[Photo] Researchers from Spain 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. (Photo courtesy of Penteriani et al.)

In a paper from the November issue of The American Naturalist, Vincenzo Penteriani, Ferm??n Otalora, and Miguel Ferrer, researchers at the Estaci??n Biol??gica de Do??ana (Consejo Superior de Investigaciones Cient??ficas, Spain), focus on the forgotten and invisible side of animal populations - the floaters. Floaters are dispersed individuals who enter the reproductive population when breeding territory or a potential mate become available.

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, Ferm??n 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."

Founded in 1867, The American Naturalist is one of the world's most renowned, peer-reviewed publications in ecology, evolution, and population and integrative biology research. AN emphasizes sophisticated methodologies and innovative theoretical syntheses-all in an effort to advance the knowledge of organic evolution and other broad biological principles.

Vincenzo Penteriani (Estaci??n Biol??gica de Do??ana, CSIC), Ferm??n Otalora (Laboratorio de Estudios Cristalogr??ficos, CSIC), and Miguel Ferrer (Estaci??n Biol??gica de Do??ana, CSIC), "Floater dynamics can explain positive patterns of densitydependence fecundity in animal populations." The American Naturalist: November 2006.