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Lecture 20

Biology 2483A Lecture Notes - Lecture 20: Persistent Organic Pollutant, Bioaccumulation, Steller Sea Lion

Course Code
BIOL 2483A
Hugh Henry

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Lecture 20
Secondary production - Heterotrophs get their energy by consuming organic compounds
that were produced by other organisms: Secondary production.
o Herbivores eat plants and algae.
o Carnivores eat other live animals.
o Detritivores eat dead organic matter (detritus).
o Omnivores eat both plants and animals.
Not all the organic matter consumed by heterotrophs becomes heterotroph
biomass. Some is used in respiration, some is egested (lost in urine and feces).
Net secondary production = Ingestion Respiration Egestion
Net secondary production in most ecosystems is a small fraction of NPP. The
fraction is greater in aquatic ecosystems than terrestrial.
Most is associated with detritivores, primarily bacterial and fungi.
Energy Flow and Food Webs
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 (detritus). In terrestrial ecosystems, only
a small portion of the biomass is consumed, and
most of the energy flow passes through the
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.
These external energy inputs are called allochthonous inputs.
Allochthonous inputs can be very important in stream ecosystems (up to 99.8%).
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
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Energy produced by autotrophs within the system is autochthonous energy.
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.
The second law of thermodynamics states that during any transfer of energy, some
is dispersed and becomes unuseable: Energy will decrease with each trophic level.
A trophic pyramid portrays the relative amounts of energy or biomass of each trophic
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
In aquatic ecosystems, the biomass pyramid
may be inverted. The primary producers are
phytoplankton with short life spans and high
Inverted biomass pyramids are more common where productivity is lowest, such as
nutrient-poor regions of the open ocean.
Phytoplankton turnover is high, associated with high growth rate and short life span
compared with phytoplankton of nutrient-rich waters.
Herbivores on land consume a much lower proportion of autotroph biomass than
herbivores in most aquatic ecosystems.
On average, about 13% of terrestrial NPP is consumed; in aquatic ecosystems, an
average of 35% NPP is consumed.
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.
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