Medium Age Oak/Pine Forest
Mature Rain Forest
Gross primary productivity
Net primary productivity
Net community productivity
Source: Odum (1987).
Source: Odum (1987).
to grow in shade usually have much higher chlorophyll concentrations in order to capture more effectively the energy available to them. This results in a lower assimilation ratio. Forests typically have about 0.4 to 3.0 g of chlorophyll per square meter, and an assimilation ratio that varies from about 0.4 to 4.0. Thinly vegetated areas such as new crops in the field, on the other hand, may have only 0.01 to 0.60 g of chlorophyll per square meter but fix from 8 to 40 g of carbon per gram of chlorophyll.
It is important to understand the relationship between the gross and net productivity, which are rates, and the biomass, or standing crop. Keep in mind that the amount of biomass present is not a measure of the gross or net productivity. As an analogy, consider a tank with water being pumped in, and with some flowing out through a hole in the bottom. The inflow is like the gross primary productivity, and the outflow is like respiration. If more is entering the tank than is draining out the hole, the difference accumulates. The rate of accumulation is like the net primary productivity. If the amount entering and leaving are equal, the amount in the tank stays the same and the system is at steady state. This would be equivalent to having zero net productivity and could occur with the tank full or almost empty. Consider a mature tropical rain forest (Table 14.2). The gross primary productivity is of such a forest is about 45,000 kcal/m2- yr. However, most of that is taken up by respiration to maintain the large biomass. Thus, the net primary productivity is essentially zero.
On the other hand, an alfalfa field is adding considerably to its biomass during the growing season. Its gross primary productivity is 24,400 kcal/m2 - yr. Respiration accounts for 9200 kcal/m2- yr, leaving a net primary productivity of 15,200 kcal/m2- yr. However, the actual yield that farmers can obtain from the field will be lower because other hetero-trophs (such as field mice) consume part of the crop. This leads to a third type of productivity, net community productivity (NCP), which is the gross primary productivity minus plant (autotrophic) respiration and minus the amount consumed by other organisms in the ecosystem (heterotrophic respiration). In the case of the alfalfa field, 800 kcal/m2- yr is consumed by heterotrophic respiration. This yields a net community productivity of 14,400 kcal/m2- yr, for an efficiency of 59%.
Primary productivity (GPP) can be limited by numerous factors, such as the amount of sunlight, availability of moisture and nutrients, soil properties, and so on. On land, moisture is the most serious limiting factor. In the open ocean, productivity is limited by nutrient availability and the ability of sunlight to penetrate the water. Ideal conditions can result in productivities that are as much as double the "typical" values (e.g., see Table 15.1); 50,000 kcal/m2-yr is thought to be the practical upper limit for any natural ecosystem. Changes in primary productivity, or comparisons to similar ecosystems, can be used as indicators of the health of an ecosystem. Worldwide, the total GPP is estimated to be about 1018 kcal/yr.
The primary productivity of the world's agriculture has only been keeping pace with increases in human population. Humans greatly increase the productivity of cultivated land by energy subsidies. This energy originates mostly from fossil fuels and takes the form of synthetic nitrogen fertilizer, pesticides, and mechanized agriculture (tilling, harvesting, etc.). Productivity of crops has also been increased by breeding new varieties. The greatly increased food production has resulted in an increase in the carrying capacity of the Earth for humans, and has been called the green revolution. However, it must be noted that the new crops are more dependent on energy subsidies than are traditional cultivars. Although the improved productivity has been important, underdeveloped countries have increased their food production as much by increasing the amount of land under cultivation. This results in destruction of forests and resulting loss in habitat for other species.
Humans have other uses for primary productivity besides as food. Cotton and flax fibers are used for clothing and wood for structures, paper, and fuel. Overharvesting of wood in poor regions of the world has led to deforestation and erosion.
14.1.2 Trophic Levels, and Food Chains and Webs
One of the most important relationships among organisms in an ecosystem is, of course, who eats whom. Ecologists discover these relationships by direct observation as well as by examining the stomach contents of animals captured from the wild. One species of bird may eat seeds, another might only eat certain types of insects, whereas a third may prey mostly on small mammals.
The energy and fixed carbon from the autotrophic producers are available to hetero-trophs in the ecosystem, and these are called the consumers. Consumers that feed directly on the producers are called primary consumers, also called herbivores (plant eaters). Those that feed on herbivores are called secondary consumers, or carnivores (meat eaters). Next may come tertiary consumers, or top carnivores, which feed on carnivores.
This structure describes the food chain, the sequential feeding relationship from primary producers to higher consumers. The feeding levels in the food chain are collectively called trophic levels. The organisms in a particular trophic level have the same number of steps in their food chain between themselves and the primary producers.
The portion of each trophic level that is not used as food by a higher level will eventually die. A parallel food chain is based on this dead material. The organisms that rely on this are called detritivores, decomposers, saprobes, or saprotrophs, and a food chain based on them is called a detritus food chain. (In contrast, a food chain based on living biomass is called a grazing food chain.) The detritus food chain starts with organisms that feed directly on dead material, including bacteria, fungus, earthworms and other soil invertebrates, and some marine crustaceans, such as lobsters and crabs. These may, themselves, be fed upon. In fact, the grazing and detritus food chains are linked at all levels. For example, an owl may eat an insectivorous (insect-eating) shrew, which in turn has fed on saprobic soil invertebrates, and the same owl may also eat an herbivorous mouse (a grazer).
Ultimately, all the dead organic material is mineralized, that is, converted back to inorganic minerals such as CO2, water, ammonia or nitrates, and salts. Along the way, the partially biodegraded biomass forms an important part of the soil and aquatic environments. The decomposers are the ultimate recyclers, making the material available for reuse by the primary producers. By releasing the minerals close to the roots of plants where they can be rapidly taken up again, they reduce the rate at which scarce minerals leach out of the ecosystem.
The energy for each trophic level is obtained from the level below. Some of the energy in a trophic level will be unavailable because it might be indigestible material, such as cellulose, lignin, chitin, or bone, or may be protected by defensive measures. Biomass may be stored, as in the case of peat formation in marshes, or it may be exported from the ecosystem, as in the case of migratory animals or tidal estuaries.
The energy input to a trophic level is the gross energy intake. Part of this is unused and is eliminated in the feces where it enters the detritus food chain. The remainder is the assimilated energy. Part of the assimilated energy is excreted with urine. A large part is used in respiration for maintenance and activity. The remainder is available for growth and reproduction, forming the net productivity of that trophic level. The production efficiency (also called transfer efficiency) of a species or of a trophic level as a whole can be defined as:
Production efficiency =--(14.1)
Typical values for production efficiency are:
Small mammals 1.5%
Other mammals 3.1%
Fish and social insects 9.8%
Other invertebrates 21-56%
Warm-blooded animals seem to pay a penalty in efficiency. The rate of respiration in animals can be related to body mass empirically by a log-log relationship. For example, for herbivorous mammals:
where R is the respiration rate in kJ/day per individual and M is the body mass in grams (1 kJ = 0.239 kcal). Cold-blooded animals may use more than an order of magnitude less energy, but the value will be strongly dependent on ambient temperature.
Although they vary widely, a rule of thumb is that the production efficiency of trophic levels is typically about 10%. Thus, if the gross primary productivity is 60 kcal/m2-day, the net primary productivity would be estimated at 6 kcal/m2 • day, and the herbivores would have a net productivity of about 0.6 kcal/m2 • day. Carnivores typically have a 20% efficiency, so net tertiary productivity would be estimated at 0.12 kcal/m2 • day.
As just described, the trophic structure of an ecosystem is commonly represented as an energy pyramid, which shows the relative proportions of energy or production in the various trophic levels. As Figure 14.1 shows, the pyramid can also be used to compare graphically quantities other than energy, such as population or standing crop. Note that the food chain is rarely longer than four trophic levels. Although this was originally thought to be due to low production efficiency, it now seems that the reason is due to the greater sensitivity of top carnivores to fluctuations in environmental conditions.
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