How important is biodiversity?

Stable unchanging climates lasting for several thousand years tend to reduce diversity, but when the climate changes to either hotter or colder by a small amount the first response is an increase in biodiversity. This is because the new conditions give rare species a chance to flourish while the established ones have not had time to decline … [I]t is important to keep in mind that biodiversity and environmental quality are not simply proportional … So rich biodiversity is not necessarily something highly desirable and to be preserved at all costs.

Source: James Lovelock, via Freakonomics