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.

No abstract

No abstract

Seabirds on islands create a circular seabird economy-they feed in the ocean, transport marine-derived nutrients onshore to their breeding colonies, and then seabird-derived nutrients runoff into the ocean, enriching nearshore ecosystems. Invasive predators reduce seabird-driven nutrient subsidies; thus, predator eradication is critical for restoring seabird islands. Terrestrial recovery of seabird nutrients following predator eradication can take mere decades or longer, but few studies have linked nearshore marine recovery to terrestrial ecosystem attributes. We tested the influence of season, seabird variables, terrestrial abiotic variables, and marine variables on nearshore algal communities surrounding islands in a New Zealand island archipelago. The link from terrestrial to marine proved strong, with over 40% of the variation in macroalgae community composition being driven by a combination of seabird and terrestrial abiotic variables. Season and depth were the strongest contributors to nitrogen enrichment. Two macroalgae species had seabird-derived nitrogen levels similar on eradicated and never invaded islands, but four more had lower seabird-derived nitrogen on eradicated versus never invaded islands. Our results suggest seabird land-sea linkages are disrupted when predators invade, and are not restored three decades after invasive predator removal in the studied archipelago. Moreover, some marine algae species are better indicators of seabird influence than others. This study provides baseline information about which abiotic and ecological variables are important when studying the linkages from land to sea in island ecosystems.