Ecology Lecture No. 20: Energy Flow & Food Webs
Thursday November 15 , 2012
-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
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.
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. Explanations For Reduced Terrestrial Herbivory:
-One hypothesis is that herbivores are constrained by predators, and never reach carrying capacity.
Predator removal experiments support this hypothesis in some ecosystems. Another hypotheis is that
autotrophs have defenses against herbivory, such as secondary compounds, spines, etc. Plants of
resource-poor environments tend to have stronger defenses than plants from resource-rich
environments. And finally, phytoplankton are more nutritious for herbivores than terrestrial plants.
Terrestrial plants have structural components such as wood, which have few nutrients, because they
need to invest energy in standing (fighting gravity), whereas phytoplankton do not.
-Trophic efficiency is the amount of energy at one trophic level divided by the amount of energy at the
trophic level immediately below it. Three types of efficiency are incorporated into trophic efficiency:
consumption efficiency (proportion of available energy that is consumed), assimilation efficiency
(Proportion of ingested food that is assimilated) and production efficiency (proportion of assimilated
food that goes into new consumer biomass).
-Consumption efficiency is higher in aquatic ecosystems than in terrestrial ecosystems. Consumption
efficiencies also tend to be higher for carnivores than for herbivores.
-Assimilation efficiency is determined by food quality and the physiology of the consumer. Food quality
of plants and detritus is low because of complex compounds such as cellulose, lignins, and humic acids
that are not easily digested and low concentrations of nutrients such as nitrogen and phosphorus.
Endotherms digest food more completely than ectotherms and thus have higher assimilation
efficiencies. Some herbivores have higher assimilation efficiencies due to mutualistic symbionts that
help them digest cellulose.
-Production efficiency is strongly related to the thermal physiology and size of the consumer.
Endotherms allocate more energy to heat production, and have less for growth and reproduction than
ectotherms. A small endotherm (e.g., a shrew), loses a greater proportion of its heat across its body
surface than a large endotherm and will have a lower production efficiency.
Food Quantity/Quality Affecting Trophic Efficiency:
-Change in food quantity or quality impacts trophic efficiency and can determine consumer population
size. Prey quant