Changes in a tropical forest support metaboliczero-sum dynamics
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Major shifts in many ecosystem-level properties of tropical forests have been observed, but the processes driving these changes are poorly understood. Over the past two decades, the forest on Barro Colorado Island, Panama (BCI) exhibited a 20% decrease in the number of trees and a 10% increase in average tree diameter. Here we show that these changes are consistent with a zero-sum constraint operating in this forest. Zero-sum constraints result when the total resource use of an ecosystem is limited by available resources. This causes increases in resource use by some groups of organisms to be compensated for by decreases in other groups. Using a metabolism-based zero-sum framework, we show that increases in per capita resource use at BCI, caused by increased tree size and increased temperature, compensated for the observed declines in abundance. This trade-off between abundance and average resource use by an individual resulted in no net change in the rate resources are fluxed by the forest. The observed changes in the forest are not consistent with other hypotheses, such as changes in overall resource availability or existing self-thinning models. The framework successfully predicts interrelated changes in size, abundance, and temperature, indicating the utility of metabolic zero-sum dynamics for understanding changes in the structure and dynamics of ecosystems.