Difference between a food chain and a food web. Food webs and chains: examples, differences

Trophic structure of biocenoses

ECOLOGY OF COMMUNITIES (SYNECOLOGY)

Populations of different species in natural conditions are combined into systems of a higher rank - communities And biocenosis.

The term “biocenosis” was proposed by the German zoologist K. Mobius and denotes an organized group of populations of plants, animals and microorganisms adapted to living together within a certain volume of space.

Any biocenosis occupies a certain area of ​​the abiotic environment. Biotope– a space with more or less homogeneous conditions, inhabited by one or another community of organisms.

The sizes of biocenotic groups of organisms are extremely diverse - from communities on a tree trunk or on a swamp moss hummock to the biocenosis of the feather grass steppe. A biocenosis (community) is not just the sum of the species that form it, but also the totality of interactions between them. Community ecology (synecology) is also a scientific approach in ecology, according to which, first of all, the complex of relationships and dominant relationships in the biocenosis are studied. Synecology deals primarily with biotic environmental factors of the environment.

Within the biocenosis there are phytocenosis– stable community of plant organisms, zoocenosis- a collection of interconnected animal species and microbiocenosis – community of microorganisms:

PHYTOCENOSIS + ZOOCENOSIS + MICROBIOCENOSIS = BIOCENOSIS.

At the same time, neither phytocenosis, nor zoocenosis, nor microbiocenosis occur in nature in their pure form, nor does biocenosis in isolation from the biotope.

The biocenosis is formed by interspecific connections that provide the structure of the biocenosis - the number of individuals, their distribution in space, species composition, etc., as well as the structure of the food web, productivity and biomass. To assess the role of an individual species in the species structure of the biocenosis, the abundance of the species is used - an indicator equal to the number of individuals per unit area or volume of occupied space.

The most important type of relationship between organisms in a biocenosis, which actually form its structure, is the food connections between predator and prey: some are the eaters, others are the eaten. Moreover, all organisms, living and dead, are food for other organisms: a hare eats grass, a fox and a wolf hunt hares, birds of prey (hawks, eagles, etc.) are able to drag away and eat both a fox cub and a wolf cub. Dead plants, hares, foxes, wolves, birds become food for detritivores (decomposers or otherwise destructors).

A food chain is a sequence of organisms in which each organism eats or decomposes another. It represents the path of a unidirectional flow of a small part of the highly effective solar energy absorbed during photosynthesis moving through living organisms and reaching the Earth. Ultimately, this chain is returned to the natural environment in the form of low-efficiency thermal energy. Nutrients also move along it from producers to consumers and then to decomposers, and then back to producers.

Each link in the food chain is called trophic level. The first trophic level is occupied by autotrophs, otherwise called primary producers. Organisms of the second trophic level are called primary consumers, the third - secondary consumers, etc. There are usually four or five trophic levels and rarely more than six (Fig. 5.1).

There are two main types of food chains – grazing (or “grazing”) and detritus (or “decomposing”).

Rice. 5.1. Food chains of biocenosis according to N. F. Reimers: generalized (A) and real (b). The arrows show the direction of energy movement, and the numbers show the relative amount of energy coming to the trophic level

IN pastoral food chains The first trophic level is occupied by green plants, the second by grazing animals (the term “grazing” covers all organisms that feed on plants), and the third by carnivores. Thus, pasture food chains are:

Detrital food chain starts with detritus according to the scheme:

DETRITE → DETRITIFOGER → PREDATOR

Typical detrital food chains are:

The concept of food chains allows us to further trace the cycle of chemical elements in nature, although simple food chains like those depicted earlier, where each organism is represented as feeding on only one type of organism, are rarely found in nature. Real food connections are much more complex, because an animal can feed on organisms of different types that are part of the same food chain or in different chains, which is especially typical for predators (consumers) of higher trophic levels. The connection between the grazing and detrital food chains is illustrated by the energy flow model proposed by Yu. Odum (Fig. 5.2).

Omnivores (humans in particular) feed on both consumers and producers. Thus, in nature, food chains are intertwined and form food (trophic) networks.

Representatives of different trophic levels are interconnected by one-way directed transfer of biomass into food chains. With each transition to the next trophic level, part of the available energy is not perceived, part is given off as heat, and part is spent on respiration. In this case, the total energy decreases several times each time. The consequence of this is the limited length of food chains. The shorter the food chain, or the closer the organism is to the beginning of it, the greater the amount of available energy in it.

Carnivore food chains go from producers to herbivores, which are eaten by small carnivores, which serve as food for larger carnivores, and so on. As they move up the predator chain, animals increase in size and decrease in number. The lengthening of the chain occurs due to the participation of predators in it. The relatively simple and short food chain of predators includes second-order consumers:

Grass (producer) -» Rabbits (consumer I order) ->

-» Fox (consumer II order).

A longer and more complex chain includes fifth-order consumers:

Pine -> Aphids -> Ladybugs -> Spiders ->

-» Insectivorous birds -> Birds of prey.

Grass -» Herbivorous mammals -> Fleas -> Flagellates.

In detrital chains, consumers are detritivores belonging to various systematic groups: small animals, mainly invertebrates, that live in the soil and feed on fallen leaves, or bacteria and fungi that decompose organic matter. In most cases, the activity of both groups of detritivores is characterized by strict coordination: animals create conditions for the work of microorganisms, dividing animal corpses and dead plants into small parts.

Detritus chains are also distinguished from pasture chains by the fact that a large number of detritivorous animals form a kind of community, the members of which are connected to each other by various trophic connections (Fig. 10.4).

Rice. 10.4.

In this case, we can talk about the existence of food webs of detritivores, separated from linear chains of predators. In addition, many detritivores are characterized by a wide range of nutrition and can, depending on the circumstances, use algae, small animals, etc., along with detritus.

Rice. 10.5. The most important connections in food webs: A - American prairie; b- ecosystems of the northern seas for herring

Food chains starting from green plants and from dead organic matter are most often present together in ecosystems, but almost always one of them dominates the other. However, in some specific environments (for example, abyssal and underground), where the existence of organisms with chlorophyll is impossible due to the lack of light, only detrital-type food chains are preserved.

Food chains are not isolated from one another, but are closely intertwined. They make up the so-called food webs. The principle of their formation is as follows. Each producer has not one, but several consumers. In turn, consumers, among whom polyphages predominate, use not one, but several food sources. To illustrate, we give examples of a relatively simple one (Fig. 10. 5a) and complex (Fig. 10.55) food webs.

In a complex natural community, those organisms that obtain food from plants occupying the first trophic level through the same number of stages are considered to belong to the same trophic level. Thus, herbivores occupy the second trophic level (the level of primary consumers), predators that eat herbivores occupy the third (the level of secondary consumers), and secondary predators occupy the fourth (the level of tertiary consumers). It must be emphasized that trophic classification divides into groups not the species themselves, but the types of their life activity. A population of one species can occupy one or more trophic levels, depending on what energy sources the species uses. Likewise, any trophic level is represented not by one, but by several species, resulting in food chains that are intricately intertwined.

Species in a biocenosis are interconnected by metabolic and energy processes, i.e., by food relationships. By tracing the food relationships between members of the biocenosis (“who eats whom and how much”), it is possible to construct food chains and networks.

Trophic chains (from the Greek trophe - food) - food chains are the sequential transfer of matter and energy. For example, the food chain of animals in the Arctic sea: microalgae (phytoplankton) → small herbivorous crustaceans (zooplankton) → carnivorous plankton-phages (worms, mollusks, crustaceans) → fish (2-4 links in the sequence of predatory fish are possible) → seals → polar bears. This food chain is long; the food chains of terrestrial ecosystems are shorter because there is more energy loss on land. There are several types terrestrial food chains .

1. Pasture food chains (exploiter chains) begin with producers. When moving from one trophic level to another, the size of individuals increases with a simultaneous decrease in population density, reproduction rate and mass productivity.

Grass → voles → fox

Grass → insects → frog → heron → kite

Apple tree → scale insect → parasite

Cow → horsefly → bacteria → phages

Detrital chains. Only decomposers are included.

Fallen leaves → molds → bacteria

Any member of any food chain is simultaneously a link in another food chain: he consumes and is consumed by several species of other organisms. This is how they are formed food webs. For example, the meadow wolf-coyote's food includes up to 14 thousand species of animals and plants. In the sequence of transfer of substances and energy from one group of organisms to another, there are trophic levels. Typically, chains do not exceed 5–7 levels. The first trophic level consists of producers, since only they can feed on solar energy. At all other levels - herbivores (phytophages), primary predators, secondary predators, etc. - the initially accumulated energy is consumed to maintain metabolic processes.

It is convenient to represent food relationships in the form trophic pyramids(number, biomass, energy). The population pyramid is a display of the number of individuals at each trophic level in units (pieces).

It has a very wide base and a sharp narrowing towards the terminal consumers. This is a common type of pyramid for herbaceous communities - meadow and steppe biocenoses. If we consider the forest community, the picture may be distorted: thousands of phytophages can feed on one tree, or aphids and elephants (different phytophages) may be at the same trophic level. Then the number of consumers may be greater than the number of producers. To overcome possible distortions, a pyramid of biomass is used. It is expressed in units of dry or wet weight tonnage: kg, t, etc.

In terrestrial ecosystems, plant biomass is always greater than animal biomass. The biomass pyramid looks different for aquatic, especially marine ecosystems. The biomass of animals is much greater than the biomass of plants. This incorrectness is due to the fact that the biomass pyramids do not take into account the duration of existence of generations of individuals at different trophic levels and the rate of formation and consumption of biomass. The main producer of marine ecosystems is phytoplankton. In a year, up to 50 generations of phytoplankton can change in the ocean. During the time until predatory fish (and especially whales) accumulate their biomass, many generations of phytoplankton will change and its total biomass will be much greater. Therefore, a universal way of expressing the trophic structure of ecosystems is productivity pyramids; they are usually called energy pyramids, meaning the energy expression of products.

Absorbed solar energy is converted into the energy of chemical bonds of carbohydrates and other organic substances. Some substances are oxidized during plant respiration and release energy. This energy is ultimately dissipated as heat. The remaining energy causes an increase in biomass. The total biomass of a stable ecosystem is relatively constant. Thus, during the transition from one trophic level to another, part of the available energy is not perceived, part is given off in the form of heat, and part is spent on respiration. On average, when moving from one trophic level to another, the total energy decreases by about 10 times. This pattern is called Lindemann energy pyramid rule (1942) orthe 10% rule. The longer the food chain, the less energy is available at the end of the chain, so the number of trophic levels can never be too large.

If the energy and bulk of organic matter decreases during the transition to the next stage of the ecological pyramid, then the accumulation of substances entering the body that do not participate in normal metabolism (synthetic poisons) increases in approximately the same proportion. This phenomenon is called the rule of biological enhancement.

Basic principles of functioning of ecological systems

Constant influx of solar energy- a necessary condition for the existence of an ecosystem.

Nutrient cycle. The driving forces of the cycle of substances are the flow of energy from the sun and the activity of living matter. Thanks to the cycle of nutrients, a stable organization of all ecosystems and the biosphere as a whole is created, and their normal functioning is carried out.

Decrease in biomass at higher trophic levels: A decrease in the amount of available energy is usually accompanied by a decrease in biomass and the number of individuals at each trophic level (remember the pyramids of energy, abundance and biomass).

We have already covered these principles in detail during the lecture.

This is a set of food chains of a community, interconnected by common food links.

cabbage ^ caterpillar ^ tit ^ hawk ^ man

For example: carrot ^ hare ^ wolf
Species with a wide range of nutrition can be included in food chains at different trophic levels. Only producers always occupy the first trophic level. Using solar energy and nutrients, they form organic matter, which contains energy in the form of the energy of chemical bonds. This organic matter, or biomass of producers, is consumed by organisms of the second trophic level. However, not all the biomass of the previous level is eaten by organisms of the next level, because
that resources for the development of the ecosystem would disappear. During the transition from one trophic level to another, a transformation of matter and energy occurs. At each trophic level of a pasture food chain, not all of the consumed biomass is used to form the biomass of organisms at that level. A significant part of it is spent on ensuring the vital functions of organisms: breathing, movement, reproduction, maintaining body temperature, etc. In addition, not all biomass eaten is digested. The undigested part of it ends up in the environment in the form of excrement. The percentage of digestibility depends on the composition of the food and the biological characteristics of the organisms; it ranges from 12 to 75%. The bulk of the assimilated biomass is spent on maintaining the vital functions of organisms, and only a relatively small part of it goes to building the body and growth. In other words, most of the matter and energy during the transition from one trophic level to another is lost, because only that part of it that was included in the biomass of the previous trophic level reaches the subsequent consumer. It has been estimated that on average about 90% is lost, and only 10% of matter and energy is transferred at each stage of the food chain. For example:
Producers ^ consumers I ^ consumers II ^ consumers III
1000 kJ ^ 100 kJ ^ 10 kJ ^ 1 kJ This pattern was formulated as the “law of 10%”. It states that during the transition from one link to another in the pasture food chain, only 10% of the matter and energy is transferred, and the rest is spent by the previous trophic level to maintain life. If the amount of matter or energy at each trophic level is diagrammed and placed one above the other, an ecological pyramid of biomass or energy is obtained (Fig. 13). This pattern is called the “rule of the ecological pyramid.” The number of organisms at trophic levels also obeys this rule, so it is possible to build an ecological pyramid of numbers (Fig. 13).
Male 1 Calves 4.5 Lucerne 2107

Pyramid of Energy

Thus, the supply of matter and energy accumulated by plants in pasture food chains is quickly consumed (eaten away), so food chains cannot be long. Usually they include 4-5 links, but no more than 10. At each trophic level of the pasture food chain, dead organic matter and excrement are formed - detritus, from which detritus chains, or chains of decomposition, begin. In terrestrial ecosystems, the process of detritus decomposition includes three stages:
The stage of mechanical destruction and partial conversion into saccharides. It is very short - 3-4 years. It is carried out by first order decomposers - macrobiota (worms, insect larvae, burrowing mammals, etc.). At this stage, there is practically no energy loss.
The stage of destruction of detritus to humic acids. It lasts 10-15 years and is still poorly studied. It is carried out by second order decomposers - mesobiota (fungi, protozoa, micro-
organisms larger than 0.1 mm). Humic acids are humus, half-destroyed organic matter, therefore, when they are formed, some chemical bonds are broken and thermal energy is released, which is dissipated in outer space.
3. The stage of destruction of humic acids to inorganic matter - biogens. It proceeds very slowly, especially in our temperate zone (hundreds and thousands of years) and has not yet been studied. It is carried out by third-order decomposers - microbiota (microorganisms less than 0.1 mm). When humic acids are destroyed, all chemical bonds are broken and a large amount of thermal energy is released, which is lost in outer space. The biogens formed as a result of this process do not contain energy; they are subsequently absorbed by producers and are again involved in the cycle of matter.
As can be seen from the above, at the level of decomposers there is a delay in life, but this should not be the case. The soil contains a reserve of humic acids that were formed a long time ago, so life is not delayed. In different ecosystems, the rate of destruction of humic acids is different. If it is less than the rate of their formation, then the fertility of the soil increases, but if on the contrary, then it decreases. That is why in the temperate zone, after the destruction of the biogeocenosis, long-term use of soil fertility is possible. In the tropics, soil fertility is sufficient for 2-3 years, and then it turns into a desert. Here, the destruction of humic acids occurs quickly. This is facilitated by high temperature, humidity and aeration. In the temperate zone, the soil contains up to 55% carbon, and in the tropics - only up to 25%. This is why tropical forests should not be cut down to prevent desertification of the planet.
Thus, the flow of energy entering the ecosystem is further divided into two main channels - pasture and detritus. At the end of each of them, energy is lost irretrievably, because plants cannot use long-wave thermal energy during photosynthesis.
The ratio of the amount of energy passing through pasture and detritus chains is different in different types of ecosystems. The loss of energy in food chains can only be replenished by the intake of new portions. This is achieved through the assimilation of solar energy by plants. Therefore, there cannot be an energy cycle in an ecosystem, similar to the cycle of matter. The ecosystem functions only due to the directed flow of energy - its constant supply in the form of solar radiation, or in the form of finished organic matter.

Any living organism chooses the conditions that are most favorable for its habitat and provide it with the opportunity to eat fully. The fox chooses a place to live where many hares live. The lion settles closer to the herds of antelope. The sticky fish not only travels attached to the shark, but also eats with it.

Plants, although they are deprived of the ability to consciously choose their habitat, mostly grow in the places that are most comfortable for themselves. Gray alder is often accompanied by nettles, which require nitrogen nutrition. The fact is that alder cohabits with bacteria that enrich the soil with nitrogen.

The food web is a kind of symbiosis

Here we are faced with a certain type of relationship. We are talking about the so-called symbiosis. It is a direct relationship in which both organisms benefit. They are also called food webs and chains. Both terms have similar meanings.

How are food chains and food webs different from each other? Separate groups of organisms (fungi, plants, bacteria, animals) constantly exchange certain substances and energy with each other. This process is called the food chain. Exchange between groups occurs when some eat others. The process of interaction between such chains is called a food web.

How organisms are interconnected

It is known that leguminous plants (clover, mouse peas, caraganas) coexist with nodule bacteria, which convert nitrogen into forms that are absorbed by plants. In turn, bacteria receive the organic substances they need from plants.

Many of the relationships described are of a specific nature. However, in every biocenosis there are relationships in which each population takes part. These are nutritional or trophic (trophos - food) relationships.

Examples of food webs and chains:

In all cases, the organism that feeds on others derives a one-sided benefit. By participating in the feeding process, all individuals of the population provide themselves with the energy and various substances necessary for their life. The population that serves as food is negatively affected by the predators that devour it.

Autotrophs and heterotrophs

Let us remember that according to the way they feed, organisms are divided into two groups.

Autotrophic (autos - itself) organisms live off an inorganic source of hydrocarbons. This group includes plants.

Heterotrophic (heteros - other) organisms live off an organic source of hydrocarbons. This group includes fungi and bacteria. If autotrophs are independent of other organisms as a source of carbon and energy, then heterotrophs in this regard are completely dependent on plants.

Competitive relations between groups

Relationships that lead to oppression of one of the partners are not necessarily related to nutritional relationships. Many weeds produce metabolites that inhibit plant growth. Dandelion, wheatgrass, and cornflower have a depressing effect on oats, rye and other cultivated grains.

In each biocenosis, populations of many species live, and the relationships between them are diverse. We can say that the population is limited in its capabilities by these relationships and must find a place that is unique to it.

The level of provision of a habitat with environmental resources determines the possibility of the existence of many niches. The number of species populations forming the biocenosis also depends on this. In the favorable climate of the steppes, biocenoses are formed consisting of hundreds of species, and in the tropical climate of forests - of thousands of species of organisms. Desert biocenoses in hot climates number several dozen species.

The spatial distribution of populations is equally variable. Tropical forests are multi-tiered, and living organisms fill the entire volume of space. In deserts, biocenoses are simple in structure, and populations are small. Thus, it is clear that the joint life of organisms in biocenoses is unusually complex. And yet, plants and animals, fungi and bacteria are united in biocenoses and exist only in their composition. What are the reasons for this?

The most important of them is the need of living organisms for nutrition and trophic dependence on each other.