Transcript population biology

POPULATION BIOLOGY
CHAPTER 5
What affects population size?
• Remember, a population is a
group of organisms belonging
to a single species that lives
in a given area.
• Answer the following
questions about what factors
can affect population size.
1. Think of a population in your
area. How might this population
grow in size?
2. How might this same population
decrease in size?
How is population size
estimated?
• For stationary organisms:
– Quadrant method
• For moving organisms:
– Mark and Recapture
– Great Turtle Race
• For people:
– Census
DEFINITIONS
• Species – groups of organisms
that are able to interbreed
and produce fertile offspring
• Population – a group of
organisms belonging to the
same species
Characteristics of Populations
• Geographic distribution
• Population Density - the
number of individuals of a
population within a given area
• Growth rate
POPULATION GROWTH
• Three factors that affect
population size:
– Number of births (birthrate)
– Number of deaths (death rate)
– Number of individuals that
leave (emigration) or enter
(immigration) a population
EXPONENTIAL GROWTH
• Exponential Growth – highest
rate of reproduction under
ideal conditions
# organisms
Exponential Growth
160
140
120
100
80
60
40
20
0
0 2 4 6 8 10 12 14 16 18
AGE OF POPULATION (HOURS)
LOGISTIC GROWTH
• Logistic growth – occurs when a
population’s growth slows or stops
following a period of exponential
growth
• Carrying capacity – maximum
number of individuals in a
population that the environment
can support
LOGISTIC GROWTH
# organisms
Carrying capacity
700
600
500
400
300
200
100
0
0
2 4 6 8 10 12 14 16 18
AGE OF POPULATION (HOURS)
QUESTIONS
1. Imagine a small island that has a population of
five rabbits. How might each of the following
factors affect the rabbit population?
a. Climate
b. food supply
c. predation
2. Now imagine another small island that has a
population of 500 rabbits. How would the same
factors affect this population?
3. Which of the factors depend on population
size?
4. Which factors do not depend on population
size?
LIMITING FACTORS
• Limiting factors –causes
population growth to decrease
– List examples
DENSITY-DEPENDENT LIMITING
FACTORS
• Limiting factors that become limiting
only when the population reaches a
certain density
– Examples:
• Competition
• Disease (contagious)
• Predation
• Parasitism
• Competition – competing for resources
such as food, space, water, sunlight,
mates etc.
– Can occur between the same species
(intraspecific) or different species
(interspecific)
– What are the general outcomes of
competition?
• Extinction
• Adaptation
• Emigration
– How do organisms avoid
intraspecific (same species)
competition?
• Life cycles (frogs and
tadpoles do not compete)
• Life span (adults die shortly
after young are produced)
• Parental care of young
• Dominance and Social
Hierarchy (chicken’s pecking
order)
•Role separation (bees and
ants)
•Behavioral and
Physiological Changes
(killing young, no mating)
•Emigration
•Territoriality (birds’
songs, a buck’s scent)
• Disease (contagious)
– The more crowded an area, the
quicker disease spreads
• Predation – hunting, killing, and eating
an animal
– Example: eagles (predators) and mice
(prey)
– Predation may be a limiting factor for
the prey population while food
availability may be a limiting factor
for the predator
– Predator-prey relationships show
cycles of increases and decreases in
both populations in response to one
another
– Predators often kill the old, weak, and
young
– Predation may keep prey population
very near carrying capacity
• Example: Moose vs. wolf on an island
in Lake Superior
• Crocs
60
2400
30
1200
0
0
1955
1975
Moose
1995
Wolves
• Parasitism – when organisms live
on or in a host; always harmful to
host
•Example: tics and lice
(parasites) live on humans
(host)
•The greater the population
density, the greater number
of parasites are passed from
one organism to another
DENSITY-INDEPENDENT
LIMITING FACTORS
• Limiting factors that limit a
population regardless of its
density
– Examples:
• Temperature (as with
ladybugs)
• Climate and natural disasters
NUMBER OF
LADYBUGS
POPULATION GROWTH
DENSITY-INDEPENDENT LIMITING
FACTOR
80
60
40
20
0
SPRING
SUMMER
AUTUMN
How fast are we growing?
Until about 500 years ago, the
world’s human population
remained fairly stable. Then, as
advances in medicine, agriculture,
and technology occurred, the
human population began growing
very rapidly. Today, the world’s
human population is greater than
6.5 billion people, and it continues
to grow, but at a slower rate.
1. The human population is increasing by
about 1.4 percent each year. Assume
that the population is 6 billion
(6,000,000,000). How large will the
population be in one year?
2. If the human population continues to
grow at a rate of 1.4 percent per year,
the population would double in size (to
12 billion people) in only 51 years!
What effect might this increase in
population have on the environment
and on other people?
HUMAN POPULATION
• World Population as of 9/15/08
(Estimate) 6,723,708,602
• World Population as of 2050
(projection) 9,404,251,020
• U.S. Population as of 9/15/08
(Estimate) 305,158,990
One birth every.................................. 7 seconds
One death every.................................. 13 second
One international migrant (net) every............ 29 seconds
Net gain of one person every..................... 9 seconds
• U.S. Population for 2050
(projection) 419,854,000
• A World Population Clock
• World Clock with extra data
•
Source for USA and World Pop data: http://www.census.gov/ipc/www/idb/worldpopinfo.html and
www.census.gov/population/www/popclockus.html
Industrial
Revolution
begins
Agriculture
begins
Plowing
and
irrigation
Bubonic
plague
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World Population reached:
1 billion in 1804
2 billion in 1927, (123 years later)
3 billion in 1960, (33 years later)
4 billion in 1974, (14 years later)
5 billion in 1987, (13 years later)
6 billion in 1999, (12 years later)
October 12, 1999 was Y6B day (the day
the world population reached 6 billion)
PATTERNS OF POPULATION
GROWTH
• Demography – study of human
populations
• Since the 1970’s the worldwide
population growth rate is
decreasing
• However, the population size is
still increasing (just at a slightly
slower rate)
• Growth rates higher in
developing countries
– Birthrates higher
– More people at reproductive
age
• Zero population growth is
when birthrate = death rate
• Birthrate, death rate, and age
structure affect population
growth rate
• Demographic transition – as
countries modernize, death rates
decrease at first without
birthrates decreasing. This causes
populations to increase rapidly
(USA 1790-1910) Later, birthrates
also decrease slowing population
growth.
• Age-structure diagrams – show
numbers of people in different age
groups
Figure 5-13: Age Distribution
U.S. Population
Rwandan Population
RAPID GROWTH
2.1% ANNUALLY
STABLE GROWTH
1.7% ANNUALLY
NEGATIVE GROWTH
-0.1% ANNUALLY
Male
Female
Population of the USA by Age and
Sex from 1950-2050 (in millions)