AP Syllabus focus:
‘Carrying capacity is the sustainable abundance of a species that an ecosystem’s total resources can support.’
Carrying capacity links population size to finite environmental resources.
This figure contrasts exponential (J-shaped) growth under unlimited resources with logistic (S-shaped) growth when resources are finite. The logistic curve levels off at the carrying capacity, illustrating how resource limitation reduces population growth rate as density increases. Source
Understanding how food, water, space, and nutrients constrain survival and reproduction helps explain why populations cannot increase indefinitely in real ecosystems.
Core idea: resources set an upper limit
This diagram shows logistic (S-shaped) population growth over time as a population approaches a carrying capacity (K). It visualizes how growth slows as resources become limiting and the population stabilizes near K rather than increasing indefinitely. Source
Populations require energy and matter to maintain metabolism, grow, and reproduce. When essential resources become scarce, individuals experience reduced survival, reduced fertility, delayed development, or increased emigration, which collectively slows population increase.
Carrying capacity (K): The sustainable abundance of a species that an ecosystem’s total resources can support over time.
Carrying capacity is not a “goal” a population always reaches; it is an outcome of environmental constraints and population resource demand.
What “sustainable abundance” means
A population near K can persist because, on average, births plus immigration are balanced by deaths plus emigration. Sustainability depends on:
Renewal rates of resources (e.g., plant regrowth, prey reproduction, water recharge)
Waste processing and detoxification capacity (e.g., oxygen availability, ammonia breakdown, salinity balance)
Habitat structure that supports foraging, nesting, and shelter
Resource limitation
Resource limitation occurs when one or more required inputs are insufficient to meet the needs of all individuals at a given population size, reducing per capita access to those inputs.
Limiting resource: A resource that is in shortest supply relative to demand and therefore restricts population size, survival, or reproduction.
A limiting resource can shift with season, life stage, or environmental conditions.
Common limiting resources for populations
These conceptual diagrams show how nitrogen and phosphorus enter aquatic ecosystems and how increased nutrient availability can change primary production and downstream biological responses. They reinforce the idea of a “limiting resource” by tracing how nutrient supply can constrain (or amplify) producer growth and ripple through the food web. Source
Food/energy supply: prey availability, plant biomass, nectar, seeds
Water: especially for terrestrial organisms and during drought
Space and shelter: territories, nesting cavities, refuges from predators or heat
Mineral nutrients: nitrogen, phosphorus, iron (often limiting for producers, indirectly limiting consumers)
Light (for producers): canopy cover, turbidity in aquatic systems
Suitable mates/pollinators: when reproductive encounters depend on density and movement
How carrying capacity emerges from resource budgets
Carrying capacity reflects the match (or mismatch) between:
Resource supply in the ecosystem (how much is available per unit time)
Resource demand by the population (how much each individual requires per unit time)
As population size increases, total demand rises. If supply cannot increase proportionally, per capita resource availability falls, causing:
Lower body condition and energy reserves
Higher susceptibility to disease and environmental stress
Reduced reproductive output (fewer offspring, less frequent breeding)
Higher mortality (starvation, dehydration, exposure)
Why K varies rather than staying fixed
Carrying capacity is best viewed as a range that can change because ecosystems change. Factors that can raise or lower K include:
Climate variation: temperature and rainfall patterns affecting productivity
Disturbance: fire, storms, flooding changing habitat and food webs
Seasonality: winter food scarcity or dry-season water limitation
Resource pulses: mast years, algal blooms, episodic nutrient inputs
Long-term habitat change: succession, soil degradation, salinisation
Population size relative to carrying capacity
When a population is below K, resources are relatively abundant per individual, supporting higher survival and reproduction. When a population is near or above K, resource competition intensifies and individuals experience stronger constraints.
Overshoot and dieback (resource-driven)
Populations can temporarily exceed sustainable levels if:
resources were initially abundant,
stored resources are used (fat reserves, seed banks),
or there is a time lag before reduced reproduction/mortality responds.
If consumption reduces future resource availability (e.g., overgrazing), the environment may support fewer individuals afterward, effectively lowering K until recovery occurs.
FAQ
They combine long-term population counts with habitat measurements.
Common approaches include:
measuring resource availability (e.g., biomass) and typical per-capita consumption
tracking survival and reproduction as density changes
using mark–recapture to estimate abundance and demographic rates over time
Estimates are usually expressed as a plausible range rather than a single value.
Yes, because local resource supply and habitat structure differ.
Examples of drivers:
soil fertility and plant productivity
water availability and shelter sites
human land use (fragmentation, grazing pressure)
Even small differences can change sustainable abundance.
A limiting resource is a required material or energy input (food, water, nutrients).
A limiting factor is any condition that restricts performance, which can include:
resources
abiotic conditions (temperature extremes)
biological conditions (pathogen pressure)
Only limiting resources directly reflect supply versus demand.
Seasonality changes both supply and demand.
Supply can drop (winter plant dormancy; dry-season water loss). Demand can rise (breeding increases energetic needs). The result is a seasonal shift in sustainable abundance, even if the habitat is unchanged.
If they use different resources or microhabitats, their effective resource supply differs.
For example:
individuals specialising on different foods
territorial individuals securing better shelter
age classes relying on distinct resources (juveniles vs adults)
This creates unequal access, so “K” experienced by subgroups can differ.
Practice Questions
Define carrying capacity and state one way resource limitation can reduce population growth. (2 marks)
Correct definition of carrying capacity as the sustainable abundance supported by total ecosystem resources (1)
One valid mechanism: reduced reproduction, increased mortality, reduced growth, or increased emigration due to scarce resources (1)
A rabbit population in a grassland increases rapidly over several months. Later, average rabbit mass decreases and fewer juveniles survive to adulthood. Explain how resource limitation relates to carrying capacity in this scenario. (6 marks)
Identifies grass/food as a limiting resource as rabbit numbers rise (1)
Links increasing population size to reduced per capita resource availability (1)
Explains decreased average mass as evidence of reduced energy intake/condition (1)
Explains reduced juvenile survival via starvation/poor condition/exposure due to limited resources (1)
Connects these effects to reduced population growth rate stabilising numbers near the carrying capacity, (1)
Notes that depends on total resource supply and can change if grass productivity changes (season/climate) (1)
