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Studying Invasion: Have We Missed the Boat?

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Studying Invasion: Have We Missed the Boat? Ecology Letters, (2005) 8: 715–721 doi: 10.1111/j.1461-0248.2005.00774.x LETTER Studying invasion: have we missed the boat? Abstract Linda M. Puth1,2* and David M. Invasive species, and the ensuing homogenization of the world’s biota, form a global Post2 problem with consequences ranging from the decline and extirpation of native species to 1School of Forestry and threats to human health. The magnitude of this issue demands a thorough understanding Environmental Studies, Yale of the invasion process, which consists of three main stages: initial dispersal, University, New Haven, establishment of self-sustaining populations, and spread. To assess the relative CT 06511, USA distribution of research effort among these stages, we conducted a literature review 2Department of Ecology and using 873 articles published in 23 major journals over the past 10 years. Of the 873 Evolutionary Biology, Yale papers, only 96 (11.0%) studied initial dispersal, and only half of these (6.2% of the total) University, New Haven, CT 06520-8106, USA were empirical. As the first stage in a contingent process, we argue that initial dispersal is *Correspondence: E-mail: the best stage during which to direct management efforts. In addition, initial dispersal [email protected] has direct relevance for fundamental ecological questions regarding community assembly and metacommunity dynamics. In so far that answering these questions and preventing invasion are goals of ecologists, the disparity in research effort noted here suggests that ecologists need to expand their efforts to include more research on initial dispersal. Keywords Colonization, community assembly, establishment, exotic species, initial dispersal, invasion, invasive species, metacommunity, metapopulation, research effort, spread. Ecology Letters (2005) 8: 715–721 contingent nature of both invasion and community assem- INTRODUCTION bly render the first stage, initial dispersal, the fundamental Following a long period of neglect since Charles Elton’s stage upon which all other stages rest. At long temporal seminal work on invasion in 1958 (Elton 1958), invasive scales, the initial dispersal stage of the invasion process is species have recently re-emerged as an important focus of the essential first filter upon which all other community ecological study, with studies of the invasion process and assembly processes operate, including subsequent diversifi- exotic species increasing considerably in the past decade cation (Webb et al. 2002). Therefore, initial dispersal is (Williamson 1996) (Fig. 1a). Invasion is a multi-step process essential to understanding the response or recovery of (Williamson 1996; Kolar & Lodge 2001; Leung et al. 2002) communities to environmental perturbations, including comprised of three phases: initial dispersal (where an climate change (Clark et al. 2003) and post-glacial retreat organism moves from its native habitat, often over long (Williams et al. 2001). At shorter temporal scales, dispersal distances, to a new habitat outside of its home range), of native species is of fundamental importance to meta- establishment of self-sustaining populations within the new populations because it mediates the dynamic balance habitat, and spread of the organism to nearby habitats. between extinction and colonization (Hanski 1999). Dis- While successful invasions, comprised of those species that persal also is central to growing research on metacommu- establish populations and spread, receive the greatest nities because it is the mechanism that links local and attention, the contingent nature of the invasion process regional community dynamics (Leibold et al. 2004) and makes initial dispersal the stage with the greatest effects on because tradeoffs between dispersal and competitive abilities other, later stages. mediate local community structure (Abrams & Wilson 2004; The process of invasion is fundamentally important for Kneitel & Chase 2004). understanding community assembly (Diamond 1975; Webb In a management context, it is during the initial dispersal et al. 2002) and metapopulation and metacommunity stage that management efforts can prevent the establish- dynamics (Leibold et al. 2004), as well as for managing the ment and subsequent, often detrimental impacts of invasive unwanted colonization and spread of exotic species. The species (Mills et al. 1993; Hobbs & Humphries 1995; Ó2005 Blackwell Publishing Ltd/CNRS 716 L. M. Puth and D. M. Post resources effectively? Here, we report on a literature review Initial dispersal (a) 120 to answer this question using 873 studies published in Establishment Spread 25 major journals over the past 10 years in two stages of 90 literature searches. 60 MATERIALS AND METHODS 30 We searched ISI Web of Science in 2004 for the time period 1995–2004 in 25 sources that we considered general science 0 and ecological journals. We included five journals focusing (b) 80 Freshwater exclusively on freshwater and/or marine systems to attempt Marine Number of articles to counter potential terrestrial biases in the more general 60 Terrestrial literature. Therefore, for the rest of the article, we will refer to two searches: one based on all 25 journals (ALL) and one 40 based on these 25 journals minus the five freshwater and/or marine journals (GENERAL). For both searches we used 20 the key words invasion OR invasive species OR exotic 0 species. We chose these three key words to effectively 95 96 97 98 99 00 01 02 03 04 survey the literature without unduly biasing our results. For Year example, using the key words ÔdispersalÕ or ÔspreadÕ might have biased our results and overestimated studies of either Figure 1 (a) Number of papers in the period 1995–2003 found in a initial dispersal or spread, respectively. Other key words literature search on ISI Web of Science using the search terms such as Ônoxious speciesÕ or ÔcolonizationÕ connote estab- invasion, invasive species and exotic species. Numbers are broken lished exotic species and could have biased our results down by stage of invasion process they describe. (b) From the against finding studies of initial dispersal, while the term same literature search, but removing all articles from study-system- ÔtransportÕ would limit our study to those organisms carried specific journals, the number of papers studying initial dispersal, distinguished by study system. by vectors. Searching with these terms garnered papers addressing both basic and applied ecological issues, inclu- ding the assembly process, metapopulation and metacom- Pimentel et al. 2000; Leung et al. 2002; Simberloff 2003). munity dynamics, the effects of new species on community Typically, only a small percentage of colonists survive the structure and ecosystem function and the management of initial dispersal stage (Lodge 1993; Kolar & Lodge 2001). invasive species. The articles included empirical studies, While some non-indigenous species may have neutral or theoretical papers, reviews, and opinion pieces. We included even beneficial impacts on native biota and ecosystems (e.g. in our search the journals Science, Nature, The American crop species and pollinators), others will become invasive Naturalist, Ecology, Ecology Letters, Ecological Applications, (species that establish populations and spread). The Ecological Entomology, Ecological Monographs, Ecological Modeling, eradication of invasive species can be extremely costly, Conservation Biology, Oecologia, Oikos, Landscape Ecology, TREE, involve potentially harmful chemicals (Myers et al. 2000), Journal of Ecology, Journal of Animal Ecology, Proceedings of the and is rarely accomplished, except on small islands, in National Academy of Sciences, The Journal of Biogeography, Diversity isolated areas, or before species have spread widely (Myers and Distributions, Journal of Experimental Marine Biology and et al. 2000; Zavaleta et al. 2001; Simberloff 2003). Even Ecology, Marine Ecology Progress Series, Limnology and Oceanog- when eradication is successful, invasive species may have raphy, Canadian Journal of Fisheries and Aquatic Sciences and caused long-lasting changes in community structure and Hydrobiologia. We included strictly freshwater and marine ecosystem function (Zavaleta et al. 2001). Thus, preventing journals to account for a perceived terrestrial bias in more unintentional introductions by targeting the initial dispersal general journals. This search (ALL) yielded 942 studies, of stage is likely to be the most effective management option. which 69 were excluded as being exclusively molecular, Because community assembly, metacommunity dynamics, medical, or biogeochemical in nature, and so not pertinent and the management of invasive species depend so critically for this exercise. upon our understanding initial dispersal, the earliest stage of After reading the abstracts, we classified each article the invasion process, we have been surprised by the lack of according to the stage of invasion it considered (initial research on this topic. Thus, we raise the question: are dispersal, establishment, spread), whether it was empirical, ecologists studying the invasion process (for management and the study system considered (terrestrial, freshwater or understanding community assembly) focusing their aquatic or marine). Where any of these categories were Ó2005 Blackwell Publishing Ltd/CNRS Studying invasion 717 400 2). Of all 873 articles (all), 440 (50.4%) focused on terrestrial Initial dispersal systems, 240 (27.5%) were freshwater studies, and 154 350 Establishment (17.6%) concentrated on marine
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