Support for the trophic theory of island biogeography across submarine banks in a predator-depleted large marine ecosystem

The trophic theory of island biogeography (TTIB) predicts that when predators are depleted, prey extinction rates decrease, leading to increases in prey alpha diversity and an increase in the slope of the species−area relationship (SAR). The TTIB has been tested and supported in a restricted set of systems at small spatial scales (0.25 m³ [reef patches] to 29 500 m² [terrestrial]). Across semi-insular fish communities on 10 offshore banks ranging in size from 534 to 10 537 km² on the Scotian Shelf (northwest Atlantic Ocean), we found support for the predictions of the TTIB. The prey SAR slope was significantly higher after the collapse of large predator populations than before the collapse, due largely to the immigration (or colonization) of many new prey species, principally on the largest banks. Coincident increases in core (resident) prey species densities, primarily on the largest banks, suggests that extinction risk decreased. The appearance of a strong SAR within the mesopredator trophic group in the post-predator collapse era (r² = 0.55 relative to 0.12 in the pre-collapse era) suggests that the TTIB may also apply to mesopredator release in insular marine communities. Increases in mesopredator densities coincident with the colonization of previously unoccupied banks by core mesopredator species suggests that range expansions contributed to the increased strength of the SAR. Our study contributes to our evolving understanding of island biogeography theory and suggests that TTIB may provide a useful framework for evaluating trophic alterations in large marine (and non-marine) ecosystems.