Nov 19, 2013
Various biomes show a wide array of diversity when it comes to the animals that live there.
In terms of ecological functioning, these animals are linked by their feeding interactions, rather
than their physical appearances or taxonomic relationships. 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
Energy Flow & Food Webs
Trophic levels describe the feeding positions of groups of organisms in ecosystems.
Each feeding category, or trophic level is based on the number of feeding steps by which it is
separated from autotrophs.
The first trophic level consists of autotrophs who
generate chemical energy from sunlight or dead
inorganic matter in an ecosystem. The second
level consists of herbivores who feed on
autotroph biomass. This category also includes
detrivores that consume dead organic matter.
The remaining trophic levels contain carnivores
that consume animals at the trophic level below
them (primary and secondary carnivores). Most
ecosystems have four or fewer trophic levels.
Omnivores (foxes) defy our attempt at grouping
organisms into simple feeding categories.
However, their diets can be partitioned to reflect
how much energy they consume within each trophic level. This partitioning is facilitated via the
use of stable isotopes.
"We are all going to be dirt in the ground"
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 detritus. The image to the right
shows that more than 50% of NPP ends up as 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 (autotrophs), and detritivores are part of the
second level (herbivores).
Much of the detritus in streams, lakes, and estuaries is derived from terrestrial organic matter.
These external energy inputs are called allochthonous inputs. These outputs include plant
leaves, stems, wood, and dissolved organic matter that fall in from adjacent terrain or flow in via
ground water. Allochthonous inputs can be very important in stream ecosystems (up to 99.8%).
However, they are not very important in lake and marine ecosystems. Although important, this
energy is lower quality due to the chemical composition of the carbon entering the system
(amount used is actually lower than input indicates).
Energy produced by autotrophs within the system is autochthonous energy (detritus in
terrestrial ecosystems comes from plants). The importance of autochthonous energy inputs
increases from the headwaters toward the lower reaches of a river. This is because 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.
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. For example, autotrophs
lose chemical energy through cellular respiration, lowering the amount of energy available to
the second trophic level.
A trophic pyramid portrays the relative amounts of energy or biomass of each trophic level
stacked from lowest to highest. They are made up of a set of rectangles (each of which
represents the amount of energy or biomass within one trophic level). These pyramids show us
how energy flows through the ecosystem. Some of the biomass at each level is not consumed,
and some of the energy is dispersed in the transfer to the next level. Therefore, the size of the
energy rectangles decreases as we move from one trophic level to the one above it.
In terrestrial ecosystems, energy (biomass production) and biomass(total amount of living or
organic matter) pyramids are similar because biomass is closely associated with energy
production. In aquatic ecosystems, the biomass pyramid may be inverted relative to the energy pyramid.
This is because the primary producers are phytoplankton which have short life spans, and high
consumption rates by primary consumers. In other words, the biomass of heterotrophs may be
greater at any time than the biomass of autotrophs. However, the energy produced by
autotrophs is still greater than that produced by heterotrophs.
Inverted biomass pyramids are more common where productivity is lowest, such as nutrient-
poor regions of the open ocean. This is because the phytoplankton in the open ocean have a
higher turnover rate than the phytoplankton in more nutrient rich waters (lakes) Thus,
phytoplankton in nutrient poor regions provide a greater energy supply per unit of time.
Energy Flow Differs Among Ecosystems
It is reasonable to assume that the flow of energy to higher trophic levels is associated with the
amount of NPP at the base of the food web. However, other factors come into play (proportion
of each level consumed by the one above, nutritional content of primary producer, efficacy of
To better understand energy flow between trophic levels, a comparison of the proportions of
autotroph biomass consumed in terrestrial and aquatic ecosystems was looked at. 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.
If herbivore production is limited by the amount
of available food, it makes sense that the proportion of autotroph biomass that terrestrial
herbivores consume could increase if they consumed more of the available biomass. Several
hypotheses have been proposed that explain the lower proportion of autotroph biomass
consumed in terrestrial ecosystems.
1. Herbivores are constrained by predators, and never reach carrying capacity. Predator
removal experiments support this hypothesis in some ecosystems
2. Autotrophs have defenses against herbivory, such as secondary compounds, spines, etc.
Plants of resource-poor terrestrial environments tend to have stronger defenses than
plants from resource-rich terrestrial environments.
3. Phytoplankton are more nutritious for herbivores than terrestrial plants. Terrestrial
plants have structural components such wood, which have few nutrients. Freshwater
phytoplankton have carbon:nutrient ratios closer to those of herbivores than to those of
Efficiency of Energy Transfer
Not all of the energy consumed by a heterotroph gets incorporated into heterotroph biomass.
Energy efficiency (output of energy per unit of energy input) is used to describe energy transfer
between trophic levels.
Trophic efficiency: Amount of
energy at one trophic level divided
by the amount of energy at the
trophic level immediately below it.
Trophic efficiency incorporates
three types of efficiency: Proportion of available energy that is
consumed (consumption efficiency).
Consumption efficiency is higher in aquatic
ecosystems than in terrestrial ecosystems.
Consumption efficiencies also tend to be
higher for carnivores than for herbivores.
Proportion of ingested food that is
assimilated by digestion (assimilation
efficiency). Once ingested, energy must be
assimilated by the digestive system before it
can be used to produce new biomass.
Assimilation efficiency is determined by
food quality. 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. Animals have carbon:nutrient ratios similar to the animals
consuming them. Assimilation efficiencies of herbivores and detritivores are 20%–50%;
carnivores are about 80%. Digestive capacity of consumers is associated with their
thermal physiology. Endotherms (generate own heat) digest food more completely than
ectotherms (rely on heat exchange with environment) and thus have higher assimilation
efficiencies. Some herbivores have mutualistic symbionts that help them digest
Ruminants (cattle, deer, camels) have a modified foregut with bacteria and
protists that break down cellulose. They have higher assimilation efficiencies
than other herbivores.
Proportion of assimilated food that goes into new consumer biomass in the form of
consumer growth or reproduction (production efficiency). A small portion must be used
for respiration associated maintenance, as well as construction of new biomass.
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. Thus, ectotherms have higher production
efficiencies. Body size in endotherms is an important determinant of heat loss, and thus
production efficiency. Body size affects heat loss in endotherms. As body size increases,
the surface area-to-volume ratio decreases. A small endotherm (e.g., a shrew), loses a
greater proportion of its heat across its body surface than a large endotherm, such as a
grizzly bear, and will have lower production efficiency.
Biomass not ingested or assimilated enters the pool of detritus
Trophic Efficiences Influence Population Dynamics
Changes in food quantity or quality impact trophic efficiency and can determine consumer
population size that can be sustained (and health of individuals in consumer populations). A study was done to examine the effects of changes in
food quality on Steller Sea Lion populations.
Steller sea lion populations in Alaska declined by
about 80% over 25 years.
They found that individual sea lions collected
during the period of decline were smaller than
individuals within the same age classes collected
before the start of the decline. There was also a
reduction in the number of pups born per
female, which resulted in a shift of the age
structure toward older individuals. Smal