Lecture 16 – Energy Flow and Food Webs
What links organism together in the context of ecological functioning is their trophic interactions – what
they eat and what eats them.
The influence of an organism on the movement of energy and nutrients through an ecosystem is determined
by the type of food it consumes, and by what consumes it.
Trophic levels describe the feeding positions of groups of
organisms in ecosystems
o Each feeding category, or trophic level, is
based on the number of feeding steps by which it is
separated from autotrophs.
o The first trophic level consists of
autotrophs (primary producers)
o Figure 20.3 All organisms not consumed
by other organisms end up as detritus. In
trophic studies, detritus is considered
part of the first trophic level, and detritivores are
grouped with herbivores in the second
Some organisms do not conveniently fit into
trophic levels. Omnivores feed at multiple trophic levels. Ex: Coyotes are
opportunistic feeders, consuming vegetation, mice, other carnivores, and old leather boots.
All organisms in an ecosystem 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.
o Detritus pool is very important.
o Figure 20.4 – (A) detritus is consumed by a multitude of organisms, including fungi and
arthropods. (B) Numerous trophic studies of both terrestrial and aquatic ecosystems
found that in most ecosystems, most of the NPP ends up as detritus. (more than 50%) (C)
Similarly, relatively little of the NNP in an ecosystem is consumed by herbivores. These
trends are stronger for terrestrial ecosystems than aquatic. In most studies, only a small
proportion of NPP is consumed by herbivores.
Much of the input of detritus into streams, lakes and estuarine ecosystems is derived from
terrestrial organic matter.
o These external energy inputs are called allochthonous inputs
Tend to be more important in stream and river ecosystems
o Energy produced by autotrophs within the system is autochthonous energy.
Auto = self
o Example: Bear Brook, a head water stream revieves 99.8% of its energy as allochthonous
inputs; the rest is NPP derived from benthic algae and mosses in the stream.
In contrast autochthonous energy accounts fro 80% of the energy in nearby
Mirror Lakes. (often lower quality)
Energy Flow among Trophic Lecels
The amount of energy transferred from one trophic level to the next depends on food quality and
consumer abundances and physiology
A trophic pyramid is a graphical representation of trophic relationships. Energy will decrease with
each trophic level. Figure 20.5 – in terrestrial ecosystems, energy and biomass pyramids are usually 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.
There is a positive relationship between NPP and the amount of biomass consumed by herbivores.
The amount of autotroph biomass consumed is higher in aquatic ecosystems than in terrestrial
Why don’t terrestrial herbivores consume more of the available biomass? Several hypotheses
o Herbivore populations are constrained by predators, and never reach carrying capacity.
o Autotrophs have defenses against herbivory
o Terrestrial plants have nutrientpoor structural materials such as stems and wood, which
are typically absent in aquatic autotrophs
Trophic Efficiency – the amount of energy at one trophic level divided by the amount of energy at
the trophic level immediately below it.
o Incorporates the proportion of available energy that is consumed,
o The proportion of ingested food
that is assimilated by the
o And the proportion of
assimilated food that goes
into producing new consumer biomass.
Figure 20.7 – Proportion of energy transferred between
trophic levels depends on efficiens of consumption,
assimilation, and production
Biomass to net primary production is secondary production.
Assimilation efficiency is determined by the quality of the food and the physiology of the
Food quality of plants and detritus is lower than animals because of complex compounds such as
cellulose, lignins, and humic acids, that are not easily digested, and low concentrations of nutrients
such as N and P.
Animal bodies have carbon: nutrient ratios similar to that of the animal consuming them, and so
are assimilated ore readily.
Assimilation efficiencies of herbivores and detrititvores vary between 2050%, carnivores are
Endotherms tend to digest food more completely than ectotherms and thus have higher
Some herbivores have mutualistic symbionts that help them digest cellulose. This gives ruminants
(cattle, deer, camels) higher assimilation efficiencies than nonruminant herbivores.
Production efficiency is strongly related to the thermal physiology and size of the consumer –
ectotherms have a higher advantage.
o Endotherms allocate more energy to heat production, and have less for growth and
reproduction than ectotherms. Table 20.1
Changes in food quantity and quality, and the resulting changes in trophic efficiency, can
determine consumer population size.
o Steller sea lions pop. in Alaska declined by about 80% in the last 25 years. Smaller body
size and decreased birth rates suggested food quantity or quality might be a problem
(Figure 20.8 shows the decline in population size)
o Various lines of evidence suggested that prey quantity was not declining. Pallock and cod
have half the fat and energy as herring. Sealions main diet is the herring. o During the period of the population decline, the sea lion diet shifted away from the
herring toward a greater proportion of Pollock and cod. (table 20.2)
Changes in the abundances of organisms at one trophic level can influence
energy flow through an ecosystem