| Census and survey data for blue-footed boobies ( Sula Sula nebouxii excisa) in Galápagos, Ecuador from 2011-2012 indicated a population reduction, probably by more than 50%. Anthropogenic effects such as introduced predators are unlikely to explain this decline, because islands with and without such factors exhibited the same low breeding. The poor reproduction seems to be linked to scarcity of food. Previous studies indicated that sardine and herring (Clupeidae) support successful breeding, but these fish were mostly absent from the diet. Elsewhere in the eastern Pacific, sardines have decreased dramatically in abundance by natural processes in the last 15 years, as part of a well-documented and apparently natural cycle. This cyclic change in abundance provides an explanation for the recent demographic changes in blue-footed boobies in Galápagos.;Land barriers have been mentioned as one of the mechanisms that promote population differentiation in pelagic seabirds, at what scale does a land barrier restrict gene flow effectively? Genetic data indicate that the Isthmus of Panamá does restrict gene flow in boobies and other seabirds. I evaluated a smaller isthmus (the Perry Isthmus) that could allow transit across Isabela Island, Galápagos. Daytime observations revealed crossings by ≥ 48 blue-footed boobies and > 2 frigatebirds (Fregata spp. ). If the Isthmus of Panamá (width = 57 km, height = 26 m above sea level) is assumed to be an effective barrier to gene flow in boobies, but the Perry Isthmus (width = 12.5 km, height 23 m) is not, then these two features bracket the minimum dimension of historical and contemporary landforms that can interrupt gene flow in this group. |
