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 rarer. 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 fertilize islands with isotopically heavy nitrogen, which means that nitrogen stable isotope analysis (δN) could provide a useful means for assessing corresponding change in ecosystem function. We quantified decadal changes in δN on eight temperate New Zealand islands subject in pairs to distinct mammal invasion and seabird restoration histories: invaded, never-invaded, invader-eradicated, and undergoing active seabird restoration. First, we investigated long-term changes in δN values on individual islands. Second, we used a space-for-time analysis to determine whether δN levels on islands from which invaders had been removed eventually recovered to values typical of never-invaded islands. On each island, soil, plants (Coprosma repens, Coprosma robusta, and Myrsine australis), and spiders (Porrhothelidae) were sampled in 2006/2007 and 2022, allowing δN change on individual islands over 16 years to be assessed. Combined, the samples from invader-eradicated islands provided a 7- to32-year post-eradication dataset. Change in δN was only detected on one island across the study period, following the unexpected recolonization of seabirds to an invaded island. Invader-eradicated islands generally had higher δN values than invaded islands; however, they were still lower than never-invaded islands, and there was no trend in δN with time since eradication. This, and the measurable increase in δN following seabird recolonization on one island, may suggest that δN change occurs rapidly following invader eradication but then slows, with δN values staying relatively constant in the time period studied here. Isotope and seabird population studies need to be coupled to ascertain whether plateauing in δN reflects a slowing of seabird population growth and subsequent basal nutrient input or whether the baseline nutrients are entering the ecosystem but then not propagating up the food web.