Population Growth
 Population: a group the same species that live in the same
place at the same time
 Resources: food, water, shelter, space and mates
 In theory populations can grow to an infinite size, but they
are limited by resources
 This causes individuals to compete for resources (remember
intraspecific and interspecific competition!).
Factors that Control Populations
 There are 4 main factors
controlling population
sizes:
 Natality - births increase the
population
 Mortality - deaths decrease
the population
 Immigration - movement of
individuals into an area
increases the population
 Emigration - movement of
individuals out of an area
decreases the population
Carrying Capacity
 Carrying capacity: maximum number of individuals of a
species that can be sustained indefinitely in a given space
 No population can grow indefinitely! Resources = Limited!!
Limiting Factors of Populations
 Factors (biotic or abiotic) which prevent population
numbers from growing too large and overrunning an
ecosystem.
 Example:
 Disease/ parasites
 disasters
 hunting & predation
 competition for resources
 (food, oxygen, nutrients)
Patterns of Population Growth
 “J” population growth curve
 Exponential growth = starts out slow and then
proceeds faster and faster
 Occurs when a population has few resource limitations
 Sudden collapse
('diebacks')
Patterns of Population Growth
 “S” growth curve
(S for sigmoid)
 Initially shows
exponential growth then
levels off at the carrying
capacity.
 Occurs when a
population has limited
resources.
 Results in stable
population
J Curve vs. S Curve
Population Dynamics
 A look at the factors that tend to increase or decrease the size
of a population
 The population size is determined by the interplay of biotic
potential and environmental resistance.
 Biotic potential- growth rate with unlimited resources
 Environmental resistance - all the factors acting jointly to
limit population growth
Biotic Potential vs. Environmental Resistance
Biotic potential
(Growth Factors)
 Favorable light,




temperature
High reproductive rate
Adequate food supply
Ability to migrate
habitats
Ability to adapt to
environmental change
Environmental resistance
(Decrease Factors)
 Too much or too little




light and temperature
Low reproductive rate
Inadequate food supply
Inability to migrate
habitats
Inability to adapt to
environmental change
Biotic Potential vs. Environmental
Resistance
Reproductive Strategies
r
Unstable environment,
K
Stable environment, density
 Small size
 Large size
 Many offspring are produced
 Few offspring produced
 Early maturity
 Late maturity (long parental
 Short life span
care)
 Long life span
 Individuals reproduce more
than once
 Type I or II survivorship curve
 Each individual reproduces
once
 Type III survivorship curve
Survivorship Curves
 Type I - high survival rate of the young, live most of
their expected life span and die in old age. (ex.
Humans)
 Type II - relatively constant death rate, could be due to
hunting or diseases. (ex. coral, squirrels, honey bees
and many reptiles)
 Type III - have many young, most of which die very
early in their life. (ex. plants, oysters and sea urchins).
Survivorship Curves
Pop Quiz
 Which type of survivorship curve does the cheetah
have?
 What does parental care have to do with the shape of
these curves?
 If you drew separate survivorship curves for female and
male cheetahs, what would they look like?
 What do you think the survivorship curve would be for
a white-tailed deer population?
Density Dependent Factors
 Density dependent factors
 Depend on the size of the population
 Effects of the factors increase as the population grows
 Act as negative feedback
 Tend to be biotic
 Two categories:
 Internal factors = Within a single species
limited resources
 reduced fertility rates
 External factors = between species
 populations of predators or prey
 diseases spread more easily in densely-populated areas

Snowshoe Hare and Canada Lynx
Density Independent Factors
 Do NOT depend on the size of the population
 Tend to be abiotic
 Effect the population regardless of its size
 Examples:
 1. Weather
 2. Earthquakes
 3. Floods
 4. Fires
 R-strategists are most
affected by these factors
Ecological Succession: Change over
Time
 Two Types of Succession
 Primary succession - An ecosystem starts from bare
rock
 Secondary succession – Ecosystem is built from a
previous ecosystem

Starts with soil
Important
Terms
 Sere: A set of stages of changes in
an ecosystem.
 A snapshot of ecosystem
 Pioneer organisms: First species
that begin to populate a sere,
typically r-strategists.
 Ex. Weeds, lichens
 Climax community: Populations
of organisms living together in a
sere where all species are in
balance.
 Ex. A mature forest, many Kstrategists
Pioneer Species
Primary Succession
 No soil in a terrestrial
system
 No bottom sediment in
an aquatic system
 Takes hundreds to
thousands of years
 Need to build up
soils/sediments to
provide necessary
nutrients
Primary succession will occur
after a volcanic eruption
Primary succession occurs after
a glacier retreats
Image source: http://www.callipygia600.com/
Primary succession occurs after
a glacier retreats
Primary succession occurs after
a glacier retreats
Glacier Bay, Alaska
Primary Ecological Succession
Lichens and
Exposed mosses
rocks
Small herbs
and shrubs
Heath mat
Jack pine,
black spruce,
and aspen
Balsam fir,
paper birch,
and white
spruce forest
community
Secondary Succession
 Begins with soil from previous
ecosystem
 Ecosystem has been
 Disturbed, Removed, or
Destroyed
 Abandoned farms
 Burned forests
 Deforestation
 A huge storm
Secondary Succession
Annual
weeds
Perennial
weeds and
grasses
Shrubs and
small pine
seedlings
Young pine forest
with developing
understory of oak
and hickory trees
Mature oak and hickory
forest
Secondary Ecological Succession in Yellowstone Following
the 1998 Fire
Conditions during succession
 Early Stages Gross
Productivity is Low
 Not many producers
 Later Stages Gross
Productivity is High
 Climax Community