EARTH SCIENCE | BIOSPHERE | ECOSYSTEMS | TERRESTRIAL ECOSYSTEMS | ISLANDS
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Invasive mammal eradications are widely used for managing island ecosystems. However, tracking the outcomes of such large-scale, whole ecosystem projects is challenging and costly, and monitoring all components of an ecosystem is near impossible. Instead, indicators of ecosystem change may provide more practical and integrated measures of ecosystem response to eradications. As high-order marine predators, seabirds subsidise island ecosystems with nutrients isotopically enriched in nitrogen. Invasive mammals have caused a global decline of seabirds on islands, reducing this nutrient subsidisation. Following eradications, nitrogen stable isotope analysis may provide a useful and resource-efficient indicator of ecosystem functional change on eradicated islands. However, isotope ratios are affected by a myriad of factors, with potential sources of variation being introduced by spatial and temporal variation in sampling, and within and between different taxa and ecosystem components. To correctly attribute isotopic change to post-eradication ecosystem function change, these confounding variables need to be understood. To address this need, we analysed stable isotopes of nitrogen in soil, plant, spider, and seabird guano samples collected at different distances from seabird colonies and at different stages of the short-tailed shearwater breeding cycle on six island sites around south-eastern Tasmania, Australia. Across these cool, temperate islands we detected no temporal variability in δ15N throughout the breeding season. However, there was notable spatial variability in δ15N values. The effects of seabird-derived nutrient subsidisation were highly localised with high δ15N values found inside seabird colonies and then rapidly decreasing from the colony boundary. Higher δ15N values also occurred in areas of higher burrow density within a colony. Variability in δ15N values also existed both within and between ecosystem components. Our results highlight the importance of context dependency when using ecological indicators and have important implications for the design, implementation and interpretation of studies employing stable isotopes as indicators for ecosystem change. We provide recommendations for designing future stable isotope studies on seabird islands.
We investigated the effects of seabird presence and seasonality on ground-active spider community structure (activity-density, family-level richness, age class and sex structure) and composition at the family-level across five short-tailed shearwater breeding islands around south-eastern Tasmania, Australia. Using 75 pitfall traps (15 per island), spiders were collected inside, near and outside seabird colonies on each island, at five different stages of the short-tailed shearwater breeding cycle over a year. 3. Pitfall traps were deployed for a total of 2,674 days, capturing 1,592 spiders from 26 families with Linyphiidae and Lycosidae the most common.
Data on the type, provenance, quantity (density), and rate of accumulation of beach-washed plastic debris were recorded on Henderson Island, a remote, uninhabited island in the South Pacific during 29 May – 15 August 2015. Henderson Island is rarely visited by humans, thus debris on the islands' beaches may act as a proxy for the adjacent South Pacific Ocean. The island was found to contain the highest density of debris anywhere in the world, up to 671.6 items/m2 (mean ± SD: 239.4 ± 347.3 items/m2 on the surface of the beaches. Approximately 68% of debris (up to 4,496.9 pieces/m2) was buried <10 cm in the beach sediment. Up to 26.8 new items/m are thought to accumulate daily.
Nitrogen stable isotope data from soil, leaf and spider samples collected from invaded, never invaded and eradicated islands around New Zealand's north island; and associated R code used to investigate the use of stable isotope analysis as a post-eradication ecosystem function assessment tool.