Organisms in food webs are grouped into categories called trophic levels. Roughly speaking, these levels are divided into producers (first trophic level), consumers, and decomposers (last trophic level).
Producers make up the first trophic level. Producers, also known as autotrophs, make their own food and do not depend on any other organism for nutrition. Most autotrophs use a process called photosynthesis to create food (a nutrient called glucose) from sunlight, carbon dioxide, and water.
In addition, smaller animals are more numerous than larger ones. Tigers and ants are both consumers in a tropical food web. However, it takes much more biomass to support a tiger population than a colony of ants. Tigers consume more food and take up a much larger space. There are many more ants than tigers in the food web of a tropical ecosystem.
Every link in a food web is connected to at least two others. The biomass of an ecosystem depends on how balanced and connected its food web is. When one link in the food web is threatened, some or all of the links are weakened or stressed. The ecosystems biomass declines.