The book "The Law of the" jungle ". In search of the formula of life "

How does life work? How does nature know how many zebras and lions should live in the savannah or how many fish should swim in the ocean? How does our body know how many red blood cells should be in the blood? Sean Carroll - an American biologist, a leading expert in the field of evo-devo - tells us an incredibly interesting story of discoveries. The hidden secrets of nature - the laws that govern the number of cells in our bodies, animals and plants in the wild. The most surprising thing about these rules is that they are similar and obey the same logic - the logic of life. Carroll talks about how knowledge of the laws of the human body stimulated the emergence of drugs and brings us to the idea that it is time to use the laws of the "jungle" to heal our sick planet. The bold and inspiring work of one of the most famous biologists and gifted popularizers of science tells about laws of life in all forms, manifestations and scales. Read this book and your view of the world will change.

THE SIZE IS IMPORTANT: WHO CAN YOU LOCK, AND WHO IS TOO BIG FOR IT?

Eating or being eaten is, in essence, animal life. In the absence of epidemic factors like the plague of cattle, it is this truth that outlines the two main mechanisms for regulating animal populations in nature. First, it is the opportunity to eat - it's about the availability of food (ascending orientation in the trophic cascade). Secondly, the possibility that a predator will eat you (descending orientation), or a combination of these two mechanisms. For any species there is a simple question: which of these two areas is more important?



In the case of most species in nature, it is much easier to formulate this question than to answer it. Long-term observations are required, and even better - experiments. Sinclair and his colleagues Simon Mduma and Justin Braceers investigated a 40-year-old array of data on the causes of mortality among mammals in the Serengeti. They found a strong correlation between body size and vulnerability for predators.



There is a clear threshold - body weight of about 150 kg. The number of small animals, which on average weigh less, depends on the activity of predators, and the number of larger animals is determined by other factors. For example, the majority of small antelopes, such as oribi (18 kg), impala (50 kg) and swamps (120 kg), are predominantly killed by predator attacks (Fig. 7.5, upper left). In principle, the smaller the animal, the more predators it hunts. For example, out of 10 species of predators living in the Serengeti (among which there is a wildcat, jackal, cheetah, leopard, hyena, and lion), at least six hunt for Oribi, and also eagles and pythons (see Fig. 7.5).

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But larger mammals, such as a buffalo, are subjected to much less pressure from predators (only lions hunt them), and almost no one hunts adult giraffes, hippos and elephants (see Figure 7.5, bottom right). Herbivores from this category, representatives of the so-called megafauna, apparently, got rid of regulation by predators, having developed a large body (and defense organs), because of which it became too difficult or dangerous to get them even for a lion. Since elephants and other animals whose size exceeds the above-mentioned threshold value do not obey downward regulation on the part of predators, it follows that their numbers should be regulated from the bottom up, that is, it depends on the availability of food.



The correlation with the size of the body is quite interesting, but was it possible to check it “in the Payne way” - “push” the Serengeti and see what happens? Yes, it was. Unfortunately, such an "impetus" was the increasingly active poaching, as well as the extermination of animals with poison. Because of this, in the northern Serengeti, from 1980 to 1987, most lions, hyenas and jackals were destroyed. Sinclair and his colleagues compared the number of herbivore populations before and after the reduction in the number of predators, as well as with indicators recorded later when the number of predators recovered. All five species of small herbivores, which they observed (oribi, Thompson gazelle, warthog, swamps and impala), became more numerous with the disappearance of predators, but the giraffe population did not increase. After the predators returned, the populations of all five of the above-mentioned small animals again decreased. Thus, it is these animals, but not giraffes, that are subject to negative downward regulation by predators.



These observations of herbivores and predators in the Serengeti make it possible to quantitatively and experimentally evaluate the conclusions that Elton logically made almost eighty years earlier (and he didn’t have the luxury of studying an ecosystem like the Serengeti): “the size of predator prey and its hunting potential, and in the downstream - the ability to catch small prey in sufficient quantities so that it could be fed. " This is how a special law is formulated: how far the size of an animal’s body can determine the vulnerability of this animal to predators.



LAW JUNGLE 4

REGULATION REGIME DEPENDS ON BODY DIMENSIONS

The body size of the animal is an important determining factor on which the mechanism of regulation of the population size in food chains depends. Populations of small animals are regulated by predators (from top to bottom), and the number of populations of larger animals depends on the availability of food (from bottom to top).



So, if it is too big to be so profitable - no one will overwhelm you - then you would think that in an ecosystem with so many predators all species would have to evolve in this direction. But this did not happen. In addition, the Serengeti is not covered with herds of buffalo or elephants. Their numbers are also regulated, but how is the regulation that affects such large animals? It turns out that although we are now trying to explain ecological regulation at the macroscale, the underlying mechanism is already known to us from molecular biology.

REGULATION ON THE PRINCIPLE OF FEEDBACK IN THE WORLD OF ANIMALS

Sinclair's research has shown that, following the enormous demographic explosion that occurred after the extirpation of the plague of cattle, the buffalo population stabilized in the 1970s. The story of elephants in the Serengeti is also a story of recovery, but after another scourge. In the XIX century. ivory was such a popular commodity that in the first half of the 20th century elephants have become a rare species. In 1958, Grzhimeki counted only 60 elephants in the southern part of the park, but in the period from the early 1960s to the mid-1970s, the population increased to several thousand animals and remained relatively stable for many years.



When Sinclair plotted an increase in the number of each species depending on the size of the population, he obtained several similar lines (Fig. 7.6). The graphs showed that the rate of increase in the number of each species was higher when the total number of the species was lower. With the growth of the population, this rate decreased, and then became negative (the population decreased). In other words, the rate of change in a population depends on its density.



This phenomenon is called "density-dependent regulation". He was already anticipated in the works of the social economist Thomas Malthus, who wrote that the population size was increasing indefinitely until something prevented this. However, suppose we have a group of large animals in a limited space, say, a herd of goats in a pasture. If the initial population size is small, then its growth rate depends only on the reproductive ability of animals. But as the number of animals increases, there is a shortage of free space or food. If the population grows so much that the ecosystem cannot satisfy its needs, then the population will shrink: it stabilizes on the maximum number of animals that can feed on a finite amount of resources.



Density-dependent regulation is a variant of negative regulation on the basis of feedback. Just as the accumulation of reaction products involving enzymes can inhibit the synthesis of these enzymes by the principle of reverse regulation, so an increase in animal populations can slow down demographic growth and even lead to a reduction in the population. Sinclair investigated how such negative reverse regulation works in the case of buffaloes, studying their fertility and mortality. He found that with the increase in the population, more and more adult individuals died from malnutrition, and not only in absolute numbers, but also in proportion.



Sinclair, Simon Mdum and their colleague Ray Hilborn found that similar density-dependent regulation restrains the number of wildebeest migrants. When their population approached a million individuals, the tendency to increase slowed down and then reversed (Figure 7.6, below). To find out which factors triggered such density-dependent regulation, they studied a 40-year-old dataset of wildebeest numbers, as well as the causes of animal death. Scientists have found that, despite the importance of predator exposure (25–30% of deaths), most of the wildebeest began to die when the population increased, from malnutrition. Having carefully studied the data on rains and phytomass in the Serengeti, Sinclair and his colleagues found out that such malnutrition correlates with a reduction in the available food (per head) in the dry season.





Fig. 7.6



Density-dependent regulation in animal populations. As the Buffalo, Elephant, and Wildebeest populations increased in the Serengeti, the rate of their increase slowed down, and then the numbers began to decrease. Illustration based on data from Sinclair et al., 2010; Sinclair, 2003; Sinclair and Krebs, 2002, performed by Leann Olds



Although the Serengeti is a boundless and abundant country, the dry season is a critical period in which the amount of green is reduced, and animals become more vulnerable. Such vulnerability manifested itself even more clearly when a natural experiment began to unfold in 1993: the strongest drought in 35 years covered the Serengeti. During the prolonged dry season, the amount of available food was a small fraction compared with the usual year. Sinclair, Mdum and Hilbourne witnessed a massive famine, when up to 3,000 wildebeads perished daily in November, and the population size fell below a million.



This is a tragic episode, but it helps to understand the important "reverse side" of the density-dependent regulation. When the population decreased, in subsequent seasons more food was available for each head of wildebeest, and the number of animals stabilized. The “beneficence” of the density-dependent regulation was that it absorbed changes in both directions: it slowed down the population explosion, and when the population began to decline, it slowed down the death of the wildebeest. Such regulation is compared with a thermostat that starts the cooling mechanism when the temperature exceeds a certain level, or is heated if the temperature falls below another predetermined level.



Food is not the only possible factor of density-dependent regulation. Predators can also hold back population growth, but when the population shrinks and production decreases, predators can switch to another, more abundant game, as a result of which the number of their main game is restored (and this species does not die out). Territorial competition between predators - for example, for comfortable dens or hunting grounds - can also have the effect of density-dependent regulation. Reverse regulation, which is based on density factors, is a common mechanism of influence on the number of animals.



LAW JUNGLE 5

DENSITY: NUMBER OF SOME ANIMALS DEPENDS ON THE DENSITY OF THEIR POPULATIONS

The population size of some animals is determined by density-dependent factors, which usually stabilize the size of the population.



We examined two main mechanisms for regulating the number of animals: the influence of predators and the availability of food. In addition, we learned how many animals avoid the game - they just get bigger. Is there any way to circumvent the shortage of food resources (at least partially)?



In fact, there is a way to solve both problems at the same time - to escape from predators and feed themselves - and it is he who explains the great action unfolding in the Serengeti.

MIGRATION: HOW TO EAT MORE BUT, BUT NOT BE EATED

Let us return to the figures that you already know well: 60,000 buffalo, more than a million wildebeest. The 450-pound buffalo is much less vulnerable to predators than the 170-pound wildebeest, but the Serengeti has much more wildebeest than the buffalo. How, besides the size of the body, do these two types differ? The first graze in one place, and the second - no.



Can migration explain such a huge difference in numbers between the most common types of resident and nomadic animals in the Serengeti? Since the two main mechanisms governing population size are the availability of food and the pressure of predators, it is important to know how migration affects each of these two mechanisms. That is what Sinclair and his colleagues did.



Migration provides obvious gastronomic benefits. The gnu run after the rains, annually making a nearly thousand-kilometer journey around the Serengeti; during the rainy season, they migrate to the plains covered with green, exceptionally nutritious, low grasses. This resource quickly disappears, but calves of willow can grow stronger on it, and sedentary species do not eat such grass. Then, when drought sets in on the plains, the wildebeest moves to high-grass savanna and light forest where more precipitation falls than on the open plains.



The predator factor in this equation deserves more thorough study. Gnu - prey of lions and hyenas. But, when I talked above about the size of the body of a different game, I specifically omitted the statistics on wildebeest. The fact is that this indicator depends on the population of the wildebeest. In the Serengeti there are two types of wildebeest: one gathers in huge nomadic herds, while others form small sedentary populations, which they spend the whole year in a particular corner of this ecosystem (close to non-drying out water sources). In such sedentary populations, 87% are associated with predator attacks, while in nomadic herds, predation accounts for only about a quarter of all deaths. Moreover, every year about 1% of individuals die in nomadic herds, while in sedentary herds - up to 10%. Consequently, the pressure of predators on each individual nomadic individual is much lower. Studying the habits of lions and hyenas, we managed to find out why they could not count on all this running meat: predators are not able to follow the herds, because they are tied to the territories where their young are growing, and the young need protection.



Due to the cumulative effect of avoiding predators and better nutrition, the density of nomadic wildebeest populations is much higher (about 64 individuals per 1 sq. Km) than the density of settled populations (about 15 animals per 1 sq. Km). The abundance of two other nomadic species in the Serengeti — we are talking about zebras (200,000 individuals) and Thompson gazelles (400,000 individuals) —when comparing with other sedentary species, also confirms that migration is associated with great advantages. In other regions of Africa, nomadic species such as ting (antelope, subspecies of swamps) and Sudanese white whale marsh goat are also at least ten times more numerous than the most common sedentary species.



Thus, migration is another environmental law, or rather, violation of the law, a way to go beyond the limits imposed by density-dependent regulation.



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