Predictions for how the steady state fertility of the Amazon basin (soil phosphorus concentrations) has changed in response to the megafaunal extinctions. This simulation is characterized by lateral diffusivity of nutrients, Φ, by mammals away from the Amazon river floodplain source. The diffusivity of nutrients through the Amazon via ingestion, transport, and eventual defecation yields a Φ value of 4.4 km2 yr−1 (based on Doughty et al.27). a Simulated steady state soil P concentrations across the Amazon Basin with the now extinct megafauna; b With the extinction of large mammals and a continued forecasted reduction in mammal body size, the percentage of original steady state P concentrations in the Amazon Basin will decrease. Here, under a series of size thresholds for the extinct megafauna, we expect a 20–40% reduction in soil steady state P concentrations. For instance, a 5000 kg size threshold removes all animals above 5000 kg and continental P concentrations are reduced by ~10%. A size threshold of 0 has all extant South American mammals. Amazonian map from MATLAB worldmap from the Global Optimization Toolbox, The MathWorks, Inc. www.mathworks.com.