Ecological Role of PreHistoric Humans

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
none given

 Have human populations altered ecosystems and extinguished species prior to developing advanced technology? We know that extinction is a natural event – is it also natural for humans to be the agents of extinction, even widespread extinctions? We examine these questions by focusing on human-induced animal extinction, particularly the extinction of North America’s megafauna at the end of the Pleistocene.

 Human-caused Plant Extinctions

Most of the available evidence (so far as I know) concerns over-hunting of animal populations. But it seems likely that "man the gardener" (or "woman...") might have significantly altered plant communities. Artificial selection for desirable plant attributes has occurred since the dawn of agriculture. Deforestation and irrigation were taking place on a grand scale by 4,000 years ago. Fire was used as a way to drive animals to killing sites, and it is thought that fires were more frequent due to Native American populations. Human-set fires are thought to be responsible for extinction of native palms on Easter Island. Human-domesticated grazers such as sheep are credited with widespread desertification in Middle East. Surely plant extinctions resulted.

 Human-caused Animal Extinctions

 Human hunting is credited with a number of extinction events between 10,000 and 50,000 years ago. Extinction of large mammals in Africa are thought to have happened earliest, and least catastrophically, because of the long association (co-evolution) with humans. Extinction of large flightless birds (moas, 13 species, 20 to 200 kg) in New Zealand is attributed to the Maori, who arrived ~1000 AD and may not have completed this extinction until ~1800. The most argued example is the Pleistocene extinction in North America, some 10,000 yrs ago, attributed to the arrival of humans in the Americas.

 The Pleistocene Extinctions

 What did the mammalian fauna of North America look like more than 10,000 years ago? (transparencies). And what did this fauna look like only a few thousand years later? (transparency). About 10,000 yrs BP, most of the large-bodied taxa then in existence went extinct. About 39 of 45 North American genera of large mammals went extinct (and slightly later, in Sout America, 56 of 153 genera of all mammals went extinct).

At about that time, humans arrives in North America, and the glaciers began their last retreat. Both are possible causes of this major extinction event.

The over-hunting argument:

 The climate change argument:

The counter-arguments:

An Alternative Model:

 Studies of large mammals in Africa illustrate two complexities of species interactions:

  1. "sequence" grazers: 600,000 wildebeast migrate through the Serengetti plains of Kenya during the spring migration. Their feeding and trampeling shortens the green grass, but also loosens and fertilizes the soil. Then, 32 days after the wildebeast migration, Thompson's gazelle migrate north, preferentially using the areas grazed by wildebeast. These are sequence grazers, part of a system finely tuned to the climate, plant productivity, and other grazers.
  2. "keystone" species: Where the elephant is abundant in Africa, it destroys much of the forest and much of the grazing land. Where it is rare, the forest closes in, and small grazers such as antelope are disadvantaged. Where elephant density is intermediate, a mixed savannah-forested area develops, and multiple grazers coexist.

Perhaps many highly-evolved specialist species occupied the tundra, grasslands and spruce-swamp-like ecosystems at the end of the Pleistocene. It is possible that the megaherbivores declined because environmental change affected the availability of certain plants, which might have shifted the dynamic equilbrium between plants, grazers and browsers. Overhunting might have been facilitated by the concentration of certain species in restricted habitats. Reduction in sequence grazers and keystone species could have precipitated other declines, in a complex interplay of all possible mechanisms.

Transparencies: 1. Human-caused animal extinctions 2. a Pleistocene bestiary 3. Pre- vs. post-Pleistocene fauna of North America