Determinants of individual foraging specialization in large marine vertebrates, the Antarctic and subantarctic fur seals
Résumé
1. The degree of individual specialization in resource use differs widely among wild populations
where individuals range from fully generalized to highly specialized. This interindividual
variation has profound implications in many ecological and evolutionary processes. A recent
review proposed four main ecological causes of individual specialization: interspecific and
intraspecific competition, ecological opportunity and predation.
2. Using the isotopic signature of subsampled whiskers, we investigated to what degree three
of these factors (interspecific and intraspecific competition and ecological opportunity) affect
the population niche width and the level of individual foraging specialization in two fur seal
species, the Antarctic and subantarctic fur seals (Arctocephalus gazella and Arctocephalus
tropicalis), over several years.
3. Population niche width was greater when the two seal species bred in allopatry (low interspecific
competition) than in sympatry or when seals bred in high-density stabilized colonies (high
intraspecific competition). In agreement with the niche variation hypothesis (NVH), higher
population niche width was associated with higher interindividual niche variation. However, in
contrast to the NVH, all Antarctic females increased their niche width during the interbreeding
period when they had potential access to a wider diversity of foraging grounds and associated
prey (high ecological opportunities), suggesting they all dispersed to a similar productive area.
4. The degree of individual specialization varied among populations and within the annual
cycle. Highest levels of interindividual variation were found in a context of lower interspecific
or higher intraspecific competition. Contrasted results were found concerning the effect of
ecological opportunity. Depending on seal species, females exhibited either a greater or lower
degree of individual specialization during the interbreeding period, reflecting species-specific
biological constraints during that period.
5. These results suggest a significant impact of ecological interactions on the population niche
width and degree of individual specialization. Such variation at the individual level may be
an important factor in the species plasticity with significant consequences on how it may
respond to environmental variability.