A very good point. Something not many people know is that we're currently living through an extinction event - the sixth great extinction event to occur in Earth's history - and it's due to human activity.
http://en.wikipedia.org/wiki/Holocene_extinction
It's a very difficult thing to estimate, but the extinction rate has been estimated to be running at 10,000 times the "background rate", up to perhaps 140,000 species per year. A very sad fact.
Indeed.
A very interesting point on that page (further lending evidence at least in theory towards the premise that humans have the capacity to enact staggeringly quick climate change) - regarding megafauna.
"Effect on methane emissions
Large populations of megaherbivores have the potential to contribute greatly to the atmospheric concentration of methane, which is an important greenhouse gas. Modern ruminant herbivores produce methane as a byproduct of foregut fermentation in digestion, and release it through belching. Today, around 20% of annual methane emissions come from livestock methane release. In the Mesozoic, it has been estimated that sauropods could have emitted 520 million tons of methane to the atmosphere annually, contributing to the warmer climate of the time (up to 10 C warmer than at present).
This large emission follows from the enormous estimated biomass of sauropods, and because methane production of individual herbivores is believed to be almost proportional to their mass.
Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in atmospheric methane. This hypothesis is relatively new.
One study examined the methane emissions from the bison that occupied the Great Plains of North America before contact with European settlers. The study estimated that the removal of the bison caused a decrease of as much as 2.2 million tons per year.
Another study examined the change in the methane concentration in the atmosphere at the end of the Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas about 13,000 BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species there. Calculations suggest that this extinction decreased methane production by about 9.6 million tons per year. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt
climatic cooling at the onset of the Younger Dryas.
The decrease in atmospheric methane that occurred at that time, as recorded in ice cores, was
2-4 times more rapid than any other decrease in the last half million years, suggesting that an unusual mechanism was at work."