Human Appropriation of Land’s Biological Production
A Human Footprint on Earth
Life on Earth depends on the conversion of solar energy into organic carbon compounds. Within the ocean and water bodies algae (seaweed, algae diatoms) are the main mechanism for converting sunlight to carbon, whilst on land this process is driven by the photosynthesis of all the plants that comprise terrestrial vegetation cover. The output of this global process is referred to as Net Primary Production (NPP). All organisms (e.g. all species of animals including humans, bacteria, fungi) depend directly and indirectly on the primary production of plants as an essential foundation of their livelihood.
Globally, humans use a disproportionate and growing component of NPP. Some is used directly for food, as inputs to animal husbandry and animal products, for energy, or for industrial purposes. Simply put, NPP is the primary source upon which humans rely to feed themselves and their domesticated animals and as a raw material for products based on plant fibre, pulp and wood and, increasingly, as a source of energy. The diversion of products of these biological processes to sustain human populations can have profound impacts on the structure and functioning of global ecosystems. This may result in ecosystem perturbations that can exceed their natural variability and dynamics and result in persistent weakening of ecosystem functioning and finally a transformation of existing ecosystems into something quite different. These transformations or state changes may result in decreased economic and ecological value. Increasing amounts of NPP claimed by humans also means that less biomass remains for sustaining other species and maintaining ecosystems now and into the future.
The proportion of terrestrial NPP consumed directly and indirectly through human land use has come to be known as Human Appropriation of Net Primary Productivity (HANPP). It can be mapped and quantified.
A map of HANPP represents a characterisation of the extent to which anthropogenic land conversion and biomass harvest of all types (i.e. not only agricultural crops) alter the natural capacity of primary biomass production (NPP0) of “undisturbed” terrestrial ecosystems under current environmental conditions (i.e. climate and soil). It has become an important and powerful measure of the impact of human land use on the natural potential to provide NPP. The adjacent graphical scheme illustrates the basic components considered in the calculation of HANPP.
The blue bar at the left represents natural undisturbed potential NPP0. The right bar displays actual ‘NPP actual’ under current land use, which is the sum of harvested NPPh and the remaining NPPt after human harvest. Under most land uses ‘NPP actual’ is smaller than potential NPP0.
The difference between NPP0 and ‘NPP actual’ denotes the loss of NPP caused by the conversion of land from natural conditions to anthropogenic land use. Finally, HANPP consists of the harvest and loss due to land conversion for human use;
HANPP = NPPh + NPPLC = NPP0 – NPPt.
To perform this calculation geographically requires explicit data on annual actual NPP, modelled potential NPP and on the development of land use, land cover and biomass harvest in different land uses. These data are now increasingly available from national to global scales, making it possible to provide global and regional maps on HANPP. The impact of human land use and the resulting appropriation of biomass or HANPP is evident in virtually all ecosystems on Earth (excluding largely unoccupied parts of the arctic, hyper-arid zones and tropical rain forests).
The total global biomass production appropriated by humans (HANPP) amounted to 15.6 GtC/yr around the year 2000, which corresponds to 23.8 % of the global potential NPP. Of this, only 6.07 GtC/yr or 38.91 % of total HANPP are really used for human requirements, while the remaining biomass included in the HANPP is lost or destroyed during harvest or a consequence of land conversion. Land conversion alone reduced global potential NPP by an estimated 9.6 % or 6.29 GtC/yr2.
When examining global HANPP numbers for different world regions and land use categories, a clearer picture of critical areas and issues appears: global croplands show the highest HANPP levels with an average of 83.5 % of potential NPP, followed by areas with high levels of built-up areas/infrastructure, with an average HANPP of 73 %. Eastern and southern Asia, Western Europe and parts of North America are characterised both by intense agriculture and by high population and infrastructure density. By most measures – and particularly HANPP – they are the most intensely used regions in the world (shown in red).
Significant increases in future biomass demand are expected. Projected growth of world population, together with changes in human diets towards animal-product consumption, are expected to drive further increases in the amount of biomass required as food and feed. Moreover, many energy scenarios also envisage increases in the amount of biomass used for energy.
Land degradation refers to reduction of land productivity as a result of overuse or over-appropriation by humans. Therefore, the ratio between harvested NPP and NPP lost due to land conversion could be used to identify critical areas of land degradation. Not all conversions provoke NPP losses as this depends on the land use.
The concept of HANPP first appeared in the early 1970s. Research on HANPP increasingly attracted attention with initial global estimates in the mid-1980s. The most recent approach to comprehensive assessments of global HANPP is based on a combination of vegetation modelling, agricultural and forestry statistics and global Geographical Information System (GIS) data derived from satellite imagery.
When putting land in economic use, humans usually replace original vegetation with human-dominated systems such as agroecosystems, managed forests or tree plantations, or, in extreme cases, total replacement of natural vegetation with built-up or urban land. As indicated in the graphical representation of the basic NPP components considered in HANPP calculations the notion of "potential vegetation" refers to the vegetation that would prevail in a defined area under current soil and climate conditions in the absence of human intervention. Land use results in a deviation from previously existing natural vegetation (potential), to another type of vegetation (actual) that is dependent on human management to adapt to changing demands and environmental conditions.
The NPP of actual and natural vegetation types are sometimes dramatically different, but usually occur in fairly restricted geographical areas. For example, if natural vegetation is totally replaced by houses and streets (urbanisation), the actual NPP is significantly reduced compared to the natural potential and HANPP becomes very high. Conversely, actual NPP may be dramatically increased and exceed the natural potential NPP in arid areas through the introduction of irrigation and fertilisation.
Basic NPP components considered in HANPP calculation
Source: redrawn after: Haberl, H. et al., 2013 [CC BY-SA].
HANPP% by continent.