Food chains and energy flow

Classification of food chains

Depending on which link serves as the starting point, ecology distinguishes two main types of food chains.

Grazing chains (predator chains) Their distinctive feature is that they begin with living plant organisms. The structure of such chains has a clear gradation by trophic levels:

• Level I (Producers): grass, trees, phytoplankton.
• Level II (Primary consumers): herbivorous animals, for example, a cow, a hare, or small zooplankton.
• Level III (Secondary consumers): carnivorous or omnivorous creatures — a fox, a whale, a human.

Detrital chains (decomposition chains) These sequences do not start with living greenery, but with dead organic substrate — detritus (Lat. detritus). This category includes animal carcasses, feces, and fallen leaves. A classic example of a detrital chain is as follows: fallen leaves are eaten by earthworms, the worms become food for small birds, and those, in turn, serve as food for birds of prey.

Features of energy flow

The most important task of any ecosystem is to ensure a stable influx of energy, without which growth, reproduction, tissue renewal, and basic vital activity of species are impossible.

The main difference between energy and mineral components lies in its route. While substances constantly participate in cycling, energy moves unidirectionally. As it moves up the trophic steps, its available quantity drops rapidly.

Here the famous 10 percent rule comes into play: only a tenth of the accumulated energy is transferred to each subsequent step of the food chain. The huge remainder is simply dissipated. That is why obtaining calories from the initial levels (from producers) is always energetically more advantageous than eating higher-order predators.

Ecological pyramids and E. Odum's example

To visually assess the efficiency of ecosystems, graphic models — ecological pyramids — are used. Their wide lower steps demonstrate that the maximum amount of energy, and therefore the greatest biomass and abundance, are always concentrated at the starting level (in producers).

There are three main types of pyramids:

1. Pyramid of numbers: reflects the population size (number of individuals) at each tier.
2. Pyramid of biomass: shows the total weight of all living matter.
3. Pyramid of energy: records the actual energy flow in calories.

A textbook example of resource losses was described by ecologist E. Odum using a simple chain: Alfalfa → Calves → Boy (12 years old).

• In the pyramid of numbers, the bottom tier is $2 \cdot 10^7$ (20 million) alfalfa plants, which feed a certain number of calves to sustain the life of just one boy.
• In the pyramid of biomass, 8,211 kg of vegetation turns into 1,035 kg of veal to ensure the existence of a child weighing 48 kg.
• In the pyramid of energy, out of $6.3 \cdot 10^9$ cal of absorbed sunlight, $1.49 \cdot 10^7$ cal is stored in alfalfa. Calves get $1.19 \cdot 10^6$ cal, and a microscopic fraction — $8.3 \cdot 10^3$ cal — settles in human tissues.

The practical conclusion is harsh: if the boy ate exclusively veal all year round, he would need 4.5 calves. And to raise them, you need to sow as much as 4 hectares of land with alfalfa.

Ecosystem balance and human impact

The laws of energy transfer dictate strict limitations for the biosphere. A huge total biomass cannot physically accumulate at the final links of food chains — there simply won't be enough resources. Therefore, the consumption of mass and energy must be impeccably balanced. If the balance collapses, an explosive growth in the population of a single species begins, which destabilizes the entire system.

Today, the main factor in the destabilization of natural ecosystems is anthropogenic impact. Humans extract colossal amounts of biomass and energy from nature to support the needs of civilization and feed livestock, grossly disrupting the natural energy equilibrium.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Ecosystem productivity: methods for assessing the rate of organic matter creation.
• Population metrics: how density and population size affect trophic relationships.
• Biomass assessment: the combined mass of organisms as a key ecological parameter.
• Species diversity: the role of the ratio of various species in maintaining food web stability.
• Anthropogenic crises: the consequences of excessive biomass extraction from natural ecosystems.

Other topics in the "General Ecology" section

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