There is no consensus among experts on the definition of the terms “biotic invasion” and “invasive species”. In all definitions the terms include the spread of organisms in the landscape, with “invasive species” relating to the organisms involved and “biotic invasion” referring to the overall process. However, while some experts refer only to those organisms that spread in an area where they would not have been present without human assistance (so-called alien or non-native species), others refer to any spreading organisms, including those that had been present in the general geographic area (e.g. a continent, country or island) without human assistance (native species). Further, some experts reserve the term “invasive” only for those species that are perceived to have negative economic or ecological impacts. Given these two dimensions, spreading species that are either only alien, or native and alien species, and that either pose a perceived problem or not, may be called invasive (for an extensive discussion of the terminology see Colautti and McIsaac, 2004*; Daehler, 2001; Davis and Thompson, 2000; Richardson et al., 2000*).
The different meanings of the terms reflect the dynamics of the research field that are discussed in this article. We therefore do not use only one definition of “invasive species” or “biotic invasion” in this article, but rather review any literature that attempts to be of relevance to the understanding and management of biotic invasions according to its own implicit definition of biotic invasion.
At present the societal problem of biotic invasions is typically framed as follows: the problem of biotic invasions has emerged because of the massive increase in global human travel and transportation over the past few centuries, which led to the introduction of large numbers of diseases, animal and plant species to new areas where they would not have been present without human assistance. It has for instance been estimated that more than 50,000 alien species have been introduced to the U.S.A. (cf. Pimentel et al., 2005*). A small proportion of alien species have the potential to spread in a landscape and to achieve large population sizes. These species are called invasive alien species and some of them are thought to lead to massive economic and ecologic costs in the areas where they were introduced (Mack et al., 2000*; MEA, 2005*; Pimentel et al., 2005*). The costs of biotic invasions in the United States alone have been estimated to amount to almost US$ 120 billion per year (Pimentel et al., 2005*), and invasive alien species were identified as one of the five major causes of species extinctions, alongside habitat destruction, over-exploitation, climate change, and pollution (MEA, 2005*). Biotic invasions are therefore nowadays considered a major driver of global environmental change (Vitousek et al., 1997) and a high priority in national and international environmental policies, including Article 8h of the Convention on Biological Diversity of the Rio Declaration (McNeely, 2001*; Mooney et al., 2005*). It is expected that the scale of the problem will increase in the future (Lodge et al., 2006*; MEA, 2005*).
An explicit research field focused on biotic invasions emerged in the late 1950s from ecology (Davis, 2006*; Richardson and Pyšek, 2008). The book entitled “The Ecology of Invasions by Animals and Plants” by Charles Elton published in 1958 is generally seen as the starting point of systematic research on biotic invasions, and the research field that emerged in response to Elton’s book is generally called invasion biology. However, it is important to note that the spread of non-native species had been addressed by naturalists since the early 19th century (Cadotte, 2006*; Kowarik, 2003*; Trepl, 1990*). In fact, several research questions that are discussed in this review as innovations of invasion biology had been addressed before 1958 and were thereafter neglected for some time. This is particularly the case for European plant ecology that had discussed human agency as a relevant causal factor of biotic invasions since the 19th century (Kowarik, 2003*; Trepl, 1990). However, because in this article we are interested in the formation of problem-oriented research as a clearly defined research field, we focus our review on the period since the publication of Elton’s book and mention earlier research only in some cases. Since the late 1990s (Mooney et al., 2005*), mainstream research on biotic invasion widened its focus and now includes a broad range of natural and social scientists. Because we are interested in the transformation of research focused on biotic invasions from an ecological sub-discipline to a multidisciplinary research field we propose and use in this article the broader term ‘invasion research’ for systematic research on biotic invasions.