25 April 2012

Gomphotheres of the Rambunctious Garden

At an event titled Human Nature? Man and the Environment at Canning House in London yesterday, Yadvinder Malhi, one of the four excellent speakers, invited his listeners to consider human impacts on the Amazon basin over a longer time frame than many of us are accustomed to do. 

Malhi described the sense of sense of awe he felt at seeing this continent-sized forest on his first visit in 1995. Contemplating the huge trees, he experienced nature in the Amazon as essentially benign. The feeling has remained with him. You do have to watch out for the odd snake, but essentially there's nothing [natural] to worry about. Even the insects aren't too bad.

His more recent experience in the West African rainforest, by contrast, was a completely different. Walking with a guide in forest in Gabon for the first time, he learned to stop every few steps and listen for forest elephants: cow-sized animals which look cute but are extremely dangerous.

The contrast led him to ask himself, why were there no elephants in the South American rainforest?The answer is that, of course, there were. Gomphotheres were common across much of the continent until about 12 to 10,000 years ago when, along with some fifty other large mammals including the Stegomastodon, the Toxodon, the Giant Ground Sloth and the Glyptodont, they became extinct.

Few dispute that humans played an important role in many of these extinctions, as well those on other continents beyond Africa from about 60,000 to 10,000 years ago. Rapid climate change may also have played a part. Quite likely it was a combination of the two pressures that did it. (In Africa animals had co-evolved with humans, and learned to be wary of them much earlier [although see this].)

What impact did the South American extinctions have on the forest? Elephants and other large mammals have significant impact on African ecosystems, and there is every reason to suppose that the same was true for their counterparts in South America. In Africa, tree cover is often more patchy where elephants are present in large numbers. On the other hand, the large seeds of some tree species may need large animals such as elephants to spread them around, and the same would have been true in the New World.

In addition, it may be that (over a very long time) big animals played a major role in the transport of nutrients flowing down the rivers to the poor soils beyond. The animals would eat vegetation on the river banks and often pooh elsewhere. Other animals would later eat the vegetation so fertilized and carry nutrients further inland. The Amazon ecosystem could still be responding to the loss of these animals-as-nutrient-transport-systems 10,000 years later.

The Amazon, said Malhi, has only really become a part of the global social-economic system since around 1950, his favoured date for the start of the Anthropocene, or at least its startling acceleration. Malhi suggested four scenarios/ways of thinking about the future Amazon:
  • 'Breadbasket' (made possible by huge inputs of fertilizers and fuel)
  • (Inter)national conservation park
  • Lungs of the world
The last scenario, from a phrase coined or popularized by Emma Marris, would, he suggested, represent compromise between the preceding visions -- one that recognizes that Amazonia was and is a human-altered system.

I wonder: what about a project to introduce and monitor the impact of a small herd of elephants on a designated and contained area of South America forest or savannah?

P.S. 26 April:  following the four speakers, some discussion focused on Brazil's land use law.

No comments: