Download Populations

Populations

What is a population?
 Population: a group of organisms all of the same
species, which interbreed and live in the same place
at the same time
 Ex: Humans in Negaunee
 Grey squirrels in Marquette
 White-tailed deer in Ishpeming
Population Growth
 There are three main factors that determine how
fast a population will grow
 1. Birth Rate
 2. Death Rate
 3. Migration Rate
 Immigration: movement into a population
 Emigration: movement out of a population
Types of growth
 There are two main types of population growth
 1. Exponential
 2. Logistic
Exponential Growth
 As a population gets larger, it grows at a faster rate
 Ex: The population doubles with each generation
 Occurs when resources are unlimited and
conditions are optimal
 Results in unchecked growth
 Cannot continue indefinitely
Exponential Growth: “J” Shaped Curve
 Starts off slow
 Eventually grows faster and faster
 Unchecked growth
 Does NOT continue indefinitely
Logistic Growth

A population cannot grow indefinitely. Eventually, certain
factors will slow population growth.

Limiting factors such as food, disease, predators, and lack of
space will cause population growth to slow down and
stabilize.
Logistic Growth: “S” Shaped Curve
 Growth starts fast but limiting factors
cause growth to slow and reach a
stable size
 Populations will slow down due to
either an increased death rate or a
decreased birth rate
 Will reach a stable level called
carrying capacity
Carrying Capacity
 Carrying capacity is the number of organisms of one
species that an environment can support
 Once the carrying capacity is reached, certain
factors work to keep population in check (including
lack of food, overcrowding, predations,
accumulation of waste)
Carrying Capacity

If a natural population overshoots
the carrying capacity, three things
can happen:

1. It will die back to the original
carrying capacity

2. It will die back but the damage
to the environment will lower
carrying capacity

3. It will become extinct
Limiting Factors
 Factors that limit the growth and size of a
population
 Two types:
 1. Density Dependent
 2. Density Independent
Bill Nye Video
 1:20-3:07, 12:05 – 13:05
 https://www.youtube.com/watch?v=XwBKV-qRIMQ
Density Dependent Factors

Depend on the size of a population

Increase in effect as the population increases

Factors such as disease, competition, predation, parasitism,
crowding stress

Usually not enough to eliminate a population
Density Independent Factors
 Affect populations regardless of density
 Usually abiotic factors
 Ex: pollution, habitat destruction, natural disasters,
weather, temperature, chemical pesticides
 Usually just brings populations below carrying
capacity but these factors have the potential to
cause a population to become extinct
Predator Prey Relationships
 Predator and prey populations follow similar
patterns
 If prey populations are high, the predator
population will also be high
 If prey populations are low, predators will have less
food and their population size will decrease
 f
Dynamics in a Population: Age Structure
 1. Very young and very old are more susceptive to
disease
 2. If there are huge numbers of young adults, the
population will grow; if there are mostly elderly, the
population will decline.
Age Structure
Sex Ratios
 In a monogamous species, the ratio of males to
females should be about equal.
 Ex: Humans
 In deer and lion groups, this is not as important,
because one male often fertilizes many females.
Behavior
1.
Territory- a defended area which insured the
occupants will have enough resources for
themselves and their offspring.
2.
Social hierarchy – wolves, chickens, social status
determines which individuals eat or breed.
The End!
 Any questions?