BIO 182 MODULE 4, HOW POPULATIONS
CHANGE OVER TIME – POPULATION ECOLOGY I
COMPREHENSIVE STUDY GUIDE & REVISION
NOTES 2026
⭐ Big Picture
Population ecology is the study of how and why populations change over time
and space.
Population ecologists ask questions like:
Why do some populations grow rapidly?
Why do others decline?
What factors control population size?
Can we predict future population growth?
This lecture introduces the basic tools biologists use to answer these questions.
Population Ecology
Definition
Population ecology is the study of how populations:
grow
shrink
move
interact with their environment
A population consists of individuals of the same species living in the same area.
Module 4: How Populations Change Over Time 1
, The Cane Toad Example
The lecture uses the Cane Toad (Rhinella marina) as its primary example.
Originally introduced to Australia to control agricultural pests, cane toads became
an invasive species because their population grew rapidly and spread throughout
the country.
Scientists studying this population would ask:
Are predators present?
Is food available?
Is disease affecting the population?
Are droughts or other climate events occurring?
These all influence future population size.
⭐ Exam Thinking
If your professor gives you an animal and asks:
"What factors could influence its future population?"
Think:
✔ Predators
✔ Disease
✔ Food availability
✔ Climate/weather
Those are the major ecological factors introduced in this lecture.
Four Ways Population Size Changes
A population changes because individuals are:
Added
Module 4: How Populations Change Over Time 2
, Births (B)
Immigration (I)
Removed
Deaths (D)
Emigration (E)
A simple way to remember it:
Population Change = (Births + Immigration) − (Deaths + Emigration)
Example
Population starts with 100 toads.
During one year:
Births = 20
Deaths = 10
Immigration = 15
Emigration = 8
Population change:
(20 + 15) − (10 + 8)
= +17
New population:
117 toads.
⭐ Exam Thinking
Whenever you see a question listing:
births
deaths
Module 4: How Populations Change Over Time 3
, immigration
emigration
Don' t panic.
Always organize them like this:
Increase
✔ Births
✔ Immigration
↓
Decrease
✔ Deaths
✔ Emigration
Then do the math.
Population Growth Model
To predict future population growth, scientists first decide on an appropriate time
interval.
Examples:
Bacteria → hours
Humans → years
The time interval depends on the organism being studied.
Intrinsic Rate of Increase (r)
One of the most important ideas in ecology is r.
Definition
r = intrinsic rate of increase
It describes how quickly a population grows or declines.
Module 4: How Populations Change Over Time 4
CHANGE OVER TIME – POPULATION ECOLOGY I
COMPREHENSIVE STUDY GUIDE & REVISION
NOTES 2026
⭐ Big Picture
Population ecology is the study of how and why populations change over time
and space.
Population ecologists ask questions like:
Why do some populations grow rapidly?
Why do others decline?
What factors control population size?
Can we predict future population growth?
This lecture introduces the basic tools biologists use to answer these questions.
Population Ecology
Definition
Population ecology is the study of how populations:
grow
shrink
move
interact with their environment
A population consists of individuals of the same species living in the same area.
Module 4: How Populations Change Over Time 1
, The Cane Toad Example
The lecture uses the Cane Toad (Rhinella marina) as its primary example.
Originally introduced to Australia to control agricultural pests, cane toads became
an invasive species because their population grew rapidly and spread throughout
the country.
Scientists studying this population would ask:
Are predators present?
Is food available?
Is disease affecting the population?
Are droughts or other climate events occurring?
These all influence future population size.
⭐ Exam Thinking
If your professor gives you an animal and asks:
"What factors could influence its future population?"
Think:
✔ Predators
✔ Disease
✔ Food availability
✔ Climate/weather
Those are the major ecological factors introduced in this lecture.
Four Ways Population Size Changes
A population changes because individuals are:
Added
Module 4: How Populations Change Over Time 2
, Births (B)
Immigration (I)
Removed
Deaths (D)
Emigration (E)
A simple way to remember it:
Population Change = (Births + Immigration) − (Deaths + Emigration)
Example
Population starts with 100 toads.
During one year:
Births = 20
Deaths = 10
Immigration = 15
Emigration = 8
Population change:
(20 + 15) − (10 + 8)
= +17
New population:
117 toads.
⭐ Exam Thinking
Whenever you see a question listing:
births
deaths
Module 4: How Populations Change Over Time 3
, immigration
emigration
Don' t panic.
Always organize them like this:
Increase
✔ Births
✔ Immigration
↓
Decrease
✔ Deaths
✔ Emigration
Then do the math.
Population Growth Model
To predict future population growth, scientists first decide on an appropriate time
interval.
Examples:
Bacteria → hours
Humans → years
The time interval depends on the organism being studied.
Intrinsic Rate of Increase (r)
One of the most important ideas in ecology is r.
Definition
r = intrinsic rate of increase
It describes how quickly a population grows or declines.
Module 4: How Populations Change Over Time 4
