Impact scores of invasive plants are biased by disregard of environmental co-variation and non-linearity

Jan Thiele,Annette Otte,Maike Isermann,Johannes Kollmann

doi:10.3897/neobiota.10.1191

Abstract

Prioritisation of high-impact species is becoming increasingly important for management of introduced species (‘neobiota’) because of their growing number of which, however, only a small fraction has substantial impacts. Impact scores for prioritising species may be affected by the type of effect model used. Recent studies have shown that environmental co-variation and non-linearity may be significant for effect models of biological invasions. Here, we test for differences in impact scores between simple and complex effect models of three invasive plant species (Heracleum mantegazzianum, Lupinus polyphyllus, Rosa rugosa). We investigated the effects of cover percentages of the invasive plants on species richness of invaded communities using both simple linear effect models (‘basic models’) and more complex linear or nonlinear models including environmental co-factors (‘full models’). Then, we calculated impact scores for each invasive species as the average reduction of species richness predicted by basic and full effect models. All three non-native species had negative effects on species richness, but the full effect models also indicated significant influence of habitat types. Heracleum mantegazzianum had uniform linear effects in all habitats, while effects of L. polyphyllus interacted strongly with habitat type, and R. rugosa showed a marked non-linear relationship. Impact scores were overestimated by basic effect models for H. mantegazzianum and R. rugosa due to disregard of habitat effects and non-linearity, respectively. In contrast, impact of L. polyphyllus was underestimated by the basic model that did not account for the strong interaction of invader cover and habitat type. We conclude that simple linear models will often yield inaccurate impact scores of non-native species. Hence, effect models should consider environmental co-variation and, if necessary, non-linearity of the effects of biological invasions on native ecosystems.

Highlights

Impacts differ in quality and quantity among introduced species (‘neobiota’)
We test for differences in impact scores between simple and complex effect models of three invasive plant species (Heracleum mantegazzianum, Lupinus polyphyllus, Rosa rugosa)
We investigated the effects of cover percentages of the invasive plants on species richness of invaded communities using both simple linear effect models (‘basic models’) and more complex linear or nonlinear models including environmental co-factors (‘full models’)

Summary

Introduction

Impacts differ in quality and quantity among introduced species (‘neobiota’). Many non-indigenous species can be considered neutral, but some have severe implications for native biodiversity, ecosystems, human health and economy (Gaertner et al 2009, Hejda et al 2009, Goodenough 2010). Regarding species richness of invaded communities, for instance, removal or seedaddition experiments have been used (Meffin et al 2010), as well as comparisons of invaded and uninvaded sites (Adams and Engelhardt 2009, Hejda et al 2009, Flory and Clay 2010, Maurel et al 2010), multi-year studies of expanding margins of invasive stands (Brewer 2008), comparisons of different stages of invasion (Prévosto et al 2006), and correlational studies of gradients of invader abundance (Isermann et al 2007, Gooden et al 2009) All these approaches have their pros and cons concerning inference about invader impacts (cf Adams and Engelhardt 2009, Meffin et al 2010)

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