Quantifying island ecosystem function recovery following invasive mammal eradication
Creators: name: Pascoe, Penelope
Publisher: name: University of Tasmania
Year: 2024

description: Seabirds nest almost exclusively on offshore islands where they act as ecosystem engineers. They provide nutrients through guano deposition, and physically alter the environment through burrowing, nesting, and trampling. Seabird populations are adversely affected by invasive mammals and because of their ecosystem engineering role, this has implications across island ecosystems. Consequentially, invasive mammal eradications on islands are a critical conservation tool for both seabird populations and island ecosystems more broadly. However, due to the complex and interacting effects of invasive species and their removal, whole ecosystem recovery is not always assured, or even definable, following eradications. Despite this, monitoring beyond the confirmation of invasive species removal is rare and there is little understanding of what monitoring tools are suitable for assessing ecosystem-level recovery. In this thesis, I investigate the utility of nitrogen stable isotope analysis (SIA<sub>N</sub>) as a means for assessing ecosystem function recovery on islands following invasive mammalian predator eradication.<br>Foraging near the top of the marine food web, seabirds fertilise islands with nutrients containing nitrogen that is enriched in the heavy isotope (δ <sup>15</sup>N), and elevated δ <sup>15</sup>N values are traceable across ecosystem components on islands with seabird colonies. On islands with invasive mammalian predators, this marine-terrestrial linkage may be degraded, reflected by reduced δ 1<sup>5</sup>N values ecosystem-wide. This makes SIA<sub>N</sub> a potentially useful monitoring tool for assessing ecosystem recovery. However, how δ <sup>15</sup>N values change over time following an eradication remains relatively unknown. Additionally, nitrogen isotope values are affected by a myriad of other environmental and physiological factors which need accounting for if this technique is to be a meaningful ecosystem monitoring tool.<br>In the first two data chapters of this thesis (Chapters 2 and 3), I investigate confounding variables that need to be accounted for to separate out the effect of time since eradication on δ 1<sup>5</sup>N change. In Chapter 2, I compile existing island SIA<sub>N</sub> datasets consisting of soil, plant, and spider samples from 28 islands around New Zealand to conduct a preliminary investigation into how island characteristics and aspects of the sampling regime influence δ <sup>15</sup>N values. I use this to design a repeat sampling study across six island sites in south-eastern Tasmania, Australia in Chapter 3 to investigate in depth the effects of spatial, temporal and ecosystem component variability on δ <sup>15</sup>N values. I found significant spatial variability. δ <sup>15</sup>N values were high in proximity to seabird colonies, where seabirds deposit guano containing isotopically elevated nitrogen however, values decreased rapidly from colony boundary. I also found δ <sup>15</sup>N variability down to species-level between ecosystem components. No seasonal variation was found, suggesting that the nutrient legacy from the summer breeding season remains present throughout the year. These results highlight the importance of context dependency when using ecological indicators such as stable isotopes, and I provide recommendations for the implementation of future studies employing stable isotopes as indicators for ecosystem change.<br>In Chapter 4, I explore in greater depth how the physical and chemical influences of seabirds propagate through the island food-web by investigating the spatial and temporal effects of seabird presence on spider communities across a sub-set of five of these Tasmanian islands. I found spider activity-density, a measure of pitfall trap capture rates, was generally higher inside seabird colonies, while family-level richness was often greater away from seabird disturbance.<br>In my final data chapter, I use the findings from Chapters 2 and 3 to develop a replicate sampling study across eight seabird islands in Cook Strait, New Zealand. Islands represented invaded, never-invaded, eradicated, and actively seabird-restored restoration treatments. Each was re-sampled 16 years on from an initial sampling event to investigate for the first time long-term changes in δ <sup>15</sup>N values on individual islands, and if δ 15N values approach levels similar to never-invaded islands. Sample δ <sup>15</sup>N did not change on the individual islands, except for one island where δ <sup>15</sup>N increased following seabird recolonisation during the study. Combining samples from eradicated islands and using space-for time substitution I also found no change in δ <sup>15</sup>N across a 7–32 year post-eradication time series – the longest investigated to date. This, and the measurable increase in δ <sup>15</sup>N following seabird recolonisation on one island, may suggest that δ <sup>15</sup>N change occurs rapidly following eradication, but then slows, with δ <sup>15</sup>N values staying relatively constant in the time period studied here.<br>Together, these chapters highlight the importance of considering confounding variables in stable isotope studies and outline recommendations for designing future studies to ensure ecologically meaningful conclusions can be drawn. They demonstrate that while a theoretical basis exists for SIA<sub>N</sub> being a useful post-eradication island ecosystem function monitoring tool, relationships between δ <sup>15</sup>N and ecosystem function change are complex and further work is required to untangle the effects of seabird population recovery rate and ecosystem function response.<br>