The claim that current species-loss rates are 100–1,000 times the background rate is a standard, widely cited figure in conservation biology. It is not an exact measurement but a robust order-of-magnitude comparison between documented recent extinctions and the long-term “background” rate inferred from the fossil record.
How the numbers are derived?
Background rate: Typically estimated at 0.1–2 extinctions per million species-years (E/MSY). A commonly used conservative value is 1–2 E/MSY (roughly 1–2 extinctions per 10,000 species per century). Some analyses place the pre-human rate even lower (0.1 E/MSY).
Recent rates: Documented extinctions since 1500 (especially among well-studied groups such as vertebrates) yield rates of roughly 100 E/MSY or higher. Ceballos et al. (2015) found that even using a relatively high background rate of 2 E/MSY and only confirmed extinctions, vertebrate losses over the last century were up to 100 timesbackground. Other estimates that include probable extinctions or groups with higher undocumented losses (e.g., many invertebrates) push the multiplier toward 1,000 times.
IPBES (2019) summarized the situation as “tens to hundreds of times higher than the average over the past 10 million years.”
Important caveats
Documented global extinctions remain a small percentage of described species (hundreds of vertebrates and a few thousand species overall in the last 500 years). The high rate reflects how few extinctions would be expected in a short interval under background conditions.
Most scientists therefore describe the present as the onset or early stages of a sixth mass extinction rather than an event that has already reached the ~75% species-loss threshold of the “Big Five.” At current rates it would still take many thousands of years to reach that magnitude, though rates could accelerate.
The comparison is strongest for vertebrates (mammals, birds, amphibians). Invertebrates and plants are far less completely assessed, so overall rates could be higher or lower depending on the group.
Population declines and local extirpations are far more widespread than global extinctions; these “biological annihilation” effects are often considered a more immediate signal of the crisis.
In short, the 100–1,000× range is a scientifically supported characterization of elevated extinction rates, not a claim that most species have already disappeared. The main drivers are habitat loss, overexploitation, invasive species, pollution, and climate change.