IB Environmental systems and societies 2.1E Population Size and Growth Notes

Population Growth: Feedback Keeps Numbers Near Carrying Capacity

1. Populations are regulated by density-dependent factors that get stronger as population density rises.
2. These factors act as negative feedback mechanisms that return a population to equilibrium around its carrying capacity (K).
3. By contrast, density-independent factors like droughts and volcanic eruptions hit a population whatever its size and do not regulate it.

Density-Dependent Factors: Regulation That Scales With Crowding

1. They create negative feedback loops that stabilize a population around its carrying capacity.
2. Competition for limited resources
   1. As density rises, individuals compete for scarce food, water, and space.
   2. This lowers growth, reproduction, and survival rates.
3. Predation
   1. Higher prey density attracts more predators and raises predator success.
   2. This self-regulating pressure keeps the prey population in check.
4. Disease and pathogen transmission
   1. Close contact in dense populations speeds the spread of infectious disease.
   2. This raises mortality and lowers reproductive success.

Negative Feedback: How Populations Self-Correct Toward K

1. Negative feedback occurs when a rise in population size triggers mechanisms that reduce growth rates and bring numbers back toward equilibrium.
2. These mechanisms stop a population exceeding the carrying capacity of its environment for long.
3. When a population overshoots carrying capacity, two things follow.
   1. Resource scarcity lowers reproduction rates.
   2. Predation and disease raise mortality.
4. These processes act as stabilizing forces that return the population toward equilibrium at K.

Coral Reef: A Predator-Prey Feedback Loop

Density-Independent Factors: Shocks That Ignore Population Size

1. These are mostly abiotic factors that affect a population regardless of its density.
   1. Natural disasters such as fires, floods, and droughts.
   2. Temperature extremes or long-term climate change.
   3. Volcanic eruptions, earthquakes, and hurricanes.
2. These can cause sudden crashes but do not regulate a population around K.