Theory 21/Paleontology & Mass Extinction /Extinction selectivity/3 min
Extinction Risk Scales With Size
Big animals are said to be at risk. The risk may follow a specific curve, and the curve's shape may name the cause.
Extinction risk in mammals scales as a power of body size rather than depending on size categorically, so the selectivity of an extinction event can be predicted from the exponent, and the exponent itself indicates which mechanism is operating.
01The evidence
- A three-quarter power scaling of extinction risk in Late Pleistocene mammals has been reported, together with a new theory of the size selectivity of extinction, which makes the exponent the object of explanation.KNAW research portal
- Body size downgrading of mammals over the late Quaternary has been documented as a systematic pattern, which is the aggregate signature a power-law selectivity would produce.Body size downgrading
02The tension
Extinction selectivity is described categorically: large-bodied species are more vulnerable. That description fits the data and cannot predict how much more vulnerable, or why the degree of selectivity differs between events. A category has no magnitude.
03The theory
If risk scales as a power of body size, then the selectivity of an event is summarised by the exponent, and different mechanisms produce different exponents. Hunting, for instance, should select more steeply than climate, because it targets large individuals directly.
That makes the exponent a diagnostic. An event with a steep exponent was probably driven by a size-biased agent; one with a shallow exponent by a process that affects all sizes but hits small populations harder.
The prediction is that the exponent should be measurable and should differ between events with different suspected drivers, and that the same driver should produce a consistent exponent across regions.
The consequence is that extinction selectivity becomes quantitative, and the cause of an event can be inferred from the shape of the selectivity curve rather than from its direction.
04What this is not
Categorical accounts say large species are at risk. This says risk follows a power law whose exponent identifies the driver, which turns a description into a diagnostic.
05Predictions
- P1Risk should scale as a power of body size rather than stepwise.
- P2The exponent should differ between hunting-driven and climate-driven events.
- P3The same driver should give a consistent exponent across regions.
06What would refute it
Selectivity that is stepwise rather than power-law, with no exponent differences between drivers, would leave the categorical account adequate.
07What it would change
It would make extinction selectivity a diagnostic tool, and change how causes are attributed.
08Evidence base
- The three-quarter power scaling of extinction risk in Late Pleistocene mammals — KNAWhttps://pure.knaw.nl/portal/en/publications/the-three-quarter-power-scaling-of-extinction-risk-in-late-pleis/
- Body size downgrading of mammals over the late Quaternaryhttps://scholar.google.fi/citations?view_op=view_citation&hl=fi&user=RrsIFt8AAAAJ&citation_for_view=RrsIFt8AAAAJ:fEOibwPWpKIC