Seabird Modulations of Isotopic Nitrogen on Islands

Abstract

The transport of nutrients by migratory animals across ecosystem boundaries can significantly enrich recipient food webs, thereby shaping the ecosystems’ structure and function. To illustrate the potential role of islands in enabling the transfer of matter across ecosystem boundaries to be gauged, we investigated the influence of seabirds on nitrogen input on islands. Basing our study on four widely differing islands in terms of their biogeography and ecological characteristics, sampled at different spatial and temporal intervals, we analyzed the nitrogen isotopic values of the main terrestrial ecosystem compartments (vascular plants, arthropods, lizards and rodents) and their relationship to seabird values. For each island, the isotopic values of the ecosystem were driven by those of seabirds, which ultimately corresponded to changes in their marine prey. First, terrestrial compartments sampled within seabird colonies were the most enriched in δ15N compared with those collected at various distances outside colonies. Second, isotopic values of the whole terrestrial ecosystems changed over time, reflecting the values of seabirds and their prey, showing a fast turnover throughout the ecosystems. Our results demonstrate that seabird-derived nutrients not only spread across the terrestrial ecosystems and trophic webs, but also modulate their isotopic values locally and temporally on these islands. The wealth of experimental possibilities in insular ecosystems justifies greater use of these model systems to further our understanding of the modalities of trans-boundary nutrient transfers.

Inflated population density of island antechinus: a case of allochthonous marine inputs leading to increased food availability?

“…15 N values for terrestrial plants and invertebrates on the island greatly exceeded levels found in mainland samples and are indicative of marine-derived nitrogen (Mizutani and Wada 1988;Farina et al 2003;Stapp and Polis 2003;Fukami et al 2006;Caut et al 2012).…”

“…Extensive research on seabird colonies has shown a significant positive effect on the productivity of island ecosystems (Smith 1978;Polis and Hurd 1996;Huxel and McCann 1998;Stapp and Polis 2003;Bancroft et al 2005;Caut et al 2012), including increased biomass/ availability of invertebrates (e.g. Sánchez-Piñero and Polis 2000; Markwell and Daugherty 2002;Caut et al 2012).…”

“…Colonial seabirds and seals are powerful vectors of marine nutrients and can significantly increase the productivity of island ecosystems (Gillham 1960;Polis et al 1997;Farina et al 2003;Caut et al 2012). Seabirds, in particular, provide nutrients, in the form of guano, discarded prey, egg remains and carcasses, which can increase greatly the concentration of nutrients in the soil and plant productivity of colony areas (Gillham 1960;Smith 1978;Anderson and Polis 1999;Bancroft et al 2005;Fukami et al 2006;Jones 2010).…”

Seabirds supply nitrogen to reef-building corals on remote Pacific islets

“…These nutrients will probably impact higher trophic level consumers such as parrot fish feeding on corals 51, 52 and even higher trophic level organisms, since baseline isotopic values have been shown to propagate up to top predators in marine and terrestrial systems 7, 53 . Assimilation of seabird-derived nitrogen by coral and zooxanthellae could have important unanticipated effects on coral health even if it is still subject to debate whether high levels of nitrogen are beneficial or not to corals 54 .…”

“…Seabird guano is enriched in 15 N relative to 14 N, partly due to the birds’ high trophic position 36 and to preferential volatilization of 14 N from guano 37 . Thus, nitrogen stable isotope ratios can be used as a useful tool to trace guano incorporation in marine food webs 7, 12 , with high δ 15 N values in animal tissues being a proxy for seabird derived nitrogen.

Figure 1Sampling sites in New Caledonia. The map was generated with ArcGIS 10.2.2 (http://www.esri.com/arcgis/aboutarcgis) and customized in Adobe Illustrator CS3 (version 13, www.adobe.com).

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