In which of the following is the ultimate source of energy for all ecosystems?

• A. Heat from the Earth
• B. Water
• C. Nutrients
• D. Light from the sun

Answer: (D)

Sunlight powers almost every ecosystem. Plants, algae, and some bacteria capture light and turn it into chemical energy through photosynthesis, storing energy in sugars that can be eaten by other organisms. As energy moves through the food web—from producers to herbivores to predators—some energy is used for life processes and the rest is lost as heat, but the original energy input comes from the sun. Water and nutrients are essential for life and growth, but they don’t provide energy themselves. Heat from the Earth does exist in some special environments, yet it isn’t the universal energy source for ecosystems. So the ultimate energy source is light from the sun.