Cliff hangers

The peregrine falcon holds a special place in the minds of environmentalists and conservation biologists. The crash in its population throughout the 1950s and 60s was linked to the rapidly increasing use of agricultural organochlorine pesticides which led to a thinning of the birds’ eggshells and dramatic breeding failure. Since the widespread ban on the use of these chemicals, populations have recovered to some extent in many areas, often as a result of intensive efforts by biologists to restock former habitats. In California, a major project was established to reintroduce the species by a range of captive breeding and rearing programmes. But a new study shows that even for highly mobile species like the peregrine, detailed knowledge of differing habitats is vital to the successful reintroduction of a species. The Californian peregrines have been classified into three subpopulations that occupy coastal, interior and urban habitats. These three subpopulations have been highly managed by the introduction of around 800 banded fledglings since 1977 as part of a statewide recovery effort. But a new study by Matthew Kauffman, Jacob Pollock and Brian Walton at the University of California, Santa Cruz, in American Naturalist (published online) suggests clear differences in sub-population survival rates, between-habitat dispersal, and density-dependent territory acquisition within the subpopulations. Such information could be vital for conservation efforts of other high-profile species of conservation concern. It is now widely recognised that habitat patchiness may strongly mediate population dynamics when individuals experience steep gradients in habitat quality, resulting in highquality ‘source’ and low quality ‘sink’ habitats. Habitat fragmentation and degradation are thought to reduce both the connectivity and regional viability of subpopulations, but often details are unknown. The researchers believe this is vital to conservation efforts. “For endangered populations with complicated source–sink dynamics it becomes essential to base management decisions and viability assessments on habitatspecific demographic rates rather than on potentially misleading indicators such as population count,” the authors say. Their results suggest that there is a lack of immigration from the urban and interior source habitats which made artificial augmentation of the coastal sink habitat critical for recovery of that subpopulation. “Our analysis illustrates the necessity of understanding spatial structure and its effect on managed populations of conservation concern. Because we treated the three habitats separately, it is clear that the coastal sub-population was a sink throughout much of the recovery and that management intervention played a critical role in facilitating its recovery.” Cliff hangers