Demographic consequences of native fox predation on Socotra cormorants on Siniya Island, United Arab Emirates

Abstract

Predation by both native and introduced terrestrial predators is a major threat to colonially breeding seabirds leading to low breeding success and negative demographic effects. The Socotra cormorant Phalacrocorax nigrogularis is a regionally endemic seabird categorized as Vulnerable by the IUCN. It breeds on islands in the southern Arabian Gulf and in the Gulf of Oman. One colony, Siniya Island, on the northern United Arab Emirates hosts about 40% of the global breeding population. Socotra cormorants are ground nesters and are vulnerable to predation by native Arabian red foxes Vulpes vulpes arabica, occuring naturally on Siniya island. We used camera trapping to estimate the population size and diet composition of foxes on the island. A total of 24 foxes were identified including adults and cubs, and cormorants and their eggs dominated the fox's diet (80%) during the breeding season. We estimated that the foxes killed >3500 cormorants during the season. The cormorant population was modelled using Vortex. The simulated population increased from 46 500 individuals (corresponding to the known population in 1995) to stabilize between 154 000 and 188 000 birds after 2010 in the absence of mortality from predation. Simulated mortality showed various degrees of population suppression and increasing extinction probabilities at higher degrees of predation. The model was more sensitive to predation of adult birds. Predation rates causing up to 35% juvenile mortality stabilized at <250 000 birds. Contrastingly, adult mortality of >25% caused the population to stabilize at <80 000 birds. Actual population estimates from 2011-2015 ranged from 79 000-124 000 birds. Thus, our model suggested that the cormorant population was suppressed due to predation at levels consistent with 20-25% adult mortality in the simulated population. Continual monitoring of the impact of foxes is essential and management of foxes may be recommended as a conservation measure if predation pressure increased.