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 of sensory organs, quick and accurate attacks on blows, agility and fast running, lightning-fast reactions, sneaking and a variety of 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 has very acute 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, feeding on a certain species, put 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 tend to different ways passive and active protection from predators. With the passive method of defense, protective coloring, hard shells, spines, and the ability to find safe places. 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.
The functioning of the complex “predator-prey” system was studied by modeling by ecologists Lotka and Volterra.
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 produces 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.
Predator as a universal breeder
Doctor of Biological Sciences Alexey SEVERTSOV, Faculty of Biology, Moscow State University. M.V. Lomonosov, Candidate of Biological Sciences Anna SHUBKINA, Institute of Ecology and Evolution named after. A.N. Severtsova
Why does this or that animal become the prey of a predator? Observational experience shows that in natural conditions it is quite difficult to assess the reasons why one or another individual becomes a victim. The attackers cannot catch any animal that is suitable for them in size; not every potential object of hunting is available to them. Consequently, there is, in the language of specialists, “selectivity of removal,” and therefore natural selection carried out by predators.
WHO IS LUCKIER THAN THE CHEETAH?
Research related to the study of this multifaceted scientific topic and carried out in nature is fraught with great difficulties. At the same time, classical field techniques are ineffective. The first problem that arises is assessing the success of the predator's pursuit of prey. In other words, it is necessary to know exactly how many chases took place and what their results were. Typically, such work is carried out in winter by tracking through the snow, i.e., simply put, studying attacks in the tracks of the animal. The method is difficult and labor-intensive, since predators can cover distances of several tens of kilometers in a day, and in hard-to-reach places, and in order to obtain reliable data, a biologist needs to count all attempts to capture a prey. In addition, the winter period is not easy for herbivores; at some moments they may simply be helpless, therefore, there is a possibility of misinterpreting the reasons for their death. The use of technical means is not always possible, and predators are rightly wary of snowmobiles following in their tracks: intensive observations can influence their behavior and territorial distribution. Therefore, estimates of hunting success (the proportion of pursuits resulting in captures) are, as a rule, quite approximate. It is known that it rarely reaches 50% of the number of attempts. For example, the cheetah, the fastest of large predators, achieves success in 25-26% of cases. Group hunting by wolves and hyena dogs is most effective - sometimes up to 40-45% of their pursuits end in capture. But before setting off in pursuit, these predators observe potential victims and often, for some reason, do not start the hunt. It is generally accepted that wild canines, having discovered potential prey, preliminarily assess the prospects of pursuit.
The second problem is that in nature it is difficult, and often impossible, to assess the reasons that made this or that individual a victim. Predators eat it, everything left uneaten goes to scavengers, and the remains decompose in destructive food chains. Therefore, in classical field studies, selectivity—the reasons why a predator chooses one or another individual—can only be assessed in general terms.
There are generally accepted approaches to studying the remains of victims. Their general condition (condition) is determined by the proportion of bone marrow fat in the tubular bones of dead animals - these structures are preserved better than others. Its reduction to 50% indicates complete depletion of fat under the skin and in the abdominal cavity. Using this method, it was shown, for example, that spotted hyenas destroy mainly wildebeests of low fatness, but not those that are about to die themselves.
There is evidence of selective destruction by wild predators of the youngest and oldest and, of course, disadvantaged (poor condition, injured, sick, exhibiting inappropriate behavior, etc.) animals. These facts confirm the selectivity of hunting, but do not make it possible to clarify either its degree or the mechanisms by which the attacker determines the availability or inaccessibility of a particular potential victim. To determine the degree of selectivity, it is necessary to analyze not the remains, but fresh prey, which is almost impossible to do in the wild.
WILD PREDATOR MODEL - SIGHTHOUNDS
To be able to study the characteristics of the prey and repeatedly reproduce the process of search, pursuit, attack and capture, we developed a hunting model of wild canids. Greyhounds were used as predators. Note that this is the only group of domestic dog breeds capable of catching prey without human assistance and without a shot. It is generally accepted that they catch up with animals that they can visually recognize in a field or in the steppe, developing high speed. Thus, they imitate the “stealth hunt” characteristic of wolves, jackals, cheetahs, hyenas and other terrestrial predators. The model has several advantages. Firstly, the pursuit takes place in open space, which makes observation easier. Secondly, despite the existence of several breeds of greyhounds - dogs of genetically different groups - there is a generally accepted unified system for describing their “work”, fixed in the rules of field testing. Finally, when the victim is captured, it is not the remains that are in the hands of the researcher, but the entire loot. True, the model is not without its shortcomings: the centuries-old selection of greyhounds was aimed at ensuring that they would pursue any prey without first assessing the feasibility of an attack.
With dogs of these breeds they hunt hares and foxes, wolves and jackals, small and medium-sized antelopes. Consequently, the speed of greyhounds is slightly higher. To find out how it affects the success of pursuit and to quantitatively describe the behavior of greyhounds, special high-frequency GPS recorders were developed. They were placed on dog collars during field training and during trials for catching wild brown hares. We recorded second-by-second coordinates, thereby determining the location, speed and direction of movement of conditional predators and their victims. It has been established that the speed of pursuit is influenced by many factors: relief and microrelief of the area, properties of the soil and vegetation, weather, etc. ( Natural conditions do not always allow both the prey and the predator to develop the maximum possible speed.) The use of GPS registration made it possible to identify a number of important points.
The speed of greyhounds, which are superior to brown hares in this indicator, is by no means as great as it seems. It ranges from 7.43 to 16.9 m/s - i.e. does not exceed 17 m/s. This is consistent with data obtained from racing English greyhounds, as well as figures established by similar methods for thoroughbred racehorses and cheetahs.
The gallop speed at hippodromes for English racehorses ranges from 7 to 20 m/s, while for a cheetah in nature it almost never reaches 26 m/s, usually amounting to 10–18 m/s. The variety of real conditions causes greyhounds to change the pace of pursuit every few seconds: even a seemingly flat field of winter wheat includes areas of soil of different density, with vegetation of different heights, micro- and macro-lowering and rising.
The range of pursuit of the brown hare by greyhounds ranges from 389 to 2674 m, which is significantly greater than that of the cheetah (average range 173 m, maximum 559 m). Of course, criteria such as speed, length, and duration of the chase are important, but they are still not enough - captures occur at different speed ranges, during pursuits of different lengths and durations.
Microorganisms on the nose of a hare (preparation by E. Naumova and G. Zharova)
Greyhounds often detect hares before they become visible (they rise from their bed to run away), i.e. able to smell them. This is clearly visible when observing dogs and is confirmed by GPS registration data - search activation can occur long (tens of seconds) before the hare begins to move.
In a number of cases, greyhounds, having started a pursuit, stop it after one or several approaches to the victim, i.e. despite the obvious superiority in speed. And often they continue to chase the hare for a distance of more than a kilometer and catch it, although not always. In other words, in the process of chasing, dogs evaluate its prospects, but they can also make mistakes.
It should be emphasized that ethical restrictions have been and remain an integral element of our experiments. Field work was carried out in late autumn - early winter to exclude the possibility of deterioration in the condition of wild animals from lack of food or other unfavorable factors. During this period, the presence of young animals, pregnant females, etc. is excluded. Experiments were carried out only in those regions where the condition of the prey species is assessed as favorable, i.e. removal of animals for subsequent research could not have a negative impact on the well-being of the populations. For comparison, we used animals shot by local hunters at the same time and in the same areas.
SELECTIVITY OF CHOICE
The highest degree of selectivity of greyhounds was shown when studying their hunting of saigas back in the 1980s. This began the study of predator-prey interactions at the individual level.
During two seasons of expeditionary work in Kalmykia (this was a period of high numbers of saigas), they studied their pursuit by greyhounds. In total they managed to catch 38 individuals. At the same time, employees of the state hunting supervision shot 40 antelopes in order to cull the sick ones. Each of these 78 individuals was subjected to a full pathological and anatomical examination by veterinarians participating in the expeditions. It turned out that all the saigas caught by the greyhounds were, to put it mildly, unhealthy. Among those shot, much fewer were identified - 33%. That is, the dogs accurately distinguished the unhealthy, which cannot be said about hunting specialists. Most of the pathologies in saigas were disorders of internal organs (heart, liver, lungs, etc.). Note that such deviations cannot be determined from the remains of victims of wolves and other predators (they primarily eat the entrails).
Over the course of 30 seasons, we studied the hunting of brown hares by greyhounds in the steppe regions, comparing the animals they hunted and those shot on the same days by local hunters. The hares did not differ in appearance and in average weight; differences were revealed only during a pathological and anatomical autopsy, when the condition was compared based on the condition of the fatty capsule of the kidneys. As a result, it turned out that the condition of the hares caught by greyhounds was much worse than that of those shot by hunters.
A microbiological study characterizing the state of stress not related to the moment of capture turned out to be promising. The amount of microflora on the surface of the nose of hares caught by greyhounds is significantly higher than that of those shot by hunters. And an examination of hares subjected to immobilization stress (placed for three days in cramped cages) showed that they had about the same number of microorganisms as those caught by dogs. This means that greyhounds catch distressed animals that are under prolonged stress.
The predator manifests himself as a breeder, and a very strict one at that - he seizes animals with a wide variety of condition deviations. At the same time, the high selectivity of the actions of predators is combined with low hunting efficiency.
MECHANISMS OF PERSECUTION
On average, 27% of greyhound launches against saiga were successful. But the success of chasing a hare varied by day and season from 0 to 70%. In tests - when 2-3 dogs were launched no closer than 25 m - the hunting success was 12% of the number of pursuits (596 greyhounds caught 35 hares in 282 pursuits). This corresponds to data obtained in the UK on the fastest breed of greyhound (Greyhound) when chasing hares of another species that were released from cages (15%). The hunting success of dogs is lower than that of wild predators, although even there the results vary depending on the season and type of prey. If all hunts of wild predators are accurately recorded, their luck is not so high. Let us recall that out of 367 cheetah pursuits, up to 26% were successful.
The choice of prey is influenced by a variety of factors: for example, when hunting rodents, feathered predator attacks an individual that is different from the majority. Perhaps some of the victims are animals that differ in movement parameters, have chosen the wrong escape strategy, have responded late to the appearance of a pursuer, etc. But there are not enough such animals for predators to feed themselves - at least in the fall, during our working season. Observers were able to predict the capture of saigas in only 5 cases out of 210 documented persecutions (2%). People are not able to predict the capture of a hare. This means that visually distinguishable signs are usually insufficient to predict whether a particular animal will be caught.
The predator takes advantage of any circumstances that increase the success of the hunt. Among its victims are individuals with a variety of health conditions. This discrepancy raises the question of the mechanisms for distinguishing between accessible and inaccessible hunting objects, i.e. unhealthy and healthy. Theoretically, there are two possible mechanisms for preliminary, distant discrimination of the availability of prey: visual and olfactory. The latter involves a change in the smell of unhealthy attack targets compared to healthy ones. The mechanism of such a change was initially demonstrated in laboratory and captive animals in work led by biologist Academician Vladimir Sokolov in the early 1990s.
IMPORTANCE OF MICROFLORA
Both the cavities and surfaces of human and animal bodies are inhabited by a large number of microorganisms - mutualists, commensals, pathogens and simply symbionts, including bacterial and yeast forms. Their number and composition are not constant: not only diseases, but also any deterioration in the condition of the macroorganism lead to the fact that the amount of microflora on the body surface increases within 2-3 days.
The number of microbes is 10 times greater than the number of macroorganism cells (shown for humans), and their number and even composition vary with changes in the physiological state, for example, with an increase in body temperature. This occurs as a result of the action of the universal generalized adaptation syndrome, better known as stress. The microflora of the cavities and surfaces of the body processes and mediates all secretions of animals and humans - that is, the smell of any macroorganism is the result of bacterial processing (which has long been used in the development of deodorants and perfumes). The intensity of the odor depends on the number and composition of microorganisms.
According to our observations in the field, greyhounds not only intensively sniff the caught hares, but they are attracted by the imprints of bacteria (cultural microflora) from the nose. Experiments have shown that the smell of cultural microflora is sufficient to change the direction and rhythm of greyhounds' movement - the dogs move towards its source. The smell of microflora is a universal modulator of the behavior of multicellular organisms. For example, the smell of cultural microflora (imprints from the skin surface) obtained from malaria patients is sufficient to change the direction of movement of its carriers, mosquitoes of the genus Anopheles.
Thus, in the course of experiments with greyhounds, it was possible to establish one of the mechanisms for remote recognition by predators of the availability or inaccessibility of the object of attack. Of course, he's not perfect. The spread of smell depends on many factors: from meteorological to biotopic and, of course, from distance.
WHAT HAPPENS BETWEEN PREDATOR AND VICTIM?
The level of well-being varies both between individuals and within each individual throughout life. These fluctuations determine changes in microflora, therefore microbiota is one of the indicators of the state of multicellular organisms. In turn, changes in the microbiota induced by the state of the host organism modulate its individual odor, which is involved in recognizing the object of attack by a predator. The most important property of victims, which increases the likelihood of their capture, is disadvantage of various etiologies. It is marked by changes in the microflora of body surfaces, which affects the smell.
Among the victims of the predator, individuals with signs of physiological deviations from the norm clearly prevail, i.e. with altered or increased microbiota. It performs a mediator function in predator-prey interactions. The use of smell ensures an increase in the differentiation of withdrawal, i.e. the participation of microbiota explains one of the mechanisms of natural selection by predators.
Not only long-term stress, but also any, even temporary, decrease in fitness becomes the reason for the selective removal of the victim - its elimination. Predators influence the number of prey, but do not regulate it. By removing disadvantaged people, they influence the qualitative composition of populations. High selectivity of removal means survival of the fittest. The main significance of selection carried out by predators is the stabilization of the population norm and an increase in the frequencies of phenotypes that have sufficient adaptability to the integral influence of the entire complex of environmental factors.
Let us emphasize: a predator is, indeed, a universal selector, and coevolution in food chains can never reach the level of complete protection of 1st order consumers (herbivores) or complete success of 2nd order consumers (predators). By selectively destroying any and all insufficiently adapted animals, predators carry out very strict natural selection. However, since it follows many and varied criteria simultaneously, it is ineffective for each of them. At the same time, it is difficult to overestimate its importance for the existence of populations of prey species. It can be compared with purifying selection at the molecular level, which occurs through the elimination of any harmful mutations. At the individual level, selection by predators stabilizes not individual traits, but the organism as a whole.
The work was supported by the Russian Foundation for Basic Research grant No. 13-04-00179, grants from the Presidium of the Russian Academy of Sciences “Biological diversity”, “ Biological resources», « Live nature».
"Science in Russia". - 2014. - No. 5. - pp. 11-17.
Predator attacks are often aimed at the weakest prey. - The impact of predators is often compensated by a decrease in intraspecific competition, but compensation is usually not complete. - A decrease in the impact of one type of predation leads to a compensating increase in another type.
If it is known that predation negatively affects individual prey (prey can be both animals and plants), then we can expect that the predation population as a whole will be negatively affected by predation. However, at the population level, these effects are not always easy to predict for the following important reasons: i) individuals killed (or damaged) do not always represent a random sample of the entire population; 2) individuals that escaped death often exhibit reactions that compensate for population losses.
Errington (1946) carefully studied muskrat (Ondatra zibethica) populations in the north central United States for a long time. He conducted censuses, recorded the deaths and movements of individuals, monitored the fate of individual descendants, and especially carefully monitored predation by the American mink (Mustela vison). Errington discovered that adult muskrats, which occupied a strong position in their individual territory, were, as a rule, not attacked by minks; but nomadic individuals that did not have their own area, or individuals that lacked water or suffered from intraspecific fights, were very often destroyed by a predator. Thus, those muskrats that were killed were those that had the least chance of survival and success in reproduction. Similar results were obtained when studying predation on other vertebrates. The most likely victims were young, homeless, sick and decrepit animals. Consequently, the impact of predation on the prey population is much weaker than might be expected.
Similar examples can be given for plant populations. In Australia, mortality of mature eucalypts caused by leaf destruction by sawflies (Perga affinis affinis) was almost entirely limited to weakened trees on poor soils or to trees suffering from root damage or altered drainage due to cultivation (Carpe, 1969).
The impact of predation may also be limited by compensatory responses of surviving individuals - most often due to reduced intraspecific competition. Thus, in an experiment in which it was shot
illp
Rice. 8.5. The net productivity of subterranean clover is a bell-shaped function of the leaf area index (LAI). With increasing illumination (J*cm-2-day~1), the optimal value of ILP increases, since light penetrates deeper into the crown, and more and more leaves appear above the compensation point. (From Crawley, 1983, after Black, 1964.)
a large number of wood pigeons (Columba palumbus), shooting did not lead to an increase in the overall level of winter mortality, and the cessation of hunting did not cause an increase in the number of pigeons (Murton et al., 1974). This happened because the number of surviving pigeons was ultimately not determined the number of individuals shot, and the availability of food and, in addition, after a decrease in population density as a result of shooting, the level of intraspecific competition and natural mortality decreased, and the influx of immigrant birds increased as they gained access to untapped food resources. (
Indeed, whenever population density is high enough to cause intraspecific competition to occur, the impact of predators on the population will be offset by a subsequent reduction in intraspecific competition. This effect is clearly seen when analyzing the bell-shaped curves of net recruitment or net productivity versus density discussed in Sect. 6.5. If the number of breeding individuals is small, then the value of net recruitment is low, as is the net productivity of plants after their partial defoliation (low leaf area index). However, the amount of net recruitment is also low when the crowding of individuals is high; and plant productivity is low where the leaf area index is high and the role of shading is high (Fig. 8.5). Therefore, if a predator or herbivorous organism exploits a population whose density corresponds to the right side of the curve, then the density of this population decreases, and “net recruitment or net productivity increases (Fig. 8.5). The rate of population recovery increases.
This effect is probably most pronounced in plant populations (especially herbaceous ones), where compensation occurs not only at the expense of surviving individuals, but also at the expense of surviving plant parts. Thus, even if defoliation has a detrimental effect on individual shoots or even on entire plants, it may not have serious consequences for the crop as a whole. Indeed, if the loss of leaves leads to an increase in the net productivity of the population, then the amount of assimilates available for the formation and maturation of seeds may increase. It was noted that grazing of wheat, rye and oats in the fall may subsequently contribute to increased seed production" (Sprague, 1954).
However, compensation is not always perfect. When 75% of emerging carrion fly (Lucilia cupfina) adults were removed daily from an experimental population, population size decreased by 40%, although some compensation did occur (Nicholson, 1954b). Similarly, when leaf removal reduced the leaf area index of the subterranean clover population to 4.5 (the left branch of the curve in Figure 8.6), there was a sharp decrease in the rate of leaf production. Consequently, the influence of predation, as a rule, leads to a compensatory weakening of intraspecific competition. But it is also obvious that the role of compensation mechanisms is limited (especially in plant populations at low densities). These problems will be discussed in more detail in Section 10.8 when For now, it should be noted that humans rely on the compensating capabilities of populations to re-harvest, but the limitations of these capabilities can push an overexploited population 1K to the point (or beyond) where the population dies out.
Compensation within a population is not always associated with a decrease in intraspecific competition. A decrease in the impact of one type of predation may lead to a density-dependent compensatory increase in another type. For example, in table. Figure 8.1 shows the results of an experiment in which the fate of Douglas flea seeds (Pseudotsuga tempziesii) planted in an open area and in an area fenced off from vertebrates was monitored (Lawrence and Rediske, 1962). As a rule, protective screens acted effectively in those cases “when they protected plantings from birds and rodents. However, at the same time, the negative impact of insects and especially fungi on seeds and seedlings increased; in general, survival rate changed relatively little. Let us emphasize once again that compensating phenomena reduce , but do not eliminate the effects of predation.
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 essential elements 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.
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 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. Closed forms Pacific Ocean turn out to be more fertile than Atlantic forms. 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. U sea fish protective devices in the same latitudes are more developed than those of 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 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 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.
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 also varies very significantly. As I.A. Paraketsov (1958) showed, 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).
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 across great depths, the dorsal spines are less developed than in related forms common 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.
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 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.
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).
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 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 Oh. In the first case, ambush predators predominate, in the second - those who steal prey. For many predators and within the same habitat, a change in the food eaten at different times of the day is clearly expressed: 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 change 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 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.
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 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.
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 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).
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. 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.
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.
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 influences of one of them on the other become weaker. When applied 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 interspecific struggle for prey. 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)