© 2007 The Authors DOI: 10.1111/j.1472-4642.2007.00430.x Journal compilation © 2007 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi 329 Diversity and Distributions, (Diversity Distrib.) (2008) 14, 329–342 BIODIVERSITY RESEARCH ABSTRACT Anthropogenic disturbance is considered a risk factor in the establishment of non- indigenous species (NIS); however, few studies have investigated the role of anthro- pogenic disturbance in facilitating the establishment and spread of NIS in marine environments. A baseline survey of native and NIS was undertaken in conjunction with a manipulative experiment to determine the effect that heavy metal pollution had on the diversity and invasibility of marine hard-substrate assemblages. The study was repeated at two sites in each of two harbours in New South Wales, Australia. The survey sampled a total of 47 sessile invertebrate taxa, of which 15 (32%) were identified as native, 19 (40%) as NIS, and 13 (28%) as cryptogenic. Increasing pollution exposure decreased native species diversity at all study sites by between 33% and 50%. In con- trast, there was no significant change in the numbers of NIS. Percentage cover was used as a measure of spatial dominance, with increased pollution exposure leading to increased NIS dominance across all sites. At three of the four study sites, assem- blages that had previously been dominated by natives changed to become either extensively dominated by NIS or equally occupied by native and NIS alike. No single native or NIS was repeatedly responsible for the observed changes in native species diversity or NIS dominance at all sites. Rather, the observed effects of pollution were driven by a diverse range of taxa and species. These findings have important im- plications for both the way we assess pollution impacts, and for the management of NIS. When monitoring the response of assemblages to pollution, it is not sufficient to simply assess changes in community diversity. Rather, it is important to dis- tinguish native from NIS components since both are expected to respond differently. In order to successfully manage current NIS, we first need to address levels of pollution within recipient systems in an effort to bolster the resilience of native communities to invasion. Keywords Anthropogenic disturbance, biological invasions, native diversity, non-indigenous species (NIS), pollution, sessile invertebrate community. INTRODUCTION Non-indigenous species (NIS) pose a significant threat to the health and productivity of marine ecosystems, through reduc- tions in native diversity, predation, habitat alteration, and competition for resources (Mack et al., 2000). Observational studies that research the distribution of exotics have identified anthropogenic disturbance as a key factor in facilitating invasion of NIS (Celesti-Grapow et al., 2006). In the marine environment, estuaries and bays are the habitats simultaneously most exposed to invasive propagules (Ruiz et al., 1997) and most prone to anthro- pogenic disturbance (Hall et al., 1998). The extent to which these two factors interact in facilitating successful invasion remains unclear. Is the increasing occurrence of NIS within estuaries and harbours primarily a result of increased inoculation rates (from a combination of vectors such as shipping, fishing and aqua- culture), or are NIS better able to take advantage of increasingly disturbed environments? This uncertainty highlights the urgent need for studies that not only record observed patterns of invasives in natural systems, but simultaneously examine the contributory mechanisms driving these patterns. Only through such studies can we hope to provide useful information to inform invasive species management. Anthropogenic disturbances can change community dynamics and facilitate the establishment of NIS through a variety of mechanisms. The most common is through increased resource School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia *Correspondence: Richard F. Piola, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia. Tel.: +61 2 9385 3447; Fax: +61 2 9385 1558; E-mail: r.piola@unsw.edu.au Blackwell Publishing Ltd Pollution reduces native diversity and increases invader dominance in marine hard-substrate communities Richard F. Piola* and Emma L. Johnston