Chapter 20 Summary

CONCEPT 20.1 Trophic levels describe the feeding positions of groups of organisms in ecosystems.

• An organism’s trophic level is determined by the number of feeding steps by which it is removed from the first trophic level, which contains autotrophs and detritus.
• Omnivores feed at multiple trophic levels, although their diets can be partitioned to reflect their consumption at each level.
• All organisms eventually end up as food for other organisms or as detritus.

CONCEPT 20.2 The amount of energy transferred from one trophic level to the next depends on food quality and consumer abundance and physiology.

• Energy and biomass pyramids portray the relative amounts of energy or biomass at different trophic levels.
• The high turnover of autotroph biomass in aquatic ecosystems can result in biomass pyramids that are inverted relative to energy pyramids.
• The proportion of autotroph biomass consumed in terrestrial ecosystems tends to be lower than that in aquatic ecosystems.
• The efficiency of energy transfer from one trophic level to the next is determined by food quality and the physiology of consumers.

CONCEPT 20.3 Changes in the abundances of organisms at one trophic level can influence energy flow at multiple trophic levels.

• Changes in the numbers and types of consumers at higher trophic levels can influence primary production through influences on the consumption of herbivores.
• Trophic cascades tend to be more apparent in aquatic ecosystems than in terrestrial ecosystems, but they have been demonstrated in complex terrestrial ecosystems as well.
• The number of trophic levels that can be sustained in an ecosystem is determined by the dispersal ability of organisms, the amount of energy entering the ecosystem through primary production, and the frequency of disturbance.

CONCEPT 20.4 Food webs are conceptual models of the trophic interactions of organisms in an ecosystem.

• Food webs are diagrams that portray the diverse trophic interactions among species in an ecosystem.
• Although trophic interactions are extremely complex, food webs can be simplified by focusing on the strongest interactions among the component organisms.
• Keystone species have greater effects on energy flow and species composition in their communities than their abundance or biomass would indicate.
• Indirect effects of a consumer on a target prey species, including effects on other species that compete with, facilitate, or modify the environment of the target species, can negate or reinforce the direct effects of consumption. These indirect effects may have important stabilizing effects on inherently unstable food webs.