6 ideas
The number of species an area can hold scales roughly with the fourth root of its area, so cutting a habitat to one-tenth of its size eventually removes about half its species. Applied to tropical deforestation rates, this rule lets extinction be estimated from habitat loss without cataloging each species, and Wilson's conservative figure is roughly 27,000 species lost per year.
The 1883 eruption wiped Krakatau clean of life. Within decades, bacteria, ferns, spiders, and wind- and sea-borne plants and animals reassembled a functioning ecosystem. The case shows that life recovers quickly from a local catastrophe because nearby source populations can disperse into it, whereas global mass extinctions leave no such reservoir.
A human-caused sixth extinction is therefore effectively irreversible on any human timescale. What is destroyed now cannot be restored by later restraint.
When a lineage reaches an environment with open niches and few competitors, such as a remote archipelago, a new lake, or a world emptied by extinction, it splits rapidly into many species specialized for different roles. The Hawaiian honeycreepers and the hundreds of cichlid fishes of Lake Victoria each descended from one or a few founders and diverged into specialized feeders.
Each species is a unique archive of genetic information refined over millions of years. Most species have never been named, let alone studied for medicines, foods, or other uses. The rosy periwinkle of Madagascar, which yielded drugs that transformed survival from childhood leukemia and Hodgkin's disease, shows that losing an unstudied species means destroying potential value before it is known.
Most speciation is allopatric: a population is split by a barrier, and the separated groups accumulate genetic differences until they can no longer interbreed if they meet again. Species are defined by reproductive isolation.