The diet, breeding, and ecological role of brown skuas Stercorarius antarcticus lönnbergi (Mathews, 1912) on Macquarie Island, following the eradication of invasive prey

Throughout 2010 and 2011, Macquarie Island underwent a substantial conservation intervention that drastically altered the state of its ecosystem. After 140 years of inhabiting the island, invasive rabbits (Oryctolagus cuniculus), black rats (Rattus rattus), and house mice (Mus musculus) were all eradicated. Brown Skuas (Stercorarius antarcticus lonnbergi) are a top-order avian predator and the island's primary terrestrial predator. For skuas, rabbits were an abundant prey species and were heavily relied upon as a food source during the summer breeding season. Prior to the arrival of rabbits, skuas most likely thrived upon vast quantities of seabirds, including burrowing petrels, as a key dietary component. However, as invasive pests severely reduced petrel numbers, rabbits became an important component of skua diet, likely replacing that of burrowing petrels. The rabbit eradication was, therefore, likely to have a negative effect on skuas; however, prior to Macquarie Island, rabbit eradication had never been attempted at this scale. Apart from the trophic changes brought about by the rabbit eradication, over 500 skuas also died of secondary poisoning after consuming poisoned rabbit carcasses during the eradication. Adaptive management and close monitoring of non-target mortality during the eradication mitigated the degree of poisoning for skuas and other native scavengers to a large extent. Nonetheless, without long-term monitoring the population level impacts would have remained unquantified. Through this thesis I investigate how the diet, breeding, and ecological role of Brown Skuas changed following rabbit eradication, while considering the contributing effects of prey-loss and secondary poisoning. First, I review the existing research into the effects of invasive prey eradications on native predator populations, focussing on eradications of invasive rabbits and rodents. Second, I assess the effect prey-loss and secondary poisoning had on skua nest density, their breeding success and diet. Third, I investigate the current diet and foraging strategies of Brown Skuas on Macquarie Island and how they influence individual breeding success. Fourth, I examine the role of skua predation in limiting the recovery of burrowing petrel prey species. Rabbits and rodents have been the target of over 692 successful island eradications. The effects of these eradications on predator populations were underreported in the literature with long-term ecological impacts such as prey loss rarely studied. Native predators were the only group studied that had more reported negative responses to an eradication than positive, highlighting the need for further studies. Using surveys of skua nest numbers and breeding success, and stable isotope analysis of feathers taken from skua chicks, I found the breeding and diet of Macquarie Island's Brown Skuas were impacted by the eradication of rabbit prey. Both nest numbers and breeding success fell following the eradication of rabbits to their lowest recorded levels on Macquarie Island. Since that time, nest numbers have slowly recovered. Stable isotope analysis revealed the isotopic niche width of skuas significantly reduced after the eradication suggesting increased competition for remaining prey. Secondary poisoning further lowered skua nest numbers below that expected by prey-loss alone. I deployed GPS units on breeding adult skuas and surveyed prey remains at the nest to investigate how the diet and foraging decisions of skuas impact their breeding success. Skuas that successfully reared chicks had foraging strategies that allowed for more time spent at the nest site and less energy expenditure. The association between nest proximity to abundant prey and breeding success was highlighted by most birds failing to rear a chick in nesting areas where skuas had historically relied heavily on rabbits in their diet. The current paucity of prey in these areas meant skuas spent longer searching for food far from the nest site and had poorer breeding success. To investigate the effect of skua predation on the recovery of burrowing petrels on Macquarie Island, I modelled the density-dependent predation rate of Brown Skuas on Antarctic Prions (Pachyptila desolata), a burrowing petrel, using burrow density as a proxy for prey density and feather 'sign' (left at the site where a skua had killed a prion) to measure predation rate. I compared this predation rate to Hollings' functional response curves and calculated an annual predation rate of skuas on prions. I found skuas followed a Type III functional response curve with low predation pressure at low prey densities. Comparing the annual predation rate with published growth rates of petrels I found evidence that skua predation, in its current form, could impact the recovery of ~3.5% of burrowing petrel species (those with the lowest growth rates). My thesis shows the eradication of an abundant rabbit population on Macquarie Island and slow recovery of some native prey species, particularly burrowing petrels, has rapidly reduced food availability for the island's skua population and reshaped their foraging landscape. This has impacts for the native predator and for native prey. I demonstrate that on isolated islands, such as Macquarie Island, where recovery of seabird prey is expected to be slower, the prey deficit left by eradication of invasive prey can have a long-term negative impact for native predators. Top-order predators are sensitive to changes in food web structure. Quantifying the multiple effects on native predators of eradicating invasive prey is integral to ensuring eradication projects have a net-positive ecosystem outcome. The ecosystem responses documented in my thesis are likely to become increasingly commonplace as eradication projects expand in their scope and scale. My thesis demonstrates the value of strategic monitoring conducted alongside large-scale conservation projects and lends support to the idea of assisted recovery following eradication projects.