 This paper examines the relationship between productivity and market structure in the U.S. stock market. It finds that when the market becomes more concentrated, productivity drops significantly. This inverse relationship between productivity and market structure is explained by the Locovolterra equations, which describe the dynamics of ecological networks. These equations suggest that increasing the strength of interactions between companies leads to increased productivity and decreased market evenness. Increasing the strength of these interactions corresponds to a decrease in interest rates, suggesting a link between interest rates and market structure. This article was authored by Hugo Fort.