Data and code for: Decadal change in seabird-driven isotopes on islands with differing invasion histories

Invasive mammal eradications are commonplace in island conservation. However, post-eradication monitoring beyond the confirmation of target species removal is rare. Seabirds are ecosystem engineers on islands and are negatively affected by invasive mammals. Following an invasive mammal eradication, the recovery of seabird populations can be necessary for wider ecosystem recovery. Seabirds fertilise islands with isotopically heavy nitrogen, which means nitrogen stable isotope analysis (δ15N) could provide a useful means for assessing corresponding change in ecosystem function. We investigated how δ15N values changed over a long-time span on eight temperate New Zealand islands with differing restoration treatments (two each of invaded, never-invaded, eradicated and undergoing active seabird restoration). First, we investigated long-term change in δ15N values on individual islands. Second, we asked whether δ15N levels values on eradicated islands progress to never-invaded island values by collectively analysing samples from islands at different times since eradication and comparing them to those on never-invaded as proxies for recovery (space-for-time substitution analysis). On each island soil, plants (Coprosma repens, C. robust and Myrsine australis) and spiders (Porrhothelidae) were sampled in 2006/07 and 2022 allowing δ15N change on individual islands over 16 years to be assessed. Combined, the samples from eradicated islands provided a 7 – 32 year post-eradication dataset for space-for-time substitution analysis. Sample δ15N change was only detected on one island across the study period, following the unexpected recolonisation of seabirds to the island. Space-for-time substitution analysis showed that while eradicated islands generally had higher δ15N values than invaded islands, they were still lower than never-invaded islands and there was no trend in δ15N with time since eradication. This, and the measurable increase in δ15N following seabird recolonisation on one island, may suggest that δ15N change occurs rapidly following eradication, but then slows, with δ15N values staying relatively constant in the time period studied here. Isotope and seabird population studies need to be coupled to ascertain if the ensuing plateauing in δ15N reflects a slowing of seabird population growth and subsequent basal nutrient input, or if the baseline nutrients are entering the ecosystem but then not propagating up the food web.