The organisms living as a group in an ecosystem as referred to as "population".

population -

A group of individuals of a single species inhabiting a specific area in a definite time period.

Population size

Population depends on population size. Population size is generally expressed as the number of individuals in a population.

There are 3 ways to express the population size,

  1. Census (Direct count)
  2. Estimate (Sampling)
  3. Index count (Indirect observation)

Census

  • Individual count of the organism.
  • Identifying every single individual separately and counting them.
  • Gives the exact population count.
  • Expensive and take a longer time.

Estimate

  • Gives density of organisms in a particular area and multiply it by the total area to determine the total number of individuals in the study area.
  • Easy to apply but may be inaccurate.
  • The mark and recapture method - estimation method.

Index count

  • Signs of animals observed (when animals themselves may not be encountered due to their secretive lifestyle)
  • Not very accurate.

Properties of the Population 


Properties are unique only to the group, not to the individual. The population has biological attributes and also have group attributes. The biological attributes relate to its life history. 
The biological group attributes are,
  • Density
  • Natality(Birth rate)
  • Mortality(Death rate)
  • Population distribution
  • Age structure
  • Sex ratio
  • Genetic composition

Population density


The number of individuals per unit of space or the biomass per unit area or volume. Animal density is referred to as crude density. The important population parameters  that affect population density are natality, mortality, immigration(moving into the population), and emigration(moving out of the area)

Major factors that influence the change in population density

Natality


This is the ability to increase population size by reproduction. This is described as maximum or physiological natality and ecological or realized natality.

  1. Physiological (maximum) natality - theoretical maximum production of new individuals under ideal conditions and is a constant for a given population.
  2. Realized (ecological) natality - the population increase under actual environmental conditions over a period of time. It's not a constant for a population but depends on several factors. 
Natality is measured as a rate (crude natality rate or specific natality rate)




Mortality


The number of animals dying within a given period of time - Mortality rate or specific mortality rate.
It reduced the population size.
Mortality that occurs under ideal conditions is described as theoretical minimum mortality (constant for population).
Mortality under given conditions is described as ecological or realized mortality (varies with population and environmental condition)



Population Distribution


Dispersal is the way individuals spread away from each other. Individual organisms within a population will occur over space and time. Population migration is the mass directional movement of large numbers of species from one location to another.
Most organisms live in their usual habitats, but some stage of their life either permanently or seasonally for their life cycle.

Major reasons for dispersal, 
  • Insufficient resources.
  • Deteriorating habitats.
  • Alleviation on inbreeding.

01) Spatial distribution


There are four  patterns of distribution
  1. Random.
  2. Regular.
  3. Clumped.
  4. Regular clumped.
Patterns of distribution 


Random distribution

  • very rare in nature.
  • It occurs where the environment is very uniform and where there is no tendency to aggregate for food or other resources.
  • The distribution is random individuals within the population have an equal chance of living anywhere within the area.

Regular distribution


  • Individuals of the population are spaced more or less evenly.
  • This distribution results from intra-specific completion among numbers of the population.

Clumped and regular clumped distribution

  • Most common distribution pattern
- Reasons -
  • Local resource differences
  • Habitat differences
  • Reproductive patterns
  • Social behavior
  • Environment changes
  • Distribution of nutrients nesting sites and water

02) Temporal distribution


Distributed in time.
This distribution related to,
  • Daily changes in light and dark
  • Lunar
  • Tidal
  • Seasonal changes
Also, involve
  • Longer periods
  • Annual fluctuations
  • Seasonal changes
  • Evolutionary changes

Population age structure


The ratio of the various age class to each other at a given time. 
Age structure is useful in,
  • Understanding and predicting population growth.
  • Periods of successful reproduction.
  • Periods of high and low survival.
Age structure is visualized by age pyramids. 

Population age pyramids for different human populations 

  1. Rapid growth
  2. Slow growth
  3. Zero growth
  4. Negative growth
The age structure can be expressed in terms of three ecological ages,
  1. Pre-reproductive.
  2. Reproductive.
  3. Post-productive.

Sex ratio


Animals distinctly separate the males and the females. The sex ratio is the ratio of males to females in a population.
The primary sex ratio also tends to be 1:1.

Generally males have a shorter life span than females.