(355d) Evolution and Robustness of the Global Agricultural-Phosphorus Trade Network
The present work focuses on characterizing the structure and understanding the systemic vulnerabilities of the global agricultural phosphorus trade network (GAPTN). We accomplish this by modeling a directed, weighted global network of annual phosphorus-containing commodity exchanges, including all synthetic phosphatic fertilizers and their chemical precursors. All commodities are further normalized to their weight content of elemental phosphorus. As such, nodes are taken to be countries participating in phosphorus trade, and edges are taken to be the aggregate of the directed trade flows of elemental phosphorus between each pair of countries, resulting in the construction of a global agricultural phosphorus trade network (GAPTN). The primary objectives of this study are to (1) characterize the network topology and evolutionary trends of the GAPTN. (2) Elucidate the sets of nodes and edges that are critical to the overall function of the network. (3) Identify those countries most dependent on the networkâ??s functioning, and outline engineering and policy measures such that they may ensure their consistent access to this essential resource.
Edges within the GAPTN display mild heterophily, preferentially connecting nodes of dissimilar degree. Based on statistical testing, the distribution of GAPTN node degrees follows a heavy-tail distribution, as does the distribution of in-, out-, and total-strength for 2014. Such distributions indicate that a small subset of all of the participating countries exchange most of the resources that comprise the system, leaving the network robust to random disruptions and highly vulnerable to inopportune ones. The potential for inopportune disruptions stems from both the price volatility of phosphatic fertilizers with respect to energy prices, and the geopolitical tension that surrounds Western Sahara & Morocco, which owns some 75% of the estimated global phosphate rock reserves. Further confirming the implications of the degree distributions, a robustness analysis of randomized versus targeted removal of nodes from the network indicates that some 80% of the networkâ??s overall activity halts when some 5% of the nodes do not participate. Results from node-level analyses reveal a small subset of countries to be particularly vulnerable to supply disruptions, even as the overall network has become more redundant and dense from 1990 to 2014. In light of these findings, both international cooperation on phosphorus management and country-level efforts to close local nutrient cycles can aid in improving food security while also minimizing the ecological impacts associated with fertilizer run-off, especially eutrophication.