From: Tim M. Blackburn, Sven Bacher, Piero Genovesi, Jonathan M. Jeschke, Sabrina Kumschick, David M. Richardson, Franz Essl, Philip E. Hulme, Ingolf Kühn, Wolfgang Nentwig, Jan Pergl, Wolfgang Rabitsch, Montserrat Vilà15, John R.U. Wilson & Petr Pyšek

Recently, we presented a method for classifying alien taxa on the basis of their deleterious impacts on the environment [1]. Our scheme uses the best available data to categorise alien taxa into a hierarchical set of impact categories, in an approach that is analogous to that of the IUCN Red List [2]. Our aim was to develop a practical method that is aligned with the mechanisms of impact identified in the IUCN Global Invasive Species Database, and which could be used to identify taxa that have different levels of environmental impact, facilitate comparisons of the level of impact among regions and taxa, facilitate predictions of potential future impacts, highlight data deficiencies, and help prioritize management actions. Despite this last aim, our scheme is not a risk management tool; while it reports on the consequences of alien incursions, it says nothing about their likelihood. Nor does it suggest how one should manage aliens [1].

Ojaveer et al. [3] pick up on our publication, and specifically on the issue of management, as the launch pad for a discussion of problems in the management of alien (non-indigenous) species in one realm, the marine. They point out that knowledge of marine biodiversity is particularly poor: in many cases we do not know whether species are alien or native (“cryptogenic” species), alien species may not be detected for decades, and there is virtually no information on the impacts of alien species in the marine realm. In consequence, they argue that basing management actions for marine aliens on evidence of impacts is a luxury that cannot be afforded. They contend that all marine aliens should be assumed to be damaging unless there is evidence to the contrary. Since it is extremely difficult to eradicate or control alien species in the marine realm, the only viable management options are prevention, or immediate eradication of new arrivals.

We are pleased that our paper has stimulated others to think about alien species management issues, but we question the need for a separate consideration of problems in the marine realm. As we noted [1], cryptogenic taxa are common in other contexts (e.g. cosmopolitan plants, stored product arthropod pests, hybrids), many alien species are not detected for decades [4, 5], and information on the impacts of most species (charismatic vertebrates and economic pests aside) is poorly documented [4]. Even for such a well-studied group as vascular plants, rigorously documented impacts were only available for 167 species at the global scale [6]. At least in Europe, the numbers and percentages of marine alien species with known impacts are not exceptionally low [7]. Our classification ensures data deficiencies are flagged up early and incorporates explicit information on uncertainties in the classification process (1). The importance of the precautionary principle in cases of species with unknown impacts is well recognised [1], as are the advantages of prevention and early eradication [8]. What is open to debate is the opportunity for these management approaches in situations where the identity or presence of aliens is unknown - attempting to eradicate all alien species once detected is unfortunately a luxury that cannot be afforded.

Ojaveer et al. [3] present a useful set of criteria for impact evaluation frameworks for the management of alien species, and a framework of their own based on these criteria. Their framework is broader than [1] in that it considers impacts on a range of values (e.g. social, economic, cultural; we are currently in the process of extending our method to socio-economic impacts, building on pre-existing approaches [9]), but is narrower within each value as it does not consider mechanisms, nor does it address how impacts measured with a variety of different metrics and on different values could be summarized or compared. The failure to consider mechanisms of impact is a surprising omission, as this is fundamental information for managing the consequences of invasions, presented by Ojaveer et al. [3] as their main aim. While we have road tested our classification against actual data, the Ojaveer et al. scheme [3] remains untested and has significant information requirements that need to be fulfilled to be a practical tool. However, Ojaveer et al. [3] do not address how their framework is workable in the absence of data, given that this is their primary professed concern for the marine realm.

The lack of data about alien taxa and their impacts is a serious shortcoming for attempts to conserve a natural world in which aliens are becoming increasingly prevalent, in all the world’s realms. Our scheme [1] provides a systematic method to identify data gaps, explicit categories for data deficient and cryptogenic species, and a method of appending uncertainty to categorisations. It can be applied to all taxonomic groups and environments, allowing rigorous and quantitative analysis and comparison of impacts across taxa and realms. Furthermore, analyses of those species for which data are available can help to understand which data deficient taxa, or indeed those with no current alien populations, might be most damaging. As with the Red List [2], increasing application of the scheme is a potentially valuable route to reducing our ignorance. Making ignorance the basis for management decisions will never be an acceptable alternative to evidence-based assessments.

References
1. Blackburn TM, Essl F, Evans T, Hulme PE, Jeschke JM, et al. (2014) A unified classification of alien species based on the magnitude of their environmental impacts. PLoS Biol 12(5): e1001850.
2. IUCN (2012) IUCN Red list categories and criteria: version 3.1. Second edition. Gland: IUCN. www.iucnredlist.org/techn...
3. Ojaveer H, Galil BS, Campbell ML, Carlton JT, Canning-Clode J, Cook EJ, et al. (2015) Classification of non-indigenous species based on their impacts: considerations for application in marine management. PLoS Biol 13(4): e1002130.
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