Occupancy and extinction probability: Are locally restricted species disproportionately extinction-prone?

Background/Question/Methods

Locally restricted (e.g., endemic) species are at greater risk of extinction than are more regionally widespread species.  This fundamental principle is dictated primarily by probability because locally restricted species lack geographic redundancy.  In the most basic case, an endemic species will be driven extinct if it is extirpated from a single location, whereas more widespread species will still occur elsewhere if extirpated from a single location.  There may also be ecological reasons that lead to locally restricted species being disproportionately extinction-prone, however.  For example, locally restricted species may be more prone to extinction because they generally occur in lower abundances and cannot benefit from rescue effects from other populations.  This combination of probabilistic and ecological factors may result in populations of geographically-rare species being subject to disproportionate local extirpation, leading to extinction rates that are higher than would be expected based on a random extinction process.  Here, we apply a null model simulation analysis to several datasets on recent mass decline and extinction events to more formally evaluate the relationship between occupancy and extinction probability.

Results/Conclusions

Our results generally indicate that the extinction rates of geographically restricted species are primarily driven by probability, suggesting that endemic species appear not to be disproportionately at risk of extirpation or regional extinction.  That is, despite the high rate of loss of geographically restricted and endemic species (with > 50% regional extinction rates), these rates do not differ substantially from what would be expected randomly.  Surprisingly, we find that more widespread species, e.g., those occupying about half of surveyed sites, are driven extinct more frequently than probability would dictate and appear to be disproportionately at risk of local and regional extinction.  If general, these results suggest that different extinction processes may be operating among species with differing range sizes, with implications to ecological theory and biodiversity conservation.