Invasive Mammals Disrupt Sea–Land Nutrient Inputs Over 169 Years on Subtropical Oceanic Islands

Abstract

Invasive mammals have severely impacted oceanic islands, reducing or extirpating seabird populations and driving the collapse of nutrient flows with lasting ecosystem effects. We investigated these dynamics on contrasting subtropical oceanic islands in the South Pacific: Rangitāhua, a large island where goats, cats, and rats caused the extinction of seabird populations, and the small mammal-free Meyer Islands, where seabird colonies remain dense. Using stable isotope analysis (δ13C and δ15N) of 408 contemporary and historical specimens spanning 169 years, across plants to invertebrates and birds, we tested hypotheses about seabird-driven nutrient coupling across space and time. Spatially, we predicted stronger seabird-driven nutrient enrichment on the mammal-free Meyer Islands than on invaded Rangitāhua. Consistent with this, Meyer Islands' soils had higher phosphorus and lower pH, and terrestrial plants and herbivorous invertebrates were enriched in δ13C and δ15N relative to their Rangitāhua counterparts. In the nearshore marine system, δ15N values—but not δ13C—were elevated in macroalgae and herbivorous and omnivorous invertebrates around the Meyer Islands compared with those around Rangitāhua, indicating stronger seabird-driven nutrient inputs to coastal food webs. By contrast, the lower values on Rangitāhua reflect slow seabird recovery more than 20 years after the eradication of invasive mammals. Temporally, we tested whether isotopic values in Rangitāhua plants and birds declined with seabird loss. Values for δ13C declined steadily from the mid-19th century to the present, consistent with reduced seabird subsidies (and possibly volcanic activity), whereas δ15N values showed no long-term change. Together, these spatial and temporal perspectives show that while seabird nutrient pathways can recover within decades on small islands, recovery on large, degraded islands can take much longer. Adjacent islets can provide recolonists but may not serve as true analogues. Restoring sea–land nutrient connectivity on large islands may require not only invasive mammal eradication but also active facilitation of seabird return.