Early-life density-dependence effects on growth and survival in subantarctic fur seals.
Résumé
Understanding the regulation of natural populations
has been a long-standing research program in ecology.
Current knowledge on marine mammals and seabirds
is biased toward the adult component of populations and
lacking are studies investigating the juvenile component.
Our goal was to estimate demographic parameters on the
pre-weaning stage of a subantarctic fur seal (Arctocephalus
tropicalis) population on Amsterdam Island, suspected
to be regulated by density-dependence. The influence of
abundance on growth parameters (length and weight) and
survival was assessed over a study period spanning 16
years. We evidenced a negative trend in population growth
rate when density increased. Density-dependence models
were favored for pup body size and mass growth. Abundance
had a clear influence on body length at high population-
density, pups grew slower and were smaller at weaning
than pups born in years with low population density.
Abundance partly explained pup body mass variation and
a weak effect was detected on pre-weaning survival. The
causal mechanisms may be increased competition for food
resources between breeding females, leading to a reduction
of maternal input to their pups. Our results suggested that
pup favored survival over growth and the development of their diving abilities in order to withstand the extreme fasting
periods that are characteristic of this fur seal population.
This analysis provides significant insight of densitydependent
processes on early-life demographic parameters
of a long lived and top-predator species, and more specifically
on the pre-weaning stage with important consequences
for our understanding of individual long-term fitness
and population dynamics.