More research for my current WIP, which has underlying environmental themes at the heart of the story, resulted in more interesting facts to share. (Researching a novel results in so much self-education it should come with its own diploma!)
Over the course of history, there are believed to be five major mass extinction events--those ending the Ordivician (440-450mya), Devonian (360-375 mya), Permian (251 mya), Triassic (205 mya) and Cretaceous (65 mya) periods.
In addition, the Cambrian mass extinction also saw considerable loss of species and could be considered a sixth.
It may be time to add number seven. The Holocene Extinction Event. In other words, right now. No cataclysmic gamma ray burst or asteroid impact triggered this event. We have met the enemy and they is us.
According to a survey in 1998 by the American Museum of Natural History, up to seventy percent of biologists believe 50% of all species could disappear in the next 100 years, making this among the most destructive and fastest extinction events on record. Some scientists believe, however, there isn't enough data available to compare current trends with the mass extinctions apparent in fossil evidence of the past.
Even so, I think it becomes apparent why environmental issues are at the forefront of so many Near Future, Futuristic, and Science Fiction stories. The environment is a major concern of our times that may profoundly effect the future of our governments, cultures and economies. (I know what you're thinking. What'dya mean, future?)