Limits on Population
can the world be taken over by one organism?
Consider this…
 Mouse litters are 6+
 It takes a mouse 6 weeks to
reach sexual maturity
 Mice can reproduce every 6
weeks
 So… why hasn't the world been
covered in mice?
 Limits on all populations!
Each Population is LIMITED
 Because there are a limited number of
resources available in an ecosystem, not
every organism born survives
 These factors are known as LIMITING
FACTORS (because they limit populations)
 Organisms basically need 3 things to
survive:
 Food
 Water
 Shelter
Biotic Potential
 maximum number of offspring a species
could produce with unlimited resources
Controlled by 4 factors
1.Birth potential
2.Capacity for survival
3.Procreation
4.Length of reproductive life
Factors That Determine Biotic Potential
Birth Potential
 maximum number of offspring per birth
 e.g. whooping crane females lay 2 eggs per
year and only 1 chick survives
Capacity for Survival
 number of offspring that reach reproductive
age
 e.g. female sea turtle lays many eggs, but
only a few offspring even reach the sea, and
few still reach sexual maturity
Factors That Determine Biotic Potential
Procreation
 The number of times that a species
reproduces each year
 e.g. elk mate only once per year (during the fall) while
farm animals will breed more than once a year
Length of Reproductive Life
 The age of sexual maturity and the number
of years the individual can reproduce
 e.g. African elephants reach sexual maturity at about
15 years of age, but may reproduce until they are 90
Limiting Factors
An environmental factor that prevents an increase in the number of
organisms in a population or prevents them from moving into new
habitats
Example: Fern
 Ideally, ferns should produce
50,000 spores per year
 Abiotic Factor
 Drier weather reduces population
 Biotic Factor
 Grazing animals reduces
population
Carrying Capacity
 maximum number of individuals of a species
that can be supported indefinitely by an
ecosystem
 i.e. the “room” available in the ecosystem
 determined by the availability of resources
(food, shelter, water)
 communities tend toward stability
 achieved when an ecosystem is in equilibrium
(i.e. balance)
Carrying Capacity
 Population can exceed carrying capacity for
a short time
 If more organisms come in, there is more
competition for space and resources
 All available
food eaten IF THE
THEREFORE,
CARRYING
CAPACITY
IS die off
 As resources
disappear,
organisms
CAN BE
becauseREACHED,
they can’t THERE
get resources
needed
A MASSIVE POPULATION
 Lack of food leadsCRASH!
to illness
 Easier prey
 Population decreases again to or below
carrying capacity
Density
 Density refers to the number of
organisms in a certain space
Low Density
BIG SPACE
 Big space to spread
out
SMALL SPACE
 Small space with a
few organisms
High Density
BGI SPACE
 Crowded
SMALL SPACE
 Crowded
Limits of Tolerance
 Living organisms need certain level of nutrients
and conditions to survive
 Law of Minimum:
 nutrient in LEAST supply limits growth
 Law of Tolerance:
 organism can survive within a certain range
of an abiotic factor (above and below this
limit… the organism cannot)
Law of Tolerance
• population of fish likely increases as water temperature is
close to optimum
• no fish survive when it’s too hot or too cold
Density Independent Factors
Affect members of population regardless
of population density… kill organisms
randomly, regardless of how many are
around
e.g. fires, floods, tornados, pesticides,
temperature, lack of rain, habitat destruction
Density Dependent Factors
 Affect population because of a large
population density
 e.g. food supply, water quality, mates,
sunlight, territory, increase predators and
disease (easier to catch a disease in a
crowded place)
 Lower food supply means energy lost to
competition
Definitions
 Limiting Factors
 Birth Potential
 Capacity for Survival
 Procreation
 Reproductive Life
 Carrying Capacity
 Density
 Law of Minimum
 Law of Tolerance
 Density Dependent
Factors
 Density Independent
Factors
Homework
1.
2.
Suppose that there is a forested park in which squirrels are reproducing very quickly
because there is so much food available. In this situation, the population will grow until it
reaches the carrying capacity. What will define the carrying capacity of the squirrel
population?
Identify the following limiting factors as either abiotic or biotic
 Wind blows the seeds of a dandelion into a pond. The seeds fail to grow.
 A population of grasshoppers eats all the available food and their numbers drop
dramatically.
 A bacterium causes a deadly disease in a herd of reindeer and some of them die.
 Plants growing beneath the trees in a forest are unable to get enough sunlight.
3.
Cockroaches are insects that reproduce very rapidly. Suggest reasons why the world is not
covered in cockroaches.
4.
Limiting factors normally control a population from expanding past its carrying capacity in a
specific area. Most of the limiting factors that would normally control the human population
have been removed through various technologies. How does this affect your quality of life?
How might it affect your children’s or grandchildren’s lives?