CH. 5
Populations
POPULATION DYNAMICS
Changes in populations result from:
• environmental stress
• changes in environmental conditions.
MAJOR CHARACTERISTICS OF A
POPULATION
1) Size
2) Density
3) Dispersion
4) Age distribution
MAJOR CHARACTERISTICS OF A
POPULATION
1) Size – number of individuals
2) Density – number of individuals in a certain
space
3) Dispersion – spatial pattern of a population
4) Age distribution – proportion of individuals of
each age in a population
GENERALIZED DISPERSION PATTERNS
• Most populations live in clumps (because of resources, protection,
predation, mating) although other patterns occur based on
resource distribution.
PARTNER WARM UP
• Partner A – What is the difference between
primary and secondary succession?
• Partner B – What are the three biotic stages of
succession?
CHANGES IN POPULATION SIZE:
ENTRANCES AND EXITS
•Populations increase - births and immigration
•Populations decrease - through deaths and
emigration
GROWTH MODELS
Biotic Potential – capacity for population growth
Intrinsic rate of increase
• rate with unlimited resources
Leads to J-shaped or exponential
growth curve
POPULATION GROWTH CURVE
Population size (N)
Exponential Growth
Time (t)
GROWTH MODELS
However, no population can grow indefinitely.
There are ALWAYS limits to population
growth.
Environmental Resistance – Factors acting to
limit population growth
GROWTH MODELS
Carrying Capacity (K)
• Determined by biotic potential and environmental
resistance
• the number of individuals of a given species that can
be sustained indefinitely in a given space.
Leads to an S-shaped or Logistic growth
curve.
POPULATION GROWTH CURVES
Carrying capacity
Population size (N)
Logistic Growth
Time (t)
EXPONENTIAL AND LOGISTIC
POPULATION GROWTH
• Populations grow
rapidly with ample
resources, but as
resources become
limited, its growth rate
slows and levels off.
WHAT HAPPENS WHEN POPULATIONS
EXCEED CARRYING CAPACITY
• Members of populations
which exceed their
resources will:
• die
• adapt
• move
POPULATION DENSITY
• Density-independent population controls
• Affect a population’s size regardless of its population density.
• Floods, fires, hurricanes, habitat destruction
POPULATION DENSITY
• Density-dependent population controls
• Factors that have a greater effect as a population’s
density grows
• Competition of resources, predation, parasitism, disease
(bubonic plague)
TYPES OF POPULATION CHANGE
CURVES IN NATURE
Population sizes may stay the same, increase, decrease,
vary in regular cycles, or change erratically.
• Stable: fluctuates slightly above and below carrying
capacity.
• Irruptive: populations explode and then crash to a more
stable level.
• Cyclic: populations fluctuate and regular cyclic or boomand-bust cycles.
• Irregular: erratic changes possibly due to chaos or drastic
change.
Number of individuals
SIMPLIFIED POPULATION CHANGE CURVES
(3) Irregular
(1) Stable
(4) Cyclic
(2) Irruptive
Time
NATURAL CONTROLS OF POPULATIONS
TYPES OF POPULATION CHANGE CURVES IN NATURE
Population sizes often vary in regular cycles when the predator and
prey populations are controlled by the scarcity of resources.
(predator-prey oscillation)
REPRODUCTIVE PATTERNS AFFECT
POPULATION
•Sexual vs. Asexual Reproduction
•K vs. R Species
REPRODUCTIVE PATTERNS AND
SURVIVAL
• Asexual reproduction
• Offspring are exact copies of a single parent
• Pros/Cons?
• Sexual reproduction
• Organisms produce offspring by combining
the gametes or sex cells from both parent
• Pros/Cons?
ADVANTAGES/DISADVANTAGES
TO SEXUAL REPRODUCTION
Disadvantages
• Females must produce twice as many offspring (because
males don’t give birth)
• Chance of genetic errors increases
• Mating entails costs
Advantages
• Provides greater genetic diversity
• Division of labor – males gather food and protect females
and young
r-Selected Species
Opportunists
• Many small offspring
• High population growth rate
• Little or no parental care
(r)
and protection of offspring • Population size fluctuates
• Early reproductive age
wildly above and below
• Most offspring die before • carrying capacity (K)
reaching reproductive age • Generalist niche
• Small adults
• Low ability to compete
• Adapted to unstable climate • Early successional species
and environmental
conditions
HOMEWORK
WARM UP 1/8/2016
Two plant species co‐occur in an oak savanna. One is fairly
long‐lived and produces a few large seeds. The other is
short‐lived and produces many small seeds.
1) Compare and contrast the advantages and disadvantages of
these two life histories.
2) Which species is most likely an r‐selected and which
species is most likely a K‐selected species? Explain your
answer in terms of reproduction rates AND the kind of
environmental conditions to which these two life histories tend
to correspond.
K-Selected Species
Competitors
•Fewer, larger offspring
•Lower population growth
•High parental care and
rate
protection of offspring
•Population size fairly stable
•Later reproductive age
and usually close to
•Most offspring survive to carrying capacity (K)
reproductive age
•Specialist niche
•Larger adults
•High ability to compete
•Adapted to stable climate •Late successional species
and environmental
conditions
Carrying capacity
Number of individuals
K
K species;
experience
K selection
r species;
experience
r selection
SURVIVORSHIP CURVES:
SHORT TO LONG LIVES
• The way to represent the age structure of a population
is with a survivorship curve.
• Type 1 = Late loss population - live to an old age.
• Type 2 = Constant loss population - die at all ages.
• Type 3 = Early loss population - die at young ages.
Percentage surviving (log scale)
Survivorship Curves
100
10
1
0
Age
Sturgeon fish
Sturgeon fish inhabit rivers, estuaries and the ocean at various stages
during their life cycle—and their lifecycle is a long one. Adults can live
up to 80 years. After spending up to 3 years in their natal rivers as
juveniles, they travel into the oceans until they reach sexual maturity.
This may take another 8-20 years, depending on the sex of the fish
(females usually take longer to reach sexual maturity) and latitude
(sturgeon in the northern rivers generally take longer to reach sexual
maturity).
Spawning typically occurs in the in the springtime. Both sexes swim
back up their natal rivers to the spawning grounds. Females lay their
eggs, which are sticky, and attach to the river’s gravel bottom—larger
females have been known to lay 800,000 to 3.5 million eggs. The males
then fertilize the eggs. The adults leave the eggs to the mercy of the
river and their well chosen spawning grounds. Of that egg mass, only
10% will survive to reproduce in the natal river again. The rest die due to
predation, water pollution or other causes.