Food web energetics and productivity of natural ecosystems

Energy flow in natural ecosystems

In a functioning natural ecosystem, there is no such thing as waste. All organisms, whether living or dead, are potentially food for other organisms. Examples of trophic interactions:

• a caterpillar eats foliage;
• a thrush feeds on caterpillars;
• a hawk can eat the thrush;
• when plants, the caterpillar, the thrush, and the hawk die, they are in turn processed by decomposers.

Organisms in natural ecosystems are involved in a complex network of many interconnected food chains, called a food web.

With each transfer from one trophic level to another within a food chain or web, work is performed, thermal energy is released into the environment, and the amount of high-quality energy used by organisms at the next trophic level decreases. Such a reduction in usable high-quality energy at each subsequent trophic level is a consequence of the second law of thermodynamics.

Thus, ecosystems exist thanks to the non-polluting and virtually eternal solar energy, the quantity and quality of which are relatively constant and abundant. Let us call this the first basic principle of ecosystem functioning.

The percentage of high-quality energy transferred from one trophic level to another ranges from 2 to 30%, depending on the types of living organisms involved and the ecosystem in which the energy transformation takes place. Ecologists have calculated that in the wild, on average, about 10% of the available high-quality chemical energy of one trophic level is transformed into available chemical energy in the organisms of the next level. The remaining energy is used to maintain the life activities of the organisms, but most of it is lost to the environment as low-quality thermal energy.

Indicators of biocenosis productivity

In the process of a biocenosis's vital activity, organic matter is created and consumed. This means that every ecosystem possesses a certain level of productivity. Productivity is assessed as the rate of matter (biomass) formation, for example, g/day, t/year.

Primary productivity of a system is defined as the biomass produced by producers per unit area per unit time. However, during the vital activity of plants, part of the created matter is used for respiration, and therefore, less biomass accumulates per unit area per unit time than was produced. Net productivity equals the rate at which plants produce chemical energy during photosynthesis, minus the rate at which plants use a portion of this energy for aerobic cellular respiration, which is necessary for their life, growth, and reproduction.

Consumers also create organic matter based on net primary productivity, but they cannot synthesize organic substances from inorganic ones themselves. The productivity of consumers is called secondary.

As stated above, secondary productivity is exceptionally low: 90–99% of energy is lost during transfer from each previous link of the trophic chain to the next.

Calculations show that 70–90 kg of fresh grass is required to produce 1 kg of beef.

Examining energy flow in ecosystems, it is easy to understand why biomass decreases as the trophic level increases. Firstly, any population of living organisms can be viewed as biomass (the total mass of living organisms), which increases each year due to the growth and reproduction of organisms while simultaneously decreasing due to their natural death and consumption by consumers. If it remains at a constant level, as is the case in a stable ecosystem, it means that primary consumers, for example, eat no more per year than what producers create. If they were to eat more (for instance, due to overgrazing), the producer population would eventually disappear. Secondly, a significant portion of the biomass consumed by consumers is not assimilated by them and returns to the ecosystem in the form of excrement. If we also consider that most of the digested food is spent on energy production, it becomes clear why the total biomass of producers is many times greater than that of herbivorous animals. The same is observed when moving to higher trophic levels. This is the second basic principle of ecosystem functioning: the greater the population biomass, the lower the trophic level it must occupy.

The productivity of ecological systems and the ratio of various trophic levels within them are expressed in the form of pyramids. The length of the rectangle is proportional to the energy flow of each level. The pyramids clearly illustrate the ratios of biomasses and their equivalent energies in each link of the food chain and are used in practical calculations to justify, for example, the areas required for crops in order to provide feed for livestock, and consequently, to meet the population's need for animal protein.

Annual primary productivity of terrestrial ecological systems is characterized by the data in Table 2.1.

of global ecosystems (according to R.H. Whittaker, 1980)

Ecosystem types Area, Net primary Total net g/m² per year, ·10⁹ t/year

1 2 3 4 5 Tropical rainforest

17 1000 –3500 2200 37.4 Temperate evergreen forests

5.0 600 –2500 1300 6.5 Boreal forests

12.0 400 –2000 800 9.6 (taiga) Shrublands

8.5 250 –1200 700 6.0 Savannas 15.0 200 -2000 900 13.5 Temperate grasslands

9.0 200 -1500 600 5.4 Tundra and alpine 8.0 10 - 400 140 1.1 Deserts and

18.0 10 - 250 90 1.6 semideserts Extreme deserts, rocks, sands, etc. 24.0 0 - 10 3 0.07 Cultivated

14.0 100 -3500 650 9.1 lands Swamps and marshes 2.0 800 -3500 2000 4.0 Lakes and streams 2.0 100 -1500 250 0.5 Continental

149 0 - 3500 773 115 ecosystems total: Open ocean 332.0 2 - 400 125 41.5 Upwelling zones 0.4 400 -1000 500 0.2

1 2 3 4 5 Continental

26.6 200 -600 360 9.6 shelf Algal beds and

0.6 500 -4000 2500 1.6 reefs Estuaries

1.4 200 -3500 1500 2.1 Marine eco-

361.0 2 - 4000 152 55 systems total Average and total 510.0 0 - 4000 333 170

The table data shows that currently the largest volume of primary production is created by forest and steppe ecological systems.

Plants 2.4∗10¹² 99.2

Animals and microorganisms 0.02∗10¹² 0.8

Plants 0.0002∗10¹² 6.3

Animals and microorganisms 0.003∗10¹² 93.7

Human life and their production activities depend on the productivity of the main biogeocenoses, on primary production and its distribution. Human nutrition is provided mainly by crops, which occupy about 10% of the land area and yield about 9 billion tons of organic matter per year, which constitutes a significant part of global resources. While there is relatively reliable data on primary productivity, there is little for other trophic levels. Determining the secondary productivity of ecosystems involves great difficulties, and only indirect data, such as biomass at various levels, are known. Data on the distribution of the total biomass of organisms on Earth are given in Table 2.2.

As can be seen, the biomass of ocean organisms is negligible compared to the biomass of terrestrial animals, plants, and microorganisms.