Leaf damage by herbivores and pathogens on New Zealand islands that differ in seabird densities

Seabirds impose a high-nutrient, high-disturbance regime on the islands on which they nest, resulting in higher nutrient cycling rates, plant nutrient uptake and leaf nutrient content. On islands off the coast of New Zealand, seabird-dominated islands support greater densities of soil- and litter-dwelling consumer biota. We predicted that islands with high seabird densities would have higher levels of leaf damage as a result of higher densities of foliar consumers (herbivores and pathogens). Damage levels on leaves of six common tree species were compared between 9 islands with active seabird colonies and 10 islands with low seabird densities resulting from invasion by predatory rats. There were no consistent differences in leaf damage by chewing, mining, or phloem-feeding herbivores across plant species; pathogen damage was lower on islands with high seabird densities than on those with low densities, but this was driven by only two of the plant species. Instead, plant species differed in which of several possible damage types responded to seabird presence, and in which plant leaf traits responded to seabird-related environmental changes. Across plant species, those with more resource-acquisitive leaf traits such as high percent nitrogen and low structural investment experienced higher levels of chewing damage (which accounted for 66–100% of all damage), but not other damage types. We conclude that the fertilisation and disturbance regimes imposed by seabirds do not lead to consistent changes in consumer damage to plants, because of variable responses by both individual plant species and different consumer groups.