Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 5. Population Waves

5.3. Types of Population Waves and Their Characteristics

Population waves essentially represent more or less regular fluctuations in population size—ranging from growth to decline—in contrast to constant, directed processes. Such fluctuations can occur around an average population density. Sharp fluctuations trigger population outbreaks followed by the depletion of the food supply. The Classification of population waves can be based on their underlying causes, although it is understood that any classification is inherently conditional, and every rule has its exceptions.

Periodic population waves. These types of numerical fluctuations are predominantly characteristic of short-lived organisms, such as microorganisms, annual plants, most insects, and others. One of the main reasons for Changes in the Abundance of certain organisms is significant differences in living conditions across different seasons of the year. For most terrestrial and freshwater ecosystems—with the exception of relatively stable ones like tropical forests and oceans—these seasonal differences drive the population fluctuations of the organisms within them. Therefore, periodic population waves can be considered a consequence of seasonal cyclic adaptations of organisms to the annual dynamics of their ecological environment.

Most organisms (such as insects) spend certain periods of the year at specific developmental stages, which shapes periodic waves. One of the adaptations of many species to seasonal environmental changes is prospective (physiological) dormancy, the primary form of which is diapause. The alternation between active life and diapause—which differ significantly in their ecological requirements—synchronizes the life cycles of animals with the seasonal dynamics of the environment. Consequently, periodic waves are not always accompanied by mass mortality; the vast majority of organisms transition into a specific developmental stage adapted to adverse conditions (while all organisms at other developmental stages typically perish).

In unicellular organisms, cysts are formed to survive unfavorable periods; in rotifers, so-called wintering (fertilized) eggs are produced, and so on. In insects, diapause can occur at any stage of The life cycle, but it is always species-specific—embrionic (egg stage), larval, pupal, or imago.

A similar phenomenon is typical for annual plants, in which seeds survive unfavorable conditions, and in some species, autumn-generation sprouts do so as well. Individual differences in the life cycle (asynchronous emergence from diapause over several years, seed banks) help mitigate sharp population fluctuations caused by unexpected deterioration in living conditions. Sharp seasonal changes in abundance are characteristic of forms such as Viruses (seasonal outbreaks of viral diseases) and r-strategists, in which high birth rates are accompanied by substantial mortality.

In K-strategists, seasonal population fluctuations are much less pronounced, and in trees, they are virtually unnoticeable. Occasionally, fluctuations in the abundance of K-strategists may be linked to their migratory activity, leading to sharp changes in numbers in certain parts of their range.

Aperiodic population waves. While periodic population waves are characterized by seasonal population dynamics, aperiodic waves exhibit multi-year (long-term) dynamics. Aperiodic population waves are primarily triggered by factors acting simultaneously within the biocenotic links of several species. This may involve a decrease in the predatory or parasitic pressure on prey, an improvement in the food supply, or the combined effect of several favorable factors. Three types of long-term population dynamics are distinguished: stable, prodromal, and eruptive.

Species with stable population numbers show only minor fluctuations within the zone of Homeostasis (primarily typical K-strategists with low migratory activity and low fecundity). Such species (large mammals, birds, certain insect species, etc.) make up about 50–60% of the total fauna species composition in a given area. Among insects, this type of population wave is characteristic of fruit and seed pests that lead a cryptic lifestyle, remaining largely unaffected by adverse weather conditions, predators, or diseases.

In the case of prodromal dynamics, favorable conditions lead to a substantial increase in abundance in certain years. Mass reproduction of such species is described by three phases: growth, peak, and thinning. An example of organisms with this type of population dynamics among vertebrates is mouse-like rodents.

For instance, the Water vole exhibits seasonal habitat shifts—reproduction occurs in flooded areas, while wintering takes place in meadow habitats. In wintering habitats during periods of population growth, the food reserves of an individual home range prove insufficient, leading to overexploitation (the reserve of underground phytomass for the following year decreases by 4.5 times compared to an unpopulated biotopes). Such a diet affects the physiological traits of the animals, causing changes in food digestibility even in reproductive habitats where food is abundant regardless of population size. These conditions impact individual viability, male competitiveness, and female fecundity, largely through the embryonic mortality of a significant portion of the latter.

One of the main causes of this dynamics in the water vole is climatic cyclicity, which determines the waterlogging of the territory. Maximum realization of reproductive potential is possible in favorable years when embryonic mortality drops to 4%, and the population reaches its peak density. The consequence of such growth is a significant destructive impact on the habitat—undermining the food base, which in turn causes a subsequent population decline.

Typical invertebrate representatives exhibiting this type of population waves include insects such as the pine tree lappet, bark beetles, longhorn beetles, and others. A distinct feature of this dynamics is typically the absence of a population depression phase.

Species with eruptive dynamics are capable, under certain conditions, of breaking free from the controlling influence of ecological factors and producing mass reproduction outbreaks. These species include r-strategists with high fecundity and strong migratory capacities. An outbreak is triggered by a prolonged deviation of basic meteorological conditions from the norm, allowing the species to maximize its reproductive potential.

Such population fluctuations are characteristic of insects, primarily certain needle- and leaf-eating forest pests. Due to their exposed lifestyle, needle- and leaf-eating pests are directly exposed to climatic factors, natural enemies, diseases, and so on. Insect mass outbreaks are divided into four phases.

The initial phase of an outbreak involves only a single generation of insects that happened to encounter optimal living conditions. Due to active reproduction, population numbers increase only slightly (no more than 2–4 fold).

In the population growth phase, which encompasses the second and third generations, overall abundance increases but is not yet high enough for the consequences of tree defoliation to be striking. During this phase, pest larvae are well-fed, with elevated levels of fat and protein, while pupae and eggs are notably large. As numbers increase, the insects disperse across the forest stand.

The outbreak culmination is characterized by a sharp surge in population density. This phase lasts 2–3 years, and due to the massive pest population, damage becomes clearly visible. Gradually, larvae begin to experience food shortages, leading to weakened condition, reduced individual fecundity, increased disease incidence, and a rise in the number of entomophages.

During the crisis phase, the pest population decreases sharply, fecundity drops to the point of complete sterility, and males begin to dominate the populations. The percentage of individuals infected with entomophages and diseases increases, and abundance drops to a minimum (depression)—marking the end of the outbreak (this phase lasts 2–3 years). Between outbreaks, the population size of these species remains at a low level, though it continues to fluctuate constantly.

Sharp population fluctuations resulting from extraordinary events. The two previous types of population waves can be termed primary, as they are inherent to almost all organisms and result from natural cyclicity. However, extraordinary events that can cause sharp numerical shifts are superimposed on these dynamics. Because they temporarily alter the natural population cycles, they can be considered secondary phenomena.

Population outbreaks in new areas are associated with the availability of a food supply and the absence of natural predators and diseases (Fig. 5.1). A distinct feature of this dynamics is that a small number of individuals arriving in new, favorable conditions produces a sharp population spike that far exceeds the normal fluctuation limits for that species. Subsequently, the food supply is rapidly depleted, natural predators appear, and this triggers a corresponding population depression. Over time, the dynamics acquire the typical pattern of long-term fluctuations.

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Fig. 5.1. Distribution map of the Colorado potato beetle (Leptinotarsa decemlineata) in France (illustration by O.V. Karnaukhov)

There are numerous Examples of such population waves, the majority of which are currently associated with human activity. These include attempts at acclimatization (muskrats in Europe, rabbits in Australia), accidental introductions (the Colorado potato beetle in Europe and most other quarantine pests), as well as synanthropization, a process through which certain species achieve extraordinary and lasting distribution (cockroaches, synanthropic flies, etc.).

A sharp decline in numbers due to natural disasters typically occurs As a result of the destruction of entire natural complexes (ranging from biogeocoenoses to landscapes). Such changes can be triggered by specific natural factors, such as prolonged heatwaves, wildfires, or, in the case of small organisms, even routine weather events like hail or prolonged rain. In any case, adverse conditions cause a significant portion of the population to perish, leading to a substantial restructuring of its genetic Structure. Anthropogenic habitat transformation is similar in its consequences to this type of population wave. The difference lies in the fact that anthropogenic impacts are always more prolonged, and the carrying capacity of the environment does not increase over time, which often leads to the extinction of the populations under these conditions.

A distinctive feature of this final type of population wave is that both the Introduction of individuals into new habitats and the catastrophic reduction in population size are associated with such ecological phenomena as the "bottleneck effect" and the "founder effect." These phenomena are frequently accompanied by a fundamental shift in the genotypic COMPOSITION OF THE populations.

Food for Thought

Thus, population fluctuations (population waves) can in themselves have evolutionary consequences for a population. Although the action of this elementary evolutionary factor is undirected and random, The process of population fluctuation can, to a certain extent, be considered manageable. For instance, the size of any population is always constrained by available resources, expressed through the environmental carrying capacity. Since this capacity can vary across seasons and years, its fluctuation serves as the foundation for population dynamics. Seasonal changes in carrying capacity (driven by wet and dry season alternations in the tropics) generate periodic population waves characteristic of nearly all organisms, whereas multi-year fluctuations lead to non-periodic population waves. METABOLISM/18.html">The Influence of additional factors capable of abruptly altering environmental carrying capacity introduces further adjustments to population dynamics.