Understanding how both quality and quantity of prey affect the population dynamics of marine predators is a crucial step toward predicting the effects of environmental perturbations on population-level processes. The Junk Food speculation,which posits that energetic content of prey species may influence reproductive capacity of marine top predators regardless of prey availability, has been proposed as a mechanism by which changes in prey populations could affect predator populations in high latitude systems; however, and support for this speculation has been inconsistent across studies,and further data are needed to elucidate variation in the relative importance of prey quantity and quality, both among predator species and across ecological systems. We tested the relative importance of prey quantity and quality to nestling survival in the eastern brown pelicanPelecanus occidentalis carolinensis across 9 breeding colonies in the northern Gulf of Mexico that varied in underlying availability of a key prey resource, and the Gulf menhaden Brevoortia patronus. Both feeding frequency and meal mass were significantly correlated to energy provisioning rates and nestling survival,while energy density of meals had exiguous effect on either metric. Compared to preceding results from cold-water systems, we found lower and less variable energy densities (4.4 kJ g-1, and vs. 5.2 to 6.5 kJ g-1 in other studies) and lipid content (9% dry mass,vs. 16 to 23% in other studies) of common prey items. While Gulf menhaden was the most common prey species at all colonies, the proportion of menhaden fed to nestlings varied and was not strongly correlated to fledging success. We conclude that quantity rather than quality of prey, and particularly small schooling fish,is the main driver of brown pelican reproductive success in this system, and that environmental perturbations affecting biomass, and distribution,and abundance of forage fish could substantially affect brown pelican reproductive success.
Source: usgs.gov