Résumé
1. Understanding the population dynamics of top predators is essential to
assess their impact on ecosystems and to guide their management. Key to
this understanding is identifying the mechanisms regulating vital rates.
2. Determining the influence of density on survival is necessary to
understand the extent to which human-caused mortality is compensatory or
additive. In wolves (Canis lupus), empirical evidence for
density-dependent survival is lacking. Dispersal is considered the
principal way in which wolves adjust their numbers to prey supply or
compensate for human exploitation. However, studies to date have primarily
focused on exploited wolf populations, in which density-dependent
mechanisms are likely weak due to artificially low wolf densities. 3.
Using 13 years of data on 280 collared wolves in Yellowstone National
Park, we assessed the effect of wolf density, prey abundance and
population structure, as well as winter severity, on age-specific survival
in two areas (prey-rich vs. prey-poor) of the national park. We further
analysed cause-specific mortality and explored the factors driving
intraspecific aggression in the prey-rich northern area of the park. 4.
Overall, survival rates decreased during the study. In northern
Yellowstone, density-dependence regulated adult survival through an
increase in intraspecific aggression, independent of prey availability. In
the interior of the park, adult survival was less variable and
density-independent, despite reduced prey availability. There was no
effect of prey population structure in northern Yellowstone, nor of winter
severity in either area. Survival was similar among yearlings and adults,
but lower for adults older than 6 years. 5. Our results indicate that
density-dependent intraspecific aggression is a major driver of adult wolf
survival in northern Yellowstone, suggesting intrinsic density-dependent
mechanisms have the potential to regulate wolf populations at high
ungulate densities. When low prey availability or high removal rates
maintain wolves at lower densities, limited inter-pack interactions may
prevent density-dependent survival, consistent with our findings in the
interior of the park.