Predator‐induced defences under tropicalisation: A biogeographic approach

Abstract

AbstractAimThe biogeography of predator‐induced defences is an understudied area of predator–prey dynamics. Range overlap with predators that induce the response and local demographics (e.g., prey abundances) are likely to be important factors for determining the biogeographic distribution of induced defences within species. However, with climate warming, range‐expanding warm‐water predators are increasingly preying upon temperate species. This is a consequence of a wider phenomenon known as tropicalisation. We aim to determine: (i) if individuals of a temperate barnacle with induced defences (‘bent morphs’) are primarily present where they co‐occur with range‐expanding warm‐water predators (muricid snails) and, (ii) if bent morphs are size‐structured within populations.LocationNorth‐eastern Pacific rocky intertidal zone (~26–40° N).TaxonTetraclita rubescens (Nilsson‐Cantell, 1931), Balanomorpha.MethodsWe use photoquadrats from sites across the range of T. rubescens to determine the biogeographic distribution of populations with bent morphs and to assess size‐structure. We use a combination of field surveys, literature, and museum occurrences to assess range overlap between cool and warm‐water predators of T. rubescens and their association with populations with bent morphs and abundance patterns of T. rubescens.ResultsBent morphs are commonly found within the equatorward portion of the species' range (where abundances are highest), in populations overlapping with range‐expanding warm‐water predators. Bent morphs primarily occur within the smaller size classes.Main conclusionsTo be partly resilient to the effects of tropicalisation, temperate prey must acclimatise/adapt to altered predator–prey dynamics. Predator‐induced defences are one way to do this. We show that bent morphs within a temperate prey species (T. rubescens) are largely restricted to populations that overlap with large‐bodied and range‐expanding warm‐water predators. This is evidence for the partial resilience of T. rubescens to tropicalisation and provides the rationale for further exploration of the eco‐evolutionary consequences of tropicalisation in this study system and others.