PART SIX • ECOSYSTEM ECOLOGY
Number of observations
Although the general model of energy flow presented in Figure
20.18 pertains to all ecosystems, the relative importance of the
two major food chains and the rate of energy flow through the
various trophic levels can vary widely among different types of
ecosystems. The consumption efficiency (In/Pn - 1) defines the
amount of available energy produced by any given trophic level
(Pn - 1) that is consumed by the next-higher level (In). Values of
consumption efficiency for the various consumer trophic levels
therefore determine the pathway of energy flow through the food
chain, providing a basis for comparison of energy flow through
different ecosystems.
Despite its conspicuousness, the grazing food chain is not
the major one in most terrestrial and many aquatic ecosystems.
Only in some open-water aquatic ecosystems do the grazing
herbivores play the dominant role in energy flow. Ecologists
Helene Cyr of the University of Toronto (Canada) and Michael
Pace of the University of Virginia compiled published measurements of herbivore consumption rates (herbivore consumption efficiency), herbivore biomass, and primary productivity
for a wide range of aquatic and terrestrial ecosystems (Figure
21.21). Although there is considerable variation in both environments, some generalizations do emerge from their analysis.
Aquatic ecosystems dominated by phytoplankton have higher
rates of herbivory (median value of 79 percent) than do those
in which vascular plants (submerged and emergent) dominate
(median value of 30 percent). In contrast, only 17 percent of
primary productivity (median value) is removed by herbivores
in terrestrial ecosystems. Therefore, in most terrestrial and
shallow-water ecosystems, with their high standing biomass
and relatively low harvest of primary production by herbivores,
the detrital food chain is dominant. In deep-water aquatic ecosystems, with their low standing biomass, rapid turnover of organisms, and high rate of harvest, the grazing food chain may
be dominant.
In terrestrial ecosystems, distinct differences in consumption efficiency and energy flow exist between forest and grassland ecosystems. Nelson Hairston of Cornell
University reviewed a wide range of studies that examined
patterns of energy flow through terrestrial ecosystems, providing a comparison of consumption efficiencies for herbivores
(primary producer S herbivore) and their predators (herbivore S carnivore) The author found an average consumption
efficiency of 3.7 percent for herbivores inhabiting deciduous
forest ecosystems, whereas herbivores inhabiting grassland
ecosystems had a value of 9.3 percent (both values lower than
the average for terrestrial ecosystems reported by Cyr and
Pace). Much smaller differences were observed for the consumption efficiency of predators inhabiting the two ecosystem
types. Predators inhabiting forests had a value of 89.9 percent,
whereas predators inhabiting grassland ecosystems had an
average value of 77 percent.
Patterns of energy flow through flowing-water ecosystems
(streams and rivers) differ markedly from both terrestrial and
standing-water ecosystems (lakes and oceans). By comparison,
9
Aquatic algae
8
7
6
5
4
3
2
1
0
(a)
0
9
Number of observations
21.12 Consumption Efficiency
Determines the Pathway of Energy
Flow through the Ecosystem
40
60
80
100
40
60
80
100
40
60
80
100
7
6
5
4
3
2
1
0
0
21
20
Terrestrial plants
18
15
12
9
6
3
0
(c)
20
Aquatic macrophytes
8
(b)
Number of observations
414
0
20
Primary production removed by herbivores (%)
Figure 21.21 Results from a review of studies that examined
rates of herbivory in different ecosystems. Histograms represent
the percentage of net primary productivity consumed by herbivores
in ecosystems dominated by (a) algae (phytoplankton), (b) rooted
aquatic plants, and (c) terrestrial plants. Number of observations refers
to the number of experiments having a given level of consumption.
Red arrows indicate the median value. Note that herbivores
consume a significantly greater proportion of phytoplankton
productivity than do either aquatic or terrestrial plants. Go to
at www.ecologyplace.com to perform a chi-square test.
(Nature Publishing Group.)
(Adapted from Cyr and Pace 1993.)
stream and river ecosystems have extremely low NPP, and the
grazing food chain is minor (see Chapter 25). The detrital food
chain dominates and depends on inputs of dead organic matter
from adjacent terrestrial ecosystems (see Section 20.4).
Figure 21.22 graphically represents the different patterns
of energy transfer in the four different ecosystems just discussed: forest, grassland, standing water, and running water.
CHAPTER 21 • ECOSYSTEM ENERGETICS
Respiration
Respiration
Respiration
415
Respiration
Grazer
system
Decomposer
system
Grazer
system
Decomposer
system
Net primary
productivity
Dead organic
matter
Net primary
productivity
Dead organic
matter
(b) Grassland
(a) Forest
Respiration
Respiration
Respiration
Respiration
Grazer
system
Decomposer
system
Grazer
system
Decomposer
system
Net primary
productivity
Dead organic
matter
Net primary
productivity
Dead organic
matter
(c) Phytoplankton community
From terrestrial catchment
(d) Stream community
Figure 21.22 General patterns of energy flow through four ecosystems: (a) forest,
(b) terrestrial grassland, (c) ocean (phytoplankton community), and (d) stream. Relative sizes of
boxes and arrows are proportional to the relative magnitude of compartments and flow.
(Adapted from Begon et al. 1986.)
21.13 Energy Decreases in Each
Successive Trophic Level
Based on the preceding discussion and the analysis presented in
Figure 21.20, we can conclude that the quantity of energy flowing into a trophic level decreases with each successive trophic
level in the food chain. This pattern occurs because not all energy
is used for production. An ecological rule of thumb is that only
10 percent of the energy stored as biomass in a given trophic
level is converted to biomass at the next-higher trophic level. If,
for example, herbivores eat 1000 kcal of plant energy, only about
100 kcal is converted into herbivore tissue, 10 kcal into first-level
carnivore production, and 1 kcal into second-level carnivore production. However, ecosystems are not governed by some simple
principle that regulates a constant proportion of energy reaching
successive trophic levels.
As we have seen thus far in our discussion, differences
in the consumption efficiency as well as the efficiency of energy conversion (assimilation and production efficiencies) exist
among different feeding groups (see Table 21.2). These differences will directly influence the rate of energy transfer from
one trophic level to the next-higher level. A measure of efficiency used to describe the transfer of energy between trophic
levels is called the trophic efficiency. The trophic efficiency
(TE) is the ratio of productivity in a given trophic level (Pn) to
the trophic level it feeds on (Pn - 1): TE = Pn/Pn - 1.
Daniel Pauly and Villy Christensen of the University of
British Columbia examined the energy transfer efficiency reported in 48 different studies of aquatic ecosystems. There is
Dry weight
(g/m2)
1.5
11
37
809
Tertiary consumers
Secondary consumers
Primary consumers
Producers
(a) Florida bog
Dry weight
(g/m2)
21
4
Consumers (zooplankton)
Producers (phytoplankton)
(b) English Channel
Figure 21.23 Biomass pyramids for the consumer food chain
of (a) a bog ecosystem in Florida and (b) the marine ecosystem of the
English Channel. The pyramid for the marine ecosystem is inverted due
to the high productivity but fast turnover of phytoplankton populations
(short life span and high rate of consumption by zooplankton).
considerable variation among studies and trophic levels, but the
mean value of 10.13 percent is close to the general rule of 10
percent transfer between trophic levels.
An important consequence of decreasing energy transfers
through the food web is a corresponding decrease in the standing
biomass of organisms within each successive trophic level. If we
sum all of the biomass or energy contained in each trophic level,
we can construct pyramids for the ecosystem (Figure 21.23).
The pyramid of biomass indicates by weight, or other means of
416
PART SIX • ECOSYSTEM ECOLOGY
measuring living material, the total bulk of organisms or fixed energy present at any one time—the standing crop. Because some
energy or material is lost at each successive trophic level, the total
mass supported at each level is limited by the rate at which energy is being stored at the next-lower level. In general, the biomass of producers must be greater than that of the herbivores they
support, and the biomass of herbivores must be greater than that
of carnivores. That circumstance results in a narrowing pyramid
for most ecosystems (Figure 21.23a).
This arrangement does not hold for all ecosystems. In such
ecosystems as lakes and open seas, primary production is concentrated in the phytoplankton. These microscopic organisms have a
short life cycle and rapid reproduction. They are heavily grazed
by herbivorous zooplankton that are larger and longer-lived.
Thus, despite the high productivity of algae, their biomass is low
compared to that of zooplankton herbivores (Figure 21.23b). The
result is an inverted pyramid, with a lower standing biomass of
primary producers (phytoplankton) and herbivores (zooplankton).
Summary
Laws of Thermodynamics 21.1
Energy flow in ecosystems supports life. Energy is governed by
the laws of thermodynamics. The first law states that although
energy can be transferred, it cannot be created or destroyed.
The second law states that as energy is transferred, a portion
ceases to be usable. As energy moves through an ecosystem,
much of it is lost as heat of respiration. Energy is degraded
from a more organized to a less organized state, or entropy.
However, a continuous flux of energy from the Sun prevents
ecosystems from running down.
External Inputs 21.5
In many aquatic ecosystems a substantial proportion of
organic carbon is derived from dead organic matter from
adjacent terrestrial ecosystems. The relative importance of
external sources of organic carbon varies widely among different aquatic ecosystems. In large rivers, lakes, and most
marine systems, the majority of organic carbon is derived
internally from photosynthesis by autotrophs. In contrast, in
smaller streams and lakes the dominant source is often external sources of organic carbon.
Primary Production 21.2
The flow of energy through an ecosystem starts with the harnessing of sunlight by green plants through a process referred
to as primary production. The total amount of energy fixed
by plants is gross primary production. The amount of energy
remaining after plants have met their respiratory need is net
primary production in the form of plant biomass. The rate of
primary production is net primary productivity, which is measured in units of weight per unit area per unit time.
Energy Allocation 21.6
Energy fixed by plants is allocated to different parts of the plant
and to reproduction. How much is allocated to each component
is a function of the plant life-form as well as the environmental conditions. The pattern of allocation will directly influence
standing biomass and productivity rate.
Terrestrial Ecosystems 21.3
Productivity of terrestrial ecosystems is influenced by climate,
especially temperature and precipitation. Temperature influences the photosynthetic rate and the amount of available water
limits photosynthesis and the amount of leaves that can be supported. Warm, wet conditions make the tropical rain forest the
most productive terrestrial ecosystem. Nutrient availability also
directly influences rates of primary productivity.
Aquatic Ecosystems 21.4
Light is a primary factor limiting productivity in aquatic ecosystems, and the depth to which light penetrates is crucial to
determining the zone of primary productivity. Nutrient availability is the most pervasive influence on the productivity of
oceans. The most productive ecosystems are shallow coastal
waters, coral reefs, and estuaries, where nutrients are more
available. Nutrient availability is also a dominant factor limiting net primary productivity in lake ecosystems. In rivers and
streams, net primary productivity is low, with inputs of dead
organic matter from adjacent terrestrial ecosystems being an
important source of energy input.
Temporal Variation 21.7
Primary production in an ecosystem varies with time. Seasonal
and yearly variations in moisture and temperature directly
influence primary production. In ecosystems dominated by
woody vegetation, net primary production declines with age.
As the ratio of woody biomass to foliage increases, more of
gross production goes into maintenance.
Secondary Production 21.8
Net primary production is available to consumers directly as
plant tissue or indirectly through animal tissue. Once consumed
and assimilated, energy is diverted to maintenance, growth, and
reproduction, and to feces, urine, and gas. Change in biomass,
including weight change and reproduction, is secondary production. Secondary production depends upon primary production. Any environmental constraint on primary production will
constrain secondary production in the ecosystem.
Efficiency of Energy Use 21.9
Efficiency of production varies. Endotherms have high assimilation efficiency but low production efficiency because they
have to expend so much energy in maintenance. Ectotherms
have low assimilation efficiency but high production efficiency;
they put more energy into growth.