The role of individual foraging specialization in the trophic relationships between seabirds and the marine environment

Abstract

Trophic relationships are central in ecology and play a crucial role in species survival, as availability of food resources varies over time and space. The spatio-temporal variation in food sources at sea has many ecological implications on marine top predators such as seabirds. However, most ecological studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent, but intra-specific variation in individual foraging strategies can be large and many apparently generalized populations are in fact composed of specialized individuals that use a small subset of the available resources over time (individual consistency). Studies on the incidence of individual specialization suggest that it may vary among species and among populations, but they are scarce, particularly for seabirds, and the mechanisms that generate inter-individual variation are not well understood. The main goal of this study is to corroborate the existence of individual specialization over time in three different species from four seabirds’ populations exploiting different marine environments. Furthermore, it investigates whether individual specialization is associated with the environment and resources exploited and assesses its ecological implications at the population and individual levels. The hypothesis of this thesis was tested using wandering albatross Diomedea exulans, Cory’s shearwater Calonectris diomedea and yellow-legged gull Larus michahellis as model seabird species, and fieldwork was conducted respectively in Bird Island (South Georgia, Antarctica), Corvo Island (Azores archipelago, Portugal) and Berlenga Island (Portugal). Two populations of Cory’s shearwater were studied, one from an oceanic environment (Corvo Island) and other from a neritic environment (Berlenga Island). A total of 199 birds were sampled during the breeding season of each species, from 2009 to 2012. Together with conventional dietary methods a combination of intrinsic and extrinsic markers were used, such as stable isotope analyses (from tissues with different turnover rates) and electronic devices (GPS loggers), to corroborate the existence of individual specialization and examine its relative incidence. Using three environmental parameters (bathymetry, sea-surface temperature and chlorophyll a concentration) to characterize marine habitats and recent developments based in a Bayesian framework, namely stable isotope mixing models (Stable Isotope Analysis in R: SIAR) to estimate individual prey consumption, and recent metrics to estimate populations’ niche width (Stable Isotope Bayesian Ellipses in R: SIBER), data were analysed to investigate ecological implications of individual specialization and its relationships with the environment and resources exploited. Patterns of individual consistency in habitat use and/or prey consumption were found in all studied species. However, these patterns varied spatio-temporally according to the population, evidencing the role of individual specialization in the foraging dynamic of these species. Inter-annual differences in the feeding ecology and foraging behaviour of birds during the breeding season were associated with the availability of food resources around the colonies, which influenced the patterns of individual consistency in Cory’s shearwater, but not in yellow-legged gulls. Results also suggest that these differences could have an impact on the reproductive performance of Cory’s shearwater and on the body condition of the yellow-legged gulls, but evidences of relationships between these ecological consequences and individual specialization at the individual level were not found. Overall, this study corroborates the existence of individual specialization over time within studied seabird populations. Therefore, such trait may be potentially widespread across several related seabird species. Such consistency, however, varied among species and populations and results showed that it was more stable in yellow-legged gulls than in Cory’s shearwaters. Fluctuations on individual consistency were related to temporal changes in the availability and predictability of resources, which means that the more specialized species may be more sensitive to that. It also corroborates that individual variability within a seabirds’ population may be regulated by small differences in breeding location (< 2km), suggesting that studies assuming a colony or sub-colonies as ecologically homogeneous in terms of foraging ecology can be biased. This study supports the hypothesis that individual specialization may have important ecological implications, such as the reduction of intra-specific competition and, consequently, a high impact on ecological processes and foraging dynamics. Further investigation is required to identify the mechanisms that generate individual specialization and its ecological implications at both population and individual level.