The following points highlight the seven main factors affecting population of living organisms. The factors are: 1. Natality 2. Mortality 3. Dispersal 4. Age Distribution 5. Biotic Potential and Environmental Resistance 6. Biotic and Abiotic Factors 7. Density Dependent and Density Independent.
Factor # 1. Natality:
The increase in number of individuals in a population under given environmental conditions is called natality. For example, birth, hatching, germination and vegetative propagation are main causes which are responsible for increase of number of individuals.
When the increase in individuals is expressed per unit time, it is called, natality rate.
Natality is Calculated as Follows:
Birth rate or natality (B) = number of births per unit time/average population
The maximum number of births produced per individual under ideal conditions of environment is called potential natality.
Natality varies from organism to organism. It depends upon the population density and environmental factors. It is a general rule that if the population density is low, the birth rate is also low, because the chances of mating between males and females are low.
On the other hand, if population density is unusually high, the birth rate may also be low due to poor nutrition, or physiological or other problems related to crowding.
The maximum or absolute natality is observed when the species exists under ideal ecological and genetic conditions. The actual number of births occurring under the existing environmental conditions is much less as compared to absolute natality. It is referred to as ecological natality or realised natality. It is not constant for population and may vary with the size of population as well as with the time.
Factor # 2. Mortality:
The loss of individuals due to death in a population under given environmental conditions is called mortality.
Mortality refers to the number of deaths per unit time.
Mortality rate = D/t where D is the number of deaths in the time t.
Mortality can be expressed as follows:
(i) Minimum or Specific or Potential Mortality:
It represents the minimum or theoretical loss of individuals under ideal or non-limiting conditions. Thus even under the best conditions individuals of a population would die of old age determined by their physiological longevity. So it is constant for a population.
(ii) Ecological or Realised Mortality:
It refers to the death of individuals of a population under existing environmental conditions. Since it varies with environmental conditions, it is never constant. The maximum mortality occurs at the egg, larval, seedling and old age.
Mortality is affected by a number of factors, such as density, competition, disease, predation and environment. Death rates vary among the species and are correlated with birth rates. When the rate of natality is equal to the rate of mortality, the population is stationary.
A birth-death ratio (Births/Deaths × 100) is called vita index.
For a population, the survival of individuals is more important than the death. The number of births in relation to the carrying capacity of the habitat is a fundamental factor influencing the mortality rate. When more young’s are born than the habitat can support, the surplus must either die or leave the area.
As the number of survivors is more important than the number of dying individuals, mortality is better expressed as survival or as life expectancy. The life expectancy refers to the average number of years the members of population have left to live.
Factor # 3. Dispersal:
The majority of organisms disperse at one time or the other during their life cycles. The individuals move into (immigration) and move out of the population (emigration), and such movements affect the size of population.
Movement of individuals into and out of the population is called population dispersal. It plays important role in the geographical distribution of organisms even to the areas previously occupied by the members of population.
Dispersal of organisms occurs for various reasons such as food, protection, prevention from overcrowding, action of wind and water, environmental factors such as light, temperature, breeding behaviour, physiological reasons or for interchange of genetic materials between the populations.
Population dispersal occurs in nature as follows:
(i) Emigration:
It is one way movement of individuals out of a population. This movement is permanent and causes spread of a species to new areas. Emigration under natural conditions occurs when there is overcrowding in the population and is generally regarded as an adaptive behaviour that regulates the population on a particular site and prevents over-exploitation of the habitat.
This type of dispersal offers new opportunity to the individuals of a population to interbreed with those of the other population leading to more genetic heterozygosity and adaptability.
(ii) Immigration:
This is one way movement of individuals into a population. It leads to rise in density of population. It may result in decreased mortality among the immigrants or decreased reproductive capacity of the individuals.
(iii) Migration:
Migration is two way mass movement of the entire population. It involves a periodic departure and return of the individuals of a population and occurs only in mobile organisms during unfavourable periods. It is shown by many birds, fishes and certain animals. In most cases, migration of population occurs for food, shelter or reproduction.
Factor # 4. Age Distribution:
Age distribution is an important characteristic of population which influences natality and mortality. Mortality usually varies with age, as chances of death are more in early and later periods of life span. Similarly, natality is restricted to certain age groups, e.g., in middle age-groups in higher animals.
The individuals of a population can be divided in three ecological stages. They are:
(i) Pre-Reproductive,
(ii) Reproductive and
(iii) Post-Reproductive.
The individuals of pre-reproductive group are young, those of reproductive group are mature and those in post-reproductive group are old.
The distribution of ages may be constant or variable. It is directly related to the growth rate of the population. Depending upon the proportion of the three age groups, populations can be said to be growing, mature or stable, and diminishing.
Rapidly increasing population contains a large proportion of young individuals, a stable population shows even distribution of individuals in reproductive age group, and declining or diminishing population contains a large proportions of old individuals.
Age Pyramids:
Age pyramid a model in which the numbers or proportions of individuals in various age groups at any given time are geometrically presented. In an age pyramid, the number of pre-reproductive individuals is shown at the base that of reproductive age group in the middle and the number of post-reproductive individuals at the top. The age pyramid indicates whether a population is expanding or stable or diminishing.
The hypothetical age pyramids are as follows:
(i) Pyramid with Broad Base:
This pyramid shows a high percentage of young individuals and an exponential growth of population due to high birth rate, e.g., yeast, housefly, etc.
(ii) Bell-Shaped Pyramid:
This type of age pyramid shows a stable population having, more or less equal number of young and middle-aged individuals and post-reproductive individuals being smallest in number.
(iii) Pyramid with Narrow Base:
This is an urn-shaped pyramid which shows increased numbers of middle-aged and old organisms as compared to young ones in the population. This shows diminishing of population.
Factor # 5. Biotic Potential and Environmental Resistance:
Every kind of organism produces more offspring than are necessary for that species to survive. The maximum reproduction rate of a population, under ideal conditions, is called biotic potential. This factor assumes that all of the young will live long enough to reproduce.
To reach its biotic potential a population must have all of the food or raw materials, water and space it needs to survive. It also needs an ideal climate and the absence of competitors and disease. For instance, some bacteria divide about every 20 minutes.
In 24 hours, the number of individuals produced would be enough to cover the entire surface of the earth to a depth of over 20 cm. However, populations do not reach their biotic potential. Many factors influence the growth rate of population. Environmental resistance means the collection of factors that reduce the growth rate of a population.
Factor # 6. Biotic and Abiotic Factors:
Biotic factors that influence population growth are living organisms or their characteristics. For example, the biotic factors that influence a population are the presence of predators or hunters, the food supply, the density of the population itself, the biotic potential of the species and disease.
Abiotic factors are non-living physical elements in the environment, such as temperature, water, humidity, light, soil, slope, air and wind.
Factor # 7. Density Dependent and Density Independent:
Density dependent factors influence a population differently if the population is crowded than if it is not crowded. The effects of predators, disease and competition vary with the concentration of the population. For example, a disease spreads more rapidly through a crowded population severely. The biotic factors influence all population than through a sparse one.
The loss of a food source affects a crowded population, regardless of their density, and is called density-independent. For example, a forest fire affects the food supply and shelter of all organisms in the areas whether they are crowded or not crowded.