BIODIVERSITY REVIEW Disentangling the relative effects of environmental versus human factors on the abundance of native and alien plant species in Mediterranean sandy shores 1; 2 1; 2 Marta Carboni 1,2 *, Wilfried Thuiller 2 , Francesca Izzi 1 and Alicia Acosta 1 INTRODUCTION Many factors govern plant community assembly and invasi- bility (Richardson & Pys ˇek, 2006). Understanding which specific features affect native plant assemblages is ever more important in light of the increasing global loss of biodiversity. In general, environmental factors that have been found to influence patterns of species richness at a regional scale include climate, landscape heterogeneity, spatial patterns, geomorpho- logic processes and level of protection (van Rensburg et al., 2002; Hawkins et al., 2003; Davies et al., 2005; Moser et al., 2005; Thuiller et al., 2006a). Moreover, human influence is nowadays one further important agent of change for species richness and diversity patterns (Maestre, 2004; Gaston, 2005). Human population density and proxies thereof have often been shown to correlate positively with species richness in response to similar factors, such as productivity (Balmford et al., 2001; Arau ´ jo, 2003; Chown et al., 2003), a congruence that suggests a marked conflict between conservation and development. However, depending on spatial scale and overall D D I 6 7 7 B Dispatch: 18.5.10 Journal: DDI CE: Samuel Chelliah Journal Name Manuscript No. Author Received: No. of pages: 10 PE: Gomathi M 1 Dipartimento di Biologia Ambientale, Universita ` degli Studi Roma Tre, V.le Marconi 446, 00146 Roma, Italy, 2 Laboratoire d’Ecologie Alpine, UMR CNRS 5553, Universite´ Joseph Fourier, Grenoble, France *Correspondence: Marta Carboni, Dipartimento di Biologia Ambientale, Universita ` degli Studi Roma Tre, V.le Marconi 446, 00146 Roma, Italy. E-mail: mcarboni@uniroma3.it ABSTRACT Aim Mediterranean coastal sand dunes are characterized by both very stressful environmental conditions and intense human pressure. This work aims to separate the relative contributions of environmental and human factors in determining the presence/abundance of native and alien plant species in such an extreme environment at a regional scale. Location 250 km of the Italian Tyrrhenian coast (Region Lazio). Methods We analysed alien and native plant richness and fitted generalized additive models in a multimodel-inference framework with comprehensive randomizations to evaluate the relative contribution of environmental and human correlates in explaining the observed patterns. Results Native and alien richness are positively correlated, but different variables influence their spatial patterns. For natives, human population density is the most important factor and is negatively related to richness. Numbers of natives are unexpectedly lower in areas with a high proportion of natural land cover (probably attributable to local farming practices) and, to a lesser degree, affected by the movement of the coastline. On the other hand, alien species richness is strongly related to climatic factors, and more aliens are found in sectors with high rainfall. Secondarily, alien introductions appear to be related to recent urban sprawl and associated gardening. Main conclusions Well-adapted native species in a fragile equilibrium with their natural environment are extremely sensitive to human-driven modifications. On the contrary, for more generalist alien species, the availability of limited resources plays a predominant role. Keywords Biological invasions, coastal dunes, diversity patterns, environmental and human determinants, exotic species, inference-based model. Diversity and Distributions, (Diversity Distrib.) (2010) 1–10 DOI: 10.1111/j.1472-4642.2010.00677.x ª 2010 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56