PS 41-138 - Community and population impacts of two range-expanding predators, the whelks Mexacanthina lugubris and Acanthinucella spirata, along the North American Pacific coast

Abstract: The climate-driven redistribution of species is a widespread consequence of global change, occurring across nearly all ecosystems and taxonomic groups. The movement of range-shifting species into novel communities can result in severe ecological impacts and, in particular, range-shifting predators are expected to have strong (and potentially cascading) impacts on lower trophic levels and community diversity. Along the Pacific Coast of North America, two species of predatory whelks, Mexacanthina lugubris and Acanthinucella spirata, are undergoing polewards expansions. Mexacanthina lugubris has recently expanded ~200 km from Baja California, Mexico, into southern California, USA, where it now co-occurs with A. spirata. Likewise, A. spirata has expanded ~250 km from Tomales Bay, California to Cape Mendocino, California. Both whelk species are known to consume acorn barnacles (Balanaus & Chthamalus spp.) and California mussels (Mytilus californianus). To determine the extent to which range-expanding predators can impact prey abundances and community diversity across their ranges, we conducted intertidal surveys in three regions: Baja California (M. lugubris native range), Southern California (M. lugubris expanded, A. spirata native range) and Northern California (A. spirata expanded range) and examined how community richness and prey abundances correlated with whelk abundances. We predicted the abundance of both prey species and overall community richness would decrease with greater whelk abundances and the impacts would be greater in higher latitudes (i.e, in the whelks’ expanded ranges). We found that higher whelk abundance was associated with lower barnacle abundances, particularly in southern California. Mussels, however, tended to be more abundant in areas with greater M. lugubris abundance (but their abundances were not related to A. spirata abundance). Similarly, we found community richness increased at greater M. lugubris abundance, but was unaffected by increasing A. spirata abundance. Altogether, our results suggest the impacts of these range-expanding predators differ depending on the response metric. These whelks may act as “drivers” of declines of some prey species (like barnacles) but “passengers” in the case where a prey species (like mussels) also creates habitat and is, itself, a driver of community diversity. These results highlight mechanisms that could lead to idiosyncrasies in the response of communities to range shifts driven by global change.