The Effects of Burrow Nesting Seabirds on Soils and Vegetation on Broughton Island, New South Wales

Abstract

<p>The offshore islands of New South Wales host millions of migratory seabirds that gather in dense colonies on islands to breed. Seabirds have the capacity to drive ecosystem function through dual roles of marine-derived nutrient subsidies via guano deposition and bioengineering through burrow-nesting. Broughton Island is managed as part of the Myall Lakes National Park estate and has experienced a range of environmental disturbances in the past decade including the introduction of invasive plants and mammals, which led to significant changes to seabird populations and native vegetation communities. In response to the threats imposed by grazing rabbits and predatory rats on seabird habitat and breeding success, these invasive animals were successfully eradicated from the island in 2009 with the goal of restoring seabird populations and plant communities. The trajectory of ecological change, however, remained largely unknown. The aims of the research presented in this thesis were to first gain scope on the effect of seabird nutrient subsidies and nesting activities on island soils and plants in colonies of the most abundant seabird species on the island, <i>Ardenna pacifica</i> (wedge-tailed shearwaters).</p> <p>The results revealed novel evidence of seabird colony soils more depleted in soil C, N and P compared to both adjacent and sloping areas of hydrological accumulation. It was also found that vegetation was distinctly different within seabird colonies and was defined by the presence of an invasive cactus, <i>Opuntia stricta</i>. This result will be the first to describe in detail how burrowing seabirds on islands with deep and sandy soils in a subtropical climate, affect their environment, thereby giving new insights onto the mechanisms driving ecosystem function and the management implications for such islands.</p> <p>Another key research aim was to elucidate the effectiveness of eradication of rats and rabbits was effective in restoring native vegetation cover and richness on Broughton Island by analysing data collected from 7 years of vegetation surveys. Overall positive effects were seen in vegetation height, species richness, and ground cover, but it may take successional plant communities longer time to recover and require additional interventions for optimal outcomes. It was concluded that positive outcomes of vegetation recovery may be confounded by areas with disturbance by burrowing seabirds, and was supported by the evidence supplied by the research comparing vegetation and soil characteristics inside and outside of seabird colonies.</p> <p>Two experimental habitat suitability models were created taking different but complementary approaches to predict preferred and projected colony habitat on Broughton Island. Both models had high accuracy at detecting suitable habitat on the island, and both models identified unoccupied areas of high habitat suitability which were used in conjunction with other results to make robust conclusions.</p> <p>Identifying the fundamental effects of seabirds on soils and plants in nesting areas provided evidence to predict how expanding seabird colonies may change the soil and vegetation environment on this distinctive island ecosystem. The spatial results, combined with the knowledge of biophysical effects on soils and vegetation from seabird colonies, identified precise areas which are predicted to experience change in vegetation and guano subsidies if seabird colonies should expand to these highly suitable areas. Since expansion of seabird colonies into suitable habitat is likely now Broughton Island is predator-free, the opportunity for effective biocontrol of weeds, and protection of habitat now exists. </p> <p>This work demonstrates how multifaceted approach using field surveys, laboratory and geospatial analyses strengthen ecological conclusions and can be applied to effective and real world conservation plans on islands experiencing ecological changes. The results will be utilised by the New South Wales National Parks and Wildlife Service to inform future island management.</p>