Energy Loss as Heat Drives Non-Cyclic Flow

Steps:

• Identify that energy enters ecosystems from the sun via producers.
• Trace energy transfers through trophic levels, where only 10% is passed on.
• Recognize that the rest dissipates as heat, per the second law of thermodynamics.
• Conclude that heat cannot be recycled back into usable energy, making flow linear.

Why D is correct:

• The second law of thermodynamics states energy transfers increase entropy, with much lost as unusable heat from organisms, preventing return to the original source.

Why the others are wrong:

• A: Energy cannot return to the sun; it's lost as heat in the environment.
• B: Energy flows from environment to living things via sunlight, not just the reverse.
• C: Energy moves from producers to consumers, regardless of size, but always with loss.

Final answer: D