Journal of Ecology 2009, 97, 393– 403 doi: 10.1111/j.1365-2745.2009.01480.x © 2009 The Authors. Journal compilation © 2009 British Ecological Society Blackwell Publishing Ltd Impact of invasive plants on the species richness, diversity and composition of invaded communities Martin Hejda, Petr Pysek* and Vojtéch Jarosík Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 Pruhonice, Czech Republic, and Department of Ecology, Faculty of Science, Charles University, Vinicná 7, CZ-128 01 Praha 2, Czech Republic Summary 1. Much attention has been paid to negative effects of alien species on resident communities but studies that quantify community-level effects of a number of invasive plants are scarce. We address this issue by assessing the impact of 13 species invasive in the Czech Republic on a wide range of plant communities. 2. Vegetation in invaded and uninvaded plots with similar site conditions was sampled. All species of vascular plants were recorded, their covers were estimated and used as importance values for calculating the Shannon diversity index H′, evenness J and Sørensen index of similarity between invaded and uninvaded vegetation. 3. With the exception of two invasive species, species richness, diversity and evenness were reduced in invaded plots. Species exhibiting the greatest impact reduced species numbers per plot and the total number of species recorded in the communities sampled by almost 90%. A strong reduction of species number at the plot scale resulted in a marked reduction in the total species number at the landscape scale, and in less similarity between invaded and uninvaded vegetation. The decrease in species richness in invaded compared to uninvaded plots is largely driven by the identity of the invading species, whereas the major determinants of the decrease in Shannon diversity and evenness are the cover and height of invading species, and differences between height and cover of invading and dominant native species, independent of species identity. 4. Synthesis. Management decisions based on impact need to distinguish between invasive species, as their effects on diversity and composition of resident vegetation differ largely. Tall invading species capable of forming populations with the cover markedly greater than that of native dominant species exert the most severe effects on species diversity and evenness. Since a strong impact on the community scale is associated with reduction in species diversity at higher scales, invaders with a high impact represent a serious hazard to the landscape. Key-words: impact, neophyte, plant community, plant cover, plant height, plant invasion, species diversity, species evenness, species richness, temperate zone Introduction Biological invasions affect biodiversity worldwide at various scales (Tilman 1999; Mack et al. 2000; Manchester & Bullock 2000; Davis 2003). Research in the past decade improved our knowledge of the patterns of invasion and substantial progress in understanding the mechanisms of invasion has been achieved (Rejmánek et al. 2005). Recently, much attention has been paid to negative effects of alien species on resident communities and functioning of invaded ecosystems (Williamson 1998, 2001; Parker et al. 1999; Byers et al. 2002; Simberloff et al. 2003) and to the mechanisms underlying these impacts (Levine et al. 2003). Ecosystem impacts are best documented for invasions by woody species and include, for example, nutrient enrichment (Vitousek & Walker 1989), water loss (Zavaleta 2000) and changed fire regimes (Brooks et al. 2004). Large-scale effects of plant invasions include the homogenization of floras, when originally different phytogeo- graphical units become similar thanks to massive plant invasions (Schwartz et al. 2006, but see Kühn & Klotz 2006). At the community level, the suppression of native plants is a phenomenon resulting from the dominance invasive species achieve in invaded habitats (Richardson et al. 1989, Pysek & Pysek 1995, Daehler 2003). Surprisingly, studies measuring *Correspondence author. E-mail: pysek@ibot.cas.cz