Population Ecology

The population consists of all the individuals of a species within a given area. The population density is the size of the population concerning per unit area at a given time.

Population density of;

India                       282/sq. km

Japan                          270/sq km

China                           70/sq km 

World                              27/sq km

Population density(D) is determined by dividing the total number of population(N) by the number of units of space (S)i.e, D=N/S. 

Characteristics of population

A. Dispersion. Every population has its own dispersion pattern.

(I) Random. When population dispersed over a wide area  without any reality

(ii) Uniform. when a population occupies a particular space

(iii) Clump. some population occur in discontinuous groups

B. Dispersal. may be through the following means:

(i) Immigration. it is a one-way movement to established areas or to an unoccupied

area or thinly populated area

(ii) Emigration. it is the one-way exit from a particular population or area which can be due to overcrowding to heavy selection pressure as a result of strong competition or may be due to unfavorable environmental effects

(iii) Migration. It is a to and fro movement of the population at regular intervals of time from a populated area. 

Three main factors that control population growth, they are natality, mortality, and survivorship 

(a) Natality (Birth rate) 

(i) It is several new young ones produced in a specific time period. 

(ii)Physiological natality (fecundity) is the theoretical number of individuals produced under ideal conditions in a specific time period

(iii)Ecological or realized natality is the actual addition of new individuals to the population Natalily varies with time and size of the population.

(b)Mortality (Death Rate)

(i)It is the number of deaths per unit time in an area.

(ii) Potential or minimum mortality is a theoretical loss of individuals in a given time period under ideal conditions. It  is constant 

(c) Ecological or realized mortality is the number of deaths in a population under an existing environmental state. It is  variable

Mortality is affected by diseases,  predation, competition, geographic conditions, etc.

The information about the distribution of deaths among various age groups of a given

important in:

(i) determining the size and composition of the population.

(ii) predicting the future development of population size and makeup and

(iii) indicating the men of control

 (c) Survivorship is the percentage of individuals living at various ages 

(d) The birth-death ratio is called vital-index and is calculated as

                          VI=births/death X100

 C. Age Distribution

(i)  It is the relative proportion of individuals of different age group in a population 

(ii) A population has 3 ecological age group in a population

(a ) Pre- Reproductive (b) Reproductive (c) Post reproductive 

(iii)  The proportion of these three age groups determine whether the population is growing, mature

(stable or diminishing 

 D. Growth Rate

(i) It can be defined as an increase in the number of individuals in a population in a specific time period 

 Yeast cells show Sigmold or S-shaped growth curve (sometimes called a logistic curve)

which has three phases

(i) Lag phase. It is the early phase of little or no growth(stabilization)

 (ii)Positive acceleration phase. Growth is faster  in this  phase

(iii) Exponential phase. This is the middle  phase of rapid or geometric growth

(iv)The negative acceleration phase. in this phase, growth slows down.

(v)Stealy phase. It  is the last  phase of  zero growth ( New cells=dying cells)

 The J-shaped growth phase is visible generally in predator-free zone Eg Reindeer population, an algal bloom Lammings of tundra, and few insects.

Phases of J-shaped growth

(a) Lag phase that shows no increase in population

(b)  Exponential phase which shows multiplication

 Zero population growth: When the number of individuals added to the population becomes the number of individuals lost from the population, then the population's size will remain the same or constant. 

 E. Strategies for survival

(i) Birth and death are crude indicators of population behavior because they disregard factors age and sex.  However, the probability of producing offsprings or death does vary with these factors. 

(ii) The relationship between age and survival is best examined by constructing a survival curve. It  can

be of three different 

Curve A

1.It is typical of most invertebrate and plant populations 

2. They have a very  much high mortality rate in the young

3. They are called r-strategies because their lifestyle is adapted to give the highest possible value of 'r ( the intrinsic rate of increase)

Curve B

It represents the rare situation in which there is an equal chance of dying at all ages represents the restoration in which there is an equal chance of dying at all ages.

 2. Example include lower animals like  hydra

Curve C

1. It is  shown by an organism that has very stable populations, close to k (the carrying capacity of the environment) 

2. They have long life spans and generation times, produce small numbers of offspring, and exhibit parental cares towards them

3. Eg Mountain sheep, elephant and man 4. 

They are called K-strate