Genetic and conservation significance of populations at the polar vs. equatorial range limits of the Pacific coastal dune endemic Abronia umbellata (Nyctaginaceae)

Abstract

Whether geographically peripheral populations are worth conserving has been hotly debated yet remains unresolved. This is especially relevant in high-latitude countries where, within their political jurisdictions, many species reach their range limits as peripheral isolates and require conservation attention, even if they are common elsewhere. In Canada, ~ 77% of “at-risk” plant species are at their northern range limit in southern Canada but more common south of the Canada-USA border. Peripheral populations might contain little genetic variation, suffer low fitness and be prone to extinction, or they might be adapted to extreme range-edge environments and thus well-poised to participate in range shifts during climate change. Abronia umbellata is endemic to coastal dunes from Baja California, Mexico to Oregon, USA but also occurs as disjunct populations designated “at-risk” in Washington, USA and British Columbia, Canada. Based on sequence variation at nine single-copy nuclear genes assayed for 94 individuals from 25 populations across the species range, these disjunct populations were very similar to range edge populations 350 and 650 km to the south in Oregon, and likely arose through recent, long-distance dispersal or fragmentation of a recently expanded range. In contrast, southern-edge populations in Baja, though not disjunct, were genetically unique and unexpectedly diverse, may currently be in decline yet receive no conservation protection. In this case, the conservation significance of range edge populations depends on which edge, and the unprotected southern edge populations seem a higher priority than those benefitting from special status at the northern range limit.