Drs. Flueck, Smith-Flueck, and Monjeau wrote a well-conceived letter in response to the paper I published with colleagues entitled “Sustainability challenges of phosphorus and food: Solutions from closing the human phosphorus cycle” (Childers et al. 2011). They present several important ways in which the extractive activities of human society, beyond the food system, affect global and regional phosphorus (P) cycling. The main premise of their response is that we did not consider the entirety of human effects on P cycling, which I acknowledge. Largely because of space limitations, we chose to focus specifically on the food system (per our title); notably, this is the largest component of human extractive uses of P and it is likely to be most malleable to sustainable solutions. Food production also dominates our use of mineral P resources (via fertilizer application). The removal of P from forests (e.g., via timber harvest), from rangelands (e.g., via feed animal harvesting), and from protected areas (e.g., via hunting) all contribute to the human P cycle. The ultimate result may well be P depletion in many of these nonagricultural ecosystems, resulting in lower yields of the products that humans harvest, but I am not aware of many situations where mineral P as fertilizer is being used to replace this extracted P. As such, these aspects of the greater human P cycle do not fit neatly into the aspects of the P cycle we reviewed, which begins with the mining of mineral P to support human food production.

An underlying tenet of both our review and Flueck and colleagues'response is the effect of human extractive activities on the global P cycle, not just on the human P cycle. The ultimate “common denominator” to the myriad sustainability challenges we face is the nexus of human population and per capita affluence, and the growth in both. We routinely hear that the human population will reach about 9 billion people by 2050, which will require a 70–100 percent increase in food production that will be fueled by P fertilizers. As per capita affluence increases, we will also require large increases in resources (including nonrenewables such as P) to support our increasing demands for “stuff,” to quote the late George Carlin. The stresses this will put on finite and dwindling biospheric resources are palpable. Why is the “increasing to 9 billion by 2050” prediction presented so often as though it were a nonnegotiable fact? Isn't this number, in fact, both negotiable and pliable? Fundamentally, we must directly address the population—affluence nexus when we envision and implement sustainable solutions to our problems, including when we consider sustainable solutions to closing the human P cycle.