Ecology: Biology of Interactions. IV-01. Populations and Their Properties
The concept of «population» — is one of the most important in biology. As is the case with key terms, it is frequently used with different meanings. According to Troyan, three approaches to defining the concept of «population» are possible: formal, concrete, and theoretical. Formal: «Popu...
Section IV. Population Ecology
IV-1. Populations and Their Properties
No man is an island, entire of itself; every man is a piece of the continent, a part of the main; if a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a manor of thy friend's or of thine own were; any man's death diminishes me, because I am involved in mankind; and therefore never send to know for whom the bell tolls; it tolls for thee.
John Donne (1572–1631)
As was stated earlier (Table III-3.1), the most important property that emerges at the population level is their potential immortality. We, as humans, together with all other creatures inhabiting the Earth alongside us, belong to populations whose history extends throughout the entire history of life on our planet. These populations have changed, adapting to shifting conditions, splitting into parts and sometimes reuniting. An enormous number of such lineages have gone extinct, clearing the way for those that survived. Tracing this path in reverse, from the present to the past, would reveal how, through the process of evolution, populations become progressively more similar and merge into a single whole, leading back to the organisms that inhabited the first ecosystems on Earth.
The concept of "population" is one of the most important in biology. As is common with key terms, it is frequently used with varying meanings.
According to P. Troyan (1988), three approaches to defining the concept of "population" are possible: formal, concrete, and genetico-evolutionary.
Formal: "A population is a group of living individuals delimited within a framework of space and time" (R. Pearl, 1937).
Concrete — essentially the same, but applied when discussing a specific field of investigation.
Genetico-evolutionary (theoretical) — a reproductive community. A totality of individuals of the same species that reproduce over a series of generations, occupy a defined habitat, and are relatively isolated from other analogous aggregations.
When speaking of populations, one may also invoke the concept of a Mendelian population — an isolated panmictic (freely interbreeding) group.
A population is "the minimal self-reproducing group of individuals of the same species that, over an evolutionarily extended period of time, inhabits a defined space, forms an independent genetic system, and shapes its own ecological niche" (A. V. Yablokov).
From the standpoint of systems theory, the justification for recognising a system at any given level is the presence within it of properties absent at the level of its separate components — emergent properties. From this perspective, the most apt approach to defining a population may be considered the one cited (though not endorsed) by A. V. Yablokov: a population is "a spatially unified group of individuals of the same species that responds as a whole to the action of various factors — a form of the species' existence under specific conditions." A consequence of this property of the population is the convergence of adaptations among its constituent individuals. Among the criteria that unify a population, one may enumerate the coexistence of individuals, the uniformity of their adaptations to the environment, the unity of seasonal cycles and population dynamics, functional cohesion, and genetic distinctiveness. Another notable characteristic of the population as a level of biosystem organisation is that it possesses its own evolutionary trajectory.
We have presented several meanings of the concept of "population," each of which may be correct when the concept is used in accordance with a particular approach — and, ideally, when that approach is explicitly stated or unambiguously understood. However, this concept is very frequently misused as well. For instance, one often encounters formulations such as "...the population of predatory fishes in the World Ocean has declined..."; this is, of course, an incorrect use of the term. Predatory fishes in the ocean are represented by a multitude of populations belonging to many species. In this example, the word "population" is used as a translation of the English word population/populations, which in English may carry the meaning of "abundance" or "numbers."
Quite often we lose sight of the fact that the Earth is inhabited not by organisms per se, but by populations.
"Several years ago, in spring, the author of these lines happened to be present during the spawning of moor frogs (Rana arvalis) near a small spawning pond — a pool among oak-covered hills. It was dusk — the hour \"between the wolf and the dog.\" Several thousand male frogs had gathered in the shallows. At this time of year they are transformed by their vivid blue nuptial colouration. The call of the male moor frog resembles the gurgling of water boiling with large bubbles. The polyphonic chorus of several thousand males merged into an extraordinary drone. This collective voice of thousands of individuals reverberated across the surrounding hills, summoning the unhurried, roe-laden females...
Approaching the spawning pool, a female finds herself a male and, together with him, deposits her clutch of eggs. Those females that arrive later than the others are compelled to spawn upon a continuous field of existing egg masses. Some of their eggs are fertilised by the milt (sperm released into the water) not of \"their own\" male, but of others — those that fathered the neighbouring egg clutches. Before long, tadpoles will hatch from the eggs; their hatching enzymes will dissolve the mucous egg membranes into fluid. Even if, during the course of their development, the water level in the spring pond falls, the liquefied eggs will flow downslope toward the receding water. And even if tens of thousands of individuals (for both tadpoles and eggs are individuals!) dry out on the margins of the pond, hundreds of thousands will reach the water, where they will continue to develop. As they grow, they will exchange a variety of chemical and physical signals. When necessary, those tadpoles that outpace the others will slow the growth of the laggards, thereby reducing competition for scarce resources. In other circumstances, conversely, the growth of larvae of different ages and unrelated lineages will prove to be synchronised — the population itself will select the strategy of its optimal development.
Thus, the dusk, the song of the frogs, and fatigue shifted the perception of the person who approached the spawning pool. She felt that she stood not before a certain number of organisms, but before an entity of a higher order — a population. A potentially immortal living object was at one of the key stages of its annual cycle. The production of new individuals will compensate for the losses among existing ones, for it is only through their flux and succession that the existence of the population is realised. One might suppose that this entity could take no notice of a human being, existing as she does at a lower level of organisation. Yet, when the entranced observer approached one of the spawning pools, the singing frogs took fright, fell silent, and hastily concealed themselves at the bottom. The voice of the population changed: it had noticed the intruder and responded to his presence...
Will you say that such an experience of perceived contact with an entity of a higher order belongs to the province of the psychiatrist rather than the biologist? Of course, you are right. But — will you believe it — such an experience is enormously helpful in imagining the population as a unified whole, rather than as an aggregate of separate individuals..." (D. A. Shabanov, 2007).
Observing how individuals of a given species inhabit a particular territory, one can often discern that there exists an entire hierarchy of population levels, wherein populations of a lower level constitute parts of larger populations. For example, salmonid fishes enter small rivers to spawn. Investigation of their biochemical characteristics allows one to identify certain markers characteristic of spawning stocks associated with the unnamed tributaries of mountain streams where they deposit their eggs. A juvenile fish hatched in such a location will, if it survives, return in time to spawn at precisely that site. The local spawning stock constitutes a low-level population. Furthermore, one can demonstrate that individuals belonging to all spawning stocks reproducing in a given river are more closely related to one another than to fish from different rivers. Salmonid fishes of a given species from the catchment of a single river constitute a higher-level population.
Some species of Pacific salmon spawn in both Asian rivers on Russian territory and in American rivers in Canada and the United States. The Asian and American stocks of such fishes also constitute populations. Ultimately, the highest-level population is the species itself — return to the genetico-evolutionary definition of a population given at the beginning of this section and you will find that the species fully satisfies it.
Thus, within the hierarchy of biosystems, the population level of organisation may correspond to several "storeys." Their number depends on the characteristics of the population biology of the species in question: the nature of their reproduction, migration, and pair formation.
For many organisms inhabiting fragmented environments, the formation of metapopulations is characteristic. A metapopulation is a persistently existing aggregate composed of subpopulations, each of which may exist for only a limited period of time. The integrity of metapopulations is maintained by the exchange of migrant individuals among subpopulations. Imagine a valley in which temporary water bodies arise and disappear. In each such water body, a subpopulation of frogs exists for some period of time. When a water body disappears, the individuals that inhabited it join other groups. Although each local group exists for only a brief interval, the metapopulation uniting them is capable of persisting indefinitely.
If the population system under consideration inhabits a given biogeocoenosis (i.e., "an ecosystem within the boundaries of a phytocoenosis"), the concept of "coenopopulation" may be applied to designate it; this term is particularly widely used in the study of plant distribution. If a population consists of groups of individuals occupying different niches (for example, terrestrial adults and aquatic larvae, etc.), the concept of "hemipopulation" introduced by V. N. Beklemishev may be applied to its constituent parts.