Population Ecology
Population Dynamics and Carrying Capacity
 Population dynamics
-study of how populations change in size, density, and age
distribution
-populations respond to their environment
-change according to distribution
Dynamics of Natural Populations
• Population growth curves
• Biotic potential - the ability to increase
population numbers
• Environmental resistance - the combination of
all the biotic and abiotic factors that limit a
population’s increase.
• Carrying capacity – the upper limit to the
population of any particular organism that an
ecosystem can support
Density Dependence And Critical Number
• Environmental resistance factors can be density
dependent.
– If population density increases, environmental
resistance becomes more intense and causes in
increase in mortality.
– If population density decreases, environmental
resistance lessens, allowing the population to recover.
– Food, Water, Disease, Predation
• Environmental factors that cause mortality can be
density independent
– A sudden deep freeze in spring
– A fire that may kill all small mammals
– Natural Disasters
Biotic Potential and Environmental
Resistance
• Environmental resistance
• Biotic Potential
– Reproductive rate
– Ability to migrate (animals)
or disperse (seeds)
– Ability to invade new
habitats
– Defense mechanisms
– Ability to cope with
adverse conditions
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Lack of food or nutrients
Lack of water
Lack of suitable habitat
Adverse weather
Predators
Disease
Parasites
Competitors
Exponential and Logistic Growth
LOGISTIC GROWTH
EXPONENTIAL GROWTH
- Rapid exp. growth followed by
steady dec. in pop. Growth
w/time until pop. Size levels off
-Population w/few resource
limitations; grows at a fixed rate
Natural Population Curves
• STABLE
– pop. Size fluctuates above or below its carrying capacity
– Stable population size
– EX: undisturbed tropical rain forests
• IRRUPTIVE
– pop. Growth occasionally explodes to a high peak then
crashes to stable low level
– EX: Algae, insects
• CYCLIC
– Fluctuations occur in cycles over a regular time period
– EX: Lynx & snowshoe hare
• IRREGULAR
– No recurring pattern in changes of population size
The Role of Predation in
Controlling Population Size
 Top-down control
- lynx preying on hares
periodically reduce the hare
pop.
 Bottom-up control
- the hare pop. may cause
changes in lynx pop.
Species Interactions
• Niche
• Competition
– Interspecific
– Intraspecific
• Symbiotic Relationships
– Mutualism
– Parastism
– Commensalism
How do Species Reproduce
• ASEXUAL REPRODUCTION
– all offspring are exact genetic copies of a single parent
– Common in single celled species (bacteria)
– Each cell divides to produce 2 identical cells
• SEXUAL REPRODUCTION
– Organisms produce offspring by combining sex cells or
gametes from both parents
– Produces offspring with combination of genetic traits from
each parent
– Provides greater genetic diversity in offspring
• DISADVANTAGES
– Males do not give birth
– Increased chance of genetic errors and defects
– Courtship & mating rituals consume time & energy
and transmit diseases
OBJ 9.10
Reproductive Patterns and Survival
 r-selected species vs. K-selected species
Fig. 9-10 p. 170
Survivorship Curves
•Shows the % of members in a pop. Surviving at different ages
LATE LOSS
-High survivorship to certain age; then
high mortality
-EX: elephants, rhinos, humans
CONSTANT LOSS
-Fairly constant death rate at all ages
-EX: songbirds
EARLY LOSS
-Survivorship is low early in life
-EX: annual plants, bony fish sp.
Age Structure Stages
• PREREPRODUCTIVE AGE
- Not mature enough to reproduce
• REPRODUCTIVE AGE
- Capable of reproducing
• POSTREPRODUCTIVE AGE
- too old to reproduce
Factors Governing Changes
in Population Size
• Four variables
– births, deaths, immigration and emigration
• Population Change = (births + immigration) – (deaths +
emigration)
• Crude Birth Rate = CBR = (births/population)*1000
• Crude Death Rate = CDR = (deaths/population) *1000
• Immigration and emigration are calculated the same way
• Crude Growth Rate = CBR = CDR
• Population Growth Rate = CGR * 100
Calculating Population Growth
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N0 is the starting population
N is the population
after a certain time, t ,
has elapsed,
r is the rate of natural increase expressed as a
percentage (birth rate - death rate) and
• e is the constant 2.71828... (the base of natural
logarithms)
Growth Curves – Two Types J or S
• Exponential growth
results in population
explosion
• Rule of 70
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to find the doubling time of a
quantity growing at a given
annual percentage rate, divide
the percentage number into 70
to obtain the approximate
number of years required to
double.
For example, at a 10% annual
growth rate, doubling time is 70
/ 10 = 7 years.
This results in a J curve
graph.