Ecosystemsgenerally refer to a specific region of the physical environment, and all theliving organisms found there. By this definition, a pond can be considered anecosystem, or a grassland, or a deciduous forest.
Most ecosystems have only four trophic levels, and the top carnivores are very rare.
At each trophic level, some energy is lost due to inefficiency and due torespiration. This meansthat one pound of hamburger actually requires 10 pounds of grain to produce,and if a person subsisted entirely on a carnivore diet, it would take nearly100 pounds of grain to support each pound of body weight. This explains why top carnivores in an ecosystem are so rare. The toxin becomes more and more concentrated as it moves up thefood chain, reaching its highest concentrations in organisms at the top of thetrophic pyramid. The network of feeding relationships in an ecosystem is called the foodweb , and these can be very complex.