Population Growth (DP IB Environmental Systems & Societies (ESS))

Carrying Capacity

• The maximum stable population size of a species that an ecosystem can support (determined by competition for limited resources) 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 individuals from surviving and reproducing result in the population no longer being able to grow in size

Density-dependent Factors & Negative Feedback Mechanisms

• Population size is regulated by density-dependent factors and negative feedback mechanisms

• Density-independent factors may influence population size

  • For example, environmental conditions like climate, temperature, rainfall patterns and soil fertility can limit the size of a population

• However, it is mainly density-dependent factors that regulate populations around the carrying capacity

  • Density-dependent factors are factors whose impact on population size varies with the population's density

Density-dependent factors

• Competition for resources:

  • As population density increases, individuals compete more intensely for limited resources like food, water and shelter

    • For example, in a forest ecosystem, as deer population density rises, competition for available food (grass, leaves, etc.) increases, placing limits on individual growth rates and overall population size

• Increased risk of predation:

  • Higher population density increases the likelihood of predators encountering prey, leading to more predation events

    • For example, in a coral reef ecosystem, as fish populations grow denser, predation by larger fish species also increases, regulating the population size of smaller fish species

• Pathogen transmission:

  • Dense populations facilitate the spread of pathogens, such as diseases and parasites, leading to increased mortality rates

    • For example, in a population of bats living in a cave, as population density increases, close contact between individuals facilitates the transmission of pathogens—this increased pathogen transmission can lead to higher mortality rates among bats, regulating the population size

Negative feedback mechanisms

• Density-dependent factors drive negative feedback mechanisms, which act to return a population to its equilibrium state, maintaining stability

  • As population density rises, factors like resource scarcity, increased predation and disease outbreaks trigger mechanisms that reduce population growth rates

Population Growth Curves

• Population growth can either be exponential or limited by carrying capacity

  • If there are no limiting factors, population growth follows a J-curve (exponential growth)

  • When density-dependent limiting factors start to operate, the curve becomes S-shaped

J-curves

• For some populations, when population growth is plotted against time, a J-curve is produced

  • A J-curve describes the growth pattern of a population in an environment with unlimited resources

• The J-curve has three distinct phases:

1. Lag phase:

   • The initial growth is slow when the population is small

2. Exponential growth phase:

   • Population growth accelerates exponentially as the number of individuals increases

   • The curve takes a J-shape due to exponential growth, as resources are not limiting the growth of the population

   • The population will continue to grow until a limiting factor such as disease or predation occurs

3. Crash phase:

   • At this point, if there has been a significant population overshoot (if the population has increased far beyond the natural carrying capacity), there may be a sudden decrease in the population, known as a population crash 

S-curves

• For most populations, when population growth is plotted against time, an S-population curve is produced

  • An S-population curve describes the growth pattern of a population in a resource-limited environment

• The S-population curve has four distinct phases:

1. Lag phase:

   • The initial growth is slow when the population is small

2. Exponential growth phase:

   • With low or reduced limiting factors, the population expands exponentially into the habitat

3. Transitional phase:

   • As the population grows, there is increased competition between individuals for the same limiting factors or resources

   • This competition results in a lower rate of population increase

4. Plateau phase:

   • The population reaches its carrying capacity and fluctuates around a set point determined by the limiting factors

   • Changes in limiting factors cause the population size to increase and decrease (these increases and decreases around the carrying capacity are controlled by negative feedback mechanisms)