 The concern for preserving nature and non-human species is addressed to some extent by making a distinction between weaker and stronger conceptions of sustainability (Beckerman 1995). Proponents of weak sustainability argue that it is acceptable to replace natural capital with human-made capital provided that the latter has equivalent functions. If, for example, plastic trees could produce oxygen, absorb carbon and support animal and insect communities, then they could replace the real thing, and a world with functionally equivalent artificial trees would seem just as good—from an economic perpective—as one with real or natural trees in it. For weak sustainability theorists, the aim of future development should be to maintain a consistently productive stock of capital on which to draw, while not insisting that some portion of that capital be natural. Strong sustainability theorists, by contrast, generally resist the substitution of human for natural capital, insisting that a critical stock of natural things and processes be preserved. By so doing, they argue, rivers, forests and biodiverse systems are maintained, hence providing maximum options—options in terms of experience, appreciation, values, and ways of life—for the future human inhabitants of the planet (Norton 2005). The Brundtland report can also be seen as advocating a form of strong sustainability in so far as it recommends that a “first priority is to establish the problem of disappearing species and threatened ecosystems on political agendas as a major resource issue” (ibid., chapter 6, paragraph 57). Furthermore, despite its instrumental and economic language, the report in fact endorses a wider moral perspective on the status of and our relation to nature and non-human species, evidenced by its statement that “the case for the conservation of nature should not rest only with development goals. It is part of our moral obligation to other living beings and future generations” (WCED 1987, chapter 2, paragraph 55). Implicit in the statement is not only a strong conception of sustainability but also a non-anthropocentric conception of the notion. Over time, strong sustainability came to be focused not only on the needs of human and other living things but also on their rights (Redclift 2004, 218). In a further development, the discourses on forms of sustainability have generally given way to a more ambiguous usage, in which the term “sustainability” functions to bring people into a debate rather than setting out a clear definition of the terms of the debate itself. As globalization leads to greater integration of world economies, the world after the Brundtland report has seen greater fragmentation among viewpoints, where critics of globalization have generally used the concept of sustainability in a plurality of different ways (Sneddon, Howarth and Norgaard 2006). Some have argued that “sustainability”, just like the word “nature” itself, has come to mean very different things, carrying different symbolic meanings for different groups, and reflecting very different interests (Redclift 2004, 220). For better or for worse, such ambiguity can on occasion allow different parties in negotiations to claim a measure of agreement. For example, commenting on the connections between agricultural systems, sustainability and climate change, one writer has argued that there is exciting scope for negotiation across different world views in working out the conditions for a future sustainable form of agriculture (Thompson 2017).