Practice question

A pyramid of energy is always upright when it expresses production or energy flux per unit area per unit time. Each trophic level receives only part of the energy processed by the level below, while respiration irreversibly disperses usable energy as heat. Biomass pyramids can invert because standing stock combines production and turnover: rapidly renewed phytoplankton may support a larger instantaneous consumer biomass. Energy flux incorporates time, so fast producer turnover cannot produce an inverted energy pyramid under consistent accounting. Pyramid shape is an accounting result with biological causes, including body-size distributions, tissue longevity, consumption, assimilation, respiration, and population turnover. A snapshot may differ seasonally, especially in plankton or annual vegetation, whereas integrated production better represents ecosystem functioning across time. The ten-percent heuristic is useful for prediction but not exact; empirical transfer efficiencies vary with food quality, ectothermy, producer defenses, and detrital routing. Higher trophic levels are often vulnerable because low energy supply produces small populations that are sensitive to habitat fragmentation and environmental variability.