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Invasive rats alter woody seedling composition on seabird-dominated islands in New Zealand

TitleInvasive rats alter woody seedling composition on seabird-dominated islands in New Zealand
Publication TypeJournal Article
Year of Publication2010
AuthorsGrant-Hoffman, MN, Mulder, CP, Bellingham, PJ

Invasive rats (Rattus rattus, R. norvegicus,R. exulans) have large impacts on island habitats throughboth direct and indirect effects on plants. Rats affect vegetationby extirpating burrowing seabirds through consumptionof eggs, chicks, and adults. These seabirds serveas ecosystem engineers, affecting plant communities byburying and trampling seeds and seedlings, and by alteringmicroclimate. Rats also directly affect plant communitiesby consuming seeds and seedlings. We studied the directand indirect impacts of rats on the seedlings of woody plantson 21 islands in northern New Zealand. We comparedseedling densities and richness on islands which differed instatus with respect to rats: nine islands where rats neverinvaded, seven islands where rats were present at the time ofour study, and five islands where rats were either eradicatedor where populations were likely to be small as a result ofrepeated eradications and re-invasions. In addition, wecompared plots from a subset of the 21 islands with differentburrow densities to examine the effects of burrowingseabirds on plants while controlling for other factors thatdiffer between islands. We categorized plant communitiesby species composition and seedling density in a clusteranalysis. We found that burrow densities explained morevariation in seedling communities than rat status. In areaswith high seabird burrow density seedling densities werelow, especially for the smallest seedlings. Species richnessand diversity of seedlings, but not seedling density, weremost influenced by changes in microclimate induced byseabirds. Islands where rats had been eradicated or that hadlow rat populations had the lowest diversity and richness ofseedlings (and adults), but the highest seedling density.Seedling communities on these islands were dominated byPseudopanax lessonii and Coprosma macrocarpa. Thisindicates lasting effects of rats that may prevent islandsfrom returning to pre-invasion states.

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