Aliens or natives: who are the ‘thugs’ in British woods?
- 224 Downloads
The invasion of native habitats by exotic, or alien, plant species has received considerable attention recently from policy, research, and practical conservation management perspectives. However, a new hypothesis for species dynamics in Britain suggests that a small number of aggressive native plant species (termed ‘thugs’) may have an equal, or greater, impact on native species and habitats than exotic species. Here, we examine this hypothesis using multivariate techniques with field-layer cover data collected during a country-wide survey of British woodlands. Multivariate analysis of these data identified a north-south gradient on the first axis, and that 20 of the 25 National Vegetation Classification woodland types were sampled within the study. The most abundant field-layer species included three of the proposed native ‘thugs’, i.e. Rubus fruticosus, Pteridium aquilinum and Hedera helix in addition to the native woodland indicator species Mercurialis perennis. Variation partitioning was used to compare the relative importance of native field-layer ‘thug’ species with invading alien shrub and tree species relative to other environmental drivers. The variation in the field-layer data-set explained by the three native ‘thug’ species was significant, but they explained a relatively small proportion of the variation relative to other environmental variables (climate, soil, management factors etc.). They did, however, explain almost four times as much variation as the three alien species that were significantly correlated with field-layer species composition (Acer pseudoplatanus, Impatiens glandulifera, Rhododendron ponticum). The results of this analysis suggest that the field-layer of British woodlands is impacted as much by native ‘thug’ species, as it is from ‘aliens’. Concern about the impact of these native ‘thug’ species has been reported previously, but their impact has not previously been compared to the impact of invading aliens. It is hoped that this analysis will do two things, first to act as a sound baseline for assessing any changing balance that should occur in the future, and second, to prompt both ecologists and conservationists to develop woodland management policies based on sound science.
Key WordsDetrended correspondence analysis National Woodland Survey native species resource assessment variation partitioning woodland field-layer species woodland herb
PMC received financial support from English Nature and CEH. We thank Dr Hugh McAllister (University of Liverpool) for technical support, Sandra Mather for the illustrations and both David Pearman (BSBI) and Kevin Walker (CEH, Monks Wood), for valuable comments on earlier versions of this manuscript. The National Woodland survey was funded by Countryside Council for Wales, Defra, English Nature, Forestry Commission, JNCC, Scottish Natural Heritage and the Woodland Trust.
- Bunce, R. G. H. & Shaw, M. W. (1973). A standardized procedure for ecological survey. J. Environm. Managem. 1: 239 – 258.Google Scholar
- CBD (2002). VI/23. Alien species that threaten ecosystems, habitats or species In: Annex I. Decisions adopted by the conference of the parties to the Convention on Biological Diversity at its sixth meeting, pp. 249 – 262. UNEP, The Hague, Netherlands.URL: http://www.biodiv.org/doc/decisions/COP-06-dec-en.pdf.
- Corney, P. M. (2006). Assessment of the factors influencing woodland field-layer vegetation composition across Britain. PhD thesis, University of Liverpool, Liverpool.Google Scholar
- DEFRA (2003). Review of non-native species policy; Report of the working group. DEFRA, London. http://www.defra.gov.uk/wildlife-countryside/resprog/findings/non-native/report.pdf.
- Dehnen-Schmutz, K., Perrings, C. & Williamson, M. (2004). Controlling Rhododendron ponticum in the British Isles: an economic analysis. J. Environm. Managem. 70: 323 – 332.Google Scholar
- EDINA (2002). Digimap online mapping service (http://edina.ac.uk/digimap). © Crown Copyright. An EDINA Digimap/JISC supplied service.
- Edwards, C., Clay, D. V. & Dixon, F. L. (2000). Stem treatment to control Rhododendron ponticum under woodland canopies. Aspects Appl. Biol. 58: 1 – 8.Google Scholar
- ERDAS (2001). ERDAS IMAGINE, version 8.5. ERDAS, inc., Atlanta, Georgia,Google Scholar
- ESRI (2000). ArcView GIS, version 3.2a. Environmental Systems Research Institute, Redlands, California.Google Scholar
- ____ (2001). Map Manager version 6.2. Environmental Systems Research Institute, Redlands, California.Google Scholar
- Grime, J. P., Hodgson, J. D. & Hunt, R. (1988). Comparative plant ecology: a functional approach to common British species. Unwin Hyman, London.Google Scholar
- Hill, M. O. (1996). TABLEFIT version 1.0, for identification of vegetation types. Institute of Terrestrial Ecology, Huntingdon.Google Scholar
- IUCN (2000). Guidelines for the prevention of biodiversity loss caused by alien invasive species. Prepared by the SSC Invasive Species Specialist Group; Approved by the 51st Meeting of the IUCN Council, Gland, Switzerland. http://www.iucn.org/themes/ssc/pubs/policy/invasivesEng.htm.
- Kirby, K. J. & Woodell, S. R. J. (1998). The distribution and growth of bramble (Rubus fruticosus L. agg.) in British semi-natural woodland and their implications for nature conservation. J. Practical Ecol. & Conserv. 2: 31 – 41.Google Scholar
- ____, Smart, S. M., Black, H. I. J., Bunce, R. G. H., Corney, P. M. & Smithers, R. J. (2005). Long-term ecological changes in British broadleaved woodland 1971 –2001. Research Report. 653. English Nature, Peterborough. http://naturalengland.etraderstores.com/NaturalEnglandShop/R653.
- Le Duc, M. G., Pakeman, R. J. & Marrs, R. H. (2000). Vegetation development on upland and marginal land treated with herbicide, for bracken (Pteridium aquilinum) control, in Great Britain. J. Environm. Managem. 58: 147 – 160.Google Scholar
- McAllister, H. A. & Rutherford, A. (1990). Hedera helix L. and H. hibernica (Kirchner) Bean (Araliaceae) in the British Isles. Watsonia 18: 7 – 15.Google Scholar
- Millennium Ecosystem Assessment (2005). http://www.millenniumassessment.org/en/About.aspx.
- Oksanen, J. (2003). Gradient Analysis of Vegetation (Gravy) software, version 0.0-21. http://cc.oulu.fi/~jarioksa/softhelp/softalist.html.
- Pearman, D. (2004a). Invading aliens — or invading natives? BSBI News 96: 41 – 42.Google Scholar
- ____ (2004b). The native plants are restless. Radio 4; Nature (25th October). URL: http://www.bbc.co.uk/radio4/science/nature_20041025.shtml.
- ____ & Lockton, A. (2004). Aliens and introductions: A discussion paper. URL: http://www.bsbi.org.uk/html/alien_invaders_.html.
- R Development Core Team (2004). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. http://www.R-project.org.
- Rackham, O. (2006). Woodlands. HarperCollins, London.Google Scholar
- Ratcliffe, D. A. (1977). A nature conservation review: the selection of biological sites of national importance to nature conservation in Britain. CUP, Cambridge.Google Scholar
- Ray, D. (2001). Ecological Site Classification, version 1.7. A PC-based Decision Support System for British Forests. Forestry Commission, Edinburgh.Google Scholar
- Rodwell, J. (ed.) (1991). Woodlands and scrub. CUP, Cambridge.Google Scholar
- Smart, S. M., Bunce, R. G. H., Marrs, R., Le Duc, M., Firbank, L. G., Maskell, L. C., Scott, W. A., Thompson, K. & Walker, K. J. (2005). Large-scale changes in the abundance of common higher plant species across Britain between 1978, 1990 and 1998 as a consequence of human activity: Tests of hypothesised changes in trait presentation. Biol. Conserv. 124: 355 – 371.CrossRefGoogle Scholar
- Sokal, R. R. & Rohlf, F. J. (1995) Biometry. 3rd edition. W. H. Freeman and Co., New York.Google Scholar
- ter Braak, C. J. F. & Šmilauer, P. (2002). CANOCO Reference manual and CANODRAW for Windows User's guide: Software for Canonical Community Ordination (version 4.5). Microcomputer Power, Ithaca, NY.Google Scholar