Kollmann, Johannes Christian

Otte, Annette

Thiele, Jan

English

Copenhagen University Research Information System

u n i ve r s i t y  o f  co pe n h ag e n  Københavns UniversitetRigorous impact assessment of invasive plant speciesThiele, Jan; Otte, Annette; Kollmann, Johannes ChristianPublication date:2008Document versionPublisher's PDF, also known as Version of recordCitation for published version (APA):Thiele, J., Otte, A., & Kollmann, J. C. (2008). Rigorous impact assessment of invasive plant species: Theoreticalconsiderations and practical applications. Poster session presented at NEOBIOTA - 5th European Conferenceon Biological Invasions, Prague, Czech Republic.Download date: 02. Feb. 2020Institute of BotanyAcademy of Sciences of the Czech RepublicFaculty of SciencesCharles University of PragueNEOBIOTA: Towards a Synthesis5th European Conference on Biological InvasionsPrague (Czech Republic), 23.-26. September 2008Book of abstractsEdited by P. Pyšek & J. PerglPoster presentation 4-24211Rigorous impact assessment of invasive plant species:theoretical considerations and practical applicationsJan Thiele1, Annette Otte2 & Johannes Kollmann11University of Copenhagen, Department of Ecology, Denmark; jat@life.ku.dk2University of Giessen, Department of Landscape Ecology and Landscape Planning, GermanyAccording to Parker et al. (1999), the impact of an invasive species can roughly be quantified as I = R × A × E, where R is the area of the invaded range, A is the invader’s average abundance, and E is the invader’s effect per individual or biomass unit. We argue that this basic formula has got someshortcomings in not accounting for non-linearity of the abundance–effect size relationship andneglecting variability in habitat quality and availability throughout the invaded range. Further, effectsof invasive species may not be unique but can be similar to those of native species.The per capita effect of an invasive plant species is likely to be correlated with its abundance. For instance, single individuals of an invader might have virtually no effect on community structure,composition or function, whereas a dense stand of the same number of individuals might have a strong effect. If this relationship is non-linear, then the product of average abundance and per capita effectgives a wrong estimate of impact. We suggest that measurement of effects on a ‘per unit area’ basismight be better suited for invasive plant species. In practical applications, non-linearity of the effect–abundance relationship could be accounted for by calculating proportional impacts for differentabundance classes of the invader, e.g. open and dominant stands.Empirical studies show that effects of invasive species can vary substantially betweendifferent invaded habitat types. For the invasive plant Heracleum mantegazzianum it was shown that it had only moderate effects in tall-herb stands, whereas it completely altered community compositionand structure of ruderal vegetation and grasslands (Thiele & Otte 2007). Hence, the measurement ofboth effects and invaded range should be habitat-specific.In the course of vegetation dynamics, particularly succession, native species as well as invasivespecies can alter community composition and structure. For instance, tall herbs may shade out smaller herbs reducing species richness of successional grasslands. If the effects of invasive and native species are redundant, the estimation of effect size based on before-after comparisons may overestimateinvader impact. Instead, a comparison of invasive vs. native effect would give a more realistic impactassessment. Hence, we suggest using differential effect estimates (invader effect–native effect). Forpractical application, we advocate a combined approach of habitat-specific field surveys and remotesensing for area-corrected quantification of the invaded range.ReferencesParker I. M., Simberloff D., Lonsdale W. M., Goodell K., Wonham M., Kareiva P. M., Williamson M. H., Von Holle B., Moyle P. B., Byers J. E. & Goldwasser L. (1999) Impact: toward a framework for understanding the ecological effects of invaders. Biological Invasions 1: 3–19.Thiele J. & Otte A. (2007) Impact of Heracleum mantegazzianum on invaded vegetation and human activities.In: Pyšek P., Cock M. J. W., Nentwig W. & Ravn H. P. (eds) Ecology and management of giant hogweed (Heracleum mantegazzianum), p. 144–156, CAB International, Wallingford.

Rigorous impact assessment of invasive plant species:Theoretical considerations and practical applications

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Rigorous impact assessment of invasive plant species:Theoretical considerations and practical applications

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