Organisms and Population Attributes

The models of population growth  are:

1. Exponential Growth: The availability of resources (food and space) is critical for population expansion. As a result of the infinite availability, the population grows exponentially. (dN/dt) is the formula for calculating the growth or decrease in population density (N) over a certain time period (t).

Let dN/dt = (b – d) XN

If (b-d) = r, then dN/dt = rN

N is the population size, b is the birth rate, d is the death rate, t is the time period, and r is the intrinsic rate of natural increase.

The parameter r is used to evaluate the influence of biotic and abiotic factors on population increase. It varies depending on the organism.

For the Norway rat, it's 0.015, while for the flour beetle, it's 0.12.

Nt = N0ert is the integral form of exponential growth, where Nt is the population density after time t, N0 is the population density at time zero (the start), r is the intrinsic rate of natural increase, and e is the base of natural logarithms (2.71828).

Any species that grows exponentially, given unrestricted resource conditions, can reach tremendous population densities in a short period of time.

2. Logistic Growth: In practice, no population of any species in nature has unrestricted resources. Individual competition and the survival of the fittest happen as a result of this. As a result, a given habitat has sufficient resources to support a limited number of people, beyond which no further expansion is conceivable. This is referred to as the species' carrying capacity (K) in that habitat. The logistic growth, also known as Verhulst-Pearl Logistic Growth, appears as a sigmoid curve when N is plotted against time t. It is calculated as dN/dt = rN (K – N/K), where N is the population density at time t, K is the carrying capacity, and r is the natural rate of increase.

This model is more realistic in nature because no population growth can continue to rise exponentially and endlessly because basic necessities will be met.

If efforts are made around the world to slow population growth and bring it to a halt, the human population growth curve will become S-shaped.

A population pyramid, often known as an age pyramid, is a graphical representation of a population's age and gender. As a result, age-sex pyramids are often known as population pyramids. Three types of population pyramids exist: 

• Expansive population pyramids indicate populations with a higher number of persons in younger age groups than constrictive population pyramids. High fertility rates and shorter life expectancies are common in populations with this morphology. Expansive population pyramids are found in many third-world countries 

• Constrictive population pyramids are named as such because they are restricted at the bottom. Younger folks make up a smaller percentage of the population. Because each successive age group gets smaller and smaller, constrictive population pyramids reflect diminishing birth rates. The demographic pyramid in the United States is constrictive.

• Stationary population pyramids illustrate that each age group has a roughly equal proportion of the population. The population is neither decreasing, nor increasing; it is steady. Austria's population pyramid is static.

The term "organism" refers to a living being. Molecules are the basic building blocks of both living and non-living objects. A living creature, on the other hand, can be distinguished from an inanimate object by its distinguishing qualities. An organism, for example, is made up of one or more cells. This structure is made up of biologically manufactured and naturally existing chemicals. Biomolecules are the name given to such molecules. Proteins, nucleic acids, lipids, and carbohydrates are examples. These biomolecules can group together to create complex particles, which then form subcellular structures. Within a cell, these subcellular structures exist. Every living organism is made up of at least one cell, hence the cell is considered the fundamental biological unit.

A mortality rate (MR) is the number of occurrences (deaths) that occur in a population of known size that is at risk of death over a given time period (usually called the recall period).

The mortality rate is calculated as follows: Total deaths during the time / (mid-period population at risk x duration of the period)

The mid-period (or midpoint) population is calculated by taking the population at the end of the period, adding one-half of the period's deaths, and removing one-half of the period's births.

Natality or birth rate is the rate at which new individuals are born in a population per unit of time. It can be statistically measured only for a population and not for an individual. An individual can only have birth but not a birth rate. The birth rate is calculated by dividing the total number of offspring produced or births by the total population in a given time. 

Population density refers to the size of the population in a given area in a given time. It is also measured to look at the conditions of the habitat, since higher population density indicates that the habitat is healthy and provides more favorable living conditions.

Population density is given by the formula:

Population Density (PD) = (Number of Individuals in a Region (N))/(Number of Unit Area in the Region (S)) = PD = N/S