IB Environmental systems and societies Topic 2.1D… | RevisionDojo

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IB Environmental systems and societies Topic 2.1D… | RevisionDojo

Carrying Capacity: The Ceiling an Ecosystem Can Sustain

What Carrying Capacity Means

Carrying capacity (K) is the maximum number of individuals of a species that an ecosystem can sustainably support given the biotic and abiotic resources available, such as food, water, space, and light.
At K the population is in dynamic equilibrium: birth rate equals death rate, so population size stabilizes.
K is set by resource availability and the interactions between organisms and their environment.

Note

K is not a fixed number.
It shifts with environmental change, resource availability, and human activity.

Factors Affecting Carrying Capacity

Biotic factors

How Biotic Factors Set K

Biotic Factor	Effect on Carrying Capacity	Example
Availability of food	Determines how many individuals can be nourished.	Number of deer supported by forest vegetation.
Predation	Controls prey population size; keeps ecosystem in balance.	Wolves limiting deer population in Yellowstone National Park.
Competition	Reduces access to resources; both intra- and interspecific.	Red squirrels declining due to competition with grey squirrels in the UK.
Disease and parasitism	Increase mortality rates and lower population density.	Rinderpest virus historically reduced African wildebeest populations.
Reproductive success	Species with lower fertility or longer gestation recover more slowly from decline.	Elephants have small offspring numbers and long gestation periods.

Example

Plants in a dense forest compete for sunlight, which caps how many can grow.

Abiotic factors

How Abiotic Factors Set K

Abiotic Factor	Effect on Carrying Capacity	Example
Temperature	Determines metabolic rate and habitability for species.	Extreme cold limits reptile populations; high heat reduces amphibian survival.
Water availability	Essential for drinking, photosynthesis, and metabolism.	Desert ecosystems support fewer large herbivores due to scarce water.
Nutrient availability	Influences plant growth and, indirectly, herbivore populations.	Nitrogen-poor soils limit plant productivity in tundra regions.
Light intensity	Affects photosynthesis, which controls food energy supply.	Low light in dense rainforests limits understorey plant growth.
pH and salinity	Restrict species that cannot tolerate certain chemical conditions.	Freshwater fish cannot survive in high-salinity estuaries.
Space and shelter	Determines breeding sites, nesting areas, and territory.	Limited nesting sites restrict seabird populations on cliffs.

Example

In arid ecosystems, water scarcity is the main limit on populations such as gazelles and acacia trees.

Lake Fish: How K Shifts With Conditions

A lake's carrying capacity for fish depends on dissolved oxygen, nutrient levels, and temperature.
Heavy organic waste input drives eutrophication, which lowers dissolved oxygen and cuts the number of fish the lake can support.
Seasonal temperature shifts change oxygen solubility, so K rises and falls through the year.

Active recall

What is carrying capacity (K)?
Why does birth rate equal death rate at K?
Name one biotic and one abiotic factor that can lower K.
Why is K not a fixed number for a given ecosystem?
In the lake example, how does eutrophication reduce carrying capacity?