In contrast to indicators such as the ecological footprint (Haberl et al. 2004b; Wackernagel et al. 2002), there is no clear-cut sustainability threshold referring to HANPP. One hundred percent HANPP clearly would be destructive because this would leave no resources for other species except those used directly by humans. It is a matter of debate how to set a meaningful lower threshold. It has been argued that human impact should be small compared with natural processes, resulting in a proposed threshold of 20 percent HANPP (Weterings and Opschoor 1992), but this number cannot be justified scientifically. This section takes stock of our knowledge on effects of HANPP on biodiversity, thus aiming at a more rational discussion on this issue.
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