BIODIVERSITY RESEARCH Invasiveness of alien plants in Brussels is related to their phylogenetic similarity to native species Carlo Ricotta 1 *, Sandrine Godefroid 2,3,4 and Duccio Rocchini 5 INTRODUCTION Increasing globalization and human mobility have facilitated the intentional and unintentional introduction of species beyond their natural geographic ranges. Such alien species are considered to be a main driver of global biodiversity loss with substantial environmental and economic impacts on invaded communities. While the potential economic cost for almost 90% of the alien species found in Europe is still unknown, the European Commission (2008) estimated the economic impact of alien species in Europe to be close to €10 billion annually, giving urgency to the scientific community to better under- stand which species are likely to have large impacts on the novel communities to which they are introduced. In this framework, urban areas usually contain the greatest proportion of alien plants; on one hand, cities act as focal point for the broad-scale introduction (both intentional and unin- tentional) of new species (Pys ˇek, 1998; Wittig, 2004; Chytry ´ et al., 2005, 2008). On the other hand, the ‘urban heat-island effect’ provides distinctive environmental conditions that have allowed many alien species with higher temperature require- ments and tolerance for arid environments to become 1 Department of Plant Biology, University of Rome ‘‘La Sapienza’’, Piazzale Aldo Moro 5, 00185 Rome, Italy, 2 National Botanic Garden of Belgium, Domein van Bouchout, 1860 Meise, Belgium, 3 Laboratory of Plant Biology and Nature Management (APNA), Department of Biology, Faculteit Wetenschappen en Bio-ingenieurswetenschappen, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium, 4 Laboratory of Complexity and Dynamics of Tropical Systems, CP 169, De´partement de Biologie des Organismes, Faculte´ des Sciences, Universite´ libre de Bruxelles, Avenue F.D. Roosevelt 50, 1050 Brussels, Belgium, 5 IASMA Research and Innovation Centre, Fondazione Edmund Mach, Environment and Natural Resources Area, Via E. Mach 1, 38010 S. Michele all’Adige, TN, Italy *Correspondence: Carlo Ricotta, Department of Plant Biology, University of Rome ‘‘La Sapienza’’, Piazzale Aldo Moro 5, 00185 Rome, Italy. E-mail: carlo.ricotta@uniroma1.it ABSTRACT Aim Understanding the processes that drive invasion success of alien species has received considerable attention in current ecological research. From an evolutionary point of view, many studies have shown that the phylogenetic similarity between the invader species and the members of the native community may be an important aspect of invasiveness. In this study, using a coarse-scale systematic sampling grid of 1 km 2 , we explore whether the occupancy frequency of two groups of alien species, archaeophytes and neophytes, in the urban angiosperm flora of Brussels is influenced by their phylogenetic relatedness to native species. Location The city of Brussels (Belgium). Methods We used ordinary least-squares regressions and quantile regressions for analysing the relationship between the occupancy frequency of alien species in the sampled grid and their phylogenetic distance to the native species pool. Results Alien species with high occupancy frequency in the sampled grid are, on average, more phylogenetically related to native species than are less frequent aliens, although this relationship is significant only for archaeophytes. In addition, as shown by the quantile regressions, the relationship between phylogenetic relatedness to the native flora and occupancy frequency is much stronger for the most frequent aliens than for rare aliens. Main conclusions Our data suggest that it is unlikely that species with very low phylogenetic relatedness to natives will become successful invaders with very high distribution in the area studied. To the contrary, under future climate warming scenarios, present-day urban aliens of high occupancy frequency are likely to become successful invaders even outside urban areas. Keywords Archaeophytes, biological invasions, neophytes, phylogenetic distance, quantile regression, randomization. Diversity and Distributions, (Diversity Distrib.) (2010) 16, 655–662 DOI: 10.1111/j.1472-4642.2010.00676.x ª 2010 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi 655 A Journal of Conservation Biogeography Diversity and Distributions