COS 214-5 - Seabird island ecosystem function recovery through an isotopic lens

Background/Question/Methods

Seabirds are island ecosystem engineers, providing nutrients through guano and modifying habitats through physical disturbance. Invasive mammals threaten seabird populations, with cascading impacts across island ecosystems. Invasive mammal eradications are undertaken on islands globally to protect seabird populations and conserve ecosystems. Ideally, long-term multi-species monitoring would provide the basis for assessing ecosystem recovery following eradications. However, due to resource limitations and a lack of tools to assess recovery at an ecosystem level monitoring is generally limited. We aimed to address this by investigated the use of stable isotope analysis as a cost- and time-efficient technique for evaluating the recovery of ecosystem function.

We collected nitrogen stable isotope data from three ecosystem components: soil, plants, and spiders, from islands around Australia, New Zealand, and the sub-Antarctic. By examining never-invaded, invaded and islands at various times since eradication, we investigated whether stable isotopes could track the progression of recovery of an island from an invaded to a never-invaded state. Isotopic data is noisy. We investigated island characteristics and sampling methods that may influence the interpretation of results. We also re-sampled previously studied islands to build a three-decade post-eradication dataset, allowing us to investigate long-term and individual island trends for the first time.

Results/Conclusions

We have found stable isotope values are generally higher on islands with greater times since eradication, but that they also vary with island size and intra-island sampling location. Despite the highly seasonal nature of colony attendance displayed by many seabird species, stable isotope values did not vary seasonally. For small islands with similar environmental characteristics, inter-island comparisons may be possible for predicting the recovery trajectories of ecosystem function. For islands that differ in their environmental characteristics, long-term repeat sampling following an eradication may be a better approach. Our long-term data set will provide the first insight into this. Our results highlight the importance of standardising collection locations between islands or longitudinally within a single island to ensure variation in stable isotope values can be attributed to the post-eradication recovery. Ultimately, we aim to provide guidelines for implementing future island stable isotope studies for understanding ecosystem process and as a resource-efficient tool in the post-eradication monitoring toolkit.