A marine biogeographic and phylogeographic break around 30°S along the shores of the southeastern Pacific coincides with a mesoscale change in coastal upwelling regime. Prior ecological and genetic evidence indicated that dispersal limitation was the leading mechanism maintaining the break. Recent evidence of large environmental heterogeneity between sites along the transitional zone provides support for a niche-based mechanism as the underlying driver. To test this hypothesis, we examined patterns of phenotypic plasticity under contrasting thermal conditions for species belonging to several taxa and multiple trophic levels, which were sourced from local populations spanning the biogeographic break. We pooled plasticity patterns with data from similar studies over multiple sites spanning the break and examined their joint spatial structure and its relationship with environmental heterogeneity. We observed that regardless of taxa and trophic level, individuals from populations within the biogeographic break showed higher phenotypic plasticity. Similarly, the spatial structure in neutral genetic markers for this diverse set of benthic species, which either spanned the transition zone or found the edge of their geographic ranges around it, hints at the presence of local adaptation. Our results suggest that evolutionary novel ecological interactions are taking place under environmental conditions that challenge species’ physiological limits. The species that span the transition zone are able to persist locally likely through a net influx of propagules; the strong selective pressure highlighted by plasticity patterns, together with our tentative evidence for local adaptation, suggests that the region may represent an area of special interest for future conservation efforts