Biology 2483A Lecture Notes - Lecture 20: Persistent Organic Pollutant, Atmospheric Circulation, Autotroph
25 Aug 2013
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Ecology Lecture No. 20: Energy Flow & Food Webs
Thursday November 15th, 2012
Introduction:
-The ecological roles of organisms are determined by their trophic interactions (what they eat and what
eats them). This determines the influence of an organism on the movement of energy and nutrients
through an ecosystem. Trophic levels describe the feeding positions of groups of organisms in
ecosystems.
Detritus & Detritivores:
-All organisms are either consumed by other organisms or enter the pool of dead organic matter known
as detritus. In terrestrial ecosystems, only a small portion of the biomass is consumed, and most of the
energy flow passes through the detritus. Dead organisms and feces are consumed by organisms called
detritivores (primarily bacteria and fungi), in a process called decomposition. Detritus is considered part
of the first trophic level, and detritivores are part of the second level.
-Much of the detritus in streams, lakes, and estuaries is derived from terrestrial organic matter. These
external energy inputs are called allochthonous inputs. Energy produced by autotrophs within the
system is autochthonous energy. Allochthonous inputs can be very important in stream ecosystems. The
importance of autochthonous energy inputs increases from the headwaters toward the lower reaches of
a river. Water velocity decreases, and nutrient concentrations increase as you go downstream.
Energy Flow Among Trophic Levels:
-The amount of energy transferred from one trophic level to the next depends on food quality and
consumer abundance and physiology.
Trophic Pyramids:
A trophic pyramid portrays the relative amounts of energy or biomass of each trophic level. Some of the
biomass at each level is not consumed, and some of the energy is dispersed in the transfer to the next
level. In terrestrial ecosystems, energy and biomass pyramids are similar because biomass is closely
associated with energy production. In aquatic ecosystems, the biomass pyramid may be inverted. The
primary producers are phytoplankton with short life spans and high turnover.
-Herbivores on land consume a much lower proportion of autotroph biomass than herbivores in most
aquatic ecosystems. There is a positive relationship between net primary production and amount of
biomass consumed by herbivores. This suggests that herbivore production is limited by the amount of
food available.
Document Summary
The ecological roles of organisms are determined by their trophic interactions (what they eat and what eats them). This determines the influence of an organism on the movement of energy and nutrients through an ecosystem. Trophic levels describe the feeding positions of groups of organisms in ecosystems. All organisms are either consumed by other organisms or enter the pool of dead organic matter known as detritus. In terrestrial ecosystems, only a small portion of the biomass is consumed, and most of the energy flow passes through the detritus. Dead organisms and feces are consumed by organisms called detritivores (primarily bacteria and fungi), in a process called decomposition. Detritus is considered part of the first trophic level, and detritivores are part of the second level. Much of the detritus in streams, lakes, and estuaries is derived from terrestrial organic matter. These external energy inputs are called allochthonous inputs. Energy produced by autotrophs within the system is autochthonous energy.
