Quantifying extinctions
Measuring extinctions over time involves both rate and magnitude.
Rate = the number of extinctions ÷ time of extinction occurrence)
Magnitude = % of species that have gone extinct
Previously, extinctions were calculated using the rate; a rapid increase would indicate a mass extinction. However, this approach assumes that extinctions always rise. In certain cases, mass extinctions are caused by decreasing origination rates (eg. the Devonian and Triassic events). Thus, palaeontologists characterise mass extinctions by the sum total of species disappearance.
When extinction rates accelerate relative to origination rates such that over 75% of species disappear within a geologically short interval—typically less than 2 million years, in some cases much less.
Assessing the magnitude of the sixth mass extinction, means comparing current extinctions against background rates and measuring whether past and projected biodiversity loss in the holocene-anthropocene epoch exceeds 75% of global species.
Too much science? Enjoy an artistic interlude of the 5 mass extinctions captured in the fossil record.
Data disparities?
It is worth noting the methodological differences in collecting data on past and present extinctions (Table 1). Currently, scientists are limited to taxonomically defined species. The comparison group for current extinctions is against 4% of the IUCN's documented species and 1% of species preserved the fossil record. Therefore, only assessing a subset of species inevitably ignores species that have not been quantified. Furthermore, the per-taxon extinction rate is biased by interval length (eg. calculating a rate over a shorter time will yield a rate that is much faster or slower than the average million-year rate, depending on increasing or decreasing species). Nonetheless, the overall trend of species loss is undeniable.
Table 1. Comparison groups for current and fossil data on species extinction (Author, 2020)
Enter E/MSY
Devised by scientists, this metric is widely used to quantify extinction rates. E/MSY asks if we had a million species, how many would we expect to go extinct in a year?
E/MSY = Extinctions per million species per year.
If we apply the background rate of 2E/MSY (2 extinctions every 1 million years), to the ~5500 species alive today, we would expect 1 mammalian extinction every 100 years. In reality, 43 threatened species (extinct in the wild) are predicted to disappear without conservation efforts, with recent extinctions measuring at 20 - 200/MSY! Moreover, we have exceeded the safe operating space according to the planetary boundaries framework, which sets 10E/MSY as the boundary for biodiversity loss.
All that stands between us and the sixth mass extinction is keeping the threatened species on earth. Otherwise, within 3 centuries, 3 out of every 4 familiar species will be gone.
The good news is that less that 1% of all species (including unnamed) are actually extinct, we still have the species from the past 12,000 years. This means we have the potential to halt extinction in its tracks. Biodiversity's hope rests upon us to stop over-exploitation and step up conservation. Next week, I shine light on a cruel taste driving extinctions.
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