SEABIRD COMMUNITY STRUCTURE ALONG A PRODUCTIVITY GRADIENT: IMPORTANCE OF COMPETITION AND ENERGETIC CONSTRAINT

Abstract

The oceanic eastern tropical Pacific supports a speciose seabird community that feeds in flocks and depends for food on schools of tunas and dolphins, which force prey to the surface. We analyzed data collected throughout an area of 40 3 10 6 km 2 of open ocean over 10 years (1979-1988) during 1136 at-sea days, from 1750 feeding flocks comprising 51 688 birds of 49 species. A PCA identified three distinct flock types based on species composition: ''Sooty Tern Flocks'' ( n 5 941 flocks) with a large number of Sooty Terns (Sterna fuscata), ''Juan-Wedge Flocks'' ( n 5 402 flocks) with a large number of Juan Fernandez Petrels (Pterodroma externa) and Wedge-tailed Shearwaters (Puffinus pacificus ), and ''Booby Flocks'' ( n 5 407 flocks) with a large number of Red-footed and Masked Boobies (Sula sula and S. dactylatra, respectively). Flock types exhibited largely disjunct distributions that mirrored patterns in thermocline depth and surface water chlo- rophyll content. Sooty Tern Flocks occurred in areas with deepest thermocline (77.6 m) and lowest chlorophyll (0.14 mg/m 3 ), Juan-Wedge Flocks in areas of intermediate ther- mocline (68.8 m) and chlorophyll (0.16 mg/m 3 ), and Booby Flocks in areas of shallowest thermocline (62.5 m) and highest chlorophyll (0.17 mg/m 3 ). These differences were sta- tistically significant. As thermocline depth and surface water chlorophyll are reliable in- dicators of surface water productivity, we conclude that Sooty Tern Flocks foraged in waters of low productivity, Juan-Wedge Flocks in waters of intermediate productivity, and Booby Flocks in waters of highest productivity. Differences in published cost-of-flight values support the hypothesis that energetic con- straint determines flock structure at the low-productivity end of the system. Sooty Terns have the lowest flight costs (4.8 W) and feed in waters of lowest productivity. Flight costs are progressively higher for those species feeding in more productive waters (9.9 W for Wedge-tailed Shearwaters, 19.0 W for Red-footed Boobies). Differences in body size sup- port the hypothesis that interference competition determines flock structure at the high- productivity end of the system. The largest species, Masked Boobies (1987 g) and Red- footed Boobies (1003 g), feed in areas of highest productivity; progressively smaller species, Juan Fernandez Petrels (430 g) and Wedge-tailed Shearwaters (388 g), and Sooty Terns (180 g), feed in areas of decreasing productivity. We hypothesize that this community is largely structured by two factors, competition and energetic constraint, that operate at opposite ends of a productivity gradient. As productivity decreases (low prey abundance), flight proficiency is increasingly important because only species with relatively low flight costs will be able to transit between prey patches. As productivity increases (high prey abundance), competitive ability is increasingly important because competitive dominants will exclude other species from feeding flocks. Our hypothesis is in accordance with em- pirical data and theoretical models designed to explain factors structuring communities for a wide variety of habitats and taxa.