Ecosystem development explained by competition within and between material cycles

Ecosystems generally develop towards increased productivity and biomass, a decreased productivity/biomass ratio, and tighter nutrient cycles. These broad-scale trends are still lacking an explanation in terms of underlying ecological processes. Here I present a simple ecosystem model to show that resource competition between organisms in a material cycle (within-cycle competition) can explain successional and evolutionary changes in functional properties of ecosystems. Competition between organisms that are involved in spatially distinct cycles (between-cycle competition) constitutes another selective force that may lead to more productive and more closed ecosystems. However, these changes in ecosystem properties are only the indirect result of selection for different functional or demographic traits in the organisms involved. Within-cycle competition maximizes resource-use intensity, while between-cycle competition maximizes the basic reproductive rate. Therefore, the two levels of competition may operate in partly contradictory directions, a complication that is ignored by holistic 'maximum principles' in ecology.