Floating lovers count too — in the health of eagle populations

The researchers' work has shown that factors affecting the survival of floaters within their settlement areas may directly influence the dynamics of the whole population. Vincenzo Penteriani, Fermín Otalora, and Miguel Ferrer looked at population data for the Spanish imperial eagle Aquila adalberti over the last century.

With less than 150 pairs in the whole Iberian Peninsula, this eagle is one of the most threatened raptors in the world. They found that extremely high mortalities of floaters in settlement areas cause a decrease in the number of breeders, due to the increasing difficulty of breeding pair formation and, consequently, a positive density-fecundity relationship in the breeding portion of the population.

The results support the novel idea that taking floater dynamics within settlement areas into consideration can illuminate inexplicable positive density-dependent patterns in breeding populations. “Population studies that ignore floater dynamics may fail to understand all the different factors influencing density-dependent population patterns,” Penteriani says.

He continues, “Clearly defining the portion of the population that shapes density-dependent patterns may help to solve some of the ambiguities that, after some seventy years of debate, still surround density-dependence and population dynamics in general.”