Which factor does NOT affect primary productivity in terrestrial ecosystems?

Primary productivity in terrestrial ecosystems is largely influenced by several environmental factors, including temperature, moisture, and nutrient availability. Temperature affects photosynthesis rates as it influences plant metabolism and growth. Moisture is crucial, as it determines water availability for plants, impacting their ability to carry out photosynthesis and grow. Nutrient availability is another vital factor since plants need essential nutrients, such as nitrogen and phosphorus, to support growth and reproductive processes.

Human population density, while impactful in other ecological interactions and pressures (such as land use changes), does not directly affect the primary productivity of ecosystems in the same immediate and biological way that the other three factors do. Therefore, it does not play a direct role in influencing the rates at which plants produce organic matter through photosynthesis, differentiating it from the other factors that are directly tied to the physiological processes of primary producers.