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Population Ecology
Population:
Group of interbreeding
members of the same
species that exist in a
specified area
Ecology:
Interactions among
organisms and between
organisms and the
environment
How Do We Describe Populations?
Demographics
 Statistics that describe a population
 Size, density and distribution of a population
Population Demographics
Population size: total number of individuals in
a population
Population density: number of individuals per
unit area or volume
Population distribution: spacing of members
within a population in a specific area
Boundary of a population: natural or arbitrary
Population Size
Plot Sampling
Uniform conditions
Mostly non mobile
species
Used to estimate population size in a defined area
based on direct counts in small portions of the area
Mark - Release - Recapture
Involves marking a number of individuals in a natural
population, returning them to that population
This technique is commonly used by fish and wildlife
managers to estimate population sizes before fishing
or hunting seasons
Patterns of Distribution
Clumped:
Attraction
Members of a
population group in
patches/ most common
Unequal distribution of
resources
Social
Reduce risk of
predation
Uniform: Active
repulsion
Even distribution
Competition for limited
resources
Plants competing for
resources/ Allelopathy
Territorial behavior in
animals
Random: minimal
influence
Uniform distribution
of resources
No pattern
Rare because of
social interactions
and varying habitats
Why does the size of
a population
change?
Interactions between
biotic and abiotic factors
cause variations in
population size
 Birth and immigration
 Death and emigration
How is population growth
measured?
Zero population growth
Time interval in which the number of births is
equal to the number of deaths
Per capita growth rate (r)=
per capita birth rate (b) – per capita death rate (d)
Exponential
growth
Population grows
by a fixed
proportion of its
total in successive
time intervals
(assuming r
remains constant)
Population growth rate (G) =
per capita growth rate (r) x number of individuals (N)
Death will slow the rate of increase but does not stop
the exponential growth
Factors Affecting Population Growth
Density-dependent factors:
Intraspecific competition (food, territory and
other resources), predation, diseases
Density-independent factors:
Weather changes, fires, habitat
destruction,….
How Has the Size of the Human
Population Changed Over Time?
For most of history, the human population
grew very slowly
Growth rate began to increase about
10,000 years ago, then soared during the
past two centuries
Expansions and Innovations
 Invention of agriculture
 Use of energy in fossil fuels
 Advances in the medical field and sanitation
 Migration
into new habitats and
expansion into new climate zones
 Development
of new technologies that
increased the carrying capacity of
existing habitats
US Census Bureau (2013)
Rank
1
Country or Area
China
Population
1,349,585,838
2
3
4
5
6
7
8
9
10
India
United States
Indonesia
Brazil
Pakistan
Nigeria
Bangladesh
Russia
Japan
1,220,800,359
316,668,567
251,160,124
201,009,622
193,238,868
174,507,539
163,654,860
142,500,482
127,253,075
Fertility and future growth
Fertility rate
Age structure:
the age distribution
of individuals
Reproductive base of
a population:
Members of the
prereproductive and
reproductive age
categories
Future Growth
 World population is expected to reach
8.9 billion by 2050, and possibly to
decline as the century ends
 China
and India each hold more than
one billion people (together, 38%
percent of the world population); the
United States is third, with 317 million