Assessing extirpation risk

Detection frequency

A useful metric for assessing extirpation risk is the detection frequency or sighting rate of a species. Species that were observed only once historically may have a different likelihood of persistence than those recorded repeatedly before disappearing from records. In other words, the number and frequency of historical observations can inform our prior belief about whether a species is likely still extant or has been extirpated.

When many records attest to a species’ historical presence, we can be more confident that it represented a well-established population. Conversely, if a species was reported only once or twice, historical populations may have been small, transient, or non-viable—or the species may still be present but overlooked due to rarity or limited sampling effort.

Sighting rates thus constitute informative prior knowledge that can be incorporated into models to estimate extinction probabilities.