Organisms and Populations — Part 1: Population attributes and growth: visual lesson, ISC Class 12 Biology

Lesson

Populations

Part 1 — the attributes of a population and how populations grow.

In this lesson you will

Define population density and describe ways of measuring it
Calculate natality, mortality and change in population size; interpret sex ratio and age pyramids
Explain exponential and logistic growth with equations and graphs, and carrying capacity

Concept

A population has attributes that an individual does not.

Table of population attributes
Attribute	Meaning
Population density (N)	Number of individuals per unit area or volume
Natality (birth rate)	Births per individual per unit time
Mortality (death rate)	Deaths per individual per unit time
Immigration and emigration	Individuals entering from or leaving to other habitats
Sex ratio	Proportion of females to males
Age structure	Proportions of pre-reproductive, reproductive and post-reproductive individuals

Concept

Counting every individual is often impossible, so ecologists use the method that suits the organism.

Table of methods of measuring population density
Method	Used for
Total count	Large, easily seen animals, such as a herd in a reserve
Quadrats	Plants and slow-moving animals: count in sample squares and scale up
Percentage cover or biomass	When numbers mislead, e.g. one huge banyan tree versus many small herbs
Indirect signs	Pug marks and faecal pellets of tigers
Mark and recapture	Mobile animals such as fish or insects