In this chapter, and moving on from the example of the globally mobile and invasive Knapweed, I explore how the idea of nativism structures both conservation policy and the publics' sanctioned relationships with nature. This is a highly contested terrain, receiving critique and debate from a wide variety of natural scientists, social scientists, activists and stakeholders. It continues to be a difficult dialogue, with tempers flaring on all sides. Debates about the definition of what is “natural”, and about the separation of humans and nature, take a specific and meaningful form in the biosecurity context. This chapter therefore considers the language and definitions used to structure nativist concerns, the suitability of classification criteria, the underpinning science, and the pragmatic justifications for nativist policies. Crucially, it discusses the discursive and political implications of the ‘nativist paradigm', the ideological assumptions and cultural motivations for nativist policies, including the degree to which the 'native good, aliens bad' discourse might be a barrier to all citizens' participation in environmental conservation, including ethnic minorities. Concrete examples draw from other research carried out in Switzerland where the issue of invasive species was specifically raised on the political agenda following the arrival of Ragweed (Ambrosia artemisiifolia), a North American plant with human health impacts. It ends with a discussion of what happens when policies are put into practice, indicating that on the ground categories are much less fixed than current academic debates might suggest.