Population Size and Carrying Capacity
- Junessa Masaya
- Apr 15
- 4 min read
Updated: May 20
HSC Biology | Free Study Notes
In this lesson
what limiting factors are
what carrying capacity means
how populations change over time
how examples can be explained using ecosystem factors
What is population size?
Population size is the number of individuals of one species in a particular area at a particular time.
Population size can change because of:
births
deaths
immigration
emigration
In ecosystems, population size is not fixed. It changes in response to environmental conditions and interactions with other organisms. Module 4 centres on these changing relationships within ecosystems.
Limiting factors
Limiting factors are factors that restrict population growth.
A population cannot increase forever because resources and conditions are limited.
Abiotic limiting factors
Abiotic limiting factors are non-living factors such as:
temperature
water availability
light
rainfall
space
The syllabus specifically includes the impact of abiotic factors on ecosystems.
Biotic limiting factors
Biotic limiting factors are living influences such as:
competition
predation
disease
food availability
symbiotic relationships
The syllabus specifically includes predation, competition, symbiosis and disease as factors affecting populations.
Why limiting factors matter
If limiting factors become stronger:
survival may decrease
reproduction may decrease
population size may fall or stop increasing
Carrying capacity
Carrying capacity is the largest population size of a species that an environment can support over time.
What this means
Carrying capacity depends on:
available food
water
shelter
breeding space
predation pressure
disease levels
abiotic conditions
Important idea
Carrying capacity is not a fixed number forever. It can change if environmental conditions change.
For example:
drought may reduce carrying capacity
more food availability may increase carrying capacity
disease outbreaks may reduce carrying capacity
Population change
Populations can change in different ways over time.
Population increase
A population may increase when:
resources are plentiful
predation is low
disease is low
conditions favour survival and reproduction
Population decrease
A population may decrease when:
resources become limited
predation increases
competition increases
disease spreads
abiotic conditions become unfavourable
Population stability
A population may become relatively stable when it is close to carrying capacity.
At this point:
births and deaths may be similar overall
resources are limiting further growth
population size may fluctuate around a similar level
How limiting factors affect carrying capacity
Example idea
If food is limited, then not all individuals can survive and reproduce successfully. This prevents the population from rising beyond what the environment can support.
Another example
If rainfall drops for a long period:
plant growth may decrease
herbivore food supply may fall
herbivore populations may drop
predator populations may later decline too
This shows that carrying capacity is closely linked to ecosystem conditions and food web relationships.
Population change in ecosystems
Module 4 is about ecosystem dynamics, so population change should always be linked to interactions in the ecosystem.
Key idea
For example:
fewer producers may reduce herbivore numbers
fewer herbivores may reduce predator numbers
disease in one population may affect other species connected through food webs
The syllabus specifically asks students to predict consequences for populations due to predation, competition, symbiosis and disease.
Examples
Example 1: Rabbit population after rain
After a period of high rainfall:
plant growth increases
food becomes more available
rabbit survival and reproduction may increase
rabbit population size rises
If rabbit numbers become very high:
food may become limited
competition increases
the population may level off or decline
Example 2: Predator and prey
If prey numbers rise:
predators may have more food
predator numbers may later increase
If predator numbers then increase too much:
prey numbers may fall
predator numbers may later fall as food becomes limited
Example 3: Disease outbreak
If disease spreads through a population:
survival decreases
reproduction may fall
population size may decline
This is directly linked to the Module 4 syllabus point about predicting consequences for
populations due to disease.
Population size and carrying capacity compared
Term | Meaning |
Population size | Number of individuals of a species in an area |
Limiting factor | Factor that restricts population growth |
Carrying capacity | Maximum population size the environment can support over time |
Worked example
Exam-style question
Explain why a population may stop increasing even when individuals continue to reproduce.
Worked answer
A population may stop increasing when it reaches carrying capacity. At this point, limiting factors such as food availability, competition or disease reduce survival or reproduction enough that the population can no longer grow overall.
Why this works
This answer:
uses the term carrying capacity
includes limiting factors
explains why growth stops
Common mistakes
Saying carrying capacity is the biggest population ever recorded, rather than the size the environment can support.
Forgetting that carrying capacity can change over time.
Treating limiting factors as only abiotic.
Describing population change without linking it to resources or species interactions.
Assuming populations always grow until they collapse, rather than often fluctuating around carrying capacity.
Quick quiz
What is population size?
What is a limiting factor?
What is carrying capacity?
Give one abiotic limiting factor and one biotic limiting factor.
Why can carrying capacity change over time?
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