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 δ(15)N 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.

Figure 1. Characteristics of the four studied islands.

Figure 2. Mean (+SE) δ¹⁵N values of the ecosystem compartments for different levels of seabird influence;

on (A) La Possession Island and (B) Bagaud Island. Within each island, each ecosystem compartment is represented by a symbol at the top and was analyzed separately: plants, arthropods and rodents. In (A) asterisks represent significant differences between bars within each compartment. Bars sharing a common letter were not significantly different in (B) based on contrast analyses. The dotted line with the seabird symbol (penguin for La Possession Island and seagull for Bagaud Island) represents the mean isotopic value of seabirds on each island.

Figure 3. Relationships between seabird δ¹⁵N values and δ¹⁵N values of each ecosystem compartment;

on (A) Surprise Island and (B) Teuaua Island. Symbols represent each ecosystem compartment: plants, seabird prey, arthropods, rats, mice and reptiles. Seabird values are shown by the upper edge of the grey polygon.