Nearshore ecosystems on seabird islands are potentially influenced by invasive predator eradications and environmental conditions: a case study at the Mercury Islands, New Zealand

Abstract

Seabirds maintain island ecosystem function by providing rich marine-derived nutrients to the islands where they nest. These nutrients are returned to the sea through runoff, fertilizing the nearshore environment. Invasive predators disrupt this bottom-up control by decimating seabird populations. While invasive predator eradications lead to terrestrial recovery on seabird islands, there is little information on the nearshore impact. We determined how nearshore macroalgae communities and seabird-derived nitrogen concentrations are influenced by predator eradications and environmental parameters (sampling depth, season, wave exposure, and runoff). This case study examined 4 islands in the Mercury Islands archipelago, representing 3 eradication histories: never invaded by mammalian predators, eradicated over 30 yr ago, and eradicated 2 yr ago. Macroalgal diversity was highest at never-invaded islands, followed by islands in order of eradication year (eradicated 30 and 2 yr ago). The amount of seabird-derived nitrogen (δ15N) in algae was higher during the rainy season and decreased with sampling depth and wave exposure. Sampling near high runoff points resulted in increased δ15N in red algae alone. Never-invaded islands had the highest δ15N in most species. With species found at both eradicated islands, the recently eradicated island had unexpectedly higher δ15N than the island eradicated over 30 yr ago. This discrepancy may be a result of the recently eradicated island’s large size and presence of streams, estuaries, and sheltered bays. Studying nearshore habitats is crucial in understanding the extent to which seabirds act as a conduit of the land-sea interface and the marine impacts of island management.