Effect of predation on prey population. Spatial distribution of populations Topic: “General and natural mortality of fish”

2014-06-02

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

In connection with the predatory way of life, predators produced various forms adaptations for catching and catching prey. These include: better development of the senses, quick and accurate attack on blows, agility and fast running, lightning-fast reaction, sneaking up and a variety of specific, relative to the living environment, adaptive features of the species (long sticky tongues attached by the front end, accurate aiming at frogs , chameleons, lizards, curved poisonous teeth in vipers, cobwebs and poisonous glands in spiders, etc.) (Fig. 9.7).

When waiting for prey, the spider usually hides near the net in a secret nest made of cobwebs. A signal thread is stretched from the center of the grid to the nest. When a fly, small butterfly, or other insect enters the net and begins to flounder in it, the signaling thread vibrates. By this sign, the spider comes out of its hiding place and pounces on prey, densely entangling it with a web. He directs the claws of the upper jaws into her and injects poison into the body. The spider then leaves the prey for a while and hides back to its innermost nest.

An interesting example of the adaptation of a predator and prey is starlings and a peregrine falcon. Peregrine Falcon, which is very sharp eyesight, catches prey in the air. Having folded its wings, it falls like a stone down onto the victim - a bird flying below, while developing a speed of up to 300 km / h. Starlings, noticing a peregrine falcon, in order to avoid its attack, instantly huddle together. 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, feeding on a certain species, put them in a certain dependence on the abundance 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.

Prey also have different ways of passive and active defense against predators. With the passive method of protection, a protective coloration, hard shells, spikes, and the ability to find safe places develop. The active method of protection is due to the development of the victims' sense organs, running speed, deceptive behavior, accompanied by the improvement of the nervous system.
The functioning of the complex system "predator-prey" was studied by the ecologists Lotka and Volterra by modeling.

Black indicates the net increase in the number of prey, and white indicates its reduction.
A - predators are inefficient, 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 prey density can lead to its regulation from the side of the predator (point a);
B - if the number of prey is limited by the capacity of the environment, then predators can effectively regulate the prey population and the equilibrium point disappears;
D - when the prey population is completely eaten away, there is no equilibrium point.
At a low predator density, the number of prey increases, and at a high density, it decreases. The natural nature of such an influence, provided for by modeling these processes in laboratory conditions, is violated in nature under the influence of various environmental factors. If, for example, a 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, the number of prey will increase. This situation often occurs in agriculture when a pest (insects, mouse-like rodents) suddenly gives a threatening outbreak of numbers. After such an outbreak, predators (birds or others) cannot regulate the pest population, so they use pesticides that can drastically reduce the number of pests and restore the regulatory effect of predators again. However, inefficient predators cannot regulate prey populations at its low density, so they insignificantly reduce the number of prey, leaving the population size near the equilibrium level, determined by the resources available in the environment.
Stabilization of the predator-prey relationship is facilitated by the inefficiency of the predator or the flight of the prey, the presence of other food resources on the territory, as well as a certain limiting effect of environmental factors (Fig. 9.9).
The reaction of a predator to an increase in the population of a prey by increasing its numbers due to the birth rate or immigration (receipt) of new individuals from other territories is called a numerical response.

A functional response is the dependence of the rate of eating a prey by an individual predator on the population density of the prey. The functional response of many predators grows more slowly at lower prey numbers than at high ones.
It is believed that the bilateral predator-prey interaction, which is characterized by a slowdown in the predator's response to an increase in the number of prey, is unstable. By limiting the growth of a population 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 concluded that:
1. A prey plant survives if it finds shelter from a predator. To confirm this, he cites 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 by her and became the main weed of pastures. Together with this weed, a beetle (Chrysolina quadrigemina) was introduced from Europe, which feeds on it. He also multiplied so quickly, in fact, exterminated St. John's wort. He remained under the cover of the forest, in the shade, where he became inaccessible. As a result, the population of the beetle also decreased.

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

3. The modern distribution of plants is driven by a predator, and not by the resistance of the plant 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 make up one of essential elements environments, to which adaptations are very diverse. Greater fecundity of fish, protection of offspring, protective coloration, various protective devices (thorns, spines, poisonousness, etc.), protective features of behavior are various forms of adaptations that ensure the existence of a species under conditions of a certain pressure of predators.

In nature, there are no fish species that would be free from more or less, but the natural impact of predators. Some species are affected to a greater extent and at all stages of ontogenesis, for example, anchovies, especially small ones, herring, gobies, etc. Others are affected to a lesser extent and mainly at 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, some species of cyprinids. Finally, the third group consists of species in which death from predators is very small even at the early stages of ontogeny. Only some sharks and rays belong to this group. Naturally, the boundaries between these groups we have identified are conditional. In fish adapted to a significant predation pressure, a smaller percentage die of old age as a result of senile metabolic disorders.

Greater or lesser protection from predators, respectively, is 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 grazing by predators can also compensate for large losses. Adaptation to a certain nature of the impact of predators is formed in fish, as in other organisms, in the process of the formation of a faunal complex. In the process of speciation, co-adaptation of predator and prey takes place. Predator species adapt to feed on a certain type 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 fertility changes and, in particular, showed that populations of the same species in low latitudes ah are more prolific than the high ones. Related forms Pacific Ocean are more prolific than the forms of the Atlantic. river fish Far East more prolific than the fish of the rivers of Europe and Siberia. These differences in fecundity are associated with different pressure of predators in these water bodies. Protective devices are developed in fish in relation to life in their respective habitats. In pelagial fish, the main forms of protection are the corresponding "pelagic" protective coloration, speed of movement, and - for protection against the so-called diurnal predators, guided by the organs of vision - flock formation. The protective value of the flock, apparently, is threefold. On the one hand, fish in a school detect a predator at a greater distance and can hide from it (Nikol'skii, 1955). On the other hand, a flock also provides a certain physical defense against predators (Manteuffel and Radakov, 1960, 1961). Finally, as noted for cod (predator) and juvenile saithe (prey), the multiplicity of prey and protective maneuvers of the flock disorientate the predator and make it difficult for him to catch prey (Radakov, 1958, 1972; Hobson, 1968).

The protective value of the flock is retained in many fish species not at all stages of ontogenesis. It is usually characteristic of the early stages: in adult fish, a schooling lifestyle, which loses its protective function, manifests itself only in certain periods of life (spawning, migration). A flock as a protective adaptation 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 flock serves as protection against diurnal predators, but facilitates the search for fish in the flock by nocturnal predators, who navigate in search of food with the help of other senses. Therefore, in many fish, for example, in herring, the flock breaks up at night and the individuals stay alone in order to gather again at dawn in the flock.

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

The development of "weapons" in fish against predators is far from being the same in different faunas. In the faunas of the seas and fresh waters of low latitudes, the "armament" is usually more intensively developed than in the faunas of higher latitudes (Table 76). In faunas of low latitudes, the relative and absolute number of fish "armed" with spines and spines is much greater, and their "armament" is more developed. In low latitudes, there are more poisonous fish than in high latitudes. At marine fish protective adaptations in the same latitudes are more developed than in fresh water fish.

Among the representatives of the ancient deep-sea fauna, the percentage of "armed" fish is incomparably less than in the fauna of the continental shelf.

In the coastal zone, the "equipment" 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 account for 67%, and far from the coast their number decreases to 44%. A somewhat different picture is observed in the region of the Gulf of Guinea. Here, in the coastal zone, the percentage of "armed" species is very low (only Ariidae catfishes), while far from the coast it increases significantly (Radakov, 1962; Radakov, 1963). A 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 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 orient themselves to the prey with the help of other sense organs (see below).

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

The relative number of "armed" fish is also different 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, there are fewer "armed" fish in the rivers of the Arctic Ocean basin than in the basin of the Caspian Sea and the Aral Sea. Different "armament" is also characteristic of fish inhabiting different biotopes. In the direction from the upper to the lower reaches of the river, the relative abundance of "armed" fish usually increases. It 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 spines and spines, and about 30% in the upper reaches. In the middle and lower reaches of the Amur, there are more than 50 "armed" species, and less than 25% in the upper reaches (Nikol'skii, 1956a). True, there are exceptions to this rule in rivers flowing from south to north in the northern hemisphere.

So, in r. The Ob, for example, fails to notice a noticeable difference in the "equipment" of fish in the upper and lower reaches. In the lower reaches, the percentage of "armed" species becomes even somewhat smaller.

The intensity or, so to speak, the power of the development of "weapons" in different zones also varies greatly. As IA Paraketsov (1958) showed, closely related species of the North Atlantic have a less developed "armament" than those of the Pacific Ocean. This can be clearly seen in the representatives of this family. Scorpaenidae and Cottidae (Fig. 53).

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

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

The intensity of consumption of "armed" fish to a very large extent also depends on how 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 shown in the stickleback experiment (Hoogland, Morris a. Tinbergen, 1956-1957). Here we have a special case of the general pattern, when, under conditions of insufficient provision of the main, most accessible food, the nutrition spectrum expands due to less accessible food, the extraction and assimilation of which requires more energy.

The behavior of prey is essential for the accessibility of "armed" fish to predators. As a rule, fish are eaten by predators during the period of their greatest activity. 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 mass appearance of juveniles, at the end of June - beginning of July (Fig. 55) (Fortunatova, 1959).

We have considered only two forms of prey protection from predators: flocking behavior and "armament" of prey, although forms of protection can be very diverse: this is the use of certain shelters, for example, digging into the ground, and some behavioral features, for example, "hook" in juveniles. saithe (Radakov, 1958), and vertical migrations (Manteuffel, 1961), and the toxicity of meat and caviar, and many other ways. The intensity of the effect of a predator on the prey population depends on many factors. Naturally, each predator is adapted to feed in certain conditions and certain types of prey. The nature of the habitat of the prey to a very large extent depends on the specificity of the predators that feed on them. In the muddy waters of the rivers of Central Asia, the main type of predators are fish that focus on prey with the help of the organs of touch and the organs of the lateral line. The organ of vision in them does not play a significant role in the hunt for victims. Examples are the great shovelnose Pseudoscaphyrhynchus kaufmanni(Bogd.) and common catfish Silurus glanis L. These fish feed both day and night. In rivers with more transparent 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.) are guided by prey mainly with the help of the organ of vision and hunt mainly in the daytime. In this zone, only, perhaps, burbot lota lota(L.), which focuses on prey mainly by smell, touch, and taste, feeds mainly at night. The same is observed in the seas. Thus, in the coastal muddy waters of the Gulf of Guinea, predators navigate mainly with the help of the organs of touch and the lateral line. The organ of vision in this biotope plays a subordinate role in predators. Farther from the coast, beyond the zone of muddy water, in the Gulf of Guinea, in the water of high transparency, the main place is occupied by predators that focus on prey with the help of the organ of vision, such as Sphyraena, Lutianus, tuna, etc. (Radakov, 1963).

The methods of hunting for predators that get food in thickets and in open waters are also different. In the first case, ambush predators predominate, in the second - catching prey by stealing. In many predators and within the same habitat, a change in the food eaten in different time days: 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 kilka Clupeonella delicatula nordm. (Kanaeva, 1956).

The nature and intensity of the impact of predators on the population peaceful fish depend on many reasons: on the abiotic conditions in which hunting is carried out, on the presence and abundance of other types 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 change the availability of prey for predators. So, for example, in reservoirs, where, as a result of significant level fluctuations, underwater vegetation disappears, in the coastal zone, hunting conditions 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 is reflected in the intensity of their prey. In this regard, the feeding conditions of predators change especially strongly if prey belonging to other, more northern faunistic complexes appear in mass numbers. So, for example, in the harvest years in the Amur for small-mouthed 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 decreases sharply (Lishev, 1950). This was observed, for example, in 1947 and to a somewhat lesser extent in 1948, and in the smelt crop failure of 1946, predators switched to feeding on other foods and their food spectrum expanded.

A similar picture is observed in the seas; for example, in the Barents Sea, in years when capelin is harvested, this fish forms the basis of cod food in 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. Kultus, leads to the fact that predators of the same faunal complex that usually feed on it switch to a large extent to feed on other, less characteristic prey, sometimes moving during the feeding period to habitats that are less common for them, where their feeding conditions turn out to be worse ( Riker, 1941).

The presence of another predator that eats the same prey, or the presence of a predator in respect of which the first predator is a prey, has a significant effect on the intensity of prey consumption by a predator.

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 their prey much faster than one predator at the same prey density. The intensity of predation increases especially if predators of different biological types prey on the fish at the same time. One of the usual ways to protect the fish from a predator is to move to another habitat where the prey is inaccessible to the predator, for example, avoiding large predators in shallow water or pressing to the bottom from 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, when juveniles of the Far Eastern salmon Myoxocephalus in the rivers flowing into the Amur estuary), the intensity of grazing increases sharply, because moving away from pelagic predators to 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.

Often, the intensity of predation by predators can change quite dramatically if the latter are themselves under the influence of a 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 in large numbers It is eaten by the chebak Leuciscus waleckii (Dyb.), and if the pike Esox reicherti Dyb., for which the chebak is the main food, is kept here in the lower reaches of the tributary, the activity of the chebak as a consumer of the migration of salmon juveniles is sharply reduced.

A similar picture is observed in the Black Sea for anchovy, horse mackerel and bonito. In the absence of bonito Pelamys sarda(Bloch) horse mackerel Trachurus trachurus(L.) feeds quite heavily on anchovy Engraulis encrassicholus L. In the case of the appearance of bonito, for which horse mackerel is a victim, its consumption of anchovy decreases sharply.

Naturally, the effect of a predator on the prey population is not carried out with the same intensity throughout the year. Usually, intense death from predators takes place during 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 with the example of smelt. Catfish Silurus glanis L. Volga delta vobla rutilus rutilus caspicus Jak. plays important role in food in 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 juvenile carp Cyprinus carpio L., rolling down from the hollows to the fore-delta, and in autumn - again vobla, coming from the sea to the lower reaches of the Volga for wintering. Thus, roach in the food of catfish matters only about two months - during the spawning run, spawning and during migration in autumn for wintering; at other times, catfish in the Volga delta practically does not feed on roach.

A different picture is observed in asp Aspius aspius(L.): it intensively eats young voblas in summer, when they run down from spawning reservoirs, mainly in the surface layers of the middle part of the river and are inaccessible to catfish, but are well accessible to asp. Behind summer months(June-July) asp eats 45% of its annual diet, and 83.3% (in terms of the number of pieces) of all food is roach juveniles. During the rest of the year, asp hardly feeds on roach (Fortunatova, 1962).

Pike, like catfish, eats mainly spawning vobla in the lower zone of the delta, where larger pikes are kept. Rolling down juveniles of roach for pike, as well as for catfish, is inaccessible (Popova, 1961, 1965).

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

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

The conditions of the impact of predators on prey vary greatly in different hydrological regime years. In river water bodies, 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 which part of the population is affected by the predator, it causes a corresponding restructuring of the structure of the prey population. It is safe to say that most predators selectively remove individuals from the population. Only in some cases this removal is not selective, and the predator takes the prey in the same size ratio as it is contained in the population. So, for example, beluga whale Delphinapterus leucas, various seals, kaluga Huso dauricus(Georgi) and some other predators eat fish out of a herd of running chum salmon without selecting certain sizes. The same is apparently observed with regard to the migration of juvenile Far Eastern salmon - chum salmon and pink salmon. Probably, cod feeding on spawning capelin is non-selective. In the majority of 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 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, prey (thorns, spines). The different accessibility of different sexes is essential. So, for example, in gobies, in sticklebacks, during the protection of the nest, males are usually eaten away by predators in larger numbers. This is noted, for example, in Gobius paganellus(L.), which is compensated by a large percentage of males in this species in the offspring (Miller, 1961). Less grazing big fish during the feeding period, compared to eating juveniles, it can often be associated with their greater caution (Milanovsky and Rekubratsky, 1960). In general, most predatory fish feed on the immature part of the prey herd. The sexually mature part of the herd, especially in large fish, is eaten away by predators in relatively small quantities. In this, the impact of predators differs from the impact of harvesting, which usually removes mainly sexually mature individuals from the population. Thus, predators (perch, catfish, pike) take fish from roach herds mainly from 6 to 18 cm in length, while fishing takes fish from 12 to 23-25 cm in length (Fig. 56).

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 effect of predators on the structure of the prey population usually affects by eating juveniles, i.e., reducing the size of recruitment, which causes an increase in the average age of the mature part of the population. What part of the whole herd of fish is eaten away by predators and what relative value of mortality a population can compensate for by reproduction, we still know very poorly. 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 on what part of the population was eaten by predators in the literature. This is hampered by the fact that it is not possible to determine the total size of either the prey population or the predator that feeds on it. However, in some cases, attempts of this kind have been made. So, Crossman (Crossman, 1959) determined that rainbow trout Salmo gairdneri Rich, eats out in the lake. Paul (Paul Lake) 0.15 to 5% of the population Richardsonius balteatus(Rich.).

Sometimes it is possible to approximately determine the ratio of natural and commercial mortality in relation to some species; for example, K. R. Fortunatova (1961) showed that predators eat only a little less roach than it is caught by fishing (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 predator-prey relationships:

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

2) when a predator eats 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, Rikker cites the feeding of predators by spawning aggregations of herring or rolling salmon fry. In this case, the number of fish eaten is determined by the duration of contact with predators.

As an example of the second type, Rikker cites eating nearby predators in the lake. Cultus of juvenile sockeye salmon, which these predators feed on throughout the year: here the intensity of predation 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 (naturally, within certain limits) on the number of prey and the number of predators. An example is the eating of juvenile Atlantic salmon by fish-eating birds in spawning rivers. As shown by Elson (Elson, 1950, 1962), regardless of the initial size of the prey population, only such an amount can survive that is provided with shelters, where the prey is inaccessible to the predator. Thus, the quantitative impact of the predator on the prey can be threefold: 1) when the amount eaten is determined by the duration of contact between the prey and the predator and the abundance and activity of the predator; 2) when the number of prey eaten depends on both the number of prey and the predator and has little to do with the time of contact; 3) the number of prey eaten is determined by the availability of the necessary shelters, i.e., the degree of accessibility for the predator. Although this classification is formal to a certain extent, it is convenient when developing a system of measures for biotic melioration.

The effect of the predator on the prey, its nature and intensity, as was said, are specific to each stage of development, just as the forms of defense are specific. In the larvae of the Chinese perch, the main organs of defense are the spikes on the gill cover, and in the fry, the spiny rays of the fins, combined with the height of the body (Zakharova, 1950). In fry of flying fish, this is swimming away from the pursuer and dispersal, 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 correct regulation of the impact of predators on a stock of commercial fish.

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. The high fecundity of fish, the protection of offspring, protective coloration, various protective devices (thorns, spines, poisonousness, etc.), protective behavioral features are various forms of adaptations that ensure the existence of a species under conditions of a certain pressure of predators.

In nature, there are no fish species that would be free from more or less, but the natural impact of predators. Some species are affected to a greater extent and at all stages of ontogenesis, for example, anchovies, especially small ones, herring, gobies, etc. Others are affected to a lesser extent and mainly at 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 species of cyprinids. Finally, the third group consists of species in which death from predators is very small even at the early stages of ontogeny. Only some sharks and rays belong to this group. Naturally, the boundaries between these groups we have identified are conditional. In fish adapted to a significant predation pressure, a smaller percentage die of old age as a result of senile metabolic disorders.

Above, we examined the regularities of changes in fertility and, in particular, showed that populations of the same species in low latitudes are more fertile than in high latitudes. The closely related forms of the Pacific Ocean are more fertile than the forms of the Atlantic. The fish of the rivers of the Far East are more prolific than the fish of the rivers of Europe and Siberia. These differences in fecundity are associated with different pressure of predators in these water bodies. Protective devices are developed in fish in relation to life in their respective habitats. In pelagial fish, the main forms of protection are the corresponding "pelagic" protective coloration, speed of movement, and - for protection against the so-called diurnal predators, guided by the organs of vision - flock formation. The protective value of the flock, apparently, is threefold. On the one hand, fish in a school detect a predator at a greater distance and can hide from it (Nikol'skii, 1955). On the other hand, a flock also provides a certain physical defense against predators (Manteuffel and Radakov, 1960, 1961). Finally, as noted for cod (predator) and juvenile saithe (prey), the multiplicity of prey and protective maneuvers of the flock disorientate the predator and make it difficult for him to catch prey (Radakov, 1958, 1972; Hobson, 1968).

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

The development of "weapons" in fish against predators is far from being the same in different faunas. In the faunas of the seas and fresh waters of low latitudes, the "armament" is usually more intensively developed than in the faunas of higher latitudes (Table 76). In faunas of low latitudes, the relative and absolute number of fish "armed" with spines and spines is much greater, and their "armament" is more developed. In low latitudes, there are more poisonous fish than in high latitudes. In marine fish, protective adaptations in the same latitudes are more developed than in fish in fresh water.

Table 76

Among the representatives of the ancient deep-sea fauna, the percentage of "armed" fish is incomparably less than in the fauna of the continental shelf.

In the coastal zone, the "equipment" 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 account for 67%, and far from the coast their number decreases to 44%. A somewhat different picture is observed in the region of the Gulf of Guinea. Here, in the coastal zone, the percentage of "armed" species is very low (only Ariidae catfishes), while far from the coast it increases significantly (Radakov, 1962; Radakov, 1963). A 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 the impossibility of hunting here for "visual predators", which concentrate in adjacent areas with clear water. In the zone with muddy water, less numerous predators are represented by species that orient themselves to the prey with the help of other sense organs (see below).

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

Rice. 53. Relationship between the growth of the preopercular spine (S) and the change in the length of the fish body (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)

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

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

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

We have considered only two forms of prey protection from predators: flocking behavior and "armament" of prey, although forms of protection can be very diverse: this is the use of certain shelters, for example, digging into the ground, and some behavioral features, for example, "hook" in juveniles. saithe (Radakov, 1958), and vertical migrations (Manteuffel, 1961), and the toxicity of meat and caviar, and many other ways. The intensity of the effect of a predator on the prey population depends on many factors. Naturally, each predator is adapted to feed in certain conditions and certain types of prey. The nature of the habitat of the prey to a very large extent depends on the specificity of the predators that feed on them. In the muddy waters of the rivers of Central Asia, the main type of predators are fish that focus on prey with the help of the organs of touch and the organs of the lateral line. The organ of vision in them does not play a significant role in the hunt for victims. Examples are the great shovelnose Pseudoscaphyrhynchus kaufmanni(Bogd.) and common catfish Silurus glanis L. These fish feed both day and night. In rivers with more transparent 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.) are guided by prey mainly with the help of the organ of vision and hunt mainly in the daytime. In this zone, only, perhaps, burbot lota lota(L.), which focuses on prey mainly by smell, touch, and taste, feeds mainly at night. The same is observed in the seas. Thus, in the coastal muddy waters of the Gulf of Guinea, predators navigate mainly with the help of the organs of touch and the lateral line. The organ of vision in this biotope plays a subordinate role in predators. Farther from the coast, beyond the zone of muddy water, in the Gulf of Guinea, in the water of high transparency, the main place is occupied by predators that focus on prey with the help of the organ of vision, such as Sphyraena, Lutianus, tuna, etc. (Radakov, 1963).

The methods of hunting for predators that get food in thickets and in open waters are also different. In the first case, ambush predators predominate, in the second - catching prey by stealing. In many predators, and within the same habitat, the change of food eaten at different times of the day is clearly expressed: for example, burbot eats inactive 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 kilka 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 types 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.

Often, the intensity of predation by predators can change quite dramatically if the latter are themselves under the influence of a 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 is eaten in large numbers by the chebak Leuciscus waleckii (Dyb.), moreover, if here in the lower reaches of the tributary there is a pike Esox reicherti Dyb., for which the chebak is the main food , the activity of the chebak as a consumer of the migration of juvenile salmon is sharply reduced.

Naturally, the effect of a predator on the prey population is not carried out with the same intensity throughout the year. Usually, intense death from predators takes place during 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 with the example of smelt. Catfish Silurus glanis L. Volga delta vobla 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 juvenile carp Cyprinus carpio L., rolling down from the hollows to the fore-delta, and in autumn - again vobla, coming from the sea to the lower reaches of the Volga for wintering. Thus, roach in the food of catfish matters only about two months - during the spawning run, spawning and during migration in autumn for wintering; at other times, catfish in the Volga delta practically does not feed on roach.

A different picture is observed in asp Aspius aspius(L.): it intensively eats young voblas in summer, when they run down from spawning reservoirs, mainly in the surface layers of the middle part of the river and are inaccessible to catfish, but are well accessible to asp. During the summer months (June-July), asp eats 45% of its annual diet, and 83.3% (by the number of pieces) of all food is roach fry. During the rest of the year, asp hardly feeds on roach (Fortunatova, 1962).

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

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

The conditions of the influence of predators on prey vary greatly in different hydrological years. In river water bodies, in high-water years, the availability of prey for predators is usually greatly reduced, and in years with low floods, it increases.

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, prey (thorns, spines). The different accessibility of different sexes is essential. So, for example, in gobies, in sticklebacks, during the protection of the nest, males are usually eaten away by predators in larger numbers. This is noted, for example, in Gobius paganellus(L.), which is compensated by a large percentage of males in this species in the offspring (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 predatory fish feed on the immature part of the prey herd. The sexually mature part of the herd, especially in large fish, is eaten away by predators in relatively small quantities. In this, the impact of predators differs from the impact of harvesting, which usually removes mainly sexually mature individuals from the population. Thus, predators (perch, catfish, pike) take fish from roach herds mainly from 6 to 18 cm in length, while fishing takes fish from 12 to 23-25 cm in length (Fig. 56).

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

If we add to this the eating of vobla fry by young predatory fish, then 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 a certain part of the prey population;

Topic: “General and natural mortality of fish“.

The dynamics of populations of an organism is a process of interaction of 3 interrelated processes: the birth, growth and loss of individuals.

Population decline is closely related to the reproduction and growth of individuals. Reproduction compensates for the loss, growth regulates both the intensity of the loss and the intensity of reproduction.

Fish with a short life cycle, maturing early, are adapted to a relatively stable mortality rate, starting from the juvenile period.

Causes of death.

Each species is characterized by a certain maximum age limit.

However, only a very small percentage of individuals die of old age; the bulk of the population dies from other causes. This mortality, caused by various causes, is compensated by the fecundity of individuals.

All causes of fish death can be subdivided:

1. from old age, including post-spawn mortality;

3. under the influence of abiotic conditions;

4. from violation of food supply;

5. as a result of a catch.

These reasons are interrelated and such division is to some extent artificial.

The value of total mortality is usually understood as the difference in the number of herds or one or another of its age groups at the beginning and end of a certain period of time.

Accordingly, the value of natural and commercial mortality is the initial number of the herd minus the number of deaths from natural causes or fish caught for a certain period of time.

For each species, not only the total mortality rate is specific, but also its distribution over individual age groups and stages of development.

In some species, the greatest death occurs at the stage of eggs, in others at the stages of a free embryo, in others at the stage of mixed nutrition or later stages. Thus, in Far Eastern salmon, the main mortality rate falls on the period of life in mounds at the stage of eggs and free embryos.

In many herds of Atlantic salmon and trout, the greatest death occurs in the first summer of life in the river after leaving the spawning mounds; in herring, anchovies, cod, and many other fish, at the stage of mixed feeding;

There are also different reasons why mass death fish at the stages of ontogeny.

At the stage of eggs and free embryos, the leading relationships and the main causes that determine death are abiotic conditions, primarily the conditions of respiration, as well as the impact of predators. With the transition to external feeding and the acquisition by the larva of the ability of active movement, the lethal effect of abiotic conditions usually decreases; I place is occupied by the influence of food supply and great importance predators retain as a factor of mortality.

Direct determinations of total mortality are feasible in rather rare cases, when it is possible to completely catch a water body from year to year and take into account all changes occurring in the population.

Currently, two groups of methods are commonly used to estimate total mortality:

1. analysis of the age composition of the population;

2. mass tagging and accounting for the return of tags.

Both methods are approximate.

The most accurate way is to compare the number of a generation of a certain year in catches with non-selective fishing gear for a number of years equal to the life expectancy of a generation. Taking the average catch of a generation per unit of fishing effort as 100% and subtracting from it the catch of this generation for the next year, expressed as a % of the catch of the previous year, we get the mortality for the year.

To determine the overall mortality, researchers use the analysis of age composition, believing that the right “shoulder” of the curve for the age composition of the catch by straining gear reflects for older age groups the ratio of age groups in the population.

In this case, the assumption is made that the initial size of generations is the same from year to year.

But there are fluctuations.

P. V. Tyurin calculates the overall mortality rate, and knowledge of the coefficients for each age group is required.

A. V. Zasosov introduces the instantaneous mortality rate,

where N is the herd size, t is the time.

The principle of determining mortality by the tagging method is as follows: it is assumed that the mortality rate in a population over a certain period of time corresponds to a decrease in the number of tagged fish in catches over this period.

Mortality of fish from old age. It is common to all organisms. Death from old age is a specific adaptive property. Within a population, the age limit may vary somewhat due to changes in food availability. If there is a lot of food, then the fish mature earlier and live less. The general pattern of mortality is specific to the species. Far Eastern salmon die after the first spawning; in Atlantic salmon, mainly males die after spawning.

Patterns of the impact of predators on the population.

All types of fish are susceptible to predators. Some species to a greater extent and at all stages of ontogeny (anchovies, herring, gobies, etc.), other species are affected to a lesser extent and mainly at the early stages of development. At later stages of development, the influence of predators weakens and disappears. This group includes catfish, sturgeons, barbels, yellow cheeks, etc.

Finally, the third group consists of species in which death from predators at the early stages of ontogenesis is small. Only some sharks and rays belong to this group. This division is conditional.

Species adapted to significant grazing by predators can also compensate for large deaths. Adaptations are formed in fish-predators and their prey mutually within one faunal complex.

In addition to fish, predators are coelenterates, mollusks, mainly cephalopods, crustaceans and insects. They mainly eat eggs and young fish.

Coastal bottom and bottom fish also have different methods of protection from predators. The main role is acquired by “weapons”.

Their development in prey fish is far from the same in different faunas. In the faunas of the seas and fresh waters of low latitudes, the “armament” is usually more intensively developed than in the faunas of higher latitudes (more in the Caspian Sea than in the Arctic Ocean). In low latitudes, there are more poisonous fish than in high latitudes. In marine fish, protective adaptations in the same latitudes are more developed than in fish in fresh water.

There are more armed fish on the shelf than in the ichthyofauna of the slope and plateau. This is seen in all oceans. If it is somewhat less, as in the Gulf of Guinea, then this is due to the greater turbidity of these waters, and the fish orient themselves with the help of other senses.

In general, the more stable the abiotic conditions of a particular zone, the higher the predation pressure in this zone. The opposite picture is observed in the direction from the depths to the coastal zone of the ocean: at depths, abiotic conditions are more stable than in the coastal zone, however, the intensity of the impact of predators is also apparently lower. Accordingly, predators of low latitudes turn out to be adapted to feed on better protected prey than predators of higher latitudes.

Naturally, the development of spines does not create absolute protection against predators, but only reduces the intensity of the predator's impact on the prey herd.

The protective value of spines and spines varies depending on the size and method of hunting by predators that eat “armed” fish, and also on the behavior of the victim.

Perch in the delta eat the most small fish, pike - larger, catfish eat the largest.

The larger the predator, the more armed fish it eats. The behavior of prey is essential for the accessibility of “armed” fish to predators. As a rule, fish are eaten by predators during the period of their greatest activity.

During the day, predators can change the set of food organisms (perkarina eats crayfish and mysids during the day, sprat at night). The nature and intensity of the impact of predators on the population of peaceful fish depend on many reasons, on the abiotic conditions in which hunting is carried out, on the presence and abundance of other types of prey.

The accessibility of the victim is of great importance. In spring, all predators of the Volga delta feed on spawning roach. Then they disperse to their ecological niches.

The intensity of feeding is influenced by the presence of other predators. For example, the appearance of bonito in the Black Sea reduces the intensity of feeding of jack mackerel with anchovy.

The different accessibility of different sexes is essential. So, for example, in gobies, in sticklebacks, during the protection of the nest, males are usually given out in large numbers, which is compensated by a large percentage of them in the litter.

Ricker (1952) distinguishes 3 types of possible predator-prey ratios:

1. when a predator eats a certain number of prey, and the rest avoids capture; Predators feed on spawning herring or rolling salmon fry. The number of fish eaten is determined by contact with the predator.

2. when a predator eats a certain part of the prey population in a limited place, a lake, for example, the intensity of grazing depends on both the number of prey and the number of the predator .;

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. Thus, the quantitative impact of the predator on the prey can be threefold:

when the amount eaten is determined by the duration of contact between the prey and the predator and depends on the activity of the predator,

when the number of prey eaten depends both on the number of prey and on the number of predators and has little to do with contact time,

the number of victims eaten is determined by the availability of necessary shelters, i.e. accessibility for predators.

Influence of abiotic factors on fish mortality.

The lethal effect of abiotic factors on the number of fish stocks is usually more pronounced at the edge of the species range or as a result of anthropogenic factors.

Development in adverse conditions leads to the development of deformities. Anthropogenic factor: drying of eggs in the downstream of dams, in reservoirs when water is discharged, discharge of toxic substances.

A significant cause of fish mortality under the influence of abiotic conditions are kills resulting from the development of putrefactive processes and the disappearance of oxygen from the water.

Low food supply as a cause of mortality.

Finally, in some cases, the deterioration in food supply leads to a prolongation of the feeding season and sometimes puts the population in unfavorable conditions (anchovy fattening in the Sea of Azov).

Direct lack of food leads to the death of fish, not only in the early stages of development. There is not always a direct relationship between the abundance of food and the size of the population (fodder anchovy).

The state of the larvae and, first of all, the supply of yolk, then the age of the parents, are of great importance.

The following regularity is outlined: at the edge of the range of the species and the faunistic complex as a whole, abiotic factors are of great importance as a cause of mortality. However, all factors interact and abiotic factors determine the magnitude of mortality often through changes in biotic relationships.

Predation- a form of trophic relationships between organisms different types, for which one of them ( predator) attacks another ( sacrifice) and feeds on his flesh, that is, there is usually an act of killing the victim.

"predator-prey" system- a complex ecosystem for which long-term relationships between predator and prey species are realized, a typical example of coevolution.

Co-evolution is the joint evolution of biological species interacting in an ecosystem.

Relations between predators and their prey develop cyclically, being an illustration of a neutral equilibrium.

1. The only limiting factor limiting the reproduction of prey is the pressure on them from predators. The limited resources of the environment for the victim are not taken into account.

2. The reproduction of predators is limited by the amount of food obtained by them (the number of victims).

At its core, the Lotka-Volterra model is a mathematical description of the Darwinian principle of the struggle for existence.

The Volterra-Lotka system, often called the predator-prey system, describes the interaction of two populations - predators (for example, foxes) and prey (for example, hares), which live according to somewhat different "laws". Prey maintain their population by eating natural resource, for example, grasses, which leads to exponential population growth if there are no predators. Predators maintain their population only by "eating" their prey. Therefore, if the prey population disappears, then the predator population then exponentially decreases. Eating prey by predators damages the population of prey, but at the same time provides an additional resource for the reproduction of predators.

Question

THE PRINCIPLE OF MINIMUM POPULATION SIZE

a phenomenon that naturally exists in nature, characterized as a kind of natural principle, meaning that each animal species has a specific minimum population size, the violation of which threatens the existence of the population, and sometimes the species as a whole.

population maximum rule, it lies in the fact that the population cannot increase indefinitely, due to the depletion of food resources and reproduction conditions (Andrevarta-Birch theory) and limiting the impact of a complex of abiotic and biotic environmental factors (Frederiks theory).

Question

So, as Fibonacci already made clear, population growth is proportional to its size, and therefore, if population growth is not limited by any external factors, it continuously accelerates. Let's describe this growth mathematically.

Population growth is proportional to the number of individuals in it, that is, Δ N~N, Where N- population size, and Δ N- its change over a certain period of time. If this period is infinitely small, we can write that dN/dt=r × N , Where dN/dt- change in population size (growth), and r - reproductive potential, a variable that characterizes the ability of a population to increase its size. The above equation is called exponential model population growth (Figure 4.4.1).

Fig.4.4.1. Exponential Growth.

It is easy to understand that with increasing time, the population grows faster and faster, and rather soon tends to infinity. Naturally, no habitat can sustain the existence of an infinite population. However, there are a number of population growth processes that can be described using an exponential model in a certain time period. We are talking about cases of unlimited growth, when some population populates an environment with an excess of free resources: cows and horses populate a pampa, flour beetles populate a grain elevator, yeast populate a bottle of grape juice, etc.

Naturally, exponential population growth cannot be eternal. Sooner or later, the resource will be exhausted, and population growth will slow down. What will this slowdown be like? Practical ecology knows the most different variants: and a sharp rise in numbers, followed by the extinction of a population that has exhausted its resources, and a gradual deceleration of growth as it approaches a certain level. The easiest way to describe slow braking. The simplest model describing such dynamics is called logistic and proposed (to describe the growth of the human population) by the French mathematician Verhulst back in 1845. In 1925, a similar pattern was rediscovered by the American ecologist R. Perl, who suggested that it was universal.

In the logistic model, a variable is introduced K- medium capacity, the equilibrium population size at which it consumes all available resources. The increase in the logistic model is described by the equation dN/dt=r × N × (K-N)/K (Fig. 4.4.2).

Rice. 4.4.2. Logistic growth

Bye N is small, the population growth is mainly influenced by the factor r× N and population growth is accelerating. When it becomes high enough, the factor begins to have the main influence on the population size (K-N)/K and population growth starts to slow down. When N=K, (K-N)/K=0 and population growth stops.

For all its simplicity, the logistic equation satisfactorily describes many cases observed in nature and is still successfully used in mathematical ecology.

#16 Ecological Survival Strategy- an evolutionarily developed set of properties of a population, aimed at increasing the likelihood of survival and leaving offspring.

So A.G. Ramensky (1938) distinguished three main types of survival strategies among plants: violents, patients, and explerents.

Violents (enforcers) - suppress all competitors, for example, trees that form indigenous forests.

Patients are species that can survive in adverse conditions (“shade-loving”, “salt-loving”, etc.).

Explorents (filling) - species that can quickly appear where indigenous communities are disturbed - on clearings and burnt areas (aspens), on shallows, etc.

The ecological strategies of populations are very diverse. But at the same time, all their diversity lies between two types of evolutionary selection, which are denoted by the constants of the logistic equation: r-strategy and K-strategy.

sign	r-strategies	K-strategies
Mortality	Does not depend on density	Density dependent
Competition	Weak	Acute
Lifespan	short	Long
Development speed	Rapid	Slow
Timing of reproduction	Early	Late
reproductive enhancement	Weak	big
Type of survival curve	Concave	convex
body size	Small	Large
The nature of the offspring	many, small	small, large
Population size	Strong fluctuations	Constant
Preferred environment	changeable	Constant
Succession stages	Early	Late

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