How are these terms related?
species
population
What they have in common:
What makes them unique
How will you remember?
community
5-1 How Populations Grow
Objectives:
1. List the characteristics used to describe a
population
2. Identify factors that affect population size
3. Differentiate between exponential and logistic
growth
Characteristics of Populations
• Four Characteristics:
1.
2.
3.
4.
Geographic range
Density and distribution
Growth rate
Age structure
Geographic Range
• Area covered by a population
– Size depends on organism and other limiting
factors
Distribution of long-tailed salamanders Eurycea longicauda
Population Density
• Refers to the number of individuals per unit
area
A
B
• Can vary greatly among species in the same
ecosystem
What is the population density?
1m
5m
10 butterflies/5 m2
= 2/m 2
Distribution
• Distribution refers to
how members of a
populations are
spaced out across the
range of that
population
• Can be described in
one of 3 ways:
– Random
– Uniform
– Clumped
Ways to Determine Population Size
1. Direct Observation- counting one by one
2. Indirect Observation- relies on signs like
tracks, empty nests
– Useful when organisms are difficult to observe
3. Sampling- uses an estimate (an
approximation of a number based on
reasonable assumptions)
4) Mark and Recapture- another way of
estimating the population
Mark and Recapture
• Scientists capture and mark members of a species and
then release them
• Later the species is captured again and the marked
members are counted to estimate the population
Total population = number marked x number captured = 5 x 4 = 20
number captured with marks
Marked Species = 5
1
Captured Species =4
Number with marks = 1
Population Growth
Population size is affected by:
1. Births and deaths
Births>Deaths= population grows
2. Immigration and emigration
– Organisms immigrate into and emigrate out of
populations to find mates, food, establish new
territories, etc.
Immigration>emigration=population grows
3. Limiting Factors
- Space, food, predators, disease, etc.
Patterns of Population Growth
• Exponential growth- occurs when the
number of organisms rises at an everincreasing rate
– Occurs when conditions are ideal
• no limiting factors affecting population
– Population reproduces at a constant rate
• i.e. binary fission (reproduction) in bacteria occurs
every 20 minutes
– Can result in a population explosion
J-shaped Curve
• Shows exponential
growth (unchecked
growth)
• Initial increase in
number of organisms is
slow because the
number of reproducing
organisms is small
• Rate increases rapidly
because the total
number of potential
reproducers increases
Patterns of Population Growth
• Logistic growth- occurs when a population’s
growth slows or stops following a period of
exponential growth
– Growth pattern of natural populations
– Due to limited availability of resources when
populations increase in size
S-shaped Curve
• Shows logistic growth
A= lag phase-small # of
reproducing individuals
B= exponential growth
C= population growth
levels off
• carrying capacity (K)-
number of organisms of a
population that a particular
environment can support
over an indefinite period of
time
– Determined by limiting factors
C
B
A
Logistic Growth
5-2 Limits to Population Growth
• Limiting factors- factors that controls the
growth of a population
– Determine the carrying capacity of an
environment
• Two types:
– Density dependent- affect increases as population
density increases
– Density independent- affect is the same
regardless of population size
Types of Limiting Factors
•
•
•
•
•
•
Density-dependent
competition
predation
herbivory
parasitism
disease
•
•
•
•
•
Density-independent
temperature
floods
drought
habitat disruption
Competition
• Constant competition for resources within
the same population (intraspecific) or
between populations (interspecific)
• Can lead to evolution (speciation)
Predation
• Cyclical in nature which increases and
decreases over time
• Important for the health of natural
population
Parasitism and Disease
• Also limit population growth
5-3 Human Population Growth
• Objectives:
1. Discuss the trend of human population
growth
2. Explain why population growth rates differ
from country to country
History of Human Population Growth
Industrial Revolution and Human
Population
• Living conditions improved and more reached
reproductive age
– Food supplies became more reliable
– Improved nutrition
– Sanitation
– Medicine
– Healthcare
• Led to exponential growth of population
Thomas Malthus
• Suggested that populations would be
regulated by war, famine, an disease
– Limiting factors:
• Competition (war)
• Limited resources (famine)
• Parasitism (disease)
Malthus’ ideas influenced Charles Darwin
Patterns of Population Growth
• Demography- scientific study of
human population
– examines characteristics of human
populations and attempts to explain
how those populations will change over
time
– Characteristics examined:
• Birth rates
• Death rates
• Age structure
Demographic Transition
• Population growth in developed countries like
US, Europe, and Japan has slowed dramatically
• these countries have completed the
demographic transition, a dramatic change in
birth and death rates
• Birth rate falls to meet the death rate and
population stops growing
Demographic Transition
STAGE I
Reproduction and death
rates are high
throughout most of
history
STAGE II
Death rates fall, but birth
rates remain high for
some time; births exceed
deaths and population
grows exponentially
STAGE III
Standard of living and
education level rises and
birthrate falls causing
population to level off
World Population Growth
Age Structure
• Population growth depends, in part, on how
many people of different ages make up a
given population.
• Demographers can predict future growth
using models called age-structure diagrams
• Age-structure diagrams show the population
of a country broken down by gender and age
group.
Age Structure Diagrams
Future Population Growth
• Considerations for predicting world
population growth:
– Age structure of each country
– Prevalence of life-threatening disease
• Prediction for 2050 is slowing growth, but still
9 billion
– Increasing population, regardless of rate, could
have negative impact on environment and global
economy
– Or, will be offset by science, technology and
changes in society