HANPP indicates how intensively a defined area of land is being used in terms of ecosystem energetics (Haberl et al. 2004b). With reference to a given territory, it reveals how much energy is diverted by humans as compared to the energy potentially available. This can be interpreted as a measure of how strongly human use of a defined land area affects its primary productivity and how much of the NPP is diverted to human uses and thus is not available for processes within the ecosystem.
HANPP has been developed in the context of the debate on global ecological changes caused by humans and their activities (Vitousek et al. 1986, 1997), and it has been linked to the issue of human influence on biodiversity (Wright 1990). It has been used in ecological economics as a biophysical indicator of strong sustainability (Mar-tinez-Alier 1998; Sagoff 1995), although the initial idea that HANPP was a straightforward indicator for ecological limits (Costanza et al. 1998; Meadows et al. 1992) was proven wrong because biomass harvest can be increased without increasing HANPP (Davidson 2000; Krausmann 2001), and neither GDP nor population size is directly constrained by HANPP (Haberl and Krausmann 2001). Such decoupling of HANPP and biomass harvest requires fossil energy input into agroecosystems (Krausmann et al. 2003; Pimentel et al. 1990) and may be associated with environmentally detrimental impacts (e.g., pesticides, nitrogen leaching, and soil deterioration). Economic growth, particularly in the transition from agricultural to industrial society, can to a large extent be decoupled from growth in biomass consumption because industrial economies rely much more on minerals and fossil energy than agricultural societies (Krausmann and Haberl 2002). On the other hand, a larger proportion of land is devoted to settlement, industry, and transport, resulting in HANPP increases. But HANPP does not generally rise with industrialization because agricultural intensification may raise the productivity of agricultural land more than biomass harvest increases (Krausmann 2001).
A straightforward interpretation of HANPP is that it is a measure of the human domination (Vitousek et al. 1997) or colonization (Fischer-Kowalski and Haberl 1997) of terrestrial ecosystems. Current dynamics of land use-induced changes in ecosystem processes are best conceived of as a process of intensification driven by population growth, changes in technology, and increasing demand for ecosystem services. But there are also areas where human use is deintensified (marginal land, reforestation). By comparing patterns and processes to be expected in the (hypothetical) potential vege tation with those that can currently be observed, the impact of human colonization on terrestrial ecosystems can be assessed.
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