Definition
Carrying capacity
Carrying capacity is the maximum number of individuals of a species that an environment can sustainably support over time.
- Carrying capacity (K) is the maximum number of individuals of a species that an ecosystem can sustainably support over time, given the availability of biotic and abiotic resources such as food, water, habitat space, and light.
- It represents a dynamic equilibrium, when birth rates equal death rates and population size stabilizes.
- It is determined by the availability of resources and the interactions between organisms and their environment.
Note
- Carrying capacity is not a fixed number.
- It can change over time due to environmental shifts, resource availability, and human activities.
Factors Affecting Carrying Capacity
Biotic factors
Definition
Biotic factors
Biotic factors are the living components of an ecosystem, such as plants, animals, and microorganisms, as well as their interactions.
| 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 have slower recovery from population decline. | Elephants have small offspring numbers and long gestation periods. |
Example
Plants in a dense forest compete for sunlight.
Abiotic factors
Definition
Abiotic factors
Abiotic factors are the non-living physical and chemical elements, such as temperature, sunlight, and soil composition.
| Abiotic Factor | Effect on Carrying Capacity | Example |
|---|---|---|
| Temperature | Determines the metabolic rate and habitability for species. | Extreme cold limits reptile populations; high heat reduces amphibian survival. |
| Water availability | Essential for drinking, photosynthesis, and metabolic processes. | 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 abiotic factor limiting population sizes of plants and animals such as gazelles and acacia trees.
Carrying Capacity in Natural Ecosystems
1. Rabbit Population in Australia
- Rabbits introduced in the 19th century reproduced rapidly due to abundant vegetation and lack of predators.
- Initially experienced exponential growth.
- Over time, food shortages and the introduction of the myxoma virus reduced population size.
- The population now fluctuates around a lower carrying capacity determined by available vegetation and disease prevalence.
2. Fish Population in a Lake
- Carrying capacity depends on dissolved oxygen, nutrient levels, and temperature.
- High organic waste input decreases oxygen through eutrophication, reducing the lake’s ability to support fish populations.
- Seasonal temperature shifts also affect oxygen solubility, altering K.
3. Human Population Growth
- Human innovation (e.g., agriculture, medicine, sanitation) has artificially raised the global carrying capacity.
- However, overexploitation of natural resources and pollution may cause long-term declines in ecosystem carrying capacities worldwide.
Self review
- Define carrying capacity and explain how it differs from population density.
- Describe how both biotic and abiotic factors influence carrying capacity.
- Using an example, explain how a population may exceed its carrying capacity and the consequences that follow.
