Population dynamics of recovering apex predators: Golden eagles in a Mediterranean landscape

Apex predators play a critical role in shaping the biological and functional diversity of ecosystems. Like in many other living groups, population dynamics of apex predators exhibit auto-regulation traits, including density-dependent processes, which can be important for limiting population numbers. However, the study of these processes is challenging due to their slow life history traits, especially when their populations are depressed. Our main objective is to describe mechanisms driving population dynamics in apex predators by documenting the relationship between population density and demographic parameters at population level and analyzing the influence of population density and other environmental factors on the reproductive parameters at territory level. We used as biological model a recovering population of golden eagles Aquila chrysaetos in a Mediterranean landscape (North Spain). We monitored yearly all known eagle pairs within the study area for 28 years, implying 1539 reproductive events in a total of 84 territories. The average density was 3.04 pairs/1000 km(2), and the reproductive success, productivity, and flight rates averaged 0.45, 0.54, and 1.20, respectively. The population increased during the study period (from 37 to 78 pairs), although we did not find any effect of density on the reproductive parameters at population level. At territory level, we found that size of territory, proportion of open habitat, and spring precipitation increased reproductive performance, while older territories performed worse than new ones. Our findings suggest that population dynamics in recovering apex-predators are driven by a complex combination of compensatory density-dependent processes, mainly operating at territorial level, and by environmental factors mainly related with resource availability and human pressure. For species with slow life history traits, population recovery seems to be facilitated by re-colonization from refuge areas, wilder but less productive, to areas with higher resource availability, once they became safer after reduction of human pressures.