In every ecosystem, heterotrophs (consumers) feed on autotrophs (primary producers). Higher order species within the heterotrophs feed on lower order species. Various feeding levels are normally grouped into trophic levels, such as primary consumers (humans and animals with no preditors), herbivors (non-meat eating feeders), and primary producers.  Decomposers and detritus feeders (consumers of animals found dead) are another important member of an ecosystem. They assist in chemical breakdown and the return of nutrients to the supply reservoir.

The food web (illustrated in Fig 9-6) describes the interconnectivity of feeding by heterotrophs and autotrophs. In terrestrial ecosystems, the biomass decreases by 90-99% at each trophic level. This is because the exploitation efficiency -- that is the amount of carbon produced by primary productivity that makes it to biomass compared to that which is wasted -- also decreases in upper trophic levels. For example, because herbivors eat plants they make use of about 20% of the carbon produced by primary productivity for their energy needs. In contrast, carnivores only exploit about 0.2% because much is lost by the trophic levels inbetween the carnivor and the primary producer.

Biodiversity is considered high not only when the number of species is high, but also when the populations among different species are fairly homogeneous. The tropics have greater biodiversity than the poles. There are two competing hypotheses to explain why. The time stability hypothesis argues that the highly stable climate of the tropics (both in terms of the seasonal cycle and the variations from year to year) compared to the poles is key. In contrast, the intermediate disturbance hypothesis says that  occasional disturbances that occur in the tropics are just what is needed to cause turnover of species. Regional variations in disturbance frequency within the tropics support the latter hypothesis.