home

Population and its main characteristics. Spatial distribution of populations What factors limit the impact of a predator on prey

Consumption of fish by other organisms, including fish, is one of the most important causes of mortality. In each species of fish, especially in the early stages of ontogenesis, predators usually constitute one of the most important elements of the environment, adaptations to which are very diverse. Greater fertility of fish, protection of offspring, protective coloration, various protective devices (thorns, thorns, poisonousness, etc.), protective behavior features are various shapes adaptations that ensure the existence of a species under conditions of a certain predator pressure.

There are no fish species in nature that are free from greater or lesser, but natural, influence of predators. Some species are susceptible to this effect to a greater extent and at all stages of ontogenesis, for example, anchovies, especially small ones, herrings, gobies, etc. Others are exposed to this effect to a lesser extent and mainly in the early stages of development. At later stages of development in some species, the impact of predators can be greatly weakened and practically disappear. This group of fish includes sturgeon, large catfish, and some types of carp. Finally, the third group is species in which death from predators and in the early stages of ontogenesis is very low. Only some sharks and rays belong to this group. Naturally, the boundaries between these groups we have identified are conditional. In fish adapted to significant pressure from predators, a smaller percentage die of old age as a result of senile metabolic disorders.

Greater or lesser protection from predators is, respectively, associated with the development of the ability to compensate for greater or lesser death by changing the rate of population reproduction. Species adapted to significant predation can also compensate big losses. Adaptation to a certain nature of the impact of predators is formed in fish, as in other organisms, during the formation of the faunal complex. During the process of speciation, coadaptation of predator and prey occurs. Predator species adapt to feed on certain types of prey, and prey species adapt in one way or another to limit the impact of predators and compensate for the loss.

Above, we examined the patterns of changes in fertility and, in particular, showed that populations of the same species in low latitudes are more fertile than in high latitudes. Closed forms Pacific Ocean turn out to be more fertile than Atlantic forms. Fishes of the rivers Far East more prolific than the fish of the rivers of Europe and Siberia. These differences in fertility are associated with different predation pressure in these water bodies. Protective adaptations are developed in fish in relation to life in their respective habitats. In pelagic fish, the main forms of protection are the appropriate “pelagic” protective coloration, speed of movement and - for protection from the so-called daytime predators that navigate with the help of their visual organs - schooling. The protective value of the flock is apparently threefold. On the one hand, fish in a school detect a predator at a greater distance and can hide from it (Nikolsky, 1955). On the other hand, the flock also provides a certain physical protection from predators (Manteuffel and Radakov, 1960, 1961). Finally, as noted in relation to cod (predator) and juvenile pollock (prey), the multiplicity of prey and the defensive maneuvers of the school disorient the predator and make it difficult for it to catch prey (Radakov, 1958, 1972; Hobson, 1968).

The protective value of the school is not preserved in many fish species at all stages of ontogenesis. Usually it is characteristic of the early stages: adult fish have a schooling lifestyle, losing protective function, manifests itself only in certain periods of life (spawning, migration). Schooling as a protective device is usually characteristic of juvenile fish in all biotopes, both in the pelagic zone and in the coastal zone of the seas, both in rivers and lakes. The school serves as protection from daytime predators, but makes it easier for nocturnal predators, who navigate the search for food using other senses, to find fish in the school. Therefore, in many fish, for example, herrings, the school breaks up at night and individuals stay alone, only to reassemble into a school at dawn.

Coastal benthic and bottom fish also have different methods of protection from predators. The main role is played by various morphological protective devices, various thorns and spines.

The development of “weapons” in fish against predators is far from the same in different faunas. In the faunas of seas and fresh waters of low latitudes, the “armament” is usually more intensively developed than in the faunas of higher latitudes (Table 76). In the faunas of low latitudes, the relative and absolute number of fish “armed” with thorns and prickles is much greater, and their “weapons” are more developed. At low latitudes and poisonous fish more than at high latitudes. In marine fish, protective devices in the same latitudes are more developed than in freshwater fish.

Table 76. Number of fish with spines and spines in different marine faunas (without lampreys and cartilaginous fish), %

Among the representatives of the ancient deep-sea fauna, the percentage of “armed” fish is incomparably lower than in the faunas of the continental shelf.

In the coastal zone, the "armament" of fish is much more developed than in the open part of the sea. Along the coast of Africa, in the Dakar region in the coastal zone, “armed” fish species in trawl catches make up 67%, and away from the coast their number decreases to 44%. A slightly different picture is observed in the Gulf of Guinea region. Here, in the coastal zone, the percentage of “armed” species is very small (only Ariidae catfish), and further from the coast it increases significantly (Radakov, 1962; Radakov, 1963). The smaller percentage of “armed” fish in the coastal zone of the Gulf of Guinea is associated with the high turbidity of the coastal waters of this area and, because of this, the impossibility of hunting here for “visual predators”, which concentrate in adjacent areas with clear water. In the area with muddy water less numerous predators are represented by species that focus on prey using other senses (see below).

The situation is similar in the seas of the Far East. Thus, in the Sea of Okhotsk there are more “armed” fish among the coastal zone than far from the coast (Schmidt, 1950). The same thing is observed along the American Pacific coast.

The relative number of “armed” fish also varies in the northern part Atlantic Ocean and in the Pacific Ocean (Clements a. Wilby, 1961): in the northern part of the Pacific Ocean the percentage of “armed” fish is much higher than in the North Atlantic. A similar pattern is observed in fresh waters. Thus, in the rivers of the Arctic Ocean basin there are fewer “armed” fish than in the Caspian and Aral Sea. Different “armament” is also characteristic of fish inhabiting different biotopes. In the direction from the upper reaches to the lower reaches of the river, the relative number of “armed” fish usually increases. This is noted in the rivers different types and latitudes. For example, in the middle and lower reaches of the Amu Darya there are about 50 fish with thorns and spines, and in the upper reaches - about 30%. In the middle and lower reaches of the Amur there are more than 50 “armed” species, and in the upper reaches less than 25% (Nikolsky, 1956a). True, there are exceptions to this rule in rivers flowing from south to north in the northern hemisphere.

So, in the river Ob, for example, it is not possible to notice a noticeable difference in the “armament” of the fish of the upper and lower reaches. In the lower reaches, the percentage of “armed” species becomes even somewhat smaller.

The intensity or, so to speak, power of the development of “weapons” in different zones Oh. As shown by I. A. Paraketsov (1958), related species of the North Atlantic have less developed “weapons” than species of the Pacific Ocean. This can be clearly seen in the representatives of the family. Scorpaenidae and Cottidae (Fig. 53).

Rice. 53. Relationship between the growth of the preopercular spine (S) and the change in fish body length (L) in representatives of Myoxocephalus (a) and Gymnacanthus (b) (according to Paraketsov, 1958): 1 - Myoxocephalus jaok Cuv. a. Val.; 2 - M. brandii Steind; 3 - M. scorpius L.; 4 - M. quadricornis L.; 5 - Gymnacanthus herzensteini Jord. et Staiks (Pacific); 6 - G. tricuspis Rnd. (Atlantic)

The same thing occurs within different zones of the Pacific Ocean. In more northern species, “weapons” are less developed than in their close relatives, but widespread to the south (Paraketsov, 1962). In species distributed at great depths, the dorsal spines are less developed than in related forms distributed in the coastal zone. This is well demonstrated in Scorpaenidae. It is interesting that at the same time, since at depths the relative sizes of prey are usually larger (and sometimes significantly) than in the coastal zone, deep-seated “armed” fish usually have larger heads and more developed opercular spines (Phillips, 1961).

Naturally, the development of thorns and prickles does not create absolute protection from predators, but only reduces the intensity of the predator’s impact on the prey herd. As M. N. Lishev (1950), I. A. Paraketsov (1958), K. R. Fortunatova (1959) and other researchers showed, the presence of spines makes fish less accessible to predators than fish of a similar biological type and shape, but devoid of thorns. This is most clearly shown by M. N. Lishev (1950) using the example of eating common and spiny bitterlings in the Amur. Protection from predators is provided not only by the presence of spines (the possibility of pricking), but also by an increase in body height, for example in sticklebacks (Fortunatova, 1959), or in the width of the head, for example in sculpins (Paraketsov, 1958). The protective value of thorns and spines varies depending on the size and method of hunting of the predator eating the “armed” fish, as well as on the behavior of the prey. For example, stickleback in the Volga delta turns out to be accessible to different predators of different sizes. Perch's food contains the most small fish, in pike - larger and in catfish - the largest (Fortunatova, 1959) (Fig. 54). As shown by Frost (1954) using the example of pike, as the size of the predator increases, the percentage of its consumption of “armed” fish also increases.

Rice. 54. Sizes of mature stickleback in the food of perch (1), pike (2) and catfish (3) (May-July 1953) (according to Fortunatova, 1959)

The intensity of consumption of “armed” fish depends to a very large extent on how well the predator is provided with food. In hungry fish with an insufficient food supply, the intensity of consumption of “armed” fish increases. This is well demonstrated in an experiment with stickleback (Hoogland, Morris a. Tinbergen, 1956-1957). Here we have special case general pattern, when, in conditions of insufficient supply of basic, most accessible food, the nutritional spectrum expands due to less accessible food, the extraction and assimilation of which requires more energy.

The behavior of the prey is essential for the accessibility of “armed” fish to predators. As a rule, fish are eaten by predators during their most active periods. This also applies to “armed” fish. For example, the nine-spined stickleback in the Volga delta is most accessible to predators during the breeding season, at the end of May, and during the period of mass emergence of juveniles, at the end of June - beginning of July (Fig. 55) (Fortunatova, 1959).

Rice. 55. Occurrence of stickleback in the food of predatory fish (May-July 1953), (according to Fortunatova, 1959)

We considered only two forms of protection of prey from predators: school behavior and “arming” of prey, although the forms of protection can be very diverse: this is the use of certain shelters, for example, burying in the ground, and some behavioral features, for example, “hook” in juveniles pollock (Radakov, 1958), and vertical migrations (Manteuffel, 1961), and the toxicity of meat and caviar, and many other methods. The intensity of the predator's impact on the prey population depends on many factors. Naturally, each predator is adapted to feed in certain conditions and with certain types of prey. The specificity of the predators that feed on them depends to a very large extent on the nature of the prey’s habitat. In the muddy waters of rivers Central Asia the main type of predators are fish that focus on prey using the organs of touch and lateral line organs. Their organ of vision does not play a significant role in the hunt for victims. Examples include the great shovelnose Pseudoscaphyrhynchus kaufmanni(Bogd.) and common catfish Silurus glanis L. These fish feed both day and night. In rivers with clearer water, catfish are a typical nocturnal predator. IN upper reaches rivers of the European North and Siberia, where the water is clean and transparent, predators (taimen Hucho taimen Pall., lenok Brachymystax lenok Pall., pike Esox lucius L.) focus on prey mainly using the organ of vision and hunt mainly during daylight hours. In this zone there are probably only burbot Lota lota(L.), which focuses on prey mainly through the senses of smell, touch and taste, feeds mainly at night. The same is observed in the seas. Thus, in the coastal turbid waters of the Gulf of Guinea, predators navigate mainly using the organs of touch and the lateral line. The organ of vision in this biotope plays a subordinate role among predators. Further from the coast, beyond the zone muddy water, in the Gulf of Guinea, in water of high transparency, the main place is occupied by predators that focus on prey using the organ of vision, such as Sphyraena, Lutianus, tuna, etc. (Radakov, 1963).

The hunting methods of predators that get food in thickets and in open waters Oh. In the first case, ambush predators predominate, in the second, those that steal prey predominate. For many predators and within the same habitat, there is a clearly expressed change in the food eaten at different times of the day: for example, burbot eats sedentary invertebrates during the day and hunts for fish at night (Pavlov, 1959). Perkarina Perkarina maeotica Kuzn. in the Sea of Azov during the day it feeds mainly on copepods and mysids, and at night it eats sprat Clupeonella delicatula Nordm. (Kanaeva, 1956).

The nature and intensity of the impact of predators on the population of peaceful fish depend on many factors: on the abiotic conditions in which hunting is carried out, on the presence and abundance of other species of prey that the same predator feeds on; from the presence of other predators feeding on the same prey; on the condition and behavior of the victim.

Abrupt changes in abiotic conditions can greatly alter the availability of prey to predators. For example, in reservoirs where, as a result of significant fluctuations in level, underwater vegetation disappears, hunting conditions in the coastal zone for the ambush predator pike sharply worsen and, conversely, favorable conditions are created for the predator of more open waters - pike perch.

Each predator is adapted to feed on a certain type of prey and, naturally, the presence or absence of other types of prey affects the intensity of their consumption. In this regard, the feeding conditions of predators change especially strongly if prey belonging to other, more northern faunal complexes appears in large numbers. So, for example, in the years when there are good harvests in the Amur for smallmouth smelt Hypomesus olidus(Pall.) in the spring, during the period of its mass appearance, all predators switch to feeding on it and, naturally, their impact on other fish is sharply reduced (Lishev, 1950). This was observed, for example, in 1947 and to a somewhat lesser extent in 1948, and in the poor smelt harvest year of 1946, predators switched to feeding on other foods and their food spectrum expanded.

A similar picture is observed in the seas; Thus, in the Barents Sea, in years with a good harvest of capelin, this fish forms the main food source for cod in the spring. In the absence or small amount of capelin, cod switches to feeding on other fish, in particular herring (Zatsepin and Petrova, 1939).

Reducing the number of prey, for example, juvenile sockeye salmon in the lake. Cultus, leads to the fact that the predators of the same faunal complex that usually feed on it switch to a large extent to feed on other prey that is less typical for them, sometimes moving during the feeding period to habitats that are less usual for them, where their feeding conditions are worse ( Ricker, 1941).

A significant influence on the intensity of a predator's eating of a prey is exerted by the presence of another predator eating the same prey, or the presence of a predator for which the first predator is the prey.

In the case of two or more predators hunting for one prey, the availability of the latter greatly increases. This was shown in an experiment by D.V. Radakov (1958), when several predators (cod) ate victims much faster than one predator at the same prey density. The intensity of grazing especially increases if the fish is simultaneously hunted by predators of different biological types. One of the usual ways of protecting fish from a predator is to go to another habitat where the prey is inaccessible to the predator, for example, leaving large predators in shallow water or being pinned to the bottom by pelagic predators, or finally jumping into the air by flying fish.

If the prey is hunted simultaneously by predators of different biological types (for example, during the migration of juvenile Far Eastern salmon, Salvelinus loaches and sculpins Myoxocephalus in rivers flowing into the Amur estuary), the intensity of grazing increases sharply, because moving away from pelagic predators into the bottom layers makes the prey more accessible to bottom predators and, conversely, moving away from the bottom into the water column increases grazing by pelagic predators.

The intensity of predation by predators can often change quite dramatically if the latter themselves are under the influence of the predator. So, for example, during the migration of juvenile pink salmon and chum salmon from the tributaries of the Amur in the lower reaches of the tributaries, it large quantities is eaten by the chebak Leuciscus waleckii (Dyb.), and if the pike Esox reicherti Dyb., for which the chebak is the main food, stays here in the lower reaches of the tributary, the activity of the chebak as a consumer of the descending juvenile salmon is sharply reduced.

A similar picture is observed in the Black Sea in relation to anchovy, mackerel and bonito. In the absence of bonito Pelamys sarda(Bloch) mackerel Trachurus trachurus(L.) feeds quite intensively on anchovy Engraulis encrassicholus L. In the event of the appearance of bonito, for which horse mackerel is a prey, its consumption of anchovy is sharply reduced.

Naturally, the influence of a predator on the prey population does not occur with the same intensity throughout the year. Typically, intense mortality from predators occurs over a relatively short period of time, when the period active nutrition predator coincides with the state of the prey when it turns out to be relatively easily accessible to the predator. This was shown above using the example of smelt. At catfish Silurus glanis L. delta Volga roach Rutilus rutilus caspicus Jak. plays important role in food in the spring, from mid-April to mid-May, when the catfish eats 68% of its annual diet; In summer, in June and July, the main food of catfish is young carp Cyprinus carpio L., rolling down from the hollows into the delta front, and in the fall - again roach, coming from the sea to the lower reaches of the Volga for the winter. Thus, roach is important in catfish food for only about two months - during the spawning run, spawning and during migration in the fall for wintering; at other times, catfish in the Volga delta practically do not feed on roach.

A different picture is observed in asp Aspius aspius(L.): it intensively eats juvenile roach in the summer, when it rolls down from spawning reservoirs, mainly in the surface layers of the core part of the river and is inaccessible to catfish, but is well accessible to asp. During the summer months (June-July), the asp eats 45% of its annual diet, with 83.3% (by number) of all food being juvenile roach. During the rest of the year, the asp almost does not feed on roach (Fortunatova, 1962).

Pike, like catfish, eats mainly roach going to spawn in the lower zone of the delta, where larger pikes stay. Rolling juvenile roach for pike, as well as for catfish, turns out to be inaccessible (Popova, 1961, 1965).

For a very limited time, cod feed on capelin. Intensive feeding of cod on capelin usually lasts about a month.

In the Amur, predators usually feed intensively on small smelt in two stages: in the spring, during its spawning, and in the fall, during its migration upstream in the coastal zone (Vronsky, 1960).

The conditions under which predators influence their prey change greatly in years with different hydrological regimes. In river reservoirs, in high-water years, the availability of prey for predators is usually greatly reduced, and in years with low floods it increases.

Predators also have a certain influence on the population structure of their prey. Depending on what part of the population the predator affects, it causes a corresponding restructuring of the prey population structure. It is safe to say that most predators selectively remove individuals from the population. Only in some cases is this removal not selective in nature, and the predator removes prey in the same size ratio as it is contained in the population. For example, beluga whale Delphinapterus leucas, various seals, Kaluga Huso dauricus(Georgi) and some other predators eat out the running chum salmon from the stock of fish without selecting certain sizes. The same is apparently observed in relation to the moving juveniles of the Far Eastern salmon - chum salmon and pink salmon. Cod probably feed non-selectively on spawning capelin. In most cases, the predator selects fish of a certain size, age, and sometimes sex.

The reasons for the selective feeding of predators in relation to prey are varied. The most usual reason- this is the correspondence of the relative size and structure of the predator to the size and structure, in particular the presence of certain protective devices, of the prey (thorns, spines). The different accessibility of different genders is essential. So, for example, in gobies and sticklebacks, while protecting the nest, it is usually the males that are eaten by predators in greater numbers. This is noted, for example, in Gobius paganellus(L.), which is compensated by the large percentage of males in the offspring of this species (Miller, 1961). Less grazing large fish during the feeding period, compared to eating juveniles, can often be associated with their greater caution (Milanovsky and Rekubratsky, 1960). In general, most predator fish feed on the immature part of the prey stock. The sexually mature part of the stock, especially large fish, is eaten by predators in relatively small quantities. In this respect, the impact of predators differs from the impact of harvesting, which, as a rule, removes mainly mature individuals from the population. Thus, from the roach herd, predators (pike perch, catfish, pike) take fish mainly from 6 to 18 cm in length, and the fishery takes fish from 12 to 23-25 cm in length (Fig. 56).

Rice. 56. Size composition of roach taken from a reservoir (according to Fortunatova, 1961): 1 - in catches; 2 - in the diet of pike perch; 3 - in the diet of pike; 4 - in catfish nutrition; 5 - in the diet of catfish, pike and pike perch (total)

If we add to this the eating of roach fry by juveniles predatory fish, then the difference will be even more significant (Fortunatova, 1961).

Thus, the impact of predators on the structure of the prey population is usually reflected through the consumption of juveniles, i.e., a reduction in the amount of recruitment, which causes an increase in the average age of the mature part of the population. We still know very little about what proportion of the entire fish stock is eaten by predators and what relative mortality rate the population can compensate for by reproduction. Apparently, this value is about 50-60% of the spawning stock in fish with a short life cycle and 20-40% in fish with a long life cycle and late sexual maturity.

There is very little quantitative data in the literature on what proportion of the population was eaten by predators. This is made difficult by the fact that it is not possible to determine the total size of either the population of the prey or the predator that feeds on it. However, in some cases attempts of this kind have been made. Thus, Crossman (1959) determined that rainbow trout Salmo gairdneri Rich, eats into the lake. Paul (Paul Lake) from 0.15 to 5% of the population Richardsonius balteatus(Rich.).

Sometimes it is possible to approximately determine the ratio of natural and fishing mortality for some species; Thus, K.R. Fortunatova (1961) showed that predators eat only slightly less roach than is caught commercially (in 1953, for example, 580 thousand centners of roach were caught, and predators ate 447 thousand centners). Ricker (1952) identifies three types of possible quantitative relationships between predator and prey:

1) when a predator eats a certain number of victims, and the rest avoids capture;

2) when a predator eats out a certain part of the prey population;

3) when predators eat all available individuals of the prey, with the exception of those that can avoid capture by hiding in places where the predator cannot get them, or when the number of prey reaches such a small value that the predator will have to move to another place.

As an example of the first case, when the number of prey does not limit the needs of the predator, Ricker cites the feeding of predators on spawning aggregations of herring or rolling juvenile salmon. In this case, the number of fish eaten is determined by the duration of contact with predators.

As an example of the second type, Ricker cites the consumption of nearby predators in the lake. The cultus of juvenile sockeye salmon, which these predators feed on throughout the year: here the intensity of grazing depends on both the number of prey and the number of predators.

Finally, the third case is when the intensity of grazing is determined by the presence of shelters and does not depend (of course, within certain limits) on the number of prey and the number of predators. An example is the consumption of young Atlantic salmon by fish-eating birds in spawning rivers. As shown by Elson (Elson, 1950, 1962), regardless of the initial population size of the prey, only such numbers can survive that are provided by shelters where the prey is inaccessible to the predator. Thus, the quantitative impact of a predator on a prey can be threefold: 1) when the amount eaten is determined by the duration of contact between the prey and the predator and the number and activity of the predator; 2) when the number of prey eaten depends on both the number of prey and predator and has little to do with the time of contact; 3) the number of victims eaten is determined by the availability of necessary shelters, i.e., the degree of accessibility for the predator. Although this classification is to a certain extent formal, it is convenient when developing a system of biotic reclamation measures.

The influence of the predator on the prey, its character and intensity, as stated, are specific to each stage of development, just as the forms of defense are specific. The main defense organs of Chinese perch larvae are the spines on operculum, and in fry - spiny rays of fins in combination with body height (Zakharova, 1950). In flying fish fry, this is swimming away from the pursuer and dispersing, and in adults, jumping out of the water.

The impact of most predators usually lasts a short period of time, both during the year and the day, and knowledge of these moments is necessary for the proper regulation of the impact of predators on a stock of commercial fish.

2014-06-02

Basic concepts and terms: predator, predation, Lotka-Volterra predation equations, numerical reaction of a predator, dynamics of the predator-prey system.
Predation is a one-way relationship between predator and prey, from which the predator benefits from coexistence with the prey, which feels the adverse effects. This particularly brutal form of interspecies relationships is one of the important factors influencing population growth.

Due to their predatory lifestyle, predators produced various forms of adaptation to catching and catching prey. These include: better development sensory organs, fast and accurate attacks on blows, agility and fast running, lightning-fast reaction, sneaking and various specific, relative to the living environment, adaptive characteristics of the species (long sticky tongues attached at the front end, precise aiming at frogs, chameleons, lizards; curved poisonous teeth in vipers, cobwebs and poisonous glands in spiders, etc.) (Fig. 9.7).

While waiting for prey, the spider usually hides near the mesh in a secret nest made of cobwebs. A signal thread is stretched from the center of the mesh to the nest. When a fly, small butterfly or other insect gets into the net and begins to flounder in it, the signal thread vibrates. According to these signs, the spider comes out of its hiding place and pounces on its prey, densely entangling it with a web. He points the claws of his upper jaws at her and injects poison into the body. Then the spider leaves the prey for a while and hides back in its secret nest.

An interesting example of adaptation between predator and prey is starlings and the peregrine falcon. The peregrine falcon, which is very sharp vision, catches prey in the air. Folding its wings, it falls like a stone onto its victim - a bird flying below, while developing a speed of up to 300 km / h. Starlings, having noticed a peregrine falcon, immediately huddle together to avoid its attack. The peregrine falcon does not dare to attack them in this state.

A characteristic feature of predators is a wide range of food. Specialization, that is, nutrition a certain type, placed them in a certain dependence on the number of this species. Therefore, most predatory species are able to switch from one prey to another, which is currently available. This ability is one of the necessary ecological adaptations in the life of a predator.

Victims also have different ways of passive and active protection from predators. With the passive method of protection, protective coloring, hard shells, spines, and the ability to find safe places develop. The active method of defense is due to the development of the victims’ sense organs, running speed, deceptive behavior, accompanied by the improvement of the nervous system.
Functioning complex system“predator-prey” was studied by ecologists Lotka and Volterra using modeling.

Black indicates the net increase in the prey population, and white indicates its decrease.
A - predators are ineffective, they insignificantly reduce the number of prey; its population remains near the equilibrium level (point c);
B - an increase in the efficiency of predators at a low density of prey can lead to its regulation by the predator (point a);
B - if the number of prey is limited by the capacity of the environment, then predators can effectively regulate the population of the prey and the equilibrium point disappears;
D - when the prey population is completely consumed, there is no equilibrium point.
When the density of the predator is low, the number of prey increases, and when the density is high, it decreases. The natural nature of this influence, provided for by modeling these processes in laboratory conditions, is disrupted in nature under the influence of various environmental factors. If, for example, severe drought or frost or an infectious disease significantly reduces the population of a predator and its numbers remain low for a long time, then, regardless of whether it recovers, there will be an increase in the number of prey. This situation often happens in agriculture when a pest (insects, mouse-like rodents) suddenly gives a threatening outbreak in numbers. After such an outbreak, predators (birds or others) cannot regulate the pest population, so they use pesticides that can sharply reduce the number of pests and restore the regulatory influence of predators again. However, ineffective predators cannot regulate prey populations when their density is low, so they insignificantly reduce the number of prey, leaving the population size near the equilibrium level, determined by the resources available in the environment.
The stabilization of the predator-prey relationship is facilitated by the ineffectiveness of the predator or the flight of the prey, the presence of other food resources in the territory, as well as a certain limiting effect of environmental factors (Fig. 9.9).
The reaction of a predator to the growth of the prey population by increasing its numbers due to the birth rate or immigration (arrival) of new individuals from other territories is called a numerical reaction.

The functional response is the dependence of the rate of consumption of prey by an individual predator on the population density of the prey. The functional response of many predators increases more slowly at lower prey abundances than at high prey abundances.
It is believed that the two-way interaction between predator and prey, which is characterized by a slowdown in the predator’s response to an increase in the number of prey, is unstable. By limiting the population growth of some species, predators play the role of regulators in the group and thereby contribute to its replenishment with other species.

Based on observations, ecologist R. Whittaker came to the conclusion that:
1. The prey plant survives if it finds shelter from the predator. To confirm this, he gives the example of St. John's wort (Hypericum perforatum), which was introduced from Europe to the western United States. It is poisonous to livestock, so it was not eaten and became the main weed of pastures. Along with this weed, a beetle (Chrysolina quadrigemina) was brought from Europe and feeds on it. It also multiplied so quickly that it actually exterminated St. John's wort. He remained under the cover of the forest, in the shade, where he became inaccessible. As a result, the beetle population has also decreased.

2. The relative stability of the plant is maintained by a predator, which prevents it from overgrowing in the pasture.

3. The current distribution of plants is driven by predators, and not by the plant’s resistance to environmental conditions.

Consumption of fish by other organisms, including fish, is one of the most important causes of mortality. In each species of fish, especially in the early stages of ontogenesis, predators usually constitute one of the most important elements of the environment, adaptations to which are very diverse. High fertility of fish, protection of offspring, protective coloration, various protective devices (thorns, prickles, poisonousness, etc.), protective behavioral features are various forms of adaptations that ensure the existence of the species under conditions of a certain pressure of predators.

Greater or lesser protection from predators is, respectively, associated with the development of the ability to compensate for greater or lesser death by changing the rate of population reproduction. Species adapted to significant predation can also compensate for large losses. Adaptation to a certain nature of the impact of predators is formed in fish, as in other organisms, during the formation of the faunal complex. During the process of speciation, coadaptation of predator and prey occurs. Predator species adapt to feed on certain types of prey, and prey species adapt in one way or another to limit the impact of predators and compensate for the loss.

Above, we examined the patterns of changes in fertility and, in particular, showed that populations of the same species in low latitudes are more fertile than in high latitudes. Closely related forms of the Pacific Ocean turn out to be more fertile than those of the Atlantic. Fish from the rivers of the Far East are more prolific than fish from the rivers of Europe and Siberia. These differences in fertility are associated with different predation pressure in these water bodies. Protective adaptations are developed in fish in relation to life in their respective habitats. In pelagic fish, the main forms of protection are the appropriate “pelagic” protective coloration, speed of movement and - for protection from the so-called daytime predators that navigate with the help of their visual organs - schooling. The protective value of the flock is apparently threefold. On the one hand, fish in a school detect a predator at a greater distance and can hide from it (Nikolsky, 1955). On the other hand, the flock also provides a certain physical protection from predators (Manteuffel and Radakov, 1960, 1961). Finally, as noted in relation to cod (predator) and juvenile pollock (prey), the multiplicity of prey and the defensive maneuvers of the school disorient the predator and make it difficult for it to catch prey (Radakov, 1958, 1972; Hobson, 1968).

The protective value of the school is not preserved in many fish species at all stages of ontogenesis. Usually it is characteristic of the early stages: in adult fish, a schooling lifestyle, losing its protective function, manifests itself only in certain periods of life (spawning, migration). Schooling as a protective device is usually characteristic of juvenile fish in all biotopes, both in the pelagic zone and in the coastal zone of the seas, both in rivers and lakes. The school serves as protection from daytime predators, but makes it easier for nocturnal predators, who navigate the search for food using other senses, to find fish in the school. Therefore, in many fish, for example, herrings, the school breaks up at night and individuals stay alone, only to reassemble into a school at dawn.

Coastal benthic and bottom fish also have different methods of protection from predators. The main role is played by various morphological protective devices, various thorns and spines.

The development of “weapons” in fish against predators is far from the same in different faunas. In the faunas of seas and fresh waters of low latitudes, the “armament” is usually more intensively developed than in the faunas of higher latitudes (Table 76). In the faunas of low latitudes, the relative and absolute number of fish “armed” with thorns and prickles is much greater, and their “weapons” are more developed. There are more poisonous fish in low latitudes than in high latitudes. In marine fish, protective devices in the same latitudes are more developed than in freshwater fish.

Among the representatives of the ancient deep-sea fauna, the percentage of “armed” fish is incomparably lower than in the faunas of the continental shelf.

In the coastal zone, the "armament" of fish is much more developed than in the open part of the sea. Along the coast of Africa, in the Dakar region in the coastal zone, “armed” fish species in trawl catches make up 67%, and away from the coast their number decreases to 44%. A slightly different picture is observed in the Gulf of Guinea region. Here, in the coastal zone, the percentage of “armed” species is very small (only Ariidae catfish), and further from the coast it increases significantly (Radakov, 1962; Radakov, 1963). The smaller percentage of “armed” fish in the coastal zone of the Gulf of Guinea is associated with the high turbidity of the coastal waters of this area and, because of this, the impossibility of hunting here for “visual predators”, which concentrate in adjacent areas with clear water. In an area with turbid water, less numerous predators are represented by species that focus on prey using other senses (see below).

The relative number of “armed” fish also differs in the North Atlantic Ocean and the Pacific Ocean (Clements a. Wilby, 1961): in the North Pacific Ocean the percentage of “armed” fish is much higher than in the North Atlantic. A similar pattern is observed in fresh waters. Thus, in the rivers of the Arctic Ocean basin there are fewer “armed” fish than in the Caspian and Aral Sea basins. Different “armament” is also characteristic of fish inhabiting different biotopes. In the direction from the upper reaches to the lower reaches of the river, the relative number of “armed” fish usually increases. This has been observed in rivers of different types and latitudes. For example, in the middle and lower reaches of the Amu Darya there are about 50 fish with thorns and spines, and in the upper reaches - about 30%. In the middle and lower reaches of the Amur there are more than 50 “armed” species, and in the upper reaches less than 25% (Nikolsky, 1956a). True, there are exceptions to this rule in rivers flowing from south to north in the northern hemisphere.

The intensity or, so to speak, power of the development of “weapons” in different zones also varies very significantly. As shown by I. A. Paraketsov (1958), related species of the North Atlantic have less developed “weapons” than species of the Pacific Ocean. This can be clearly seen in the representatives of the family. Scorpaenidae and Cottidae (Fig. 53).

Naturally, the development of thorns and prickles does not create absolute protection from predators, but only reduces the intensity of the predator’s impact on the prey herd. As M. N. Lishev (1950), I. A. Paraketsov (1958), K. R. Fortunatova (1959) and other researchers showed, the presence of spines makes fish less accessible to predators than fish of a similar biological type and shape, but devoid of thorns. This is most clearly shown by M. N. Lishev (1950) using the example of eating common and spiny bitterlings in the Amur. Protection from predators is provided not only by the presence of spines (the possibility of pricking), but also by an increase in body height, for example in sticklebacks (Fortunatova, 1959), or in the width of the head, for example in sculpins (Paraketsov, 1958). The protective value of thorns and spines varies depending on the size and method of hunting of the predator eating the “armed” fish, as well as on the behavior of the prey. For example, stickleback in the Volga delta turns out to be accessible to different predators different sizes. Perch has the smallest fish in its food, pike has larger ones, and catfish has the largest ones (Fortunatova, 1959) (Fig. 54). As shown by Frost (1954) using the example of pike, as the size of the predator increases, the percentage of its consumption of “armed” fish also increases.

The intensity of consumption of “armed” fish depends to a very large extent on how well the predator is provided with food. In hungry fish with an insufficient food supply, the intensity of consumption of “armed” fish increases. This is well demonstrated in an experiment with stickleback (Hoogland, Morris a. Tinbergen, 1956-1957). Here we have a special case of a general pattern, when, in conditions of insufficient supply of basic, most accessible food, the nutritional spectrum expands due to less accessible food, the extraction and assimilation of which requires more energy.

We considered only two forms of protection of prey from predators: school behavior and “arming” of prey, although the forms of protection can be very diverse: this is the use of certain shelters, for example, burying in the ground, and some behavioral features, for example, “hook” in juveniles pollock (Radakov, 1958), and vertical migrations (Manteuffel, 1961), and the toxicity of meat and caviar, and many other methods. The intensity of the predator's impact on the prey population depends on many factors. Naturally, each predator is adapted to feed in certain conditions and with certain types of prey. The specificity of the predators that feed on them depends to a very large extent on the nature of the prey’s habitat. In the turbid waters of the rivers of Central Asia, the main type of predators are fish that focus on prey using the organs of touch and lateral line organs. Their organ of vision does not play a significant role in the hunt for victims. Examples include the great shovelnose Pseudoscaphyrhynchus kaufmanni(Bogd.) and common catfish Silurus glanis L. These fish feed both day and night. In rivers with clearer water, catfish are a typical nocturnal predator. In the upper reaches of the rivers of the European North and Siberia, where the water is clean and transparent, predators (taimen Hucho taimen Pall., lenok Brachymystax lenok Pall., pike Esox lucius L.) focus on prey mainly using the organ of vision and hunt mainly during daylight hours. In this zone there are probably only burbot Lota lota(L.), which focuses on prey mainly through the senses of smell, touch and taste, feeds mainly at night. The same is observed in the seas. Thus, in the coastal turbid waters of the Gulf of Guinea, predators navigate mainly using the organs of touch and the lateral line. The organ of vision in this biotope plays a subordinate role among predators. Further from the coast, beyond the zone of turbid water, in the Gulf of Guinea in water of high transparency, the main place is occupied by predators that focus on prey using the organ of vision, such as Sphyraena, Lutianus, tuna, etc. (Radakov, 1963).

The hunting methods of predators that obtain food in thickets and in open waters are also different. In the first case, ambush predators predominate, in the second, those that steal prey predominate. For many predators and within the same habitat, there is a clearly expressed change in the food eaten at different times of the day: for example, burbot eats sedentary invertebrates during the day and hunts for fish at night (Pavlov, 1959). Perkarina Perkarina maeotica Kuzn. in the Sea of Azov during the day it feeds mainly on copepods and mysids, and at night it eats sprat Clupeonella delicatula Nordm. (Kanaeva, 1956).

In the case of two or more predators hunting for one prey, the availability of the latter greatly increases. This was shown in an experiment by D.V. Radakov (1958), when several predators (cod) ate victims much faster than one predator at the same prey density. The intensity of grazing especially increases if the fish is simultaneously hunted by predators of different biological types. One common way for a fish to protect itself from a predator is to move to another habitat where the prey is out of reach of the predator, such as avoiding large predators in shallow water, or being pressed to the bottom from pelagic predators, or finally, flying fish jumping into the air.

The intensity of predation by predators can often change quite dramatically if the latter themselves are under the influence of the predator. So, for example, during the migration of juvenile pink salmon and chum salmon from the tributaries of the Amur in the lower reaches of the tributaries, they are eaten in large quantities by the chebak Leuciscus waleckii (Dyb.), and if the pike Esox reicherti Dyb., for which the chebak is the main food, lives here in the lower reaches of the tributary , the activity of the chebak as a consumer of rolling juvenile salmon is sharply reduced.

Naturally, the influence of a predator on the prey population does not occur with the same intensity throughout the year. Typically, intense mortality from predators occurs over a relatively short period of time, when the period of active feeding of the predator coincides with the state of the prey when it is relatively easily accessible to the predator. This was shown above using the example of smelt. At catfish Silurus glanis L. delta Volga roach Rutilus rutilus caspicus Jak. plays an important role in food in the spring, from mid-April to mid-May, when the catfish eats 68% of its annual diet; In summer, in June and July, the main food of catfish is young carp Cyprinus carpio L., rolling down from the hollows into the delta front, and in the fall - again roach, coming from the sea to the lower reaches of the Volga for the winter. Thus, roach is important in catfish food for only about two months - during the spawning run, spawning and during migration in the fall for wintering; at other times, catfish in the Volga delta practically do not feed on roach.

For a very limited time, cod feed on capelin. Intensive feeding of cod on capelin usually lasts about a month.

In the Amur, predators usually feed intensively on small smelt in two stages: in the spring, during its spawning, and in the fall, during its migration upstream in the coastal zone (Vronsky, 1960).

The reasons for the selective feeding of predators in relation to prey are varied. The most common reason is the correspondence of the relative size and structure of the predator to the size and structure, in particular, the presence of certain protective devices of the prey (thorns, spines). The different accessibility of different genders is essential. So, for example, in gobies and sticklebacks, while protecting the nest, it is usually the males that are eaten by predators in greater numbers. This is noted, for example, in Gobius paganellus(L.), which is compensated by the large percentage of males in the offspring of this species (Miller, 1961). The smaller consumption of large fish during the feeding period compared to the consumption of juveniles can often be associated with their greater caution (Milanovsky and Rekubratsky, 1960). In general, most predator fish feed on the immature part of the prey stock. The sexually mature part of the stock, especially large fish, is eaten by predators in relatively small quantities. In this respect, the impact of predators differs from the impact of harvesting, which, as a rule, removes mainly mature individuals from the population. Thus, from the roach herd, predators (pike perch, catfish, pike) take fish mainly from 6 to 18 cm in length, and the fishery takes fish from 12 to 23-25 cm in length (Fig. 56).

If we add to this the consumption of roach fry by juveniles of predatory fish, the difference will be even more significant (Fortunatova, 1961).

1) when a predator eats a certain number of victims, and the rest avoids capture;

2) when a predator eats out a certain part of the prey population;

The influence of the predator on the prey, its character and intensity, as stated, are specific to each stage of development, just as the forms of defense are specific. In the larvae of Chinese perch, the main defense organs are the spines on the gill cover, and in the fry, the spiny rays of the fins in combination with the height of the body (Zakharova, 1950). In flying fish fry, this is swimming away from the pursuer and dispersing, and in adults, jumping out of the water.

There is a huge number of studies in the biological literature in which these systems are either observed in nature or simulated using “model” populations in the laboratory. However, their results often contradict each other: in some experiments oscillations are observed, in others they are not; either the system collapses quite quickly (the predator dies, but the prey remains, or the prey dies, followed by the predator); or the predator and prey coexist for a long time. Apparently, things are not so simple in this very simple ecological system. The question naturally arises: under what conditions is this community stable, what mechanisms ensure this stability? In this chapter, using mathematical models of predator-prey communities, we will try to answer this question.

Another problem is being actively discussed, which can be briefly formulated as follows: “Can a predator regulate the number of prey?” Naturally, the prey population (in the absence of a predator) has its own internal regulatory mechanisms (for example, intraspecific competition or epizootics) that limit the growth of its population.

But in this case, if the population-limiting factors did not act (or did so at sufficiently large numbers) and the prey population grew exponentially in the absence of a predator, would the influence of the predator lead to the stabilization of the entire system as a whole? Are both species remaining limited in number and are one or both going extinct? The answers to these questions constitute the solution to the problem “can a predator regulate the number of prey.”

Finally, last question: “Will random environmental disturbances lead to the collapse of the predator-prey system or will both persist?” - is considered within the framework of the analysis of equations of the predator-prey system with random perturbations of parameters.

Predation

Often the term “predation” is used to define any consumption of some organisms by others. In nature, this type of biotic relationships is widespread. Their outcome determines not only the fate of an individual predator or its prey, but also some important properties of such large ecological objects as biotic communities and ecosystems.

The significance of predation can only be understood by considering the phenomenon at the population level. The long-term connection between the populations of predator and prey gives rise to their interdependence, which acts like a regulator, preventing too sharp fluctuations in numbers or preventing the accumulation of weakened or sick individuals in populations. In some cases, predation can significantly weaken the negative consequences of interspecific competition and increase the stability and diversity of species in communities. It has been established that during the long-term coexistence of interacting species of animals and plants, their changes occur in concert, that is, the evolution of one species partially depends on the evolution of the other. Such consistency in the processes of joint development of organisms different types called coevolution.

Fig.1. Predator catching up with its prey

Adaptation of predators and their prey in joint evolutionary development leads to the fact that the negative influence of one of them on the other becomes weaker. In relation to a population of predator and prey, this means that natural selection will act in opposite directions. In a predator, it will be aimed at increasing the efficiency of searching, catching and eating prey. And in the prey - to favor the emergence of such adaptations that allow individuals to avoid detection, capture and destruction by a predator.

As the prey gains experience in avoiding the predator, the latter develops more effective mechanisms for catching it. In the actions of many predators in nature there seems to be prudence. For a predator, for example, it is “unprofitable” for the complete destruction of the victim, and, as a rule, this does not happen. The predator destroys first of all those individuals that grow slowly and reproduce poorly, but leaves individuals that are fast growing, fertile, and hardy.

Predation requires a lot of energy. During hunting, predators are often exposed to danger. For example, large cats often die when attacked, for example, in collisions with elephants or wild boars. Sometimes they die from collisions with other predators during interspecies struggle for the loot. Feeding relationships, including predation, may cause regular periodic fluctuations in the population size of each of the interacting species.

Predator-prey relationship

Periodic fluctuations in the number of predators and their prey have been confirmed experimentally. Ciliates of two species were placed in a common test tube. Predatory ciliates quickly destroyed their victims, and then themselves died of starvation. If cellulose (a substance that slows down the movement of predator and prey) was added to the test tube, cyclic fluctuations began to occur in the numbers of both species. At first, the predator suppressed the population growth of the peaceful species, but subsequently it began to experience a lack of food resources. As a result, there was a decrease in the number of the predator, and consequently a weakening of its pressure on the prey population. After some time, the growth in the number of victims resumed; its population increased. Thus, favorable conditions arose again for the remaining predatory individuals, which responded to this by increasing the rate of reproduction. The cycle repeated itself. Subsequent study of the relationships in the “predator-prey” system showed that the stability of existence of both the predator and prey populations increases significantly when mechanisms of self-limitation of population growth (for example, intraspecific competition) operate in each of the populations.

What is the significance of predator populations in nature? By killing the weaker ones, the predator acts like a breeder selecting seeds that produce the best seedlings. The influence of the predator population leads to faster renewal of the prey population, since rapid growth leads to earlier participation of individuals in reproduction. At the same time, the victims' consumption of their food increases (rapid growth can only occur with more intense food consumption). The amount of energy stored in food and passed through a population of rapidly growing organisms also increases. Thus, exposure to predators increases the flow of energy in the ecosystem.

As a result of the selective destruction by predators of animals with a low ability to obtain food for themselves (slow, frail, sick), the strong and hardy survive. This applies to the entire animal world: predators improve (qualitatively) the populations of their prey. Of course, in livestock-raising areas it is necessary to regulate the number of predators, since the latter can cause harm to livestock. However, in areas not accessible to hunting, predators must be conserved to benefit both prey populations and the plant communities that interact with them.

Fig.2. Tongue-eating woodlouse (Cymothoa exigua)