Energy Transfer and Decay.
This is why you lose biomass and energy every time you move up a trophic level.
1) Energy from the Sun is the source of energy for nearly all life on Earth.
2) Plants use a small percentage of light energy from the Sun to make food during photosynthesis. This energy is stored in the substances that make up cells of plants, and it works it's way through all the animals in the food web as animals eat plants and each other.
3) Respiration, supplies energy for all life processes, including movement. Most energy is eventually lost to the surroundings as heat. This is especially true for mammals and birds, whose bodies must be kept at a constant temperature which is normally higher than their surroundings.
4) Some of the material that make up animals is inedible (i.e. bone), so it doesn't pass to the next stage…