The primary productivity of an ecological system is defined as the rate at which radiant energy is converted by the photosynthetic and chemosynthetic activity of producer organisms (chiefly green plants) to organic substances.
It is important to distinguish the four successive steps in the production process as follows:
1. Gross primary productivity (GPP) is the total rate of photosynthesis, including the organic matter used up in respiration during the period of measurement. This is also known as total photosynthesis.
2. Net primary productivity (NPP) is the rate of storage of organic matter in plant tissues that exceeds the respiratory use, R, by the plants during the period of measurement. This is also termed net assimilation. In practice, the amount of plant respiration is usually added to measurements of net primary productivity to estimate gross primary productivity (GPP = NPP + R).
3. Net community productivity is the rate of storage of organic matter not used by heterotrophs (that is, net primary production minus heterotrophic consumption) during the period under consideration, usually the growing season or a year.
4. Finally, the rates of energy storage at consumer levels are referred to as secondary productivities. Because consumers use only food materials already produced, with appropriate respiratory losses, and convert this food energy to different tissues by one overall process, secondary productivity should not be divided into gross and net amounts. The total energy flow at heterotrophic levels, which is analogous to the gross productivity of autotrophs, should be designated assimilation and not production.
In all these definitions, the term productivity and the phrase rate of production may be used interchangeably. Even when the term production designates an amount of accumulated organic matter, a time element is always assumed or understood (for instance, a year in agricultural crop production). Thus, to avoid confusion, one should always state the time interval. In accordance with the second law of thermodynamics, the flow of energy decreases at each step due to the heat loss occurring with each transfer of energy from one form to another.
High rates of production, in both natural and cultured ecosystems, occur when physical factors are favorable, especially when energy subsidies (such as fertilizers) from outside the system enhance growth or rates of reproduction within the system. Such energy subsidies may also be the work of wind and rain in a forest, tidal energy in an estuary, or the fossil fuel, animal, or human work energy used in cultivating a crop.
In evaluating the productivity of an ecosystem, one must consider the nature and magnitude not only of the energy drains resulting from climatic, harvest, pollution, and other stresses that divert energy away from the production process but also of the energy subsidies that enhance it by reducing the respiratory heat loss (the “disorder pump-out”) necessary to maintain the biological structure.
The key word in the preceding definitions is rate. The time element—that is, the amount of energy fixed in a given time—must be considered. Biological productivity thus differs from yield in the chemical or industrial sense.
In industry, the reaction ends with the production of a given amount of material; in biological communities, the production process is continuous in time, so a time unit must be designated (the amount of food manufactured per day or per year, for example). Although a highly productive community may have more organisms than a less productive community, this is not so if organisms in the productive community are removed or “turn-over” rapidly.
For example, a fertile pasture being grazed by livestock is likely to have a much smaller standing crop of grass than a less productive pasture not being grazed at the time of measurement. Biomass or standing crop present at any given time should not be confused with productivity. Students of ecology often confuse these two quantities.
Usually, one cannot determine the primary productivity of a system or the production of a population component simply by counting (or censusing) and weighing the organisms present at any one moment, although net primary productivity can be estimated from standing crop data when living materials accumulate over a period of time (such as a growing season) without being consumed (as in cultivated crops, for example).
Only about half of the total radiant energy of the Sun is absorbed, and at most about 5 percent (10 percent of the energy absorbed) can be converted by gross photosynthesis under the most favorable conditions. Then, plant respiration appreciably reduces-—usually by about 20 to 50 percent—the food available for heterotrophs.
During the peak of the growing season, especially during long summer days, as much as 10 percent of the total daily solar input may be converted into gross production, and from 65 up to 80 percent of this may remain as net primary production during a 24-hour period. Even under the most favorable conditions, however, these high daily rates cannot be maintained over the annual cycle, nor can they achieve such high yields over large areas of farmland, as is evident when they are compared with the annual yields actually obtained nationwide and worldwide.
The relationship between gross and net productivity in natural terrestrial vegetation varies with latitude. The percentage of gross productivity that becomes net primary production is highest at cold latitudes and lowest at hot latitudes, presumably because more respiration is required to maintain the biomass in the Tropics.
The other way in which humans increase food production does not necessarily involve an increase in gross productivity, but rather the genetic selection for an increase in the food-to-fiber ratio or harvest ratio. For example, a wild rice plant may put 20 percent of its net production into seeds (enough to ensure its survival), whereas a cultivated rice plant is bred to put as much as possible (50 percent or more) into seeds—the edible part. This grain-to-straw dry weight ratio has been increased several-fold in most crops.
The downside is that the engineered plant does not have much energy available to produce anti-herbivore chemicals (in order to defend itself), so more pesticides have to be used in the cultivation of highly bred varieties.
What has been termed the Green Revolution involves the genetic selection for engineered crop varieties with high harvest ratios that are adapted to respond to massive energy, irrigation, and nutrient subsidies. Those who think that developed countries can upgrade the agricultural production of less developed countries by supplying seeds and agricultural recommendations do not realize that the less developed countries cannot afford the necessary energy subsidies.
Thus, the Green Revolution so far has benefited the economically rich countries more than the economically poor countries. This situation is documented in a dramatic way in Figure 3-5, which compares the trends in agricultural production (since 1950) of those countries with the highest production with those countries having the lowest production of three major food crops: corn, wheat, and rice. The yields have increased two to threefold in the economically rich countries (United States, France, and Japan) but barely at all in the economically poor countries (India, China, and Brazil).
In the 1960s, plant geneticists developed new varieties of wheat and rice that gave yields two to three times those of traditional varieties. Indeed, Norman Borlaug received the Nobel Prize in 1970 for his leadership in the development of these new varieties.
This advance in crop breeding was heralded as the beginning of the Green Revolution. It was poorly understood at that time that the need for an increased use of subsidies (such as optimal fertilization and irrigation) that had to accompany these new varieties would negate many of their benefits. This is one of several examples where discoveries judged worthy of Nobel Prizes resulted in unanticipated environmental consequences at a later date.
To take another example, Fritz Haber, a German chemist, received the Nobel Prize in 1918 for his discovery of a catalytic process (termed the Haber process) for synthesizing ammonia from nitrogen and hydrogen. Currently, the human alteration of the global nitrogen cycle is one of the major environmental problems confronting society. Likewise, the discovery of the pesticide DDT during World War II helped to control mosquitoes and thereby greatly reduced the number of human deaths caused by malaria.
In fact, the benefits of DDT seemed so tremendous that Paul Muller, a Swiss chemist, was awarded the Nobel Prize in 1948 for its discovery. Proponents of the widespread use of DDT (and other chlorinated hydrocarbon pesticides) failed to understand the long-term ramifications of this discovery (such as the biological magnification of these compounds up the food chain).
It was not until 1962 that Rachel Carson’s Silent Spring brought attention to and began to document the ecological effects of these large-scale biocide applications. The message is that what appears to be a breakthrough discovery at one point in time may result in major ecological consequences at a later point in time.
Concept of Energy Subsidy:
High rates of primary production in both natural and cultivated ecosystems occur when physical factors (such as water, nutrients, and climate) are favorable, and especially when auxiliary energy from outside the system reduces maintenance costs (enhances disorder dissipation). Any such secondary or auxiliary energy that supplements the Sun and allows plants to store and pass on more photosynthate is termed an auxiliary energy flow or energy subsidy.
Wind and rain in a rain forest, tidal energy in an estuary, and fossil fuel used in the cultivation of crops are examples of energy subsidies; all of these enhance production by plants and also benefit animals adapted to make use of auxiliary energy.
For example, tides do the work of bringing nutrients to marsh grass and food to oysters, as well as taking away waste products, so the organisms do not have to expend energy for these jobs, and can use more of their production for growth (another example of natural capital at work).
High productivity and high net-gross productivity ratios in crops are maintained by large inputs of energy involved in cultivation, irrigation, fertilization, genetic selection, and pest control. The fuel used to power farm machinery is just as much an energy input as sunlight; it can be measured as calories or horsepower diverted to heat in the performance of the work of crop maintenance. In the United States, the energy subsidy input into agriculture increased tenfold between 1900 and the 1980s, from about 1 to 10 calories input per calorie of food harvested.
The relationship between inputs of fossil fuels, fertilizers, pesticides, and work energy needed to produce 1 calorie of food energy is shown in Figure 3-6; the doubling of crop yield requires approximately a tenfold increase in all these inputs. Genetic selection for food-to-fiber ratio is the other way in which crop yields have been increased. The ratio of grain-to-straw dry weight for wheat and rice, for example, has been increased from 50 percent to almost 80 percent during the past century.
H. T. Odum was one of the first ecologists to state the vital relationships among energy input, selection, and agricultural productivity. He wrote the following:
In a real way the energy for potatoes, beef, and plant produce of intense agriculture is coming in large part from the fossil fuels rather than from the Sun. The food we eat is partly made of oil.
High temperatures (and high water stress) generally require a plant to expend more of its gross production energy in respiration. Thus, it costs more to maintain the plant structure in hot climates, although C4 plants have evolved a photosynthesis cycle that partly circumvents this restraint imposed by hot and dry climates. The general relationship between gross and net production of natural vegetation as a function of latitude was shown in Figure 3-4. These ratios apply to C3 crops such as rice as well.
Natural communities that benefit from natural energy subsidies are those with the highest gross productivity. The role of tides in coastal estuaries and marshes benefiting from an optimal tidal or other water flow subsidy has about the same gross productivity as an intensively farmed Iowa cornfield.
As a general principle, the gross productivity of cultivated ecosystems does not exceed that found in nature. We do, of course, increase productivity by supplying water and nutrients in areas where those are limiting (such as deserts and grasslands). Most of all, however, we increase net primary and net community production through energy subsidies that reduce both autotrophic and heterotrophic consumption and thereby increase the harvest.
There is one other important point to be made about the general concept of energy subsidy. A factor under one set of environmental conditions or at a low level of intensity may act as an energy subsidy but under other environmental conditions or at a higher level of input can act as an energy drain that reduces productivity. For example, flowing water systems, such as those in Silver Springs, Florida, tend to be more fertile than standing water systems, but not if the flow is too abrasive or irregular.
The gentle ebb and flow of tides in a salt marsh, a mangrove estuary, or a coral reef contributes tremendously to the high productivity of these communities, but strong tides crashing against a northern rocky shore subjected to ice in winter and heat in summer can be a tremendous drain. Swamps and riverine forests subjected to regular flooding during the winter and early spring dormant period have a much higher production rate than those flooded continually or for long periods in the growing season.
In agriculture, tilling the soil helps in a Temperate Deciduous biome, but not in the Tropics, where the resulting rapid leaching of nutrients and loss of organic matter can severely stress subsequent crops. Finally, certain types of pollution, such as treated sewage, can act as a subsidy or as a stress depending on the rate and periodicity of their input. Treated sewage released into an ecosystem at a steady but moderate rate can increase productivity, but massive, irregular dumping can almost completely destroy the ecosystem as a biological entity.
The Subsidy-Stress Gradient:
A factor that under one set of environmental conditions or input level acts as a subsidy can under another set of environmental conditions or at a higher input level act as an energy drain or stress that reduces productivity. Too much of a good thing (too much fertilizer, too many cars) may be as serious a stress as too little, as humans often belatedly come to realize. The concept of a subsidy-stress gradient is illustrated in Figure 3-7.
If the input or perturbation (from perturbare = “to disturb”) is poisonous, the response will be negative at any input level. If, however, the input involves usable energy or materials, productivity or other measures of performance may be enhanced. As the level of subsidy input increases, the ability of the system to assimilate it can reach saturation; performance will then decline, as shown in the model.
For example, a small amount of nitrogen fertiliser applied to a lawn will increase growth and improve the health of the lawn; too much nitrogen fertilizer will metabolically “burn up” the lawn or kill the grass. As the subsidy begins to turn into stress, the variance increases, as shown by the error bars in Figure 3-7, and the system begins to oscillate out of control until replaced by another system more tolerant of the perturbation or until viable life is no longer possible.
In summary, just about everything that civilization does have a mixed effect on the natural environment and on the quality of human life. Humans can enrich as well as degrade the environment. Very frequently, this is a matter of temporal and spatial scale. We often enhance ecosystem response or quality at low levels of input but degrade both function and quality at high levels of input. A little bit of heat, CO2, or phosphate may increase the productivity of a body of water if these inputs are limiting under natural conditions, but large amounts of these same inputs may depress basic functions, adversely affecting particular species and reducing water quality for human use.
Humans rarely recognize when increasing returns of scale (which most economists like to talk about) turn to decreasing returns of scale (which most economists do not like to talk about). In other words, most humans have difficulty determining when enough is enough.
A corollary to energy subsidy is the concept of source-sink energetics, in which excess organic production by one ecosystem (a source) is exported to another, less productive ecosystem (a sink). For example, a productive estuary may export organic matter or organisms to less productive coastal waters in a process termed outwelling. Accordingly, the productivity of an ecosystem is determined by the rate of production within it, plus that received one import, or minus that exported from a source system.
At the species level, one population may produce more offspring than are needed to maintain it, with the surplus moving to an adjacent population that otherwise would not be self-sustaining (Pulliam 1988). Also, at the population level, the metapopulation concept is based on the observation that the survival of a species in a small landscape patch (semi-isolated from other similar habitats) may depend more on the immigration and emigration of individuals into and out of the patch than on births and deaths within the patch.
Chlorophyll and Primary Production:
Gessner (1949) observed that the amount of chlorophyll “per square meter” tends to be similar in diverse communities. This finding indicates that the content of the green pigment in whole communities is more uniform than in individual plants or plant parts. The whole is not only different from the parts, but it cannot be explained by them alone. Intact communities containing various plants—young and old, sunlit and shaded—are integrated and adjusted, as fully as local factors allow, to the incoming solar energy, which, of course, impinges on the ecosystem on a “square- meter” basis.
Shade-adapted plants or plant parts tend to have a higher concentration of chlorophyll than light-adapted plants or plant parts; this property enables them to trap and convert as many scarce light photons as possible. Consequently, the use of sunlight is highly efficient in shaded systems, but the photosynthetic yield and the assimilation ratio are low.
Algal cultures grown in weak light in the laboratory often become shade-adapted. The high efficiency of such shaded systems has sometimes been mistakenly projected to full-sunlight conditions by those who would feed humankind from mass cultures of algae. However, when the light input is increased to increase yield, the efficiency goes down, as it does in any other kind of plant.
Productivity and Biodiversity:
In low-nutrient natural environments, an increase in biodiversity seems to enhance productivity, as indicated by experimental research in grasslands. However, in high-nutrient or enriched environments, an increase in productivity increases dominance and reduces biodiversity. In other words, a biodiversity increase may increase productivity, but a productivity increase almost always decreases biodiversity a two-way street.
Furthermore, nutrient enrichment (for example, eutrophication, nitrogen fertilization, and run-off) brings on noxious weeds, exotic pests, and dangerous disease organisms, because these kinds of organisms are adapted to and thrive in high-nutrient environments.
When coral reefs are subjected to human-induced nutrient enrichment, we observe an increase in the dominance of smothering, filamentous algae and the appearance of previously unknown diseases, either of which can quickly destroy these diverse ecosystems that are so beautifully adapted to low-nutrient waters. Another example is the red tide that results in periodic massive fish death in Florida estuaries.
The red tide microorganism, a dinoflagellate, produces a toxin, presumably as a self-defense against being eaten. At its normal density, not enough toxin is being produced to adversely affect fish, but when estuaries become polluted, the dinoflagellate population sometimes “blooms” (sudden large increase in abundance) resulting in mass dying of fish.
It may be stretching this principle too far, but we can suggest that humans, in their efforts to increase productivity to support increasing numbers of people and domestic animals (which in turn excrete huge amounts of nutrients into the environment) are causing a worldwide eutrophication that is the greatest threat to ecosphere diversity, resilience, and stability—essentially a “too much of a good thing” syndrome.
Global warming, which results from CO2 enrichment of the atmosphere, is one aspect of this overall perturbation, whereas nitrogen enrichment is increasingly responsible for worldwide disorder in both aquatic and terrestrial environments. We have here a dilemma or paradox, in which our efforts to feed and produce market goods and services for ever-increasing numbers of people is becoming a major threat to the diversity and quality of our environment.
Measurement of Primary Production:
Primary production is best estimated by measuring gaseous exchange—oxygen production or carbon dioxide uptake. This is most easily done in water. In standing water (ponds, lakes, oceans), measuring the diurnal changes in oxygen concentration, can be used to estimate both gross and net production. In flowing waters, the upstream-downstream method, involving diurnal measurement of oxygen change at points upstream and downstream, is often effective. Measuring changes in CO2 with the radioactive isotope 14C is widely used, especially in marine environments.
Measuring gaseous exchanges is much more difficult on land. Measuring the gradient in CO2 concentration of the air from the ground to the top of the vegetation, as was first tried by Transeau (1926) in a cornfield and by Woodwell and Whittaker (1968) in a forest, is widely used today, especially with crops. It is very difficult to measure gross production in large-biomass ecosystems such as forests because it is impractical to try to put the whole forest in a transparent bag or tent (which would have to be cooled, as the air in the bag would heat up rapidly!), although this has been tried on individual trees or limbs.
Accordingly, most productivity measurements on land vegetation are estimates of net production obtained by summing annual leaf, trunk, and root growth.
For a class exercise, collecting leaf fall with large boxes placed on the forest floor provides a simple method of measuring species diversity and estimating productivity if one takes into consideration latitudinal variations in:
(1) The ratio of leaves to wood production; and
(2) The ratio of gross to net production. For the North Temperate Zone, the ratio of annual leaf fall to net production is about 1 to 4 g/m2. Very close estimates of primary production on large landscapes, regions, and globally are obtained by combining remote sensing of the color of the landscape by satellite with “ground truth” measurements.
Thus, a bright green terrestrial landscape or dark green water indicates very productive ecosystems. On land, yellow-green indicates moderate levels and brown very low levels of productivity.
Clear blue water indicates low productivity. Quantitative values are obtained by matching color with local quantitative measurements on the surface. Aerial or satellite infrared photography is also often effective. The brighter the infrared, the more productive the landscape. Again, such remote sensing must be calibrated by actual quantitative measurement on the ground.