Seabird sentinels: The barometer for island conservation in a changing world
Seabirds are the most threatened group of marine animals, 53% of Procellariiformes (albatrosses, petrels, storm petrels, shearwaters, fulmars, and prions) are experiencing population declines; thus, their protection is a global conservation priority. On land, seabirds are vulnerable to invasive mammalian predators, due to physiological, behavioural and demographic traits, such as extended immaturity, low reproductive output and colonial nesting habits. At-sea, seabirds are impacted by human activities; suffering increasing levels of mortality from fisheries bycatch, pollution, and climate change. Land-based conservation actions, such as predator eradication have been the focus of conservation efforts. Currently, gaps remain in our understanding about the recovery of seabirds on islands following predator eradication. Moreover, little is known about the population level impacts of extrinsic stressors. In this thesis, I am to contribute to understanding if invasive predator control will deliver enduring benefits for seabirds and their island ecosystems. To do this, I investigate the factors that influence seabirds returning to islands after predator eradication, how island ecosystems are recovering, new ways of measuring the abundance and recovery of seabirds to islands, and how seabird recovery is affected by intensifying marine threats.
In chapter 2, I evaluate the recovery of seabird colonies (n=97) in the Hauraki Gulf, New Zealand; a seabird diversity hotspot (27 species) which has a long history of predator eradications. I conduct a comparative analysis of seabird assemblages on islands with three predator histories: never invaded, eradicated of predators, and with invasive mammalian predators present. I found islands cleared of predators show recovery of seabirds over time and had more unique seabird taxa than islands that never had predators. However, recovery appears to be influenced by a suite of site- and species-specific factors. While time following the eradication is an important factor, space, demographic traits and population dynamics may have a stronger influence on the passive recovery and recolonisation of seabirds to islands. As such, consideration of additional conservation management actions may be necessary to facilitate seabird island recovery. Seabirds influence island flora and fauna communities through soil disturbance from burrowing, and from subsidies of marine derived nutrients via guano, failed eggs, prey remains, and corpse deposition to their terrestrial breeding grounds. In chapter 3, I quantify seabird nutrient distribution on islands where seabirds are recovering. I investigate the ecological factors that may influence the accumulation and distribution of seabird nutrient enrichment to plants using a cross island comparison, on islands with three predator histories: never invaded by non-native predators, cleared of predators (approximately 30 years ago), and newly eradicated (<2 years). I found that there is a strong relationship of soil and leaf variables with seabird burrow density, but there is variability in the ecological influences on seabird nutrient distribution and accumulation in vegetation. While seabird nutrient influences can be detected rapidly in some foodweb components of island ecosystems, the overall enrichment of the ecosystem, which is a key driver of ecosystem function and composition, may take longer to recover. While seabirds are a global conservation priority, only a fraction of seabird species or their island habitats are consistently monitored, and in many instances, the monitoring is spasmodic, or insufficient to detect informative changes to populations that are useful for adaptive management strategies. In chapter 4, I capitalise on the established relationship between soil-foliar nutrients and spectral reflectance to investigate if seabird nutrient enrichment can be detected in the spectral reflectance of island plant species with a controlled experiment and field testing. I found that nutrient enrichment from seabirds can be detected in the spectral reflectance of pōhutukawa, a common island canopy species, in experimental conditions; however, in field testing the relationship is less apparent. While more work is needed to refine methods, our results suggest that there is potential to use spectral reflectance as a proxy measure for seabird abundance. The large spatial distribution of seabirds at-sea means they can be exposed to multiple anthropogenic stressors, such as fisheries bycatch, pollution, and climate change, which can be cumulative or interactive in nature. Quantifying the impacts of individual or interactive marine stressors on a seabird species is challenging because of the ambiguity of detecting at-sea mortality, and is confounded by demographic factors (e.g., reproductive factors; age, success, frequency), ecological noise, and how sub-lethal threats potentially manifest at the population level. In chapter 5 of this Thesis, I explore how these threats may affect the population recovery of seabirds after predator eradication using a theoretical modelling approach. Using a model specifically developed for data-limited species, I calculated the intrinsic population growth rate, and the limit of annual mortality for each population for 81 Procellariiformes. I found the mortality limits were commensurate with IUCN Red List categories, and that body size and spatial distribution are good predictors of the risk of population collapse from marine threats. Furthermore, I found a high phylogenetic signal of the sensitivity of species analysed to demographic impacts by at-sea threats, implying that the model may also help inform other closely related species that have not yet been evaluated.