Emerging issues | Invasive alien species: a growing but neglected threat? 20 Invasive alien species: a growing but neglected threat? Sarah Brunel, Eladio Fernández-Galiano, Piero Genovesi, Vernon H. Heywood, Christoph Kueffer and David M. Richardson Biological invasions are one of the five major causes of biodiversity loss as global human travel and trade have moved, and continue to move, thousands of species between and across continents. Some species of alien origin have a high probability of unrestrained growth which can ultimately lead to environmental damage. An alien species — animal, plant or microorganism — is one that has been introduced, as a result of human activity, either accidentally or deliberately, to an area it could not have reached on its own. A common definition of the term 'invasive' focuses on its (negative) impact, while other definitions consider only rate of spread and exclude considerations of impact. Despite the growing amount of legislation being adopted at the global scale, biological invasions continue to grow at a rapid rate, with no indication yet of any saturation effect. Decision-making in this area is very challenging. The overall complexity of the problem, its interdisciplinarity, the scientific uncertainties and the large number of stakeholders that need to be informed and involved, together demand governance actions that are difficult to see emerging at the regional scale (as in the EU), let alone globally. It is widely agreed that preventing biological invasions or tackling them at a very early stage is the most efficient and cost-effective approach. Harmless species can be confused with harmful invasive species, however, leading to a waste of resources. Even more seriously, harmful invaders can be mistaken for innocuous species — so-called 'invaders in disguise' — and no appropriate action may be taken to counter the threats they pose. Even with a very good risk assessment system, new outbreaks of invasive alien species could still occur, necessitating a system of rapid early warning and effective eradication response. The decision on where to draw the line on the acceptable environmental risks versus the introduction of new species or new communities that may carry invasive alien species then becomes a value judgement. There is lively debate within the scientific community regarding the most appropriate strategies for managing invasive alien species. Governments and institutions charged with making decisions have access to considerable knowledge on the topic, but the lack of rules of interactions between multiple parties regularly thwarts effective decision-making. 486 Late lessons from early warnings: science, precaution, innovation Emerging issues | Invasive alien species: a growing but neglected threat? 20�1 Introduction There is nevertheless a lively debate within the scientific community regarding the most appropriate Biological invasions are one of the five major causes strategies to adopt for managing invasive alien of biodiversity loss, alongside habitat destruction, species. Aspects of this debate offer important over-exploitation, climate change and pollution pointers to dimensions that need to be better studied (Millennium Ecosystem Assessment, 2005). Global and the measures needed to make the phenomenon human travel and trade have moved, and continue of biological invasion better understood by all to move, thousands of species between and across stakeholders. Invasive alien species represent a continents (McNeely, 2001). Only a small proportion growing threat, in particular in a globalised world, of alien species become established, some of as they are introduced and spread by people these spread, and a small subset produce major (McNeely, 2001). This is more a human-driven ecological, economic or social effects (generally environmental problem than a strictly biological termed 'impacts'). Ecological impacts include local one. Inherent uncertainties remain about what extinction or a reduction in the diversity of native species will be introduced and which will become species, and various types of ecosystem-level invasive. Consequently, making decisions is very changes such as modifications of nutrient cycling challenging. The overall complexity of the problem, or water quality. As an example, in an analysis of its interdisciplinarity, the scientific uncertainties, 680 recent animal extinctions worldwide, causes and the large number of stakeholders that need were compiled for about 25 % of these. Ninety-one to be informed and involved, together demand (54 %) included invasive species among the causes governance actions that are difficult to see emerging of extinction, and in 34 cases they were the only at the regional scale (as in the EU), let alone at the known cause (Clavero and Garcia-Barthou, 2005). global scale. Lessons already learnt in different parts For example, feral cats on islands are responsible of the world need to be considered when seeking to for at least 14 % of global bird, mammal, and reptile improve management regimes for IAS in different extinctions in recent times (Medina et al., 2011). All environments and at different scales. types of organism, from micro-organisms, including microbes and diseases, to mega-herbivores, can become invasive, and all can cause impacts on 20�2 The difficulty of defining invasive biodiversity and ecosystem functioning. All types alien species of ecosystem are affected: terrestrial, freshwater and marine. The terminology applied to organisms involved in biological invasions is complex, often confusing, Biological invasions are receiving increased attention and there is no universally accepted definition in many countries, some of which have already of IAS (Riley, 2005; Falk-Petersen et al., 2006; set in place comprehensive legislation or national Richardson et al., 2011a); all this has serious strategies to deal with various aspects of invasions, practical consequences. The term 'alien' (exotic, in particular concerning their management foreign, non-indigenous, non-native) requires a (e.g. Australia, Great Britain, Mexico, New Zealand, geographical, biogeographical or ecological context South Africa and USA) (Pyšek and Richardson, to have a useful meaning — an alien species is one 2010). As regards the management of invasive alien which has been introduced as a result of human species (IAS), it is widely agreed that preventing assistance, either accidentally or deliberately, to biological invasions or tackling them at a very an area it could not have reached on its own. The early stage is the most efficient and cost-effective area in question has to be specified since a species approach. For example, the cost of eradicating 'may be alien to any definable area, e.g. continents, weeds can increase at least 40 times if action is not islands, bio- or eco-regions, or any political entity taken promptly (Harris and Timmins, 2009), and in (e.g. countries, states, provinces)' (Lambdon et al., most cases eradication quickly becomes unfeasible 2008). The terms alien and invasive both have (Genovesi, 2007). Preventive management calls for a political overtones. The Convention on Biological precautionary approach, because prompt response Diversity (CBD) requires its Parties, through often does not permit full assessment of the risks Article 8(h) to 'prevent the introduction of, control connected to a newly detected invasion (Genovesi or eradicate those alien species which threaten et al., 2010). Despite widespread agreement on these ecosystems, habitats and species' and uses the term principles, the full range of problems associated 'invasive alien species' to refer to such species and with IAS still lacks political recognition and too few defines these as 'alien species whose introduction government-coordinated actions are in place, or are and/or spread threaten biological diversity'. The effective, in most parts of the world, including in the term 'invasive' is therefore defined by the CBD in European Union. terms of (negative) impact, while other definitions Late lessons from early warnings: science, precaution, innovation 487 Emerging issues | Invasive alien species: a growing but neglected threat? employ ecological and biogeographical criteria, is provided by the DAISIE database (http://www. i.e. invasive species are defined as alien species that europe-aliens.org/) and its List of Species Alien in sustain self-replacing populations, often in very Europe although there are still significant gaps in large numbers at considerable distances from the coverage and distribution and many problems of site of introduction such as in natural areas, and accurate identification remain to be resolved. For explicitly exclude considerations of impact (see example, even well-known plant invaders such as Richardson et al., 2011a; Blackburn et al., 2011 and Heracleum mantegazzianum and Fallopia japonica are references therein). Other definitions also include frequently confused with related species, and the economic impacts, for example on agriculture or invasive hybrids often referred to as Rhododendron the use of amenities, and social impacts such as on ponticum have now been named R.× superponticum human health. (Cullen, 2011). In other taxa such as the genus Opuntia it is often unclear which species are In general, the biogeographical concept is more involved in particular invasions, with experts widely used in the academic world, whereas the disagreeing, for example, about whether O. maxima impact concept is widely used by decision makers