The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 25

sources, fueling its basic functioning. The energy flow
in an ecosystem is directly related to its trophic structure.
By examining energy flow, however, we are focusing on
the sources of the energy and its movement within the
structure, rather than on the structure itself.
Energy flows into an ecosystem as a result of the
capture of solar energy by plants, the producers of the
system. This energy is stored in the chemical bonds of
the biomass that plants produce. Ecosystems vary in their
ability to convert solar energy to biomass. We can measure
the total amount of energy that plants have brought into
the system at a point in time by determining the standing
crop or biomass of the plants in the system. We can also
measure the rate of the conversion of solar energy to
biomass: this is called gross primary productivity , which is
usually expressed in terms of kilocalories per square meter
per year. When the energy plants use to maintain themselves
is subtracted from gross primary productivity, a measure of
the ecosystem's net primary productivity is attained.
Herbivores (primary consumers) consume plant bio-
mass and convert it into animal biomass, and predators
and parasites (secondary and higher-level consumers) who
prey on herbivores or other consumers continue the bio-
mass conversion process between trophic levels. Only a
small percentage of the biomass at one trophic level, how-
ever, is converted into biomass at the next trophic level.
This is because a large amount of energy is expended in
maintaining the organisms at each level (as much as 90%
of the consumed energy). In addition, a large amount of
biomass at each level is never consumed (and some of
what is consumed is not fully digested); this biomass (in
the form of dead organisms and fecal matter) is eventually
broken down by detritivores and decomposers . The
decomposition process releases (in the form of heat) much
of the energy that went into creating the biomass, and the
remaining biomass is returned to the soil as organic matter.
In natural ecosystems, the energy that leaves the sys-
tem is mostly in the form of heat, generated in part by the
respiration of the organisms at the various trophic levels
and in part by the decomposition of biomass. Other forms
of energy output are quite small. The total energy output
(or energy loss) of an ecosystem is usually balanced by
the energy input that comes from plants capturing solar
energy (Figure 2.2).

TABLE 2.1
Trophic Levels and Roles in a Community
Type of
Organism

Trophic
Level

Physiological
Classification Trophic Role

Plants Producers First Autotrophic

Herbivores First-level consumers Second Heterotrophic

Predators and
parasites

Second-level (and
higher) consumers

Third and
higher

Heterotrophic

S TABILITY

Over time, the species diversity, dominance structure, vege-
tative structure, and trophic structure of a community usu-
ally remain fairly stable, even though individual organisms
die and leave the area, and the relative sizes of populations
shift. In other words, if you were to visit and observe a
natural community and then visit it again 20 years later,
it would probably appear relatively unchanged in its basic
aspects. Even if some kind of disturbance — such as fire
or flooding — killed off many members of many species
in the community, the community would eventually
recover, or return to something close to the original con-
dition and species composition.
Because of this ability of communities to resist change
and to be resilient in response to disturbance, communities
— and the ecosystems of which they are a part — are
sometimes said to possess the property of stability. The
relative stability of a community depends greatly on the
type of community and the nature of the disturbances to
which it is subjected. Ecologists disagree about whether
or not stability should be considered an inherent charac-
teristic of communities or ecosystems.

FUNCTIONING OF NATURAL ECOSYSTEMS

Ecosystem function refers to the dynamic processes occur-
ring within ecosystems: the movement of matter and
energy and the interactions and relationships of the organ-
isms and materials in the system. It is important to under-
stand these processes in order to address the concepts of
ecosystem dynamics, efficiency, productivity, and devel-
opment, especially in agroecosystems where function can
determine the difference between the success and failure
of a particular crop or management practice.
The two most fundamental processes in any ecosystem
are the flow of energy among its parts and the cycling of
nutrients.

N UTRIENT C YCLING

In addition to energy, organisms require inputs of matter
to maintain their life functions. This matter — in the form
of nutrients containing a variety of crucial elements and
compounds — is used to build cells and tissues and the
complex organic molecules required for cell and body
functioning.
The cycling of nutrients in ecosystems is obviously
linked to the flow of energy: the biomass transferred

E NERGY F LOW

Each individual organism in an ecosystem is constantly
using energy to carry out its physiological processes, and
its sources of energy must be regularly replenished. Thus
energy in an ecosystem is like electricity in a home: it
is constantly flowing into the system from outside