Earth Science: Biology – Ecology – Populations
The totality of individuals of a species that form a reproductive community within a delineated space is called a population.
Each population has a uniform gene pool, which can be different from the gene pool of another population of the same species, due to different climatic zones. Population size is subject to constant changes.
Earth Science: Biology – Ecology – Growth and regulation
Factors that determine growth rate (the difference between birth and death rates) include biotic (animate) factors and abiotic (inanimate) factors, external and internal, and density-dependent and density-independent factors.
If all essential resources are present in sufficient quantities for the growth of a population, there will be—after an initial phase-exponential growth. This is because the progeny of a generation will produce new progeny, while the death rate will re- main stable. Yet, normally the growth of a population is under the influence of various environmental factors, especially by limitations of food sources.
Therefore, after a re- productive period with exponential growth, there will be a stationary phase during which developing and dying organisms balance each other out, until the die-off phase sets in, when more organisms die than new ones are born. Such growth sequences can readily be observed in a laboratory where bacterial culture can grow on a well-defined nutrient medium.
The density of a population is often the regulatory mechanism within a population, in other words the number of individuals of a species that inhabit a certain area per unit area. For in- stance, strongly growing populations quickly use up all food sources, resulting in a decline of the growth rate. Other density-dependent factors can also influence population growth, for example intraspecific competition, enemies (parasites or predators), infectious diseases, as well as social stress.
Furthermore, density-independent factors, such as interspecific competition or gradual climatic influences and their consequences, usually have additional effects on population density. Among animals, the dominance of one species can also lead to a predator-prey relationship. In such cases, the dominant species (predator) uses the individuals of the inferior species (prey) as food, so that there is a dependency between predator and prey in respect to population density.
Nevertheless, the total inventory of the prey population remains largely unaffected, provided available food resources of the respective biotope permit this.
BASICS
REPRODUCTIVE SELF-RESTRAINT Some animals have developed regulatory mechanisms to prevent over-population.
For example, birds of prey may only feed the first-born chick and let the others starve