Ecology: Biology of Interaction. 4.16. Regulation of Population Size
The abundance of every population is controlled by multiple negative feedback loops. When population size begins to grow, it is constrained by shortage of its own resources, by switching of polyphagous predators to feeding on it, by overreproduction of specialized predators, and by increasing parasitic infections.
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4.15. Ecological strategies
D. Shabanov, M. Kravchenko. Ecology: The Biology of Interaction Chapter 4. Population Ecology
4.17. (addendum) Strategies of intraspecific interaction
4.16. Population size regulation In this chapter, we have examined the types of interactions in which a population can be involved. All these interactions are components of the ecological niche of a population and a species. All of them influence which strategy of a given species and its specific population will be optimal. Can the strategies of different populations belonging to the same species differ? Fundamentally, they are similar, but changes in species strategy caused by external conditions are possible. Recall: when grown together for a relatively short time, populations of prey and parasitoid "learn" to develop in a sufficiently stable regime without sharp population fluctuations (Fig. 4.12.2). In natural conditions, it is not two species that adapt to each other, but mutual adaptations gradually accumulate in their entire network, allowing them to avoid sharp population fluctuations. These adaptations manifest at the level of heredity of individual organisms that make up the population and are passed down from generation to generation. And what is wrong with sharp population fluctuations? If a sharp decline in population size, caused by interactions between populations, coincides, for example, with a period of bad weather or a population boom of a competitor, the population may simply die out. Extinct populations disappear from the face of the Earth, and their place is taken by descendants of those populations that managed to survive due to certain adaptations. Think about it: at both the organismal and population levels, our world is inhabited entirely by the descendants of winners! Every organism in the infinite chain of its ancestors has managed to produce viable offspring. Any population continues the succession of continuous ancestral populations that have managed to survive any adverse influences. However, this does not mean that population fluctuations necessarily have an adverse effect on it. Some populations are adapted to cyclical changes in their numbers, and this is as natural for them as stable numbers are for other populations (Fig. 4.16.1). Let's consider two well-studied examples of population fluctuations. When large territories of Alaska and Northern Canada were being settled in the 19th century, one of the first categories of people to arrive in these lands were fur trappers. The pelts of killed animals were bought by the Hudson's Bay Company, whose accounting records became an important document for population ecology. As can be seen, the numbers of both interacting species exhibited cyclical fluctuations – approximately those predicted by the Lotka-Volterra equations (Fig. 4.16.2). However, as has become clear in recent decades, the population dynamics of fur-bearing animals were related not only to their interactions with each other but also to external factors, particularly climatic changes affecting the food base of hares. [IMG_1] Fig. 4.16.1. Three different types of population dynamics [IMG_2] Fig. 4.16.2. One of the best-documented cases of predator and prey population fluctuations – the dynamics of lynx and hare populations based on data from fur trappers working for the Hudson's Bay Company The second example we will consider concerns the population size of the Asian locust (Fig. 4.16.3). The population of this herbivorous insect is usually relatively low, but sometimes it suddenly increases dramatically. The reasons for this are not fully known; the eleven-year cycle of solar activity plays a role in this phenomenon. During population booms, the character of individual development changes; they develop not as solitary sedentary individuals but as gregarious migratory ones. Huge locust swarms set off to conquer new territories, bringing devastation, hunger, and death... [IMG_3] Fig. 4.16.3. Periodic sharp population outbreaks are characteristic of the Asian locust In this chapter, we have examined relationships between populations that can affect their numbers. We can conclude that the size of each population is controlled by a multitude of negative feedback loops. When the population size begins to increase, it is restrained by its own resource scarcity, the switching of polyphagous predators to feed on its individuals, the excessive reproduction of specialized predators, and the increase in parasitic infections. All these factors "return" the population size to its previous level and reduce the pressure on it when its size is low. Depending on how significant the deviation of the population size from the norm is, a greater or lesser number of regulating factors will be involved in its return (Fig. 4.16.4). [IMG_4] Fig. 4.16.4. Ranges of population size fluctuations within which different regulation mechanisms operate We have already mentioned that sharp fluctuations can lead to population extinction. How do different methods of population regulation affect its interaction with competitors? For example, a population whose size is regulated by resource scarcity or competition from another species consists mainly of "underfed" individuals. If a population boom of a certain species causes the widespread prevalence of parasites (epizootics and epiphytotics), most individuals of this species are in a weakened state due to parasites. In contrast, the vast majority of individuals of a species whose population size is regulated by predators may be in optimal physiological condition! After this, you should no longer be surprised by the periodic fluctuations in the numbers of many populations. It is the populations whose numbers remain constant that should be surprising. For example, in one small village in the Netherlands, amateur ornithologists have been observing barn swallows for several centuries. And throughout this time, the number of swallows in the village has remained practically constant – slightly more than fifteen nests, plus or minus one or two nests! How is this possible? So far, we have considered the regulation of population size by external mechanisms. However, this parameter can also be effectively controlled by internal population causes: interactions between the individuals that constitute it.
4.15. Ecological strategies
D. Shabanov, M. Kravchenko. Ecology: The Biology of Interaction Chapter 4. Population Ecology
4.17. (addendum) Strategies of intraspecific interaction