How local extinction changes rarity: an example with Sonoran Desert fishes

That spatially rare species may be predisposed to extinction is a common tenet of ecology. However, the opposite side of the relationship – how extinction alters spatial rarity – remains little explored. We used an extensive biodiversity database to contrast patterns of spatial rarity of a biogeographic assemblage of native Sonoran fishes before and after an extensive, decades‐long wave of extirpations. Focusing on 25 fish species native to the Lower Basin of the Colorado River, we analyzed two key aspects of spatial rarity: 1) species range sizes (expressed as kilometers of stream reach occupied) and 2) species’ co‐occurrence patterns. Native fish species that were spatially rare historically suffered disproportionate losses in occurrences. However, endemic species did not suffer increased losses relative to non‐endemic (but still native) species of comparable rarity. Species’ geographic range sizes were concordant through time, with spatially rare species remaining rare after extensive extirpations relative to species that were historically more widespread. In contrast, extirpations greatly disrupted patterns of species co‐occurrence on both local and regional scales. Over 50% of the species pairs that historically co‐occurred (in the same 5 km reach) no longer co‐occur anywhere in the Lower Basin, and species pairs that infrequently co‐occurred in historic times suffered greater proportional losses than did more widely co‐occurring pairs. Such changes in the relationship between spatial rarity and species richness deserve attention because they inhibit conservation planning (decreasing the efficiency of reserve design) and reduce interaction diversity altering opportunities for long‐term co‐evolutionary change.