4. Synthesis . Disturbance is a weak predictor of invasion. To increase predictive power, we need to consider multiple variables (both intrinsic and extrinsic to the site) simultaneously. Variables that describe the changes sites have undergone may be particularly informative.

3. We use data from 200 sites around the world to provide a broad test of the hypothesis that invasions are better predicted by a change in disturbance regime than by disturbance per se . Neither disturbance nor change in disturbance regime explained more than 7% of the variation in the % of cover or species richness contributed by introduced species. However, change in disturbance regime was a significantly better predictor than was disturbance per se , explaining approximately twice as much variation as did disturbance.

2. Three main conclusions emerge from our review: (i) Many lines of research, including the search for traits that make species good invaders, or that make ecosystems susceptible to invasion, are yielding idiosyncratic results. To move forward, we advocate a more synthetic approach that incorporates a range of different types of information about the introduced species and the communities and habitats they are invading. (ii) Given the growing evidence for the adaptive capacity of both introduced species and recipient communities, we need to consider the implications of the long‐term presence of introduced species in our ecosystems. (iii) Several foundational ideas in invasion biology have become widely accepted without appropriate testing, or despite equivocal evidence from empirical tests. One such idea is the suggestion that disturbance facilitates invasion.

Introduction We have come a long way in our understanding of introduced species in the century since publication of the Journal of Ecology began. One hundred years ago, introductions were widely celebrated, and acclimatization societies were busy ‘enriching’ the flora and fauna in many regions world‐wide. Some of the more notable achievements of acclimatization societies include introducing starlings and house sparrows to the United States (in an attempt to introduce all the birds mentioned in Shakespeare’s works to New York’s Central Park; Marzluff et al. 2008), introducing brushtail possums to New Zealand (to establish a fur industry; Cowan 1992) and distributing ornamental plant species such as Lantana camara and Miconia calvescens to gardens world‐wide (Meyer 1996; CRC Weed Management 2003). Many species (e.g. cane toads in Australia) were introduced as biocontrols, and many more species were introduced accidentally (e.g. zebra mussels). As a result of both intentional and unintentional introductions, introduced species now make up a substantial part of the vascular flora in most places (about half in Hawaii, New Zealand and the Cook Islands, 21% in Britain, 24% in Canada, 10–30% in several mainland USA states, 12.5% in Europe and 10% in Australia; Vitousek et al. 1996). The ecological and socio‐economic costs of invasive species have become increasingly apparent. Introduced species are thought to be the second greatest threat to native diversity (at least in the US; Wilcove et al. 1998), and the combined annual costs of introduced species have been estimated to exceed US$336 billion for just the United States, United Kingdom, Australia, South Africa, India and Brazil (Pimentel et al. 2001). The applied nature of invasion biology makes it attractive to researchers and funding agencies alike, and as a result, the field is enormous and progressing at great speed. Over 10 000 papers have been published in the field of invasion biology in the last 30 years (Gurevitch et al. 2011). In such a rapidly growing field, it is worthwhile to ask which lines of research are yielding important advances, whether there are important questions being overlooked, and whether there are areas where our efforts are yielding poor returns. In the first part of this paper, we aim to give an overview of progress in our understanding of invasion biology (particularly in relation to plants), highlighting some promising directions for future research and some areas that we believe could be scaled back.

Propagule pressure The available evidence suggests that high propagule pressure facilitates invasions (Von Holle & Simberloff 2005; Eschtruth & Battles 2009), and proxies for propagule pressure such as human population density and proximity are strong predictors of invasions (Pyšek et al. 2010; Vilà & Ibáñez 2011). However, relatively few studies have quantified propagule pressure in introduced plants (Simberloff 2009). The often‐overlooked importance of propagule pressure has been suggested as an explanation for the high level of idiosyncrasy observed in studies of invasions (Lockwood, Cassey & Blackburn 2005). The first priority for this field is to collect more empirical data on propagule pressure in different ecosystems. Studies that quantify the relationship between propagule pressure and invasion at large scales seems likely to yield interesting results, as do quantifications of the relative importance of propagule pressure under different circumstances.

Conclusions from literature review Many lines of investigation, including the search for species’ traits that are associated with high invasiveness, the search for features of communities that make them susceptible to invasions and the search for generalities about the effects of introduced species, are yielding idiosyncratic results. This idiosyncrasy highlights the need to gather detailed information to understand the biology of particular introduced species. However, since we cannot study every introduced species separately, we need to continue the search for generalizations. We join a growing chorus, suggesting that our approach to invasion biology has been too simplistic. Despite wide recognition that there will be no one explanation that fits all biological invasions (Davis, Grime & Thompson 2000; Gurevitch et al. 2011), relatively few studies actually consider a range of factors/theories simultaneously. Rather than focussing on one factor at a time, we need to find ways (including multivariate analyses) to synthesize information about the recipient habitats/communities, the characteristics of both resident species and the invaders, demographic processes, propagule pressure, the differences between current conditions and those with which the resident species evolved, evolutionary change in both native and introduced species, plasticity, and feedbacks and interactions between different species and processes (Lockwood, Cassey & Blackburn 2005; Moles, Gruber & Bonser 2008; Ordonez, Wright & Olff 2010; Gurevitch et al. 2011). By incorporating a range of different types of information, we hope that it will be possible to cut through the apparent idiosyncrasies and predict the circumstances under which species and ecosystems will respond in different ways. This will not be simple, but even combining information about three or four processes (e.g. differences between climatic conditions in home and introduced ranges, evolutionary change in the new range and demographic processes) would represent a major advance. It is common for researchers to specialize in one branch of ecology (e.g. invasion ecology, tropical biology, molecular ecology), and to feel that the literature and ecological community is so huge even within each sub‐discipline that there is no way we could keep up with developments in ecology as a whole. To bring together widely, divergent lines of evidence about invasions will therefore require researchers with different types of expertise to work together. This has a strong parallel with calls for invasion ecologists to stop treating invasion as a process separate from the rest of ecology (e.g. Davis et al. 2005) and (i) use introduced species to help us understand fundamental processes in ecology (e.g. community assembly, succession, species’ distributions and evolution), and (ii) use knowledge and techniques from other parts of biology (e.g. demography) to improve our understanding of invasion biology (Sax et al. 2007). Thus, we see large multiauthor collaborations and big data syntheses as an important part of the future of invasion biology. Since eradicating all introduced species is an unachievable or impractical goal in many ecosystems (Hobbs et al. 2006; Davis et al. 2011), we also need to consider the consequences of the long‐term presence of some introduced species. After a few hundred years in an ecosystem, both introduced species and resident natives will have had time to adapt to the new conditions. Local herbivores, pollinators, seed dispersers, seed predators and pathogens are likely to be interacting with the introduced species, making them part of food webs and other ecological networks (e.g. Pyšek et al. 2011). Does there come a point where we should stop fighting introduced species and simply accept them as plants that provide a range of ecosystem services, or is this opening the door to disaster? Debate on this topic is fierce (Davis et al. 2011; Simberloff 2011). We might gain some insight about the future trajectories of invasions by looking at long‐invaded places such as Europe, and by studying the long‐term effects of previous biotic interchanges (e.g. the joining of North and South America). Many of the ideas presented in the literature have become reified without ever being subjected to formal testing (see Slobodkin 2001). Other ideas have been subjected only to weak tests that were the best available at the time, but which have been superseded in recent decades as a result of the increasing availability of large datasets and sophisticated analytical methods. Yet other ideas have been accepted as truths despite contradictory empirical evidence. Thus, one of the most important goals for the future is searching for any untested or inadequately tested assumptions underlying our understanding of invasion biology (the same applies to many other fields in ecology). For the researcher, providing the first test of a broadly accepted idea is a win/win situation. If the dogma is right, the first empirical test will likely go on to be a citation classic. If the dogma is wrong, then the first empirical test will overturn our traditional understanding of the field, thus stimulating the development of new theoretical ideas and many other empirical tests. Following this goal of testing dogma, we tackle one idea that has become widely accepted despite equivocal evidence in empirical tests: the idea that disturbance facilitates invasion.

Overall conclusions The field of invasion biology shares a few characteristics with the species it seeks to understand. The number of both papers and introduced plants in the field has been increasing exponentially through time, and the impact of both the plants and the research on them can be (but is not always) very high. Invasions are complex processes, and simple approaches that focus on one factor at a time (such as the traits of invaders or recipient communities) have had limited success. We believe that the best way to further our understanding of invasions will be to adopt more holistic approaches that incorporate several different types of information simultaneously, especially information about the ways conditions have changed. This will not be simple to achieve, but we have an army of enthusiastic ecologists who want to understand invasions, so it seems likely that we will make substantial progress relatively quickly. Finally, invasion biologists (like other ecologists; Cooper 1926) need to take a good hard look at the fundamental tenets of the discipline and ensure that our understanding is built on hard evidence rather than assumptions, or on theories that have equivocal empirical support. Invasion biology is a big field, but there are still plenty of opportunities for new, exciting and urgently needed science.

Acknowledgements Thanks to Romina Lasagno (INTA), Wade Tozer & Ian Wright for help in the field. Thanks to Eduardo Estrada, Arturo Mora, Eduardo Alanís, Guadalupe Martínez‐Ávalos, Chris Woolmore and the Department of Conservation (Project River Recovery) New Zealand, and the Uganda Forest Department for providing data and/or access to plots, and to Owen Price for fire frequency maps for Sydney. Thanks to David Gibson and three anonymous reviewers for comments on the manuscript. D.S.’s data were collected while he was employed at the Department of Plant Sciences, Oxford (UK). P.B.R. and J.C.‐B. thank the National Science Foundation Long‐Term Ecological Research programme for funding (DEB‐0080382), AH thanks the Estonian Science Foundation (grant no 7610) and the European Regional Development Fund (Centre of Excellence FIBIR) for support, D.S. thanks the British Government’s Department for International Development Forestry Research Programme for funding (R4737), M.M.M. thanks the Teresa Heinz Scholars for Environmental Research for funding, and A.T.M. thanks the Australian Research Council for funding (DP0984222).

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