Monday, September 03, 2012

Of Megafauna And Men

During the Pleistocene epoch, colossal relatives of many familiar present-day creatures roamed the earth. Some of these formidable megafauna species vanished abruptly (in what paleontologists like to call blitzkriegs) while some faded out gradually. Once and Future Giants: What Ice Age Extinctions Tell Us About the Fate of Earth's Largest Animals by Sharon Levy is a safari through the Pleistocene and the (ongoing) Holocene epochs acquainting us with behemoths then and now. In the author's own words:
This book tells the story of the megafauna and us. It is a tale of human coexistence and clashes with giant animals, past and present, and our responsibility toward them in the future.
Just as the struggles of surviving megafauna offer clues about how and why their lost cousins perished, Pleistocene extinctions offer lessons that can be critical for the conservation of megafauna living today. 

Book review: Once and Future Giants by Sharon Levy

Proponents of the Pleistocene Overkill Theory contend that human overhunting is what triggered the mass extinction events. Levy writes:
Fossil pollen, microscopic charcoal and spores of a fungus that feeds on dung piles left by large herbivores began to tell a complicated story of extinction. Data from an array of sites pointed to a population collapse among large animals, followed by landscape transformation and forest fire, suggesting the first people may have played a role in mastodon and mammoth extinction long before the sudden climate shifts marking the end of glacial time 13,000 years ago.
The most compelling indicators of Pleistocene overkill in North America are the great many Mammoth Kill sites of the Clovis tribe of Paleo-Indians. Excavations at these sites (such as the Lehner site in Arizona, which I have promptly added to my favorite destinations list!) have revealed razor-sharp Clovis stone points along with remnants of bones from large game. 

Evidence of overhunting outside of North America appears to be minimal. Europe too lost a number of megafauna species at the end of the Ice Ages, including the woolly rhino, the mammoth, and the Irish elk. Archeological evidence from Northern Europe and Asia indicates hunting of various types of smaller deer in preference over larger animals. This notion is further confirmed by Pleistocene art: “There are very few instances of woolly rhino or mammoth drawn with spears sticking out of them. With horse, bison, ibex, reindeer, there are lots of images of those animals speared, blood coming from the mouth." But human involvement is often not limited to hunting. In the case of aurochs and tarpans (giant cousins of cows and horses respectively that roamed Europe and appear in the celebrated cave paintings of Lascaux and Chauvet),  it is believed the patterns of human domestication of subgroups of these animals may have affected their habitat and limited their grazing space and thus may have led indirectly to the demise of the larger, feral varieties. 

I found Levy's coverage of Australian megafauna particularly enthralling. During the Pleistocene, Australia was populated by its own unique collection of oversized marsupials, monotremes, reptiles, and flightless avians. These include the Diprotodon (hippopotamus-sized wombat), the Procoptodon  (over six feet tall short-faced kangaroo species), Thylacoleo carnifex (the ferocious marsupial lion), Megalania (giant monitor), Wonambi (giant constrictor), and Genyornis (large flightless bird). A wealth of megafauna fossils were excavated from limestone deposits in the Naracoorte Caves National Park (added to my destinations). However, so far we have not found any kill sites or any other direct evidence of megafauna overhunting in Australia. That said, it is possible that the early Aborigine settlers of Australia may have disrupted megafauna life in indirect ways. The Aborigine practice of setting controlled bushfires may be particularly cuplable. Burning often led to replacement of extant plant species by fire-resistant plants such as Spinifex, inedible to most herbivores, thus depleting the food sources for megafauna. The discovery of fire-scorched Genyornis eggshells hints that these bushfires may have had a role to play in the rapid disappearance of the giant birds (especially in light of the fact that they disappeared within a time period too short for pointing fingers at climate change).

Human involvement in the demise of large species is much clearer in more recent extinction events. The effects have been particularly drastic in isolated ecosystems such as Mauritius, Madagascar, and New Zealand, where the first human settlers arrived in recent years and completely upturned the ecological balance. The moa, a colossal, flightless, slow-breeding emu-relative disappeared within less than 200 years of Maori arrival in New Zealand. The gigantic Haast's eagle, "the largest predatory bird known to science," which preyed on the moa disappeared along with the moa (which I mentioned in my New Zealand travelogue). The disappearance of Madagascar's giant tortoises and giant skinks, for example, had a ripple effect. Syzygium mamillatum is an endemic plant on this island that bears conspicuously large, green fruits on its bark. The fruits use to be a food source for the giant tortoises and giant skinks, which in turn would disperse seeds and enable the plant to propagate. Today, in the absence of the seed dispersers, this plant is critically endangered. The Thylacine or the marsupial tiger of Australia vanished from the mainland soon after the possible introduction of the dingo, a wild dog species, by the Aborigines. It survived till the 20th century on the island of Tasmania. Its extinction is often attributed to direct killing by humans (as they were a threat to domestic animals) and to competition from wild dogs. Domesticated animals often disturb their wild counterparts. In Africa, with the old Masaai practice of mobile herding, elephants "once routinely traded feeding grounds with Maasai herders and their cattle. Now elephants are confined within the park while traditional herders and cattle are banished, leading to a drop in biodiversity both inside and outside park boundaries."

Although much of our knowledge of Pleistocene megafauna extinctions is based on hypothesis and conjecture, one common element in many of the mass extinction events is the plummeting of genetic diversity preceding demise. 
Mitochondrial DNA reveals a striking shift in mammoth genetics that took place about 45,000 years bp, long before the species began the steep decline in numbers that would mark the beginning of the end. ... At the moment of crisis, when the glaciers retreated and a wave of Arctic-adapted people flooded north in their wake, the mammoth’s fate may already have been sealed by this genetic loss. ... From the study of the woolly rhinoceros and its rare living cousins, the lost moa of New Zealand, and Australia’s extinct Thylacine, the researchers see a clear pattern emerging.
It thus appears that, in many of the cases, humans interfered with megafauna directly or indirectly at a time when these beasts were most vulnerable. Levy thinks understanding what happened to extinct megafauna is critical, not just in the same romantic way that spurs our fascination for the aurochs, mammoths, and tarpans painted on the walls of the Chauvet and Lascaux caves, but also in a practical way that teaches us important lessons for determining the future of extant animal species, both feral and domesticated:
Despite our high technology, our clouds of fossil fuel fumes and our endless miles of fence line, perhaps we are not so different from the Ice Age artists who made portraits of cave lions, mammoths, and aurochsen. A fascination with large animals is an ancient and abiding human trait, a part of our nature that may help us survive our own excesses. ... We can never go back to those times, but understanding our long and complicated relationship with megafauna can help us go forward. 

No comments:

Post a Comment