Some of you might have stumbled upon the recent evidence that the drastic drop in population occurred in many "megafauna" lineages in Siberia ~37,000 years ago (10,000 years prior to the arrival of humans in large numbers to the region). This is interesting to me in light of the fact that many paleoanthropologists have attempted to pin the mass extinctions of large mammals in Australia and the New World on humans, but recent moa data in combination with this data is suggestive (though definitely not conclusive) of a pattern where modern humans are exonerated of blitzkrieg mass-killings. It seems plausible to me that many large mammals would go through cycles in terms of the density of the breeding population through their range, and the difference with the arrival of humans is that we might have prevented the "bounce back" that would normally have been inevitable. Similarly, I have read that the penguins and other animals have exploded in population with the decline in the whale species because of human hunting, and some ecologists wonder if the whale population can ever regain their "natural" position within the ecosystem now that it seems to have re-equilibrated.