Population balance and basic metrics
The number of individuals in any population is unstable and constantly regulated by the ratio of two opposing processes:
- Natality (Birth rate) — the number of individuals born per a certain unit of time.
- Mortality (Death rate) — the number of individuals perishing or dying over the same unit of time. For example, in the Russian Federation, the mortality rate is about 14.2 per 1,000 people (for comparison, in the USA and Japan these figures are significantly lower).
The difference between these indicators forms the population growth (Birth rate minus Death rate). If the birth rate is higher, the population grows; if the mortality rate is higher, the population size decreases.
Qualitative characteristics of population size
A simple count of individuals does not provide a complete picture. To accurately assess the viability of a group, specific population indicators are used:
- Minimum population size. This is a critical threshold specific to each species. If the number of individuals falls below this level (on average around 200–300 units), the population degrades and dies out because self-maintenance mechanisms are disrupted.
- Effective population size. Far from all members of a group participate in reproduction. This indicator takes into account only those individuals that reproduce directly. A striking example is the human population, where approximately 10–15% of married couples face infertility.
- Form of the population rank. This is the minimum possible group (sometimes just a pair of individuals) that has the potential to multiply over several generations and create an entirely new, full-fledged population.
Age structure and its types
Age structure is understood as the ratio of various age groups within a population. It depends on total life expectancy, the age of sexual maturity onset, and reproduction intensity. Analyzing the structure makes it possible to predict the future of the population.
There are three main types of age structures, visualized as pyramids:
- Progressive type (growing population). Characterized by a colossal proportion of children (juvenile stages) and a very low proportion of older age groups. The shape of the pyramid resembles a classic triangle with a wide base (which is driven by a high birth rate). The type of reproduction is expanded.
- Stationary type (stable population). Distinguished by an equilibrium between the proportion of juvenile and older groups. Reproduction here is simple, and the pyramid takes the shape of a bell.
- Regressive type (dying population). The proportion of children is catastrophically small, while elderly and old individuals predominate. Reproduction is narrowed (depopulation is underway), and the graph shape is an inverted bell.
Human age-sex pyramids
The age-sex pyramid graphically displays the population structure, dividing males and females by age. From the shape of the pyramid, one can immediately determine the demographic state of a country:
- Rapidly growing population. The pyramid has a very wide base (a huge number of children) and a sharp tapering toward the top. Typical for countries with a high birth rate (e.g., Nigeria).
- Slowly growing population. The base of the pyramid is widened, but not much more than the middle part. Typical for countries like the USA.
- Declining population growth. The pyramid has a narrowed base (few children) and a clear expansion in middle and older age cohorts. Observed in European countries, for instance, in Germany.
Sex ratio dynamics
The sex composition of a population is the ratio of male to female individuals. In humans, this indicator undergoes significant changes during ontogenesis (individual development):
- Primary (genetic) ratio. Formed at the moment of fertilization. The established proportion is: 112 girls to 100 boys.
- Secondary ratio (at birth). By the time of birth, the proportion changes due to intrauterine development features: 100 girls are born to 106 boys.
- Tertiary ratio (in adults). At ages 16–18, the figures level out to strictly 100 young women to 100 young men. However, due to different life expectancies, by age 80 a sharp skew occurs: there are only 50 grandfathers for every 100 grandmothers.
Briefly: answers to topical questions
What ecological and social factors influence human birth and death rates?
Birth and death rates are linked to population dynamics: population size depends on the ratio of birth and death rates, and population size is determined by the equilibrium between biotic potential and environmental resistance.
Factors promoting population growth:
- birth rate;
- capacity for dispersal;
- ability to occupy new habitats;
- defense mechanisms;
- ability to withstand adverse conditions.
Environmental resistance factors limiting population growth:
- shortage of food, water, and suitable habitats;
- adverse weather conditions;
- diseases;
- parasites;
- competitors;
- predators.
As environmental resistance increases, the population growth rate decreases due to a lowered birth rate and an increased mortality rate.
It is easy to remember the types of pyramids by their shape: the "Triangle" grows upward (progressive type), the "Bell" just stands evenly (stationary type), and the "Inverted Bell" falls (regressive type).
Class and exam questions
What is effective population size and why is it calculated?
This is the number of individuals directly participating in reproduction. It is calculated to assess the real reproductive potential, since not all individuals produce offspring (e.g., due to age or infertility, which reaches 10–15% in humans).
At what age is the human sex ratio equal to 1:1?
The sex ratio levels out (100 young women to 100 young men) by age 16–18. Prior to this, boys predominate, and at an older age, the female sex begins to dominate.
What does the age pyramid of a declining population look like?
It has the shape of an inverted bell. It is characterized by a low proportion of the child population and a high proportion of elderly people, indicating narrowed reproduction (depopulation).
What else to study on this topic
- Mathematical models of population dynamics
- Causes of changes in sex ratio during ontogenesis
- Impact of external factors on age structure
- Mechanisms of effective population size formation
- Comparison of birth rates across different countries
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