Carrying Capacity (SL IB Environmental Systems & Societies (ESS))

• The maximum stable population size of a species that an ecosystem can support is known as the carrying capacity
• Every individual within a species population has the potential to reproduce and have offspring that will contribute to population growth
  • In reality, however, there are many abiotic and biotic factors that prevent every individual in a population from making it to adulthood and reproducing

• This ensures the population size of each species is limited at some point (i.e. the carrying capacity of that species is reached)
  • This is why no single species has a population size that dominates all other species populations on Earth, with the exception of humans (as we have managed to overcome many of the abiotic and biotic factors that could potentially limit the population growth of our species)

• The graph below shows the population growth of a population of lions
  • The point at which the graph starts to flatten out (plateau) is the carrying capacity of this population
  • At this point, the environmental (abiotic and biotic) factors that stop all individuals from surviving and reproducing result in the population no longer being able to grow in size

An example graph showing the population growth of a population of lions and the point at which the carrying capacity of this population has almost been reached

• Carrying capacity is determined by factors such as:
  • Resource availability
  • Interactions between species
  • Environmental conditions

Resource Availability

• The carrying capacity of an environment for a species is influenced by the availability of essential resources such as food, water, shelter, and space
  • For example, a grassland ecosystem may have a carrying capacity for a specific number of herbivores based on the amount of grasses available for grazing

Interactions Between Species

• The presence of predator-prey relationships, competition for resources, and symbiotic interactions among species can influence the carrying capacity of an environment
  • For example, the carrying capacity of a freshwater ecosystem for fish species may be influenced by the presence of predators, availability of prey, and competition for food resources
• In a stable community, the numbers of predators and prey rise and fall in cycles, limiting the carrying capacity of both predator and prey populations
• The graph below demonstrates some of the key patterns of predator-prey cycles:
  • The number of predators increases as there is more prey available
  • The number of prey then decreases as there are now more predators
  • The number of predators decreases as there is now less prey available
  • The number of prey increases as there are now fewer predators
  • The cycle now repeats

An example of a graph used to model a predator-prey cycle between the Canadian lynx and the snowshoe hare

Environmental Conditions

• Factors like climate, temperature, rainfall patterns, and soil fertility can affect the carrying capacity of species within an environment
  • For example, a forest ecosystem with favourable environmental conditions may have a higher carrying capacity for a certain bird species due to the availability of suitable nesting sites (e.g. the temperature, humidity and rainfall is suitable for incubating eggs and raising chicks)

Estimating Carrying Capacity

• Scientists use various methods to estimate the carrying capacity of an environment for a given species
  • These methods include field observations, population surveys, mathematical modelling, and data analysis
  • By studying population trends, resource availability, and species interactions, researchers can make informed estimates of carrying capacity

• However, estimating carrying capacity becomes challenging when it comes to human populations due to several reasons:

• Technological advancements:
  • Human societies have the ability to modify their environment and overcome traditional carrying capacity limitations through technology
  • For example, the development of agriculture and irrigation techniques has allowed humans to increase food production and support larger populations beyond what the natural environment could sustain

• Cultural and social factors:
  • Human population dynamics are influenced by cultural norms, social behaviours, and economic factors
  • These can affect fertility rates and migration patterns, for example, making it difficult to accurately predict or estimate carrying capacity for human populations

• Changing lifestyles and consumption patterns:
  • Human populations are characterised by varying lifestyles and consumption rates, which can significantly impact resource demands and environmental impacts
  • For example, urbanised societies with high levels of consumption may strain the carrying capacity of their surrounding areas due to increased resource demands and waste generation

• Adaptive capacity:
  • More so than any other species, humans have the ability to adapt and innovate in response to changing environmental conditions
  • This adaptability can affect carrying capacity by influencing resource use efficiency and the development of technological solutions

Balancing Population and Resources

• The concept of population growth dynamics refers to how humans interact with their environment to change in number over space and time
• The global population has grown exponentially over the past 200 years
  • In 1800 it was 1 billion
  • In November 2022 it reached 8 billion
• Due to humans’ ability to resist the limiting environmental factors (such as disease and food supply)
  • Humans have overcome these by finding medicines and vaccines to reduce or control rates of disease, and developing technologies to increase food supply to allow for population growth
• Continued population growth puts pressure on scarce resources
  • The balance between population and resource use determines a place’s standard of living
  • Careful management of population and resources is needed to maximise income per capita
  • Countries aim to achieve a perfect balance between population and resources, known as optimum population 
  • An imbalance between population and resources leads to overpopulation or underpopulation

The relationship between population, resources and standard of living

• If resources are consumed at sustainable rates, a larger population may be supported
• Countries going through industrialisation tend to consume and waste resources at unsustainable levels which leads to a lower carrying capacity
• Technological innovation can either lead to:
  • Increases in supply of resources such as energy and minerals, increasing carrying capacity
• Or:
  • Improved resource use efficiency, increasing carrying capacity
• Wealthier countries usually have a larger carrying capacity than poorer countries because:
  • They export waste to poorer countries
  • They import products from poorer countries
  • This means that although poorer countries use fewer resources, they are supporting the resource use of the richer countries

Two different scenarios showing population response to carrying capacity