The conditions for life on Earth have been in constant flux since the appearance of the first single-celled organisms and now the Earth is populated by an estimated one to six billion species, according to a recent study (Larsen et al 2017). Homo sapiens – a latecomer on the evolutionary tree – was merely a marginal species until about 12,000 years ago when the beginning of a period of climate stability (the Holocene) was favourable to the development of agricultural societies (Feynman and Ruzmaikin 2007). Currently, life on Earth is experiencing a new mass extinction event (Ceballos et al 2015), which differs to a normal background rate of extinction that accounts for an ongoing adaptation and replacement of species. What distinguishes this period of mass extinction from previous ones is the fact that this instance is of anthropogenic origin; that is, caused by humans (Wilson 2016). Besides that this makes this kind of extinction conceptually different from past extinction events, this causality has also normative implications, and constitutes the fundamental problem against which the argument for an account of ecological justice is developed in this book.

To clarify, regarding species extinctions it is possible to distinguish between four different processes of extinction (final, hybridisation, transformation, allopatric speciation). Here the primary interest is in the kind of extinction that is usually implied in the everyday meaning of the term. That is, a final extinction which entails not only the disappearance of the species but also of the related phyletic branch of the evolutionary tree which stands in contrast to other extinction processes. That means that, for example, the disappearance of a species does not go hand in hand with the creation of a daughter species (see Delord 2007). Moreover, the seriousness of this current mass extinction event which is linked to its conceptual differences to non-anthropogenic mass extinctions, which makes it unprecedented (Aiken 1998), as well as the magnitude of mass extinction events in general, generate a need to act. That, in turn, is underlined by a crisis terminology and (more or less) aptly being termed the sixth mass extinction, or the biodiversity crisis.