Download Population Dynamics

Population
Dynamics
Population Dynamics
• Changes in population size and the factors
that regulate populations over time.
Population Density
• The number of individuals per unit area volume.
volume
• Example:
the number of oak trees per km2 in
a forest.
Estimation of Population Size
1.
2.
3.
Individual counts (not always practical)
Transects
Mark-recapture method:
method
N= marked individual X total catch second time
recaptured marked individuals
Example: pond turtles 1992: 18
1993: 34 (12)
1994: 30 (18)
Estimation of Population Size
Example: pond turtles
1992: 18
1993: 34 (12)
1994: 30 (18)
1992-93: N = 18(34) = 51 turtles
12
_________________________________________
1993-94: N = 34(30) = 56.7 turtles
18
_________________________________________
So: 51 + 56.7 = 107.7 = 53.9 turtle estimation
2
Dispersion
• The distribution of individuals within
geographical population boundaries.
boundaries
• Three examples of dispersion patterns:
1. Clumped
2. Uniform
3. Random
Dispersion Patterns
1. Clumped
a. individuals are aggregated in patches
b. unequal distribution of resources in the
environment.
Example:
Example trees around a lake or pond
Dispersion Patterns
• 2. Uniform
a. individuals are evenly distributed
b. interactions among individuals of a
population
Example: creosote bushes in the desert
Dispersion Patterns
• 3. Random
a. unevenly distributed
b. random dispersion is rare
Example:
Example clams in a mud flat
Understanding Population Growth
1. Exponential Growth Model
a. exponential growth: rate of expansion
(growth) of a population under ideal
conditions.
b. simple equation: G=rN
G: growth rate of population
N: number of individuals in the pop
r: intrinsic rate of increase
(range between -1 & +1)
Exponential Growth
• Example: bacteria
produces a J-shaped curve
Number of
individuals
(N)
Time
Understanding Population Growth
2. Logistic Growth Model
a. logistic growth: environmental factors
that restrict the growth of a population
(called: population limiting factors)
factors
(K-N)
b. equation: G=rN
K
G:
N:
r:
K:
growth rate of population
number of individuals in the pop
intrinsic rate of increase
carrying capacity
K: Carrying Capacity
• The maximum stable population size that a
particular environment can support over a
relatively long period of time (K).
(K)
Logistic Growth
• Example: turtles in a pond
K
produces a S-shaped curve
Number of
individuals
(N)
Time
Question
• What if?
• N = almost 0:
exponential growth (K/K = 1, then G=rN)
• N = almost K (carrying capacity):
growth rate is approaching zero
population growth (ZPG)
Factors That Limit
Population Growth
1. Density-Dependent Factors:
a. Population-limiting factors whose effects
depend on population density.
density
b. The greater the pop, the greater the effects.
Examples:
1.
2.
3.
4.
Limited food supply
Plagues
Predation
War
Factors That Limit
Population Growth
2. Density-Independent Factors:
a. Population-limiting factors (abiotic) whose
occurrence is not affected by pop density.
density
b. the greater the pop, the greater the effects
Examples:
1.
2.
3.
4.
Earthquakes
Fires
Hurricanes
Freeze in the fall
Boom and Bust Species
• A rapid increase (boom) in a population
followed by a sharp decline (bust).
(bust)
• Examples:
a. Daphnia in a pond
number of
individuals
in pop.
boom
Time
bust
Boom and Bust Species
• Sometimes species (carnivore) depend on
other species (prey) for food.
• Example: Snowshoe hare and lynx
number of
individuals
in pop.
Years
Evolution Shapes Life Histories
• Life History:
Series of events from birth through
reproduction to death.
death
• Two basic types of life history strategies:
1. Opportunistic life history
(r-selected species)
2. Equilibrial life history
(K-selected species)
1. Opportunistic life history
(r-selected species)
Characteristics:
1. maturing time:
2. life span:
3. mortality rate:
4. times female is reproductive:
5. age at first reproduction:
6. size of offspring:
7. parental care:
short
short
often high
usually once
early
small
none
2. Equilibrial life history
(K-selected species)
Characteristics:
1. maturing time:
long
2. life span:
long
3. mortality rate:
often low
4. times female is reproductive:
often many
5. age at first reproduction:
late
6. size of offspring:
large
7. parental care:
often extensive
Life History Examples
r-selected species:
1. Garden weeds
2. Insects
3. Desert annuals
K-selected species:
1. Humans
2. Oak trees
3. Elephants
Survivorship Curves
• Type 1 survivorship curve:
High survival rates until old age.
Example:
humans
% of
survivors
% of maximum life span
Survivorship Curves
• Type 2 survivorship curve:
Intermediate between the extremes.
extremes
Example:
% of
survivors
squirrel
% of maximum life span
Survivorship Curves
• Type 3 survivorship curve:
High mortality rates as larvae but
decreased mortality at later ages.
Example:
% of
survivors
oysters
frogs
insects
% of maximum life span
Human Population Growth
• Age structure
Proportions of individuals of a population in
different age groups.
groups
A typical population has three main age groups
(age structure).
1.
Pre-reproductive
2.
Reproductive
3.
Post-reproductive
Age Structure
Post-reproductive
Reproductive
Pre-reproductive
Human Population Growth Today
• Human population as a whole is growing
exponentially.
exponentially
• Has doubled (doubling-time) three times in the
last three centuries (doubled the carrying
capacity several times).
• Is now >6 billion, might reach ~8 billion by 2020.
2020
Question:
• What are the reasons for the increase in
human population?
population
• Answer:
1.
Improved health
2.
Technology
3.
Decreased death rates
Question:
• What is Zero Population Growth (ZPG)?
• Answer:
a. birth rate equals death rate.
rate
b. intrinsic growth (r) = 0
Question:
• What are two ways the human population
can reach ZPG?
1. Limit the # of offspring per couple
a. Reduces family size
b. Voluntary contraception
c. Family planning
2. Delay reproduction
(late 20’s instead of early 20’s)