sailors who discovered the island (Martin and Steadman 1999).

In this blog post I look closer at the island of Madagascar.


This paper by Burney et al (2004) discusses how islands can be used to better understand megafauna extinction. Madagascar is interesting because it is the last place on Earth where megafauna went extinct prehistorically – extinctions in which humans had some part to play in most other parts of the world occurred much earlier, during the late Pleistocene or even earlier in Australia. Madagascar offers a relatively fresh record of paleoecological change, since humans only arrived in the late Holocene, about 2,000 radiocarbon years ago. Very little is known about how or why this group of Iron Age people came to Madagascar. Evidence of the first humans can be shown by human-modified megafauna bones, such as cuts on the fossilized bones which show removal of flesh from bone by a sharp object.

Very little is known about the late Pleistocene biota in Madagascar. The amount of data increases greatly for megafauna in the mid-Holocene, where conditions for fossil formation probably became more favourable. For example, most lakes and swamps along the coastline formed only after around 5,000 radiocarbon years ago. Nevertheless, there were major climatic changes in in the late Pleistocene and pre-human Holocene, most of which were survived by most of the megafauna. Although there have been range shrinkages, there were no extinctions. Of the 9 genera of extinct lemurs dated, only one is not securely dated to the human period. Some examples of major climate change are as follows: 
  •      20,000 radiocarbon years ago (LGM): widespread dessication occurred. Lake Alaotra, a large lake in humid eastern Madagascar, was dramatically reduced in area if not completely dry during that period. 
  •      10,000 calendar years BP: At another site called Trtrivakely, pollen evidence shows the nearly complete replacement of heath vegetation with wooded grassland.
A drastic decline in megafauna, as shown by a huge decrease in Sporormiella in sediments at 1700 radiocarbon years BP (within a few centuries of first human contact), was observed. Sporomiella is a fungus that grows in the dung of large plant-eating mammals, and it releases spores which are preserved in sediments. The presence of these spores is used as a proxy for the presence of megafauna. Humans could have hunted these megafauna or altered their habitat. Before humans arrived, these herbivores had very few predators other than large crocodiles. Although there is very scant evidence for direct human hunting of megafauna, as in many other continents where Pleistocene megafauna extinction has occurred, another way in which humans could have contributed to the decline is through altering the existing fire regime by further increasing fire incidence through burning for settlement and agriculture and through hunting of plant-eating megafauna. The decline of large herbivores such as giant hippos caused ground litter to accumulate, feeding more fires. This can be shown by charcoal peaks above background values, first occurring in the South West where humans first settled, and then spreading outward over Madagascar. Nevertheless, the extinction pattern on Madagascar does not support a Blitzkrieg hypothesis. There is an overlap of around 2,000 years from earliest human evidence to the last occurrence of extinct megafauna.

The chart (Burney et al 2004) shows a summary of events in Madagascar:

My Thoughts
The evidence from Madagascar is indeed intriguing and I feel it does make the argument for human factors in the extinction of megafauna more compelling. Madagascar’s physical geography and vegetation is very similar to Africa such that it is referred to as an ‘Africa in miniature’, and it is probably safe to assume it went through similar climate changes and vegetation responses as Africa. The megafauna on this island certainly survived all these before the humans came, after which they experienced dramatic decline and finally, extinction. The fact that it is an island is important; in a previous post I mentioned the reason for why Africa still has such a large diversity of megafauna left is that it is larger and probably provided more refugia for megafauna. Madagascar probably provided more limited refugia for the stressed populations of megafauna.     

Burney, D. A. et al (2004) ‘A chronology for late prehistoric Madagascar’, Journal of Human Evolution, 47, pp. 25-63.

Martin, P. S. and Steadman, D. W. (1999) ‘Prehistoric extinctions on islands and continents’ in MacPhee, R. D. E. (ed.) Extinctions in Near Time: Causes, Contexts and Consequences, New York: Kluwer Academic/Plenum, pp. 17-50

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