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Restoring ecosystems degraded by an over-abundance of herbivores

Herbivores shape the world's vegetation, and when populations expand (e.g. because natural predators are eliminated) they can have major influences on plant communities. We have studied how over-abundant populations of large herbivores change forest dynamics, particularly focusing on the outcomes of conservation management. In particular, culling has become increasingly common to control herbivore numbers, but little is known as to whether it restores native plant communities.

Our studies have been focused in New Zealand, which provides a natural experiment for studying the impacts of large herbivores. Deer were introduced into New Zealand in the 19th century, spreading rapidly and reaching a peak population of ca. 8 million animals by the 1950s1. The New Zealand Department of Conservation is strongly concerned by the impacts that these invasive herbivores are having on native ecosystems, and support research into this topic. By analysing data from ~200,000 trees in 1546 permanent plots (a national forest inventory), we showed that highly-preferred plant species (e.g. Griselinia littoralis) were virtually eliminated from forest understories2,3. We found that New Zealand’s native forests were not in equilibrium: over a period of 18 years, the number of small trees declined by 18%4. Some of this decline is attributable to regeneration failure as a result of deer browsing, although other factors contribute to the pattern5,6,7. Our work in Fiordland National Park showed that deer have remarkably varied diets (based on analyses of gut contents of 426 red deer)8 and cause most damage to forests in winter, when snow makes the preferred alpine grassland habitats inaccessible9. Using 40-year datasets collected in grasslands, shrublands and forests, we have shown that deer-preferred species are recover slowly following reduction culling10,11, almost certainly because the animals feed exclusively on preferred-species when their densities are low but switch to less-preferred species whenever food becomes limited at higher densities2,11. We are using process-based models to explore the long-term impacts of deer on New Zealand forests, and to test whether culling deer will lead to rapid recovery of damaged forests.

Invasive herbivores have yet to cause the extinction of any New Zealand plants. We have discovered two mechanisms that might explain why the plants are resilient to damage by novel herbivores. We have found that the species that deer prefer to eat are also the fastest to recover afterbeing browsed12, indicating a trade-off between resistance and resilience to herbivores13. Deer-preferred species were also found to benefit from growing in patches dominated by deer-avoided species, presumably because deer visit these patches less frequently14. Many New Zealand species contain high concentrations of secondary compounds with anti-feedant properties (i.e. phenolics and tannins12). We found that foliar phenolic concentration was negatively correlated with nitrogen concentrations, supporting the idea that proteins and phenylpropanoids compete for a common precursor, while soil phosphorus availability had no detectable effect on phenolics15.

  1. Forsyth, D.M., Wilmshurst, J.M., Allen, R.B., & Coomes, D.A. (2010) Impacts of introduced deer and extinct moa on New Zealand ecosystems. New Zealand Journal of Ecology 34, 48–65. click
  2. Coomes, D.A., Allen, R.B., Forsyth, D.M. & Lee, W.G. (2003) Factors preventing the recovery of New Zealand forests following control of invasive deer. Conservation Biology, 17, 450-459. click
  3. Forsyth, D.M., Coomes, D.A., & Nugent, G. (2003) Framework for assessing the susceptibility of management areas to deer impacts. Science for Conservation Series, Issue 213. 39 pages. Department of Conservation, Wellington, New Zealand.
  4. Coomes, D.A., Duncan, R.P., Allen, R.B., & Truscott, J. (2003) Disturbances prevent stem size-density distributions in natural forests from following scaling relationships. Ecology Letters, 6, 980-989. click
  5. Husheer, S.W., Coomes, D.A., & Robertson, A.W. (2003) Long-term influences of introduced deer on the composition and structure of New Zealand Nothofagus forests. Forest Ecology and Management, 181, 99-117. click
  6. Husheer, S.W., Robertson, A.W., Coomes, D.A., & Frampton, C.M. (2006) Herbivory and plant competition reduce mountain beech seedling growth and establishment in New Zealand. Plant Ecology, 183, 245-256. DOI link
  7. Wright, D. M., Tanentzap, A. J., Flores, O., Husheer, S. W., Duncan, R. P., Wiser, S. K., … Coomes, D.A. (2012) Impacts of culling and exclusion of browsers on vegetation recovery across New Zealand forests. Biological Conservation, 153, 64-71.
  8. Tanentzap, A.J., Bee, J.N., Lee, W.G., Lavers, R.B., Mills, J.A., Mark. A.F., & Coomes D.A. (2009) The reliability of palatability estimates obtained from rumen contents analysis and a field-based index of diet selection. Journal of Zoology. 278, 243–248. DOI link
  9. Bee, J.N., Wright, D.M., Tanentzap, A.J., Lee, W.G., Lavers, R.B., Mark, A.F., Mills, J.A., & Coomes, D.A. (2010) Spatio-temporal feeding selection of red deer in a mountainous landscape. Austral Ecology, in press. DOI link
  10. Coomes, D.A., Mark, A.F., & Bee, J. (2006) Animal Control and Ecosystem Recovery. In: Biological Invasions in New Zealand (eds R.B. Allen & W.G. Lee), pp. 339-354, Springer Verlag.
  11. Tanentzap, A.J., Burrows, L.E., Lee, W.G., Nugent, G., Maxwell, J.M., & Coomes, D.A. (2009) Landscape-level vegetation recovery from herbivory: progress after four decades of invasive red deer control. Journal of Applied Ecology 46, 1064–1072. DOI link
  12. Tanentzap, A. J., Lee, W. G., Dugdale, J. S., Patrick, B. P., Fenner, M., Walker, S., Coomes, D.A. (2011). Differential responses of vertebrate and invertebrate herbivores to traits of New Zealand subalpine shrubs. Ecology, 92(4), 994-999.
  13. Bee, J. N., Kunstler G. & Coomes, D.A. (2007) Resistance and resilience of New Zealand tree species to browsing. Journal of Ecology 95, 1014–1026. DOI link pdf (password needed)
  14. Bee, J.N, Tanentzap, A.J., Lee, W.G., Lavers, R.B., Mark, A.F., Mills, J.A., Coomes, D.A. (2009) The benefits of being in a bad neighbourhood: plant community composition influences red deer foraging decisions. Oikos, 118, 18-24. DOI link
  15. Wright, D.M., Jordan, G.J., Lee, W.G., Duncan, R.P., Forsyth, D.M., & Coomes, D.A. (2010) Do leaves of plants on phosphorus-impoverished soils contain high concentrations of phenolic defence compounds? Functional Ecology 24, 52-61. DOI link
  16. Tanentzap, A. J., Bazely, D. R., Koh, S., Timciska, M., Haggith, E. G., Carleton, T. J., Coomes, D.A. (2011). Seeing the forest for the deer: Do reductions in deer-disturbance lead to forest recovery?. Biological Conservation, 144(1), 376-382.
  17. Bee, J.N., Tanentzap, A.T, Lee, W.G., Lavers, R.B., Mark, A.F., Mills, J.A., Coomes, D.A , Influence of foliar traits on forage selection by introduced red deer in New Zealand. Basic and Applied Ecology, DOI link.
  18. Bellingham, P.J. & Coomes, D.A. (2003) Grazing and community structure as determinants of invasion success by Scotch broom in a New Zealand montane shrubland. Diversity and Distributions, 9, 19-28. click