Aim: The 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.

Location: North-eastern Pacific rocky intertidal zone (~26-40ºN)

Taxon: Tetraclita rubescens (Nilsson-Cantell, 1931), Balanomorpha

Methods: We use photo quadrats 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.

Results: Bent 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 conclusions: To be partly resilient to the effects of tropicalisation, temperate prey must acclimatize/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.

The datasets come from analysis of photo quadrats taken in the field from sites spread across the geographic range of Tetraclita rubescens.  Size data of each barnacle and morph were taken using the software programme ImageJ. We also used occurrence records from iNaturalist to supplement our datasets. The methods section of the paper provides more detail.