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4
Materials and Energy
Organisms depend on the input of energy (in the form of sunlight or
high-energy molecules), water, and mineral nutrients for metabolism and
growth. Human societies depend on harvesting organisms or parts of them,
so many of our perturbations of ecosystems are intended to maintain or
increase production of organisms. Methods of increasing production of
organisms useful to people include domestication, increasing the propor-
tion of production that comes from species of economic value, removing
predators, reducing competition, and increasing supplies of the resources
that support production (e.g., by fertilization, irrigation, and modification
of microclimates). Some valued ecosystem traits, such as processing of
wastes and generation of aesthetically attractive landscapes, depend on
the total production of species in the system, but often only a portion of
total ecosystem production is of immediate value. Environmental problem-
solving is often aimed at increasing allocation of production to particular
forms, such as wood production in forests or seed production in agricul-
ture.
The overall determinants of biological production in terrestrial ecosys-
tems are generally well known. The rate of photosynthesis depends on
solar radiation, the availability of water in the soil, abundance of mineral
nutrients, and temperature. All these are commonly manipulated to in-
crease productivity of agroecosystems.
Productivity is tied to fluxes of energy and matter. Sometimes it is
measured in units of mass per unit time, as when one assesses the growth
of vegetation or livestock. Sometimes it is measured in units of energy
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62 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
per unit time, especially when the objective is to estimate the efficiency
with which one form of biomass (say, a plant) is converted to another (an
animal). Viewed as energy transformation, productivity conforms to the
laws of thermodynamics; viewed as material flux, it exhibits conservation
of mass. Hence, the measurement of nutrients and nutrient fluxes is a
logical companion to productivity studies.
Production, as opposed to productivity, is the material produced and
hence is measured in units of biomass. It can be differentiated into gross
production and net production. Not all gross production (PG) is available
for harvest, because materials and energy are expended metabolically by
living organisms through respiration (R) to maintain themselves. Net pro-
duction (PN) is the portion of gross production that exceeds maintenance
(PG = PN + R). In most ecosystems, NP corresponds to the amount of
material that can be harvested or processed by detritivores.
Measuring the production of entire ecosystems is difficult, and only
incomplete data are available on even the most intensively studied eco-
systems. Nonetheless, enough is known to enable us to see general patterns
in the allocation of the products of photosynthesis to compartments in the
systems. For example, interesting patterns are revealed by examining ratios
of energy stored in long-lived tissues to energy stored in short-lived tissues
of plants. If the ratio is high, much energy is tied up in tissues that remain
intact for most of the life of the plant, such as trunks, branches, and large
roots of trees and shrubs and rhizomes of some perennial herbs and grasses.
The patterns revealed by such a comparison are as follows (Jordan, 19711:
· The ratio of production of wood (long-lived) to production of litter
(short-lived) in forests generally increases as solar energy available during
the growing season decreases that suggests the increasing importance
of energy storage in low-energy environments.
· The ratio of wood production to litter production decreases as pre-
cipitation decreases that suggests that increases in size are less valuable
than the production of photosynthetically active tissue in dry environments.
An important implication of these patterns is that a higher proportion
of energy tied up in long-lived tissues means that less energy is available
for use by other organisms in the community, because long-lived tissues
are not heavily used as food.
Consideration of ratios directs attention to trade-offs between different
patterns of allocation. A plant, for example, can allocate a quantity of
energy to wood, leaves, flowers, pollen, nectar, fruits, or defensive chem-
icals. An increase in the allocation to one comes at the expense of allocation
to at least one of the others. Domestication involves major changes in
allocation patterns to increase the energy devoted to products of value to
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MATERIALS AND ENERGY
63
people (Snaydon, 1984). The trade-off often expresses itself as a reduction
of resistance to harsh climatic conditions, predators, parasites, and path-
ogens. Protection from these agents must be provided by people e.g.,
through weeding, application of pesticides, and killing of predators.
PERTURBATIONS AND PRODUCTIVITY
Natural perturbations such as floods, droughts, windstorms, and f~re-
can cause extensive and profound changes in an environment, sometimes
even the elimination of living organisms. Perturbations caused by people
can be just as extensive geographically, and their cumulative effects can
be profound, e.g., conversion of forest to croplands or cropland to resi-
dential areas.
Productivity often increases as a result of perturbation, but decreases
as perturbation becomes more severe (Odum et al., 19791. Sulfur dioxide
and nitrous oxide at low concentrations in the air over a cornfield can
increase production, because of the fertilizing effect of the sulfur and
nitrogen. But, as their concentrations increase, productivity of the corn-
field decreases, eventually to far below the preperturbation level.
Common physical perturbations are those imposed deliberately in land
management, e.g., logging or selective harvesting of forests (Chapters 19
and 23), channeling or damming to alter water flow (Chapter 21), and
mining (Chapter 181. The disturbance might be simply the harvesting of
the species of interest (Chapter 121. Development projects can also in-
advertently affect species (Chapter 161.
Effects of perturbations caused by chemicals can be more subtle than
those caused by physical alterations. Pollutants that have been taken up
by photosynthesizers can be passed on to grazers or detritivores, stored
in those organisms, and concentrated (as in the case of some radionuclides
and fat-soluble substances). When the grazers are eaten by predators or
scavengers, the pollutant burden is passed along. Other predators or scav-
engers might eat those and thus continue the process of concentration.
Eventually, pollutant concentration can be high enough to cause significant
biological effects (Chapter 221.
CHEMICAL PATHWAYS AND BIOLOGICAL CONCENTRATION
Biological concentration of organic chemicals, especially hydrophobic,
lipid~soluble ones, can be extreme. For example, polychlorinated biphen-
yls (PCBs) can accumulate to more than 100,000 times the concentration
in water. Eating 0.5 kg of Lake Erie fish can cause as much PCB intake
as drinking 1.5 x 106 L of Lake Erie water. Such pollutants cycle both
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64 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
through the inorganic parts of the environment and through the biota, and
their presence can affect primary and secondary productivity directly or
indirectly. The DDT case study (Chapter 24) shows how a pollutant can
affect populations by altering reproductive performance.
The pollutants that follow a decomposition pathway might appear, in
some form, in the sediments of a water body or in the soils. Some metals
released into the environment are transformed by bacterial action and
become either more or less toxic to higher organisms. For example, rel-
atively insoluble inorganic mercury, which is not highly toxic, can be
methylated and become biologically mobile and highly toxic.
Effects of chemical perturbations become evident when death or illness
results from excessive concentrations of toxic materials. Chemical per-
turbations can also affect ecosystem processes and patterns. For example,
increased input of phosphorus often results in eutrophication of lakes, with
substantial increases in primary productivity and changes in species com-
position (Chapter 201.
NUTRIENT FLUXES
The major elements of organisms e.g., carbon, nitrogen, oxygen,
sulfur, and phosphorus are involved in massive global cycles that involve
both living and nonliving components of the biosphere. Except for phos-
phorus, these elements can be present as gases, and the magnitudes of
their movement are being substantially modified by human activity. For
example, the anthropogenic fluxes of sulfur today are approximately equal
to those of the natural sulfur cycle (Andreae and Raemdonck, 1983; Franey
et al., 19831. Changes in global biogeochemical cycles affect environ-
mental problem-solving at specific sites and are beyond the control of
managers. But they are the most important of the cumulative effects of
individual projects, because the perturbations are largely the sum of the
inputs of specific projects. One of the most challenging problems in main-
taining high environmental quality is to find ways of reducing undesirable
effects of individual projects on biogeochemical cycles. This requires
dealing with the cumulative impacts of many projects.
Patterns and rates of nutrient fluxes have traditionally been studied along
with biological productivity, especially in agricultural systems where re-
lations between nutrients and productivity were first demonstrated. Con-
struction of "nutrient budgets" for ecosystems by tabulating the incomes
and outputs of various elements is an application of the first law of ther-
modynamics, i.e., net energy changes in a system are determined by the
inputs and outputs, whatever the internal pathways or mechanisms. Nu-
trient conditions in both Lake Washington (Chapter 20) and Southern
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MATERIALS AND ENERGY
65
Indian Lake (Chapter 21) were predicted essentially in this way. Attaching
biological relevance to nutrient values, however, requires an understanding
of physiological processes. The importance of phosphorus as a funda-
mental and controllable limiting nutrient was central to arguments about
Lake Washington, for instance. Among the elements that constitute major
proportions of cellular mass (carbon, nitrogen, oxygen, and phosphorus),
phosphorus is unique in not having a gaseous atmospheric phase. For this
reason, plant production in lakes is often limited by rates of supply of
phosphorus and can be manipulated over long periods. Because the po-
tential for plant production rises and falls with the availability of phos-
phorus in the water, growth decreases if phosphorus is removed from the
water and its input is lessened. (The rate of decrease depends on the size
and shape of the lake and the flushing rate of water.) Although early work
on phosphorus in lake waters discounted its importance as a regulating
nutrient (Juday and Birge, 1931; Juday et al., 1928), experience with
eutrophic systems showed its pivotal role by the middle of the twentieth
century (Hasler, 19471. Domestic sewage is especially rich in phosphorus
from animal wastes and the polyphosphate complexes that are commonly
used as builders or surfactants in laundry detergents.
Relationships between production and light are sometimes more im-
portant than those between production and nutrients. For example, South-
ern Indian Lake remained thoroughly mixed after impoundment, and
suspended solids from an eroding shoreline increased abiotic turbidity and
diminished light penetration (Chapter 21~. Lake Washington was moving
to a somewhat similar situation of low water transparency during 1963
and 1964, the peak years of enrichment (Chapter 204.
In Southern Indian Lake, the element that received most attention after
impoundment was not the one that had been targeted by the impact as-
sessments. Impact studies had considered phosphorus because it is often
a limiting nutrient in freshwater lakes, but mercury emerged as more
important. The environmental problem became chemically much less like
that of Lake Washington than like the problem with DDT (Chapter 241.
In Southern Indian Lake, the deleterious material was not manufactured
and added artificially to the system, but was released as a consequence
of the manipulation.
INTERACTIONS AMONG PRODUCTIVITY, BIOMASS,
AND NUTRIENTS
Development of our understanding of productivity, biomass, and their
relations to nutrients was spurred in part by practical concerns. During
the 1960s, a debate arose about the causes of lake eutrophication and the
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66 KINDS OF ECOLOGICAL KNOWLEDGE AND THEIR APPLICATIONS
most potent management options (Edmondson, 1974; Likens, 1972; Na-
tional Research Council, 1969). That phosphorus and phosphorus loading
were basic for controlling productivity in eutrophic basins was shown by
convincing logic and careful analyses of cases in which loading was altered
by design (e.g., Edmondson, 1969, 1972; Schindler, 19741.
Current models are based on findings that are embodied in quantitative
relations among hydrological conditions, basin structure, and nutrient and
chlorophyll concentrations in lakes (Chapra and Reckhow, 1983; Dillon
and Rigler, 1974; Vollenweider, 1969, 19764. These models help to relate
physical and chemical circumstances to productivity and trophic state.
They are relevant both to applied issues and to studies of the constraints
on energy transfer among trophic levels and of the magnitude of the
production base. A key feature of the models, however, is not so much
what they explain, but what they do not. They cannot predict algal bio-
mass, chlorophyll, or related entities even to within an order of magnitude
when a set of heterogeneous basins are compared. Substantial residual
variability in lake productivity cannot be explained by nutrient loading
(Carpenter and Kitchell, 1984; Harris, 19801. For single lake basins or
lake districts, the fits are usually far better, particularly when variation in
externally derived nutrient loading is large (Edmondson and Lehman,
1981; Schindler et al., 1978), but even in such basins, changes in chlo-
rophyll and water transparency can be extreme, despite relatively constant
nutrient loads (Edmondson and Litt, 19824.
INDEXES OF ECOSYSTEM FUNCTIONING
Productivity is used as an index of perturbation in a number of ways.
In aquatic ecosystems, following the ideas of Eppley and Peterson (1979)
and Harrison (1980), primary production can be considered as a sum of
production based on "new" nutrients and production based on "regen-
erated" nutrients. The "new" portion of production is the primary carbon
fixation that is supported by externally supplied nutrients, e.g., from
upwelling, chemical weathering, and stream or overland runoff. The "re-
generated" portion of production is the availability of nutrients in situ
from excretion and decomposition of cells. Production that is based on
regeneration cannot be harvested without compromising the production
base itself. Measuring "new" and "regenerated" nutrients is not easy,
but the data needed are not much different from those needed to construct
quantitative nutrient loading models (Vollenweider, 1969, 1975, 19761.
The techniques for doing so have become very sophisticated (Reckhow,
1979).
The cycling index (Finn, 1976, 1978) is a useful measure of the effects
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MATERIALS AND ENERGY
67
of perturbations on nutrient fluxes in ecosystems, especially terrestrial
ones. The cycling index is the ratio of the amount of nutrients recycled
in an ecosystem per unit time to the amount of nutrients moving through
the system. For most undisturbed terrestrial ecosystems, the index is be-
tween 0.6 and 0.8. The 20-40% of the nutrients lost must be made up
either from the atmosphere or from weathering of bedrock, if the ecosystem
is to remain in a steady state. In agricultural or other disturbed systems,
the index is often much lower, and productivity is maintained by fertil-
ization.
CONCLUSIONS
Our efforts to extract materials from ecosystems, to use them as waste
processors, and to manage them for productivity or maintenance of species
richness often conflict. Managing the conflict requires understanding of
the determinants of biological productivity and of how those determinants
are influenced by human-induced perturbations. The most difficult changes
to predict are those involving dynamics of important nutrients, and most
surprises result from incorrect prediction of the effects of projects on the
behavior of nutrient elements. The same problem is central in understand-
ing the influence of human activities on global air circulation patterns and
climate issues addressed only peripherally here. Changes in behavior of
nutrients seldom attract attention, because they are often not directly per-
ceivable by the unaided senses, although they do become obvious, for
example, when they impair visibility in scenic areas.
Other conflicts arise over decisions to divert energy flow patterns in
ecosystems toward species of indirect value to people and away from
species with no current value. Indeed, much of the difficulty in preserving
species richness is related not to overexploitation of species, but to con-
version of the ecosystems to uses that are incompatible with preservation
of the community of species originally present.
Representative terms from entire chapter:
southern indian
