Food chain
A food chain can be thought of as a transfer of food from trophic level to trophic level (fig. 49.1). Food chains rarely are unbranched since several different primary consumers may feed on the same plant species and a primary consumer may eat several species of plants. The feeding relationships are usually woven into elaborate food webs within an ecosystem.

Ecological Pyramids
1. A pyramid of productivity has trophic levels stacked in blocks proportional in size to the energy acquired from the level below. Food chains are usually bottom heavy since only 10% of energy is transferred.

2. A biomass pyramid has each tier symbolising the total dry weight of all organisms in an ecosystem's levels at any given time. Biomass represents chemical energy stored in the organic matter of a trophic level. Most narrow sharply from producers at the base to top-level carnivores at the top. Some aquatic systems are inverted since producers can have high turnover rates. They grow rapidly but are consumed rapidly, leaving little standing biomass. Biomass of top-level carnivores is usually small compared to the total biomass of producers and lower-level consumers.

Food chains are limited to 3-5 links due to the multiplicative loss of energy. This also limits the biomass of top-level carnivores that can be supported. Only about 1/1000 of the chemical energy fixed by photosynthesis flows through a food web to a tertiary consumer, only 3 -5 trophic levels can be supported since biomass at the apex is insufficient to support another level. The fact that top-level consumer biomass is concentrated in a small number of individuals can be reflected in a pyramid of numbers where each tier represents the number of organisms at each trophic level. Predators (top-level consumers) are highly susceptible to extinction when their ecosystem is disturbed due to their small