Misleading criticisms of invasion science: a field guide

Invasion science is the study of the causes and consequences of the introduction of organisms to the areas outside their native ranges. It concerns all aspects relating to the transport, establishment and spread of organisms in a new target region, their interactions with resident organisms, and the costs and benefits of invasion with reference to human value systems. ‘Invasion science’ is a more appropriate name for the broad domain than ‘invasion ecology’ or ‘invasion biology’ because of the importance of engaging with many disciplines other than biology and ecology in understanding and managing invasions (Richardson et al., 2011). The scientific study of invasions has become increasingly popular, as indicated by the explosive growth of publications and academic books on the topic over the past two decades (Simberloff, 2004; Richardson & Py sek, 2008). Aspects of invasion science now feature in virtually all textbooks and synthetic monographs of ecology, conservation biology, biogeography and evolution. Another metric of the burgeoning impact of research on invasions is its coverage in the most highly cited journals in many disciplines: Figure 1 shows this for ecology over the past 15 years. Clearly, there has been a growing recognition that research on invasions is invaluable for understanding how most ecosystems work. Studies of invasions have yielded novel insights on key ecological concepts, including inter alia the diversity–stability relationship, trophic cascades, keystone species, the role of disturbance in community assembly, ecological na€ivet e, ecological fitting, rapid evolution, island biogeography, ecosystem engineering and niche construction. The field has also contributed concepts of its own (e.g. propagule pressure, biotic resistance, invasional meltdown, enemy release) that have stimulated productive research of both theoretical and applied importance. A key motivation for studying invasions is their environmental impact. Non-native species are far more likely to have ecological and socio-economic impacts than do those native species that, for various reasons, undergo range expansions or increase in abundance to become ‘weedy’ (Simberloff et al., 2012). The negative impacts of non-native consumers are far greater than those of native consumers (e.g. Paolucci et al., 2013). Numerous studies demonstrate the role of invasions as a driver of species loss at local and regional scales (e.g. Wyatt et al., 2008; Burghardt et al., 2010; Baider & Florens, 2011; Roy et al., 2012; Gilbert & Levine, 2013), even where other confounding stressors are at play (e.g. Light & Marchetti, 2007; Hermoso et al., 2011). Evidence points to non-native species as a major cause of global animal extinctions (Clavero & Garc ia-Berthou, 2005; Clavero et al., 2009). They also raise the extinction likelihood of native plant populations; the substantial time-lags inherent in these population extinctions are frequently ignored, resulting in spurious conclusions on the magnitude of invasions as eroders of plant biodiversity (Gilbert & Levine, 2013). Non-native species are frequently implicated as components of a lethal cocktail of stressors on biodiversity (van der Wal et al., 2008; Schweiger et al., 2010; Blaustein et al., 2011). Even where other stressors have already diminished native populations, invasions can accelerate these declines (e.g. Ricciardi, 2004). Finally, invasions also disrupt key ecological processes. Many such disruptions are subtle (e.g. Stinson et al., 2006) and may take decades to unfold or for their implications to manifest, as in the case of plant–animal mutualisms (Traveset & Richardson, 2006; Davis et al., 2010; Sekercioglu, 2011). The societal importance of biological invasions is illustrated by the growing socio-economic costs of invasions to agriculture, forestry, aquaculture, apiculture, technological (e.g. water supply) systems and human health, as well as potentially myriad positive and negative effects on ecosystem services (Cook et al., 2007; Pejchar & Mooney, 2009; Py sek & Richardson, 2010; Rothlisberger et al., 2012). Thus, it is not surprising that invasions are increasingly viewed as an issue of national security (e.g. Penman, 1998; Meyerson & Reaser, 2003; Chomel & Sun, 2010; Ricciardi et al., 2011).