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Biological invasions cause many impacts that differ widely in how they are perceived. We argue that many conflicts in the valuation of the impacts of alien species are attributable to differences in the framing of the issue and implicit assumptions--such conflicts are often not acknowledged. We present 13 principles that can help guide valuation and therefore inform the management of alien species. Seven of these relate to the science domain, representing aspects of change caused by alien species that can be measured or otherwise assessed using scientific methods. The remaining six principles invoke values, risk perception, and environmental ethics, but also cognitive and motivational decision biases. We illustrate the consequences of insufficient appreciation of these principles. Finally, we provide guidance rooted in political agreements and environmental ethics for improving the consideration of the consequences of these principles and present appropriate tools for management decisions relating to alien species. [ABSTRACT FROM AUTHOR]

Environmental problem solving needs science but also inevitably requires subjective judgment. Science can help in dealing with subjectivity, because scientists have long experience developing institutions and practices to address the subjective and value-laden choices that are essential to scientific progress. Democracy has also developed approaches to the problem. The underlying principles can be applied to environmental policymaking. This article explores these issues in the context of decisions about environmental risks, drawing on the work of the National Research Council and other sources. It suggests some guidelines for risk deliberation--including broad-based participation, commitment to scientific quality, explicit attention to values, transparency of deliberative processes, and rules for closure and reconsideration--and recommends that an experimental approach be employed to learn how best to use deliberative methods. [ABSTRACT FROM AUTHOR]