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

Abstract

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.

Keywords: chronosequence; ecosystem function; ecosystem recovery; eradication; rodents; seabird islands.

FIGURE 1

Conceptual framework for the use of nitrogen stable isotope analysis as a tool for monitoring. Graphic design: Stacey McCormack (Visual Knowledge).

FIGURE 2

Study island locations and respective restoration treatments in Cook Strait, Aotearoa/New Zealand. Each island was sampled in 2006/2007 and 2022. Takapourewa and Kuru Pongi have never been invaded by rodents; Moutiti and Tawhitinui are still invaded by rodents. Te Hoiere, Mana, Wakaterepapanui, and Nukuwaiata have all undergone rodent eradications (Mus musculus—2014, Mu. musculus—1990, Rattus exulans and Rattus norvegicus—1999, and R. exulans—1994, respectively). Mana and Te Hoiere have also had active seabird restoration projects, commencing in 1993 and 1991, respectively.

FIGURE 3

(A) Mean change in nitrogen stable isotope (δ¹⁵N) values between 2006/2007 and 2022 sampling events (±1 SD) for replicate soil, plant (Coprosma repens, Coprosma robusta, Myrsine australis), and spider samples combined for each study island. The broken circle highlights Moutiti/Victory Island which, following seabird recolonization, was no longer considered a representative invaded control island and thus omitted from panel (B). (B) Change in replicate soil, plant, and spider samples displayed separately and broken up by island restoration treatment (facets), sample type (colors), and seabird colony status at the sampling location (shapes). For both panels, islands/samples falling below Y = 0 (bold line) have decreased in δ¹⁵N between the 2006/2007 and 2022 sampling events. Icon illustrations: Penelope Pascoe.

FIGURE 4

Nitrogen stable isotope (δ¹⁵N) values for exactly replicated Coprosma repens, soil, and spider samples collected 7–32 years following invasive rodent eradication (points) compared to mean ± 1 SD δ¹⁵N values for invaded (orange) and never‐invaded (green) islands. Invader‐eradicated island samples were collected outside seabird colonies to reflect whole‐island dynamics. Icon illustrations: P. Pascoe. *Predicted years to reach δ¹⁵N levels like a never‐invaded island from Jones (2010a).