The fossil record preserves some species much better than others, and the fact that we can’t find a species after a particular point may indicate it disappeared entirely, or just became a fair bit rarer.
“We start by asking how many species are known and how many remain undescribed. We then consider by how much human actions inflate extinction rates. Much depends on where species are, because different biomes contain different numbers of species of different susceptibilities. Biomes also suffer different levels of damage and have unequal levels of protection. How extinction rates will change depends on how and where threats expand and whether greater protection counters them.”
“The species we know best have large geographical ranges and are often common within them. Most known species have small ranges and such species are typically newer discoveries.”
Pimm’s new estimate is that the background rate is 0.1 extinction per million species per year. The rate today is between 100 and 1000 extinctions per million species per year.
The Permian-Triassic extinction event occurred approximately 252 million years ago and wiped out an astonishing 96% of all marine species and 70% of all terrestrial vertebrate species.
While we have already lost more species than in many of the more minor events, Pimm believes a combination of habitat protection, captive breeding and action on climate change can avoid a sixth mass extinction.