Fundamentals of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 5. Population Waves

5.1. Population Size Dynamics and Its Causes

Fluctuations in the Abundance of a given species at any stage of development, or of the entire complex as a whole over the course of a year or several years, are referred to as population dynamics. These changes may be driven either by spontaneous processes occurring within the population itself or by external abiotic and biotic factors.

Seasonal population peaks generally align with the number of generations. When generations are numerous, the Developmental Stages of living organisms typically overlap. In long-lived organisms, population peaks are more subdued, although fluctuations can still be substantial (and somewhat delayed). A certain correlation exists between population numbers across successive developmental stages; however, within The life cycle, this correlation is relative and constrained by the condition that numbers at subsequent stages cannot exceed those of preceding stages (barring instances of immigration).

Despite the fact that all populations are in a state of constant flux, such fluctuations are bounded: on the one hand, population size cannot increase indefinitely, while on the other, species extinction occurs relatively rarely.

The upper limit of population size is determined by the available resources supporting a given population—that is, its environmental carrying capacity. The lower limit may reach the point of complete local extinction, after which population recovery is possible through immigration from surviving neighboring populations.

As noted previously, the size of any population at a given time depends on the status of four fundamental components: birth rate, death rate, emigration, and immigration. In certain populations,

The Significance of one or even several of these factors may be nearly negligible, yet no population relies on any factors beyond these four. None of these four population characteristics ever remain constant and all are subject to various conditions. It is precisely through these parameters that diverse ecological factors exert their influence on population dynamics.

Birth rate depends primarily on the biotic potential of a species, which is its capacity for geometric population growth (reproductive potential). Measured in terms of average litter or brood size, it is evident that r-strategists initially possess a significantly higher potential than K-strategists. High fecundity often reflects severe environmental resistance, resulting in the mortality of a large proportion of offspring before they reach adulthood. At the same time, high biotic potential can trigger periodic population outbreaks if environmental conditions become favorable for its realization.

Organismal mortality can be attributed to a wide range of causes. First and foremost, it should be borne in mind that potential immortality is restricted to unicellular organisms, whereas all other life forms perish sooner or later. Consequently, THE ECOLOGICAL AND evolutionary significance lies in the death of individuals before they can leave offspring. Survival potential in this context represents the degree of resistance a species exhibits against unfavorable environmental factors. It depends directly on ecological valence, which is the capacity of a species to thrive across a diverse range of conditions.

In ecology, the factors driving specific population size dynamics are often divided into density-dependent and density-independent ones (acting as regulatory or modifying mechanisms). The former primarily include population enemies (such as predators, parasites, and diseases), shortages of food or other resources, and self-regulation systems. Determining precisely which factors play a decisive role under specific conditions is quite challenging. The impact of abiotic factors is generally considered to be density-independent.

By significantly altering Organism mortality and fecundity rates, abiotic factors can drive temporal changes in population size. Furthermore, the composition and impact of abiotic factors can differ radically between terrestrial and aquatic ecosystems, while similar variations may also occur across different latitudes or even seasons for the exact same environment.

For small organisms, phenomena like microclimate acquire great significance. For instance, Temperature can vary by 5-19 C even within a single plant, and by 20 C or more across a small patch. The impact of adverse temperatures begins to depend on population density, because the higher this metric, the harder it is for an individual to find suitable optimal living conditions, leaving the population increasingly exposed to adverse factors.

Similar fluctuations can also apply to overall humidity (sometimes exceeding microclimatic temperature variations). For example, right above the soil surface within dense grass cover, relative humidity can reach nearly 100%, whereas directly above the vegetation canopy (about 40 cm above the ground) it drops to no more than 50%. This even affects the daily activity patterns of organisms, which are forced to conserve moisture as much as possible. Therefore, such a division of ecological factors into density-dependent and density-independent is somewhat arbitrary: any factor influencing population size can exert either a modifying (destabilizing) or a regulatory effect depending on conditions.

Biotic factors affecting population size primarily stem from interactions among different organisms and can be classified into five main types: competition, predation, parasitism, mutualism, and detritivory. Competition essentially occurs when one organism consumes a resource that would otherwise be accessible to another. In other words, one creature deprives another of that resource (or leaves it in short supply), impairing the latter's growth, reducing its reproductive output, and increasing its probability of mortality. Such competition can be either intraspecific or interspecific.

In many cases, competing individuals do not interact directly, but instead respond to resource depletion caused by the presence and activity of other organisms. Competition also intensifies with an increase in the frequency of interactions among competing individuals, making it density-dependent. By influencing birth and death rates, intraspecific competition regulates population size, keeping density relatively stable.

Predation can be interpreted very broadly: ranging from one animal consuming another to partial consumption followed by the regeneration of lost body parts (grazing-type feeding). The latter applies to herbivory as well as ectoparasitism,

which is quite a controversial category. The Effect of a predator on an individual organism is invariably negative, but for Selection/30.html">The population as a whole, this is not entirely true. In some cases, impaired or diseased individuals are consumed, which can help improve the genetic COMPOSITION OF THE prey population.

There are no predators capable of utilizing all types of prey, as evolution always establishes certain limitations, including dietary ones. Mutual influences between predator and prey population densities are frequently observed, yet this interaction is almost never self-sufficient and requires accounting for the impact of other factors.

Particular problems arise when defining parasitism, especially regarding its evolutionary origins from predation. The vast majority of species and individuals on Earth (more than half) belong to various groups of parasites or disease-causing pathogens. Consequently, virtually no organisms are entirely free from parasitic forms across all stages of their development.

The specific organizational traits and life cycles of parasitic forms shape the particulars of their evolution, as well as The Development of the host-parasite system. The population densities of both parasites and their hosts can significantly affect the population dynamics of both groups of species. This situation is particularly evident during population outbreaks of certain organism groups, as discussed in further detail below.

Overall, it is nearly impossible to determine with absolute certainty which processes triggered specific population fluctuations. First, we may remain unaware of all factors affecting a given population, making it difficult to pinpoint the most critical ones. Second, it is impossible to isolate the effects of different factors because their impact is always complex and integrated: one factor can influence the action of another, amplifying or dampening it.