Impacts of Endangered Seabirds on Nutrient Cycling in Montane Forest Ecosystems of Hawai‘i

Abstract

Allochthonous nutrient flow from marine sources via seabirds to the terrestrial habitats where they nest can impact resident organisms and neighboring ecosystems. Seabird populations are decreasing both in Hawai‘i and globally, yet little is known about what is being lost from the ecosystems where they traditionally nested in large numbers. Given the marked decline in seabirds, we hypothesized that current sparsely populated seabird colonies in wet montane ecosystems of Hawai‘i contribute minimally to nutrient availability, but that this small contribution should still be reflected in vegetative uptake of soil N and in plant community composition. Soil nutrient availability on Kaua‘i was assessed using ion-exchange resin probes. Plant and soil uptake of marine-derived nitrogen was determined using δ 15N values in soil and foliage of the two dominant species using a two-end member N isotope mass balance mixing model. To determine if the added nutrients impacted the plant community, we also compared canopy cover (total and by dominant species) and species richness between treatments. Soil in seabird areas had more available ammonium, but nitrate and total inorganic N did not differ between sites. The dominant canopy tree, Metrosideros polymorpha, derived 28% of foliar N from marine sources; this value was 15% for the dominant understory plant, Dicranopteris linearis. Plant species composition was not influenced by presence or absence of seabirds. Because N plays a large role in net primary productivity, use of marine-derived N by native plants under even limited seabird populations is likely important to functioning of these ecosystems.