| Human impacts on ecosystems have long been evaluated in the IPAT framework (Impact = Population x Affluence x Technology). Heated debate ensued on which predictor to address. Here I use IPAT to develop predictive models for global human appropriation of net primary production (HANPP), which is a direct estimate of anthropogenic change in plant growth (NPP). NPP is the basis of terrestrial ecosystem energetics and is vital to carbon cycling, watershed hydrology and nutrient retention, wildlife habitat, and renewable resources for humans. Year 2000 data are used 5-arc-minute resolution among 153 countries among 7 regions. Models representing alternative hypotheses were compared using the Akaike Information Criterion.
Both presence/absence and quantity of HANPP are represented well by models, which included ecological effects and country areas as covariates and countries within regions as random effects. Besides those very important effects, presence/absence of HANPP (i.e., a logistic model) is most plausibly predicted by a positive population effect and negative GDPpc effect (total R2 = 0.87). Likewise, quantitative HANPP was most plausibly predicted (total R2 = 0.63) with positive population effect and a marginal, negative effect of greenhouse gas emissions.
6.1 B people lived in the year 2000; today we number 7.8 B. The long-running IPAT-based debate about which factor is The Culprit must end. Among the IPAT factors, population size has a larger effect but all three IPAT factors play a role. Results here also highlight the spatial heterogeneity of global HANPP and global trade in plant-based products as a driver of our collective impact. |