Food web. Trophic levels
TO Among the most important relationships between organisms are food relationships. You can trace countless paths of movement of matter in an ecosystem, in which one organism is eaten by another, that by a third, etc. A series of such links is called a food chain. Food chains intertwine and form a food (trophic) web.
Food chains are divided into two types. One type of food chain starts with plants and goes to herbivores and then to predators - this is the grazing chain.
Relatively simple and short food chain:
grass → rabbit → fox
(producer) (consumer (consumer)
I order) II order)
Another type starts from plant and animal remains to small animals and microorganisms, and then to predators - this chain of decomposition (detritus).
So, all food chains begin with producers. Without their continuous production of organic matter, the ecosystem would quickly eat itself and cease to exist.
Food connections can be likened to the flow of nutrients and energy from one trophic level to another.
The total mass of organisms (their biomass) at each trophic level can be measured by collecting or capturing and then weighing appropriate samples of animals and plants. At each trophic level, the biomass is 90-99% less than the previous one. Let’s say the biomass of producers in a meadow area of 0.4 hectares is 10 tons, then the biomass of phytophages in the same area will be no more 1000 kg. Food chains in nature usually include 3-4 links; the existence of a larger number of trophic levels is impossible due to the rapid approach of biomass to zero.
Most of the energy received (80-90%) is used by organisms to build the body and maintain vital functions. At each trophic level, the number of individuals progressively decreases. This pattern is called ecological pyramid . An ecological pyramid reflects the number of individuals at each stage of the food chain, or the amount of biomass, or the amount of energy. These quantities have the same direction. With each link in the chain, organisms become larger, they reproduce more slowly, and their number decreases.
Different biogeocenoses differ in their productivity, the rate of consumption of primary products, as well as various food chains. However, all food chains are characterized by certain patterns regarding the ratio of consumed and stored products, i.e. biomass with the energy contained in it at each of the trophic levels. These patterns are called the “rules of the ecological pyramid.” There are different types of ecological pyramids, depending on what indicator is used as its basis. Thus, the biomass pyramid displays the quantitative patterns of transfer of mass of organic matter along the food chain. The energy pyramid displays the corresponding patterns of energy transfer from one link in the power chain to another. A pyramid of numbers has also been developed, displaying the number of individuals at each of the trophic levels of the food chain.
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.
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 energy available.
Carnivore food chains go from producers to herbivores, which are eaten by small carnivores, which serve as food for larger predators, etc. As
As animals move up the chain of predators, they increase in size and decrease in number. The relatively simple and short food chain of predators includes consumers of the second order:
A longer and more complex chain includes consumers of the fifth order:
The lengthening of the chain occurs due to the participation of predators in it.
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 according to the following scheme:
In most cases, the activities of both groups of detritivores are characterized by strict coordination: animals create conditions for the work of microorganisms, dividing animal corpses and dead plants into small parts.
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 food web 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 simple (Fig. 9.3, a) and complex (Fig. 9.3, b) food networks.
In a complex natural community, those organisms that
which receive 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.
Consider a diagram of the flow of energy in a simple (unbranched) food chain, including three (1-3) trophic levels (Fig. 9.4).
For this particular ecosystem, the energy budget was estimated as follows: L=3000 kcal/m2 per day, L A =1500, i.e. 50% of L, P N = 15, i.e. 1% of LA,
Rice. 9.3. Critical connections in American prairie food webs ( A) and northern sea ecosystems for herring ( b),
A- according to Ricklefs, 1979; b - from Alimov, 1989.
Rice. 9.4. Simplified energy flow diagram,
showing three trophic levels
in a linear food chain (after: Odum, 1975).
Consecutive energy flows: L- general lighting, L A - light,
absorbed by vegetation ( I- received or
absorbed energy), P G - gross primary production,
P N - pure primary production, R- secondary products (consumer-
tov), NU - not energy used, N.A.- not assimilated
energy released by consumers (released with excrement), R-energy.
The numbers below are the order of energy lost during each transfer.
P2 = 1.5, i.e. 10% of P N' , And R 3= 0.3 kcal/m2 per day, i.e. 20% of the previous level. At the first trophic level, 50% of the incident light is absorbed, and only 1% of the absorbed energy is converted into the chemical energy of food. Secondary production at each subsequent trophic level of consumers is about 10% of the previous one, although at the level of predators the efficiency may be higher.
Items of energy receipt and consumption, i.e. Energy balance can be conveniently considered using a universal model that is applicable to any living component of the system, be it a plant, animal, microorganism, or individual, population, trophic group (Fig. 9.5). Not all energy entering biomass (/) is converted. Part of it ( N.A.) is not included in metabolism. For example, food can pass through the digestive tract without being metabolized.
Rice. 9.5. Components of a “universal” model
flow of energy (after: Odum, 1975).
Explanation in the text.
bolism, and part of the light energy passes through the plants without being absorbed. The used, or assimilated, portion of the energy ( A) spent on breathing ( R) and production of organic matter ( R). Products can take various forms: G– growth, or increase in biomass; E– assimilated organic matter excreted or secreted (simple sugars, amino acids, urea, mucus, etc.), S-reserve (for example, fat deposits that can be reassimilated later). The return path of stored products is also called the “work loop”, since this is the part of the production that provides the body with energy in the future (for example, a predator uses the energy of stored substances in order to find a new victim). Remaining minus E part of the product is biomass ( IN). Summing up all items of energy receipt and consumption, we obtain: A=I-NA; P = A-R; P=G+E+S; B = P-E; B = G + S.
The universal energy flow model can be used in two ways. First, it may represent a population of a species. In this case, the channels of energy flow and connections of a given species with others make up a diagram of the food web with the name of individual species at its nodes (Fig. 9.6). The procedure for constructing a network diagram includes: 1) drawing up a diagram of the distribution of populations by trophic levels; 2) connecting them through food connections; 3) determination using a universal model of the width of energy flow channels; in this case, the widest channels will pass through populations of polyphagous species, in this case through populations of mayflies, midges and mosquitoes (Fig. 9.6).
Rice. 9.6. Fragment of the food web of a freshwater reservoir.
Second, a universal energy flow pattern can represent a specific energy level. In this embodiment, the biomass rectangles and energy flow channels represent all populations supported by a single energy source. Typically, foxes eat partly plants (fruits, etc.), partly herbivores (hares, field mice, etc.). If we want to emphasize the aspect of intra-population energy, then the entire population of foxes must be depicted as one rectangle. If it is necessary to distribute the metabolism of a fox population into two trophic levels, according to the proportion of plant and animal food, then two or more rectangles should be constructed.
Knowing the universal model of energy flow, it is possible to determine the ratio of energy flow values at different points of the food chain. Expressed as a percentage, these ratios are called environmental efficiency. Depending on the objectives of the study, the ecologist studies certain groups of environmental efficiencies. The most important of them are discussed below.
The first group of energy relations: B/R And P/R. Part of the energy spent on breathing, i.e. on maintaining the structure of biomass, is high in populations of large organisms (people, trees, etc.) Under severe stress R increases. Magnitude R significant in active populations of small organisms, such as bacteria and algae, as well as in systems that receive energy from outside.
Second group of relations: A/I And R/A. The first of them is called the efficiency of assimilation, the second is the efficiency of tissue growth. The efficiency of assimilation varies from 10 to 50% or more. It can be either very small, as in the case of the use of light energy by plants or in the assimilation of food by detritivorous animals, or very large, as in the case of the assimilation of food by animals or bacteria that feed on high-calorie foods, such as sugars or amino acids.
The efficiency of assimilation in herbivorous animals corresponds to the nutritional properties of their food: it reaches 80% when eating seeds, 60% of young foliage, 30-40% of older leaves and 10-20% or even less when eating wood, depending on the degree of its decomposition. Animal foods are easier to digest than plant foods. The efficiency of assimilation in predatory species is 60-90% of the food consumed, with species that eat insects being at the bottom of this series, and those eating meat and fish at the top. The reason for this situation is that the hard, chitinous exoskeleton, which accounts for a significant portion of the body weight in many insect species, is not digestible. This reduces the efficiency of assimilation in animals that feed on insects.
The efficiency of tissue growth also varies widely. It reaches its greatest values in cases where the organisms are small and the environmental conditions in which they live do not require large expenditures to maintain the temperature optimal for the growth of organisms.
And finally, the third group of energy relations: R/V.
In cases where R is estimated as speed, R/V represents the ratio of production at a particular point in time to biomass: P/B = B/(VT) = T - 1, where T - time. If the integral production is calculated for a certain period of time, the value of the ratio R/V is determined taking into account the average biomass for the same period of time. In this case the relation R/V - the quantity is dimensionless; it shows how many times the production is greater or less than biomass. The ratio of productivity to biomass can be considered both within one trophic level and between neighboring ones.
Comparing productivity P t and biomass Bt within one trophic level (t), note S-shaped nature of the change P t within a certain range of changes Bt. For example, at the first trophic level, production increases slowly at first, since the leaf surface is small, then faster and at high biomass density - again slowly, because
Photosynthesis in conditions of significant shading of the leaves of the lower tiers is weakened. At the second and third trophic levels, with a very small and very large number of animals per unit area, the ratio of productivity to biomass decreases, mainly due to a decrease in the birth rate.
The ratio of productivity of the previous trophic level ( P t -1) to the biomass of the present ( Bt) is determined by the fact that phytophages, eating up part of the plants, thereby contribute to the acceleration of their growth, i.e., phytophages, through their activity, contribute to plant productivity. A similar influence on the productivity of first-order consumers is exerted by predators, which, by destroying sick and old animals, contribute to an increase in the birth rate of phytophages.
The simplest dependence of the productivity of the subsequent trophic level is (P t +1) from the biomass of the present (At t). The productivity of each subsequent trophic level increases with the growth of the biomass of the previous one. Ratio Р t +1 /B t shows, in particular, what the amount of secondary production depends on, namely from the magnitude of primary production, the length of the food chain, the nature and amount of energy brought from outside into the ecosystem.
The above reasoning allows us to note that the size of individuals has a certain influence on the energy characteristics of the ecosystem. The smaller the organism, the higher its specific metabolism (per unit mass) and, therefore, the lower the biomass that can be maintained at a given trophic level. Conversely, the larger the organism, the greater the standing biomass. Thus, the “yield” of bacteria at a given moment will be much lower than the “yield” of fish or mammals, although these groups used the same amount of energy. The situation is different with productivity. Since productivity is the rate of biomass growth, small organisms have advantages here, which, thanks to a higher level
metabolism have higher rates of reproduction and renewal of biomass, i.e., higher productivity.
A food chain consists of organisms of different species. At the same time, organisms of the same species can be part of different food chains. Therefore, food chains are intertwined, forming complex food webs covering all ecosystems of the planet.[...]
A food (trophic) chain is the transfer of energy from its source - producers - through a number of organisms. Food chains can be divided into two main types: the grazing chain, which starts with a green plant and goes on to grazing herbivores and predators, and the detrital chain (from the Latin abraded), which starts from the breakdown products of dead organic matter. In the formation of this chain, a decisive role is played by various microorganisms that feed on dead organic matter and mineralize it, again converting it into the simplest inorganic compounds. Food chains are not isolated from one another, but are closely intertwined with each other. Often, an animal that consumes living organic matter also eats microbes that consume non-living organic matter. Thus, the routes of food consumption branch, forming so-called food webs.[...]
A food web is a complex interweaving in a community of food chains.[...]
Food webs are formed because almost any member of any food chain is also a link in another food chain: it consumes and is consumed by several species of other organisms. Thus, the food of the meadow wolf-coyote includes up to 14 thousand species of animals and plants. This is probably the same order of magnitude in the number of species involved in eating, decomposing and destroying the substances of a coyote carcass. [...]
Food chains and trophic levels. By tracing the food relationships between members of the biocenosis (“who eats whom and how much”), it is possible to build food chains for various organisms. An example of a long food chain is the sequence of inhabitants of the Arctic sea: “microalgae (phytoplankton) -> small herbivorous crustaceans (zooplankton) - carnivorous planktivores (worms, crustaceans, mollusks, echinoderms) -> fish (2-3 links in the sequence of predatory fish are possible) - > seals -> polar bear.” Terrestrial ecosystem chains are usually shorter. A food chain, as a rule, is artificially isolated from a really existing food network - a plexus of many food chains. [...]
A food web is a complex network of food relationships.[...]
Food chains imply a linear flow of resources from one trophic level to the next (Fig. 22.1a). In this design, interactions between species are simple. However, no system of resource flows in BE follows this simple structure; they are much more reminiscent of a network structure (Fig. 22.1, b). Here, species at one trophic level feed on several species at the next lower level, and omnivory is widespread (Fig. 22.1c). Finally, a fully defined food web may exhibit a variety of features: multiple trophic levels, predation, and omnivory (Figure 22.1, [...]
Many food chains, intertwined in biocenoses and ecosystems, form food webs. If the general food chain is depicted in the form of building blocks, conventionally representing the quantitative ratio of the energy absorbed at each stage, and stacked on top of each other, you get a pyramid. It is called the ecological pyramid of energies (Fig. 5).[...]
Food chain and food web diagrams. Dots represent species, lines represent interactions. Higher species are predators of lower ones, so resources flow from bottom to top.[...]
In the first type of food web, the flow of energy goes from plants to herbivores, and then to higher-order consumers. This is a grazing network, or a grazing network. Regardless of the size of the biocenosis and habitat, herbivorous animals (terrestrial, aquatic, soil) graze, eat up green plants and transfer energy to the next levels (Fig. 96).[...]
In communities, food chains intertwine in complex ways to form food webs. The food composition of each species usually includes not one, but several species, each of which in turn can serve as food for several species. On the one hand, each trophic level is represented by many populations of different species; on the other hand, many populations belong to several trophic levels at once. As a result, due to the complexity of food relationships, the loss of one species often does not upset the balance in the ecosystem.[...]
[ ...]
This diagram not only illustrates the interweaving of food relationships and shows the three trophic levels, but also reveals the fact that some organisms occupy an intermediate position in the system of the three main trophic levels. Thus, caddisfly larvae that build a trapping net feed on plants and animals, occupying an intermediate position between primary and secondary consumers.[...]
The primary source of human food resources were those ecosystems in which he could exist. The methods of obtaining food were gathering and hunting, and with the development of the manufacture and use of more and more advanced tools, the share of hunting prey increased, which means the share of meat, that is, complete proteins, in the diet. The ability to organize large stable groups, the development of speech, which makes it possible to organize the complex coordinated behavior of many people, made man a “superpredator”, occupying the top position in the food webs of the ecosystems that he mastered as he settled across the Earth. Thus, the only enemy of the mammoth was man, who, together with the retreat of the glacier and climate change, became one of the reasons for the death of these northern elephants as a species. [...]
[ ...]
Based on a study of 14 food webs in communities, Cohen found a remarkably consistent ratio of the number of prey "types" to the number of predator "types" of approximately 3:4. Further evidence supporting this ratio is provided by Bryand and Cohen, who studied 62 similar networks. A graph of such proportionality has a slope of less than 1 in both fluctuating and constant media. Using "types" of organisms rather than actual species usually produces less than objective results, but while the resulting prey/predator ratio may be an underestimate, its consistency is remarkable.[...]
In BE, many (but certainly not all) food webs have large numbers of primary producers, fewer consumers, and very few top predators, giving the network the shape shown in Figure 1. 22.1, b. Omnivores in these systems may be rare while decomposers are abundant. Food web models have provided a potential basis for fruitful analyzes of resource flows in both BE and PE. Difficulties arise, however, when trying to quantify resource flows and subject network structure and stability properties to mathematical analysis. It turns out that many of the necessary data are difficult to identify with certainty, especially for organisms that function at more than one trophic level. This property does not create the main difficulty in studying resource flows, but it seriously complicates the analysis of stability. The claim that more complex systems are more stable - because destruction of one type or flow path simply transfers energy and resources to other paths rather than blocking the path for the entire flow of energy or resource - is still hotly debated.[...]
Analysis of large numbers of industrial food webs can thus reveal characteristics not shown in other approaches. In the ecosystem project in Fig. 22.5, for example, network analysis may reflect a missing sector or type of industrial activity that has the potential to increase connectivity. These topics provide a rich area for detailed research.[...]
Within each ecosystem, food webs have a well-defined structure, which is characterized by the nature and number of organisms represented at each level of the various food chains. To study the relationships between organisms in an ecosystem and to depict them graphically, they usually use ecological pyramids rather than food web diagrams. Ecological pyramids express the trophic structure of an ecosystem in geometric form.[...]
Of some interest is the length of food chains. It is clear that the decrease in available energy during the transition to each subsequent link limits the length of food chains. However, energy availability does not appear to be the only factor, since long food chains are often found in infertile systems, such as oligotrophic lakes, and short ones in very productive, or eutrophic, systems. Rapid production of nutritious plant material can stimulate rapid grazing, resulting in energy flow being concentrated in the first two to three trophic levels. Eutrophication of lakes also changes the composition of the planktonic food web “phytoplankton-large zooplankton-predatory fish”, turning it into a microbial-detrital microzooplankton system that is not so conducive to maintaining sport fisheries.[...]
Given a constant energy flow in a food web, or chain, smaller terrestrial organisms with high specific metabolism create relatively less biomass than larger ones1. A significant part of the energy is spent on maintaining metabolism. This rule “metabolism and size of individuals”, or the rule of Yu. Odum, is usually not implemented in aquatic biocenoses, taking into account the actual living conditions in them (under ideal conditions it has universal significance). This is due to the fact that small aquatic organisms largely support their metabolism due to external energy from their immediate environment.[...]
Soil microflora has a well-developed food web and a powerful compensation mechanism based on the functional interchangeability of some species with others. In addition, thanks to the labile enzymatic apparatus, many species can easily switch from one nutrient substrate to another, thereby ensuring the stability of the ecosystem. This significantly complicates the assessment of the impact of various anthropogenic factors on it and requires the use of integral indicators.[...]
[ ...]
First of all, randomized food webs often contain biologically meaningless elements (for example, loops of this type: A eats B, B eats C, C eats A). Analysis of “meaningfully” constructed networks (Lawlor, 1978; Pimm, 1979a) shows that (a) they are more stable than those considered and (b) there is no such sharp transition to instability (compared to the above inequality), although stability still falls from increasing complexity.[...]
| 21.2 |  |
Of course, yes, if not as part of biogeocenoses - the lower levels of the ecosystem hierarchy - then, in any case, within the biosphere. People get food from these networks (agrocenoses - modified ecosystems with a natural basis). Only from “wild” nature do people extract fuel - energy, basic fish resources, and other “gifts of nature.” V.I. Vernadsky’s dream of the complete autotrophy of humanity still remains an irrational dream1 - evolution is irreversible (L. Dolo’s rule), as is the historical process. Without true autotrophs, mainly plants, a person cannot exist as a heterotrophic organism. Finally, if he were not physically included in the food webs of nature, then his body after death would not be subject to destruction by decomposer organisms, and the Earth would be littered with unrotten corpses. The thesis about the separation of humans and natural food chains is based on a misunderstanding and is clearly erroneous.[...]
In ch. 17 analyzes ways of combining different groups of consumers and their food into a network of interacting elements through which matter and energy are transferred. In ch. 21 we will return to this topic and consider the influence of food web structure on the dynamics of communities as a whole, paying special attention to features of their structure that contribute to stability. [...]
Four examples will suffice to illustrate the basic features of food chains, food webs, and trophic levels. The first example is the region of the Far North, called the tundra, where relatively few species of organisms live that have successfully adapted to low temperatures. Therefore, food chains and food webs here are relatively simple. One of the founders of modern ecology, British ecologist Charles Elton, realizing this, already in the 20-30s of our century began studying the Arctic lands. He was one of the first to clearly outline the principles and concepts associated with food chains (Elton, 1927). Tundra plants - lichen ("deer moss") C1a donia, grasses, sedges and dwarf willows form the food of caribou in the North American tundra and its ecological counterpart in the Old World tundra - reindeer. These animals, in turn, serve as food for wolves and humans. Tundra plants are also eaten by lemmings - fluffy short-tailed rodents that resemble a miniature bear, and tundra partridges. Throughout the long winter and short summer, arctic foxes and snowy owls feed mainly on lemmings. Any significant change in lemming numbers is reflected at other trophic levels, as other food sources are scarce. This is why the numbers of some groups of Arctic organisms fluctuate wildly, from superabundance to near extinction. This often happened in human societies if they depended on one or more few sources of food (remember the “potato famine” in Ireland1).[...]
One of the implications of the resilience hypothesis, which can be tested in principle, is that in environments with less predictable behavior, food chains should be shorter, since only the most elastic food webs appear to persist in them, and short chains have elasticity higher. Briand (1983) divided 40 food webs (based on the data he collected) into those associated with variable (positions 1-28 in Table 21.2) and constant (positions 29-40) environments. There were no significant differences in the average length of maximum food chains between these groups: the number of trophic levels was 3.66 and 3.60, respectively (Fig. 21.9). These provisions still need critical verification.[...]
In addition, the modeling results become different when it is taken into account that consumer populations are influenced by food resources, and those do not depend on the influence of consumers (¡3,/X), 3(/ = 0: the so-called “donor-regulated system” ), In this type of food web, stability is either independent of complexity or increases with it (DeAngelis, 1975). In practice, the only group of organisms that usually satisfies this condition are detritivores.[...]
However, such a strict picture of the transfer of energy from level to level is not entirely realistic, since the trophic chains of ecosystems are complexly intertwined, forming trophic networks. For example, the phenomenon of “trophic cascade,” where predation causes changes in the density, biomass, or productivity of a population, community, or trophic level along more than one lineage of a food web (Pace et al. 1999). P. Mitchell (2001) gives the following example: sea otters feed on sea urchins, which eat brown algae; the destruction of otters by hunters led to the destruction of brown algae due to the growth of the urchin population. When otter hunting was banned, the algae began to return to their habitats.[...]
Green plants convert the energy of photons from sunlight into the energy of chemical bonds of complex organic compounds, which continue their path through the branched food networks of natural ecosystems. However, in some places (for example, in swamps, at the mouths of rivers and seas), some of the organic plant matter, having fallen to the bottom, is covered with sand before it becomes food for animals or microorganisms. In the presence of a certain temperature and pressure of ground rocks for thousands and millions of years, coal, oil and other fossil fuels are formed from organic substances or, in the words of V.I. Vernadsky, “living matter goes into geology.”[...]
Examples of food chains: plants - herbivores - predator; cereal-field mouse-fox; food plants - cow - man. As a rule, each species feeds on more than one species. Therefore, food chains intertwine to form a food web. The more closely organisms are connected through food webs and other interactions, the more resilient the community is to possible disturbances. Natural, undisturbed ecosystems strive for balance. The state of equilibrium is based on the interaction of biotic and abiotic environmental factors.[...]
For example, the destruction of economically important pests in forests with pesticides, the shooting of part of animal populations, and the catching of certain species of commercial fish are partial interferences, since they affect only individual links of food chains, without affecting food networks as a whole. The more complex the food web and the structure of the ecosystem, the less significant such interference is, and vice versa. At the same time, the release and discharge into the atmosphere or water of chemical xenobiotics, for example, oxides of sulfur, nitrogen, hydrocarbons, fluorine compounds, chlorine, heavy metals, radically changes the quality of the environment, creates interference at the level of producers as a whole, and therefore leads to complete degradation of the ecosystem: since the main trophic level - producers - dies.[...]
Energy-dependent carrying capacity = (/gL -)/kV. Energy diagram of a primitive system in Uganda. D. The energy scheme of agriculture in India, where the main source of energy is light, but the flow of energy through livestock and grain is regulated by man. D. Energy network of highly mechanized agriculture. High yields are based on a significant investment of energy through the use of fossil fuels, which perform work previously done by humans and animals; in this case, the food web of animals and plants that had to be “fed” in the two previous systems falls out.[...]
A number of attempts have been made to mathematically analyze the relationship between the complexity of a community and its stability, in most of which the authors came to approximately the same conclusions. A review of such publications was given by May (1981). As an example, consider his work (May, 1972), demonstrating both the method itself and its shortcomings. Each species was influenced by its interactions with all other species; The quantitative effect of the density of species / on the growth of the number i was assessed by the indicator p. In the complete absence of influence it is equal to zero, in two competing species Рс and Pji are negative, in the case of a predator (¿) and prey (/) Ру is positive, and jjji is negative.[...]
Acid precipitation causes lethal effects on life in rivers and reservoirs. Many lakes in Scandinavia and eastern North America have become so acidic that fish cannot not only spawn in them, but simply survive. In the 70s, fish completely disappeared in half of the lakes in these regions. The most dangerous is the acidification of shallow ocean waters, leading to the impossibility of reproduction of many marine invertebrate animals, which can cause a rupture in food networks and deeply disrupt the ecological balance in the World Ocean.[...]
Models of donor-controlled interactions differ in a number of ways from traditional models of predator-prey interactions of the Lotka-Volterra type (Chapter 10). One important difference is that interacting groups of species characterized by donor-controlled dynamics are thought to be particularly resilient and, further, that this resilience is in fact independent or even increasing from increases in species diversity and food web complexity. This situation is completely opposite to that in which the Lotka-Volterra model is applicable. We will discuss these important issues regarding food web complexity and community resilience in more detail in Chap. 21.
