A population is defined as a group of individuals of the same species living and interbreeding within a given area. The most fundamental demographic parameter is the number of individuals within a population (Lebreton et al. 1992). Population density is the total size that is generally expressed as the number of individuals in a population.
It is the average number of individuals in a population per unit of area or volume. Population density is defined as the numbers of individuals per unit area or per unit volume of environment. Density may be numerical density (number of individuals per unit area or volume) when the size of individuals in the population is relatively uniform, as mammals, birds or insects or biomass density (biomass per unit area or volume) when the size of individuals is variable such as trees.
The distribution between crude density and ecological density becomes important because the patterns of distribution of individuals in nature are different and organisms in nature grow generally clumped into groups and rarely as uniformly distributed. Where D is population density; n is the number of individuals; a is area and t is unit time.
Population dispersion or spatial distribution
In nature, due to various biotic interactions and influence of abiotic factors, the following three basic population distributions can be observed. Most populations exhibit this dispersion to some extent, with individuals aggregated into patches interspersed with no or few individuals. Such aggregations may result from social aggregations, such as family groups or may be due to certain patches of the environment being more favourable for the population concerned.
Population Age structure
This is because younger people are more likely to reproduce; while older people have higher rates of dying. The proportion of individuals in each age group is called age structure of that population. The ratio of the various age groups in a population determines the current reproductive status of the population, thus anticipating its future.
The relative duration of these age groups in proportion to the life span varies greatly with different organisms.
Population Age pyramid
Population Age pyramid is 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 shape of age-pyramid changes with the change in the population age distribution over a period of time.
In rapidly growing young populations birth rate is high and population growth may be exponential as in yeasty house fly, Paramecium, etc. Under such conditions, each successive generation will be more numerous than the preceding one, and thus a pyramid with a broad base would result (Fig. A). Such an un-shaped figure is obtained when the birth rate is drastically reduced the pre-reproductive group dwindles in proportion to the other two age groups of the population.
Usually, a rapidly expanding population will have larger reproductive cohorts, stable populations show a more even distribution of age classes, and rapidly declining populations have large older cohorts (Lebreton et al. 1992). Although a population's age structure is not always pyramidal in shape, most populations have younger cohorts that are larger than older cohorts.
Natality (birth rate)
Birth rate or Natality (B/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. It is also called reproductive or biotic potential, absolute natality or maximum 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 usually low, the birth rate is also. This is so because the chances of mating between males and females are low. If population density is unusually high, the birth rate may also be low due to poor nutrition or physiological or psychological problems related to crowding. There are distinguished two types of natality:. Also called as absolute or potential or physiological natality, it is the theoretical maximum production of new individuals under ideal conditions which means that there are no ecological limiting factors. and that reproduction is limited only by physiological factors. It is a constant for a given population. This is also called fecundity rate. Also called realized natality or simply natality, it is the population increase under an actual, existing specific condition. Thus it takes into account all possible existing environmental conditions. This is also designated as fertility rate. Where N = initial number of organisms. n = new individuals in the population. Further, the rate at which females produce offsprings is determined by the following three population characteristics:. a) Clutch size or the number of young produced on each occasion. b) The time between one reproductive event and the next and (c) The age of first reproduction.
Mortality (death rate)
Mortality rate (d) = D/t where D is the number of deaths in the time t
- Vital index and survivorship curves
- Fecundity
- Sex Ratio
- Biotic Potential
- Life tables
- Survivorship Curves
Because 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 a population have left to live. Fecundity describes the number of offspring an individual or a population is able to produce during a given period of time (Martin 1995).
In demographic studies, fecundity is calculated in age-specific birth rates, which may be expressed as the number of births per unit of time, the number of births per female per unit of time, or the number of births per 1,000 individuals per unit of time. However, since every ecosystem implements constraints on its populations, ecologists prefer to measure realized (or ecological) fecundity, which is the observed number of offspring produced in a population under actual environmental conditions. Thus, ecologists measure the number of males and females within a population to construct a sex ratio, which can help researchers predict population growth or decline.
For example, stable populations may maintain a 1:1 sex ratio and therefore keep their growth rate constant, whereas declining populations may develop a 3:1 sex ratio favoring females, resulting in an increased growth rate. When the environment is unlimited, the specific growth rate (i.e., the population growth rate per individual) becomes constant and maximum for the existing conditions. The value of the growth rate under these favourable conditions is maximal, is characteristics of a.
It may be designated by the symbol r which is the exponent in the differential equation for population growth in an unlimited environment under specific physical conditions. The Overall population growth rate under unlimited environmental conditions (r) depends on the age composition and the specific growth rates due to reproduction of component age groups. When a stationary and stable age distribution exists, the specific growth rate is called the intrinsic rate of natural increase or r max.
He defined it as “the inherent property of an organism to reproduce to survive, i.e., to increase in numbers. It is a sort of algebraic sum of the number of young produced at each reproduction, the number of reproduction in a given period of time, the sex ratio and their general ability to survive under given physical conditions.” Thus with the term of biotic potential, one is able to put together natality, mortality and age distribution. Life expectancy (ex), thus, is the mean time between any specified age and the time of death of all individuals in the age group.
If it could be assumed that all members of an original population have the same capacity for survival (environmental effects for the moment are ignored), plotting the number of surviving individuals against time would produce a survivorship curve in the form of a right angle. There are three general types of survivorship curves which represent different natures of survivors in different types of population These are graphs that represent the number of individuals still alive at each age.
Thus, all the members born at the same time live out the full physiological life span characteristic of the species and all die at about the same time.
Type II: Parents produce moderate numbers of offspring and provide some parental care. Deaths occur more uniformly
Population Growth Rate: Population growth rate (r) is how fast a population changes in size over time. A positive
Population growth may also be affected by people coming into the population from somewhere else (immigration, i) or leaving the population for another area (emigration, e).
Shaped and S-Shaped Population Growth Curves
- K-Selected and r-Selected Species: Species can be divided into two basic types when it comes to how their populations grow
Here in the first phase there is no increase in population size because it needs some time for adjustment in the new environment. This increase in population is continued till large amount of food materials exist in the habitat. After some time, due to increase in population size, food supply in the habitat becomes limited which.
When the population remains static over the years, it is said to be non-fluctuating. Sometimes seasonal changes occur in the population and there are additions to the population at the time of maximum reproduction and losses under adverse climatic conditions. When the population of a species shows regular ups and downs over the years, it is called annual cyclic variation.
It appears in the form of a sigmoid curve with regular drops in population after peaks. When the change in population density does not occur at regular intervals or in response to any obvious environmental factor, it is. In this there is a sudden exponential or logarithmic increase in population density in short time followed by equally quick drop in population density due to deaths, and final return to normal level or even below that level.
