Human impacts on energy flow through natural ecosystems and implications for species

Photosynthetically fixed energy ultimately supports the great diversity of species that inhabit the world's ecosystems. Globally, natural terrestrial ecosystems fixed about $2800\cdot 10^{18}$ joules of energy per year (net) prior to significant human impacts. In recent years, humans have diverted or prevented about 20% to 30% of this energy from flowing through natural ecosystems by maintaining croplands (15%) and urban areas (1.8%) and by grazing livestock (2.3%). Habitat degradation, mainly in the form of desertification, has also caused a reduction (4%). Energy flow can be related to numbers of species with species-energy curves, which are relationships between species richness and the total energy flow in different regions, similar to species-area curves. By back-extrapolating along a species-energy curve, the observed reduction in natural energy flow due to human activity can be used to make a quantitative prediction of species endangerment. Given 20% to 30% reduction in energy and a species-energy curve exponent (z) between 0.10 and 0.20, we expect the endangerment of 2% to 7% of the world's terrestrial species. Projections to the year 2000 indicate that 3% to 9% of the world's species may be extinct or endangered by that time. These estimates are probably conservative.