The stability of urban agriculture: Hysteresis and spatio-temporal flux
Urban agriculture is a world-wide phenomenon that has arisen several times in history following severe economic crises. The stability of these systems has varied greatly from location to location, with some like in the United States occupying strong, yet brief moments in history (Victory Gardens following WWI and II), while others remaining permanent fixtures that in some cases become part of a nation’s identity (Cuba, Germany). This research tests how basic ecological patterns are altered by the physical as well as socio-political constraints of urban garden ecosystems. We take two basic biological components of urban gardens: biological control and soil quality, and test how their stability in the face of management-driven spatio-temporal fluctuations contributes to the overall success of urban agriculture movements. We develop a Lotka Volterra predator-prey model and present a theoretical framework that posits empirical results of soil quality under changing management types in the context of several socio-political scenarios for urban agriculture.
We found that control-agent pest systems are prone to hysteresis, implying that the conversion of vacant lots to urban gardens may have considerably different implications than the conversion of natural lands to agriculture. The use of chemical (CaCO3) in contrast to organic (earthworm castings) nitrogen sources left consistent legacy effects that built in the soil causing transition from a conventional to organic agriculture regime to be sharp, and irreversible. Specifically, transitioning from organic worm castings to conventional CaCO3 resulted in greater biomass, germination rates and nitrogen leaching than when the same trays of soil were transitioned from conventional to organic regimes. We suggest that this experimentally derived exhibition of hysteresis could explain why urban gardens (or other less intensive agriculture models) are difficult to stabilize in different cities and political climates. We show that strong, nonlinear subsidies in favor of industry may promote the creation of unstable mega-farms that continue to degrade the environment and unstable urban gardens/small scale farms that are abandoned. However, changing government subsidies just slightly in favor of low input/small-scale gardens (like urban agriculture) may avoid hysteresis and create low input, low ecological-cost gardens that are dynamically stable.