Top-down and bottom-up forces interact at thermal range extremes on American lobster

Exploited marine populations are thought to be regulated by the effects of fishing, species interactions and climate. Yet, it is unclear how these forces interact and vary across a species' range. We conducted a meta-analysis of American lobster (Homarus americanus) abundance data throughout the entirety of the species' range, testing competing hypotheses about bottom-up (climate, temperature) vs. top-down (predation, fishing) regulation along a strong thermal gradient. Our results suggest an interaction between predation and thermal range - predation effects dominated at the cold and warm extremes, but not at the centre of the species' range. Similarly, there was consistent support for a positive climate effect on lobster recruitment at warm range extremes. In contrast, fishing effort followed, rather than led changes in lobster abundance over time. Our analysis suggests that the relative effects of top-down and bottom-up forcing in regulating marine populations may intensify at thermal range boundaries and weaken at the core of a species' range.