The last large-scale mass extinction of animal and plant species, which wiped out the dinosaurs 65 million years ago, also had positive consequences: it was followed by the expansion and diversification of mammals and, subsequently, by the appearance of human beings. It was as if the old branches had been lopped off the tree to make way for the new shoots. Why, then, should we regard the current reduction in biodiversity as a source of alarm?
Our planet has already experienced five similar large-scale episodes, caused by natural catastrophes, climate change, or the impact of meteorites. The most extreme, at the end of the Primary Era, some 250 million years ago, wiped out more than 80% of species then in existence. Each time, after millions of years, biodiversity re-establishes itself and is enriched with a whole array of new families and genera.
In between two crises, a natural cycle occurs involving the appearance, evolution, and extinction of living species. It is estimated that one vertebrate species will disappear in this way every 50 to 100 years. In the course of the last 400 years, however, 151 species of higher vertebrates have become extinct-in other words, one species every 2.7 years. Today, one bird in eight, one mammal in four, and one-third of all amphibians is at risk. If we look at the living world as a whole, the extinction rate is some 1,000 times faster than we would naturally expect. The problem is not that certain species are disappearing: it is the speed at which this is happening.
This is why it is thought that the planet has now entered its sixth large-scale mass extinction of plant and animal species. Unlike previous extinction events, which have occurred over thousands or even millions of years, this one is taking place over decades or centuries; the process is so rapid that ecosystems are becoming incapable of adapting to it.
The other difference is that this particular event can be attributed to one single species-human beings. Unlike the dinosaurs, however, mankind also possesses the means to counteract this phenomenon.