The Flow of Energy in Ecosystems - Productivity, Food Chain, and Trophic Level

As was explained in the last subchapter, the flow of energy is an essential feature of every ecosystem, since all living systems are open systems. They depend on a steady supply of energy in order to keep up the structural organization and all life-preserving functions. According to the second law of thermodynamics, each system strives for the state of highest entropy. The inversion, i.e. the coming into being of systems that are poor in entropy or are, in other words, rather well organized, requires a steady supply of energy from the surrounding. Since only plants are able to use light energy, they have a key position in every natural ecosystem.

The amount of biomass decreases drastically from one trophic level to the next one as the flow of energy is a directed process and as an optimal ten percent of the biomass of the previous tropic level can be used in the next higher one. As a consequence, four, at the most five trophic levels exist in nature. The biomass of the sum of all carnivores is always smaller than that of the herbivores, and that of the herbivores again is smaller than that of the plants. The result is a food pyramid, whose shape depends on the productivity and the species composition of the respective ecosystem. The linear succession of the single elements (producers, consumers of the first order, ...) is called the food chain. The conditions in natural ecosystems are nevertheless mostly more complex than that, so that food web is a better way to describe the actual state. Food pyramids are usually based on biomass, not that often also on the number of species or individual organisms.

More than 90 percent of the total biomass of the earth is made up by plants, only a few percent are allotted to all other groups of organisms. A comparison of the number of existing species in contrast shows that about ten times more animal than plant species exist.

Under certain conditions, inverse food pyramids occur. The reason is either a spatial or a time shift in the appearance of the single system elements.

Inverse Pyramids: The number of parasites is larger than the number of hosts. This is due to the fact that the parasites are far smaller than the hosts, so that every host harbours several parasites.

A number of animals live from dead plants or plant remnants. The biomass of living animals is therefore in some winters higher than that of the living plants. The relation is just the other way round during all other seasons. A spatial separation is typical for the deep sea, because deep sea animals live in depths were no photosynthesis and thus no plants can occur anymore. These animals live from a constant 'rain' of dead plants, almost all of them single-celled algae. In order to determine the size of the energy flow in an ecosystem, the input of energy, i.e. the amount and quality of the sunlight have to be determined first.

The Emission Spectrum of the Sun and the Filter Effect of the Biosphere. White curve: sunlight, yellow curve: after filtering by clouds, green curve: as transmitted by the vegetation. The abscissa depicts the wave sphere from 0.1 - 10 µm, a logarithmic scale. The ordinate gives the light intensity (according to D. M. GATES, 1965).

The illustration shows that those pigments that make optimal use of the available wave length of the light were selected in the course of plant evolution. Clouding over acts as a heat shield, it absorbs UV and infrared. Both are of no use for photosynthesis. Visible light is far less absorbed. The vegetation filters visible light. Bright red light does hardly reach the deeper areas, since chlorophyll and phytochrome absorb it. Germinating plants need exactly these wave lengths to control germination and growth meaning that a dense vegetation inhibits the development of new plants.

The average energy of the sun of the whole earth's surface is 2 calories (cal)/ square centimeter (cm²) / minute (min). It is also called the solar constant. Seasonal fluctuations as well as differences in the exposition (southern or northern slope) cause differences in the order of magnitude. The solar energy reaches thus a mean value of 3,000 - 4,000 kilo calories (kcal)/ m² (120,672 kilo joule (KJ)/ square meter (m²) / day) or 1.39 KJ/ m²/ second. A part of this energy is reflected by the earth surface and is thus not available for biosynthetic processes. The remains are called the net radiation that is 1 million kcal/m²/ year over the sea between 40° north and 40° south. The net radiation over land is 0.6 million kcal/ m²/ year.

Evaporation and movements of the air are important factors that cause the largest part of this originally enormous amount of energy to be given off into space. Without this, the earth would rapidly overheat and life would become impossible. On the other hand, the sun energy and the resulting increase in temperature are the main reasons for the existence of climatic zones as well as of seasonal and daily fluctuations.

The yearly production of biomass is an estimated 164 billion tons (R. H. WHITTAKER and G. E. LIKENS, Cornell University, 1975). A third of it is produced in the oceans, two thirds in terrestrial ecosystems.

Biomass and rate of production are two different things. The rate of production or productivity means an amount of energy bound in a certain period of time. Existing biomass alone allows only under certain conditions to determine approximate values of the turnover of energy. Each step in metabolism loses energy as heat. Therefore, it is distinguished between gross and net production. The net production is the amount that remains after the energy that is lost due to respiration has been subtracted.

Among the difficulties of estimating the productivity is the fact that it can often not be distinguished whether a system (in this case a plant) is in a steady state or in a growth phase. A continuous decrease of growth results in a decrease of respiration, too, because the maintenance of structures requires lesser energy than their production.

The determination of the photosynthetic activity that is measured as the amount of emitted oxygen tells little about the productivity as a part of the oxygen is consumed by respiration and another part is required for photorespiration. The activity of photorespiration is directly correlated to the amount of available light. The single factors can be captured separately under controlled lab conditions and with a large experimental effort. From the data captured under these conditions, proportional factors can be determined. The biomass is often ascertained by measuring the amount of carbon. The factors of conversion are:

10 kcal ~ 2g dry substance ~ 1g carbon

The flow of energy is usually given as kcal/ gram (g) dry weight. Natural ecosystems are optimized for a high turnover, while ecosystems influenced by humans, especially agricultural areas, are optimized for an as high as possible rate of net production.

Climax associations, like the tropical rain forest are characterized by a high productivity at an almost constant biomass. An ecosystem that is in a succession is in a very different state. A bog, for example, grows by the continuous deposition dead Sphagnum.

Only about 5 percent of all available sun energy is conserved as chemical energy in the biomass of plants. A theoretically optimal 80 percent of this energy can be used by the organisms of the next higher trophic level. In reality, the amount is usually far smaller. The major part of the biomass of woods, for example, is made up of wood that is extremely unfavourable for animal feeding. Only a few specialists can use it. The relation between gross and net production as well as the flow of energy through the system can be demonstrated especially well with the model system wood.

The available amount of light is usually no limiting factor. This is different with the supply of water. Just think of the meagre plant growth in deserts as compared to the lush vegetation of the tropics. This difference is certainly not due to light as the amount of light in the usually cloudless deserts is far larger than that of the clouded tropics. Irrigated desert areas yield a higher gross production than areas with a lower light intensity. The high losses due to respiration during warm nights lead to a higher energy consumption than in cooler areas. The rate of net production is thus lowered. This explains, why the yield per hectare of rice harvests in equatorial areas is always lower than that in temperate zones.

The net primary production may fluctuate considerably. The lowest value for 'open sea', for example, is two kcal/g, the highest is 400, the lowest net primary production of cultivated areas is 100, the highest is 4000. The values fluctuate roughly fivefold in most other types of vegetation. In order to calculate the energy fixation from the net primary production, a calorific value (kcal/g) is used. It is 4.5 (kcal/g). Grassland has the lowest value (4.0 kcal/g), while the open sea has 4.9 kcal/ g.