The balance between predators and prey in a mixed seabird colony: managing biodiversity and the conservation of rare species

Abstract

Increases in seabird population sizes on islands, resulting from reduced mortality and/or increased reproductivity, has resulted from the control of avian predators when predation was confirmed to be the main driver of seabird decline. A substantial conflict in conservation management arises when the control operation targets a protected predator. Tensions and divisions between regulators and conservation managers can be exacerbated when the prey species is of high conservation concern. This latter situation underpins this research study based on Coquet Island, UK, where the core of conservation management is targeted at mitigating predation on Roseate Tern (RT) by breeding Large Gulls (LG), represented by Lesser Blackbacked Gull Larus fuscus (LBBGU) and Herring Gull L. argentatus (HGU). In recent years, the conservation status of HGU has changed to red and LBBGU to Amber categories in the light of increasing concern of notable decline for unknown reasons in the UK. Hence, it is important to have evidence on which to base management strategies for conserving Roseate Terns which minimises the conservation conflicts between prey and predators, all of which are of conservation concern. A complete understanding of predator-prey relationships relies on determining the dynamic stability of prey and predator populations, and how the food web containing predator-prey pairs responds to environmental influences and other indirect effects. This research investigates the main drivers of LG predation activity over the RT colony during the breeding season. A particular dilemma in estimating the impact of predation by breeding LG on the Coquet Island RT colony resulted from the presence of loafing LG, either non-breeding subadult birds or birds from other colonies, using the intertidal area around the island. Therefore, to test the hypothesis that LG breeding on the island also used the island’s seabird colonies as a foraging resource, indirect (Camera traps and dietary analysis of LG pellets) and direct (Observation of predation activity, foraging range estimation from tracking technologies) methods were used. The results of this study suggest that the frequency of LG events over the RT colony increased towards the end of the breeding season in relation to the number of loafing LG in the intertidal area, and was influenced by tidal state and decreased during the period of RT chick biomass availability. This study provided evidence that LG breeding on the island also used the reserve as part of their foraging territory. The outputs of the tracking data were compatible with the outputs of the pellet analyses which showed a high utilization of available prey from the reserve. LG on Coquet Island utilizing all types of prey sources with no difference between of breeding

or roosting LG with respect to the range of prey types. Indirect evidence that LG predation would be a threat to the small colony of RT was based on the finding that other tern species nesting on Coquet Island were identified using combined molecular and morphological techniques as prey in LG pellets collected from the breeding and roosting LG on Coquet Island. In addition, a study of laser hazing carried out as part of the thesis work shows that this is an efficient non-lethal deterrent for LG management on Coquet Island. Overall, the results of this study provide evidence showing how small numbers of LG may be allowed to breed on Coquet Island by managing the timing of their breeding in relation to the arrival and breeding of RT, and by efficient deterrence of loafing, non-breeding birds. Such an approach will facilitate a wider understanding of how to resolve conservation conflicts between protected predator and protected prey species.