Marine Subsidies Produce Cactus Forest On Desert Island

Abstract

Marine nutrient subsidies can impact terrestrial plant biodiversity. In island systems, seabird guano, rich in nitrogen, is a large proponent of such marine subsidies. In the Gulf of California Midriff Islands, seabird guano is commonplace on bird islands, such as San Pedro Mártir, where the large columnar cactus, cardón (Pachycereus pringlei), is the dominant plant biomass. We propose that a chain of interactions across the land-sea interface yield an allochthonous input of nitrogen in the form of seabird guano that fuels the production of one of the densest cactus forests in the world. Fish, seabird, guano, soil, and cardón samples were taken from the island for stable δ15N isotope ratios, which were compared to other Gulf islands and terrestrial ecosystems throughout the range of the cardón. The cactus forest on this seabird island showed elevated marine derived nitrogen isotope ratios relative to mainland and non-bird islands. δ15N values increased at each trophic level, illustrating a trophic cascade between the marine and island ecosystems driven by marine upwelling. Our findings demonstrate that marine nutrient deposits stimulate terrestrial plant production that is absent in mainland ecosystems and non-seabird islands, elucidating the integral nature of nutrient movement across the land sea interface.