Food webs are defined by their biomass. Biomass is the energy in living organisms. Autotrophs, the producers in a food web, convert the suns energy into biomass. Biomass decreases with each trophic level. There is always more biomass in lower trophic levels than in higher ones.
Because biomass decreases with each trophic level, there are always more autotrophs than herbivores in a healthy food web. There are more herbivores than carnivores. An ecosystem cannot support a large number of omnivores without supporting an even larger number of herbivores, and an even larger number of autotrophs.
A healthy food web has an abundance of autotrophs, many herbivores, and few carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass.
The biomass of an ecosystem depends on how balanced and connected its food web is.
“…there are fewer foxes than rabbits; which makes sense because a fox must eat several rabbits to get enough energy in order to survive.”
“…the amount of energy available to the top-level consumer is tiny compared to that available to primary consumers. For this reason, it takes a lot of vegetation to support higher trophic levels. This explains why most food chains are limited to three or four levels; there is simply not enough energy at the top of an energy pyramid to support another trophic level. For instance, lions and killer whales have no natural predators; the energy stored in populations of these top-level consumers is not enough to feed yet another trophic level.”
SUNNY MEADOWS ACTIVITY– changing populations in an ecosystem- http://puzzling.caret.cam.ac.uk/game.php?game=foodchain