Decadal change in seabird‐driven isotopes on islands with differing invasion histories

• Published in: Ecological applications Vol. 35; no. 4; pp. e70030 - n/a
• Main Authors: Pascoe, Penelope P., Bartlett, Mitchell, Shaw, Justine, Trebilco, Rowan, Weldrick, Christine K., Jones, Holly P.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.06.2025
• Subjects: Animals; Birds - physiology; Charadriiformes - physiology; chronosequence; Ecosystem; ecosystem function; ecosystem recovery; eradication; Introduced Species; Islands; New Zealand; Nitrogen Isotopes - analysis; rodents; seabird islands; Time Factors; New Zealand; ecosystem function; seabird islands; chronosequence; eradication; rodents; ecosystem recovery
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1002/eap.70030

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 (δ15N) could provide a useful means for assessing corresponding change in ecosystem function. We quantified decadal changes in δ15N 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 δ15N values on individual islands. Second, we used a space‐for‐time analysis to determine whether δ15N 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 δ15N 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 δ15N 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 δ15N values than invaded islands; however, 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 recolonization on one island, may suggest that δ15N change occurs rapidly following invader 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 whether plateauing in δ15N 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.