Resource partitioning within a tropical seabird community: new information from stable isotopes

摘要

Characteristics of the tropical oceanic environment (low productivity, little seasonality) and poor diversity of tropical seabird foraging methods and prey relative to temperate and polar species suggest that tropical seabirds overall encompass a narrow range of isotopic niches, with large overlaps among species. To test this hypothesis, we examined the stable carbon (δ~(13)C) and nitrogen (δ~(15)N) isotopic composition of blood and feathers of 5 seabird species from Europa Island, Mozambique Channel. While differences were small, blood δ~(13)C and δ~(15)N values characterized 5 distinct and non-overlapping trophic niches during the breeding period. Seabirds used 2 distinct foraging areas (δ~(13)C), one used by sooty tern Sterna fuscata and white-tailed tropicbird Phaethon lepturus and one used by red-footed booby Sula sula and great Fregata minor and lesser F. ariel frigatebirds. Seabird species overall encompassed less than 1 trophic level (δ~(15)N), which is in agreement with a diet mainly based on flying fish and squid. Feather δ~(13)C and δ~(15)N values showed that the trophic structure of the community was different during the breeding and non-breeding (moulting) periods, suggesting a shift in the feeding ecology when adult birds were no longer central-place foragers. The stable iso-tope method underlined sex-related (red-footed booby) and age-related (great frigatebird immatures and adults) feeding strategies. It also suggested that breeding adults could feed themselves on different prey than those given to their chicks (sooty tern). Within the tropical pelagic ecosystem, seabirds overall shared the same trophic level as large predatory fishes (albacore, yellowfin and skipjack tunas), but they had lower δ~(15)N values than the deeper-dwelling bigeye tuna and swordfish. We conclude that analyzing stable isotope values in blood and feathers appears to be a promising alternative method for investigating food and feeding ecology of tropical seabirds year round, and for determining sex- and age-related differences in their foraging strategies. A limitation of the method is the lack of information on marine isoscapes; future studies aimed at isotopically characterizing the tropical marine environment could help to associate consumer signatures to geographic origins.