Ecology: Biology of Interaction. IV-16. Population Size Regulation
Population abundance can be regulated by abiotic factors, resource limitation, competitors, predators, parasites, and internal population mechanisms. Different regulatory mechanisms operate in different abundance ranges and determine the characteristic type of population dynamics.
IV-16. Population Size Regulation In this section, we will examine 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 this species and its specific population will turn out to 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. Remember: when grown together for a relatively short time, populations of prey and parasitoid "learn" to develop in a fairly stable regime without sharp population fluctuations (Fig. IV-11.2). In natural conditions, it is not two species that adapt to each other, but in their entire network, mutual adaptations gradually accumulate, allowing them to avoid sharp population fluctuations. These adaptations manifest at the level of heredity of individual organisms within the population and are passed down from generation to generation. And what is bad about sharp population fluctuations? If a sharp decline in population size, caused by interactions between populations, occurs, for example, during a period of bad weather or a population boom of a competitor, the population can simply go extinct. Populations that go extinct disappear from the face of the Earth, and their place is taken by descendants of populations that managed to survive due to certain adaptations. Imagine: at both the organismal and population levels, our world is populated entirely by descendants of winners! The entire infinite chain of ancestors of each organism has managed to produce viable offspring. Any population continues the continuity of a continuous sequence of ancestral populations that have managed to survive any adverse influences. However, this does not mean that population fluctuations always 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. IV-16.1). Let's consider two well-studied examples of population size changes. When in the 19th century the vast territories of Alaska and Northern Canada were being settled, one of the first categories of people to arrive were fur hunters – fur gatherers. The pelts of killed animals were bought by the Hudson's Bay Company, whose accounting records became an important document for population ecology. As we can see, the numbers of both interacting species showed cyclical fluctuations – approximately those predicted by the Lotka-Volterra equations (Fig. IV-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 hare's food base. Fig. IV-16.1. Three different types of population dynamics Fig. IV-16.2. One of the best-documented cases of predator and prey population fluctuations is the dynamics of lynx and hare populations based on data from fur hunters working for the Hudson's Bay Company. The second example we will consider concerns the population size of the Asian locust (Fig. IV-16.3). The population of this herbivorous insect is usually relatively low, but sometimes it suddenly increases dramatically. The causes of this are not precisely known; the eleven-year cycle of solar activity plays a role in this phenomenon. During a population boom, the nature of individual development changes; they develop not into solitary sedentary individuals but into migrating swarms. Huge locust swarms set off to conquer new territories, bringing devastation, hunger, and death. Fig. IV-16.3. Periodic sharp population outbreaks are characteristic of the Asian locust. This section has examined the 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 a population's size begins to grow, it is restrained by the scarcity of its own resources, by the shift of polyphagous predators to feeding on its individuals, by the excessive reproduction of specialized predators, and by the increase in parasitic infections. All these factors "return" the population size to its former 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 regulatory factors will be involved in its return (Fig. IV-16.4). Fig. IV-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 population regulation methods 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 some species causes the widespread prevalence of parasites (epizootics and epiphytotics), most individuals of this species are in a weakened state due to parasites. However, the vast majority of individuals of a species whose population size is regulated by predators may be in optimal physiological condition! After what has been said, you should not be surprised by the periodic fluctuations in the population sizes of many populations. Rather, populations whose numbers remain constant 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 swallow population 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 its individuals.