Incorporation of the invasive mallow Lavatera arborea into the food web of an active seabird island
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- Hawke, D.J. & Clark, J.M. Biol Invasions (2010) 12: 1805. doi:10.1007/s10530-009-9591-4
This study investigated the role of the invasive mallow Lavatera arborea in the terrestrial ecosystem of a flourishing seabird island in SE New Zealand using natural abundance stable isotope ratios (13C/12C; 15N/14N, reported as δ13C and δ15N). Foliage samples of L. arborea came from transects encompassing three distinct environments (plateau, slope, storm-washed flat) across the island. Samples of potential marine nutrient sources (beach-cast kelp; seabirds using the island) were also collected, to contextualise the L. arborea data. Samples of invertebrate taxa (exotic and indigenous) from multiple ecosystem guilds were hand-collected; a bee, a sap-sucking Homoptera, a litter-feeding tenebrionid beetle, various carrion-feeding flies, a predatory carabid beetle, a salticid spider, and (from a seabird cadaver) Dermestes sp. exuviae. Discarded skins from the gecko Hoplodactylus maculatus were collected from moulting sites. Highly enriched δ15N values showed that L. arborea from all three environments utilised seabird N, even though breeding seabirds were absent from the storm-washed flat. The isotopic signatures of the Homoptera, and the tenebrionid and carabid beetles could be accounted for entirely by food webs based on L. arborea. Bee and salticid spider isotopic signatures could be accounted for by varying contributions from L. arborea. The flies and Dermestes were (as expected) linked to carrion from either the island or the adjacent mainland. In contrast, gecko data indicated direct dependence on seabirds, although the exact relationship was unclear. Our study therefore showed that L. arborea is an integral part of the terrestrial ecosystem of the island across multiple trophic levels from pollinators to top-level predators.