Seabirds deposit guano and disturb vegetation on islands all over the world, yet we do not know to what extent seabird impacts are similar across systems, and to what extent they are determined by system-specific characteristics such as geographic location, moisture regime, or seabird species. We compared the impact of seabird density on soil and plant chemical properties across seven different island systems: two in New Zealand (9 and 22 islands), and one each in the Northern Line Islands (South Pacific, 15 islands), Gulf of California (Mexico, 14 islands), Gulf of Maine (4 islands), French Mediterranean (9 islands), and Stockholm Archipelago (Sweden, 22 islands). Soil data (pH, %N, %P, and δ¹⁵N) were available for 3-6 island systems per variable, and leaf chemistry data (%N, δ¹⁵N, %C, δ¹³C, and %P) were available for 26 plant species and 3-6 systems per variable.

Results/Conclusions

All response variables showed significant differences in means between island systems. Soil pH, %N, and %P showed strong relationships with seabird density across all islands (negative for pH and positive for %N and %P).  However, there were no interactions between density and island systems. Values for δ¹⁵N increased with seabird density across all island systems, but much more strongly in the Gulf of California. Plant responses to seabird density were much more variable than soil responses.  Leaf chemistry increased with seabird density in all systems, but the slope was steeper for islands in the Stockholm Archipelago than elsewhere. Leaf % C and %P differed strongly between systems but seabird density did not explain variation. Leaf δ¹⁵N increased with seabird density, with more rapid increases for the Stockholm Archipelago islands. Leaf δ¹³C increased for three out of five island systems and decreased for one. Plants with different growth forms (trees, shrubs, graminoids, and forbs) showed significant differences in %N, %C, and δ¹⁵N, but there were no interactions between growth form and seabird density.  We conclude that changes in soil chemistry in response to seabirds are highly predictable regardless of soil type, but that plants growing in these soils differ in responses between islands systems. These results suggest that other factors (e.g., soil moisture and light availability) are constraining responses to changes in soil chemistry, and that these limiting factors differ between island systems.