, Volume 163, Issue 2, pp 449–460 | Cite as

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

  • Madeline N. Grant-Hoffman
  • Christa P. Mulder
  • Peter J. Bellingham
Community ecology - Original Paper


Invasive rats (Rattus rattus, R. norvegicus, R. exulans) have large impacts on island habitats through both direct and indirect effects on plants. Rats affect vegetation by extirpating burrowing seabirds through consumption of eggs, chicks, and adults. These seabirds serve as ecosystem engineers, affecting plant communities by burying and trampling seeds and seedlings, and by altering microclimate. Rats also directly affect plant communities by consuming seeds and seedlings. We studied the direct and indirect impacts of rats on the seedlings of woody plants on 21 islands in northern New Zealand. We compared seedling densities and richness on islands which differed in status with respect to rats: nine islands where rats never invaded, seven islands where rats were present at the time of our study, and five islands where rats were either eradicated or where populations were likely to be small as a result of repeated eradications and re-invasions. In addition, we compared plots from a subset of the 21 islands with different burrow densities to examine the effects of burrowing seabirds on plants while controlling for other factors that differ between islands. We categorized plant communities by species composition and seedling density in a cluster analysis. We found that burrow densities explained more variation in seedling communities than rat status. In areas with high seabird burrow density seedling densities were low, especially for the smallest seedlings. Species richness and diversity of seedlings, but not seedling density, were most influenced by changes in microclimate induced by seabirds. Islands where rats had been eradicated or that had low rat populations had the lowest diversity and richness of seedlings (and adults), but the highest seedling density. Seedling communities on these islands were dominated by Pseudopanax lessonii and Coprosma macrocarpa. This indicates lasting effects of rats that may prevent islands from returning to pre-invasion states.


Rattus rattus Rattus norvegicus Ecosystem engineer Seedling community Predator eradication 



For permission to work on the islands they own or for which they are kaitiaki (guardians), we thank the following iwi: Ngāti Hako, Ngāti Hei, Ngāti Manuhiri, Ngāti Paoa, NgātiPuu, Ngāti Rehua, and Ngātiwai, as well as the Ngamotuaroha Trust, the Ruamāhua Islands Trust, John McCallum, Oho Nicholls, Bryce Rope, and the Neureuter family. We thank the New Zealand Department of Conservation for facilitating our visits to the island they administer. We also thank Karen Boot, Tadashi Fukami, Larry Burrows, Ewen Cameron, Aaron Hoffman, Richard Parrrish, Rob Chapelle, Gaye Rattray, Brian Karl, David Wardle, Holly Jones, Nora Leipner, Walter Hoffman and Anders Hoffman for all of their help. We are grateful to Ewen Cameron for providing the list of whole-island flora needed to calculate whole-island species richness. Comments by Henrik Moller and an anonymous reviewer improved the manuscript. This study was supported by the US National Science Foundation (DEB, 0317196), Marsden Fund of the Royal Society of New Zealand, the New Zealand Department of Conservation, the New Zealand Foundation for Research, Science, and Technology (Sustaining and restoring biodiversity OBI), and the Teaching Alaskans, Sharing Knowledge (TASK)/NSF Graduate Teaching Fellows in K-12 Education Program. The experiments performed for this research abide by all current laws within the countries in which they were performed.

Supplementary material

442_2009_1523_MOESM1_ESM.doc (46 kb)
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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Madeline N. Grant-Hoffman
    • 1
    • 4
  • Christa P. Mulder
    • 1
    • 2
  • Peter J. Bellingham
    • 3
  1. 1.Department of Biology and WildlifeUniversity of Alaska FairbanksFairbanksUSA
  2. 2.Institute of Arctic BiologyUniversity of Alaska FairbanksFairbanksUSA
  3. 3.Landcare ResearchLincolnNew Zealand
  4. 4.National Landscape Conservation System, Grand Junction Field OfficeBureau of Land Management ColoradoGrand JunctionUSA

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