Geophysicist Sergey Zimov had an essay in the May 6 edition of Science concerning the ecological pressures that may have changed the Arctic ecosystem at the end of the Pleistocene. I missed it at the time, but courtesy of a reader I include some quotes here. About Zimov:
In 1989, Zimov initiated a long-term project known as "Pleistocene Park," which he now is pursuing with a number of partners. The goal of the project is to reconstitute the long-gone ecosystem of the Pleistocene epoch that supported vast populations of large animals including mammoths, horses, reindeer, bison, wolves, and other large predators. If the effort succeeds in the park, Zimov and his co-workers would like to see the ecosystem restored over much larger areas in an effort to stave off what otherwise could be a massive release of carbon that now is sequestered in the permafrost but that could be released into the atmosphere as global temperatures rise. His hunting of mammoth remains in the tundra and his bold vision of controlling and restoring ecosystems have earned him coverage in books, documentaries, and other media.
The interesting thing to me is what Zimov has to say about the forces internal to the ecosystem that may have led to its decline:
The physiological traits associated with Holocene vegetation partially explain the vegetation changes that coincided with loss of the Pleistocene megafauna. Plant transpiration accounts for most of the water loss from landscapes, and high transpiration rates are associated with more productive plants. Rates of water loss must therefore have been high in the north when productive Pleistocene meadow and steppe vegetation prevailed. As a result, vast amounts of water were sucked up from the ground, resulting in dry conditions, while the plants themselves sequestered nutrients to drive their own productivity.
Holocene vegetation, in contrast, is dominated by unproductive moss and shrubs. This type of vegetation does not transpire enough moisture to dry out the soil. Moss does not even have roots. This leads to wet conditions conducive to the growth of mosses, which account for a substantial proportion of the northern Siberian biomass. Water-saturated soils inhibit decomposition of biomass and therefore the availability of nutrients to support plant growth. What's more, mosses insulate the ground efficiently--a 20-cm layer of moss prevents the underlying frozen soil from thawing. This also has the effect of sequestering nutrients and preventing their cycling through the ecosystem. All of these factors indicate that moss communities, once they are in place, create and sustain their own environment and do not depend so much on particular climate conditions.
They are quite vulnerable to physical disturbance, however, and this is where their ecological connection to herbivores comes in.
When mosses are destroyed on loess soils, the site becomes overgrown with grasses within 1 to 2 years. The grasses then dry out the soil through their high transpiration rates, creating a steppe-like ecosystem. But when herbivore populations are low, grass productivity begins to decrease within a few years, because grass litter accumulates on the soil surface, shading and insulating the soil. In turn, soil fertility declines. As a result, shrubs and mosses, which have lower nutrient requirements than grasses, ultimately become dominant (Zimov 2005:797).
Thus, Zimov believes that the herbivores themselves maintained the grasslands, and when human predation became a severe enough pressure on the herbivores, the ecosystem lost its major regulator.
I can imagine that substantial hunting pressure may have induced an ecological collapse. A reduction in the population density of large herbivores would have allowed them to become less mobile, possibly removing their influence from large areas of the tundra-steppe. The invasiveness of the moss-lichen tundra ecosystem then replaced the grasses in such areas. These high-moisture plants might have colonized the areas originally covered by glaciers more rapidly anyway, so that climate change, human predation, and plant ecology all interacted in the terminal Pleistocene.
On the other hand, I wonder where fire comes in, since it is a major reason for the maintenance of many grassland ecosystems today. Perhaps grassland staves off forest because of fires, but cannot stave off moss?
In any event, Zimov is persuasive when he writes about the non-simultaneity of climate changes and extinctions:
Twenty years ago, scientists explained the disappearance of numerous animals in the northern grasslands very simply--the arid steppe climate changed into a humid one, and when the steppe vanished so did the steppe's animals. In short, the moist Holocene climate was a catastrophe for them. In the last few years, however, a growing accumulation of radiocarbon dates of animal remains has been suggesting a different story. It appears now that mammoths survived the Pleistocene-Holocene shift. For the first 7000 years of the Holocene, they persisted on Wrangell Island in the Arctic Ocean. Bison, horses, and musk oxen also lived in the north of Siberia in the Holocene. Horses and musk oxen lived there even up to historical times (Zimov 2005:797).
But overall, if climate were the only controlling factor, the total pasture productivity and the number of herbivores should have increased in the Holocene. Support for this view comes from the climate history that is chronicled in the Greenland ice sheet. It shows a sharp warming and dramatic increase of precipitation ~14,700 years ago, leading to conditions that resemble the present climate. Even so, in the north of Siberia, mammoth populations soared at this time (Zimov 2005:798).
The question for me is not whether people could have reduced the number of mammoths enough for this to happen. The mammoth tusks that show repeated pregnancies in females, presumably caused by humans killing the offspring, are enough to persuade me of that. The question is whether the mammoths themselves were really enough to maintain the whole ecosystem. Looking at the activity of elephants in Africa, I can see the possibility of it. But it is surely a difficult problem to wrap the mind around.
The essay goes into some detail about the Pleistocene Park project and its attempt to reconstruct the ecosystem. The main struggle is in reducing hunting pressure, which is still depressing wild herbivore populations in Siberia.
If they get it going, I wonder if it would be useful to bring in elephants. It probably wouldn't take too long to get furry ones, with some selection for cold.
Meanwhile, don't even mention bringing back the Neandertals. A little selection and they might evolve into more of us!
Zimov SA. 2005. Pleistocene Park: return of the mammoth's ecosystem. Science 308:796-798. Summary