Trophic interactions and ecosystem management at the New Zealand Subantarctic Islands
Ecosystem-based management (EBM) has become an increasingly popular concept for government agencies to incorporate into management planning strategies. The basic idea behind EBM is that an ecosystem remains intact, resilient and productive in the long-term, to provide for ecological, social, cultural and economic benefits. The problem that decision makers face is that there is often little information regarding the structure and functioning of ecosystems upon which to base meaningful decisions. A further complication is that governance of the environment is highly sectoral both across government and within agencies. This often leads to fractured management between the terrestrial, freshwater and marine environments, potentially risking biodiversity loss and the stability of ecosystems. Small oceanic islands may potentially be model ecosystems for undertaking ecological studies, due to their constrained spatial extent and often unmodified condition. The New Zealand Subantarctic Islands, which are remote and largely unmodified, provide a natural laboratory to study the structure and functioning of ecosystems. I undertook stable isotope and water nutrient sampling to describe the trophic structure, trophic interactions and the drivers of the Antipodes and Bounty Islands, two of the islands in New Zealand’s Subantarctic region. These islands have high conservation value and are an important area for breeding seabirds and marine mammals, but there have been no studies at these islands to understand how they function and what the connections are between the terrestrial and marine environments. Using the stable isotope signatures of nitrogen (δ¹⁵N) and carbon (δ¹³C) from a wide range of common marine and terrestrial species at both islands, I described the trophic structure of each island. I found that the islands had a similar number of trophic levels and that omnivory was present beyond secondary consumers and below top level predators. Antipodes Island had a more complex food web than the Bounty Islands, but both islands showed strong linkages between the terrestrial and marine environments at both a local scale and with habitats beyond the sovereignty area of New Zealand. A basic two-source mixing model was used to determine the carbon sources that were important at each island. It was found that the Antipodes Island marine communities were influenced by phytoplankton, but that kelp was also an important contributor of carbon to consumers’ diets. In contrast, at the Bounty Islands, phytoplankton was the sole carbon source in marine communities. Terrestrial species at both islands had a marine-derived carbon component to their diets, with Antipodes Island terrestrial species incorporating a combination of terrestrial-derived and marine-derived carbon. The Bounty Islands’ terrestrial species were completely reliant on marine-derived carbon that was linked to phytoplankton. To further test the diets of species, Isosource was used to reconstruct the diets of the most common marine invertebrates and terrestrial species, again demonstrating strong marine-terrestrial links. To determine if there was any correlation between the distance from shore, water nutrient concentrations and phytoplankton stable isotope signatures, samples were collected in open ocean sites across the Campbell Plateau and within 12 nautical miles of each island. It was found that the nitrate levels of Antipodes Island water samples decreased with distance towards the island and that nitrate and dissolved reactive phosphorous levels increased with distance towards the Bounty Islands. This research has clearly demonstrated that there is a strong link between the marine and terrestrial realms at both islands and at spatial scales beyond the islands. The current management of the islands requires this new information to be taken into consideration in future management planning, so that trophic connections are maintained across realms. Further work is required across government and within agencies to bring legislation, policy and science into an integrated framework across sectors. This will allow environmental managers to reduce threats at the ecosystem level to minimise biodiversity loss and the risk of degradation of ecosystems, to protect New Zealand’s long-term biodiversity, social, cultural and economic prosperity.