IB Biology Topic C4.1.6: Negative Feedback Control of Population Size by Density-dependent Factors Notes

Density-Dependent Factors Push a Population Back Toward Its Carrying Capacity

1. As a population grows denser, factors such as competition, predation, and disease act more strongly and raise the death rate or lower the birth rate.
2. These density-dependent factors form a negative feedback loop that holds the population near its carrying capacity.

Carrying capacity is the maximum population a habitat can sustain, set by its limiting resources (covered in C4.1.5).

Negative Feedback Counteracts Change to Restore Stability

1. Negative feedback is a response that opposes a change and returns a system toward a stable level.
2. When a population rises above its carrying capacity, density-dependent factors push it back down.
3. When it falls below, those pressures ease and the population recovers.

Three Density-Dependent Factors Strengthen as a Population Grows

1. Intraspecific competition:
   1. More individuals share the same food, water, and space, so each gets less.
   2. Weaker competitors die or fail to reproduce, which slows population growth.
2. Predation:
   1. A dense prey population is easier to find and feeds more predators.
   2. Rising predator numbers then cut the prey population back down.
3. Disease and parasites:
   1. Close contact in a dense population lets pathogens spread faster.
   2. More deaths lower the density, which then slows further spread.

The Feedback Loop in Four Steps

1. Population rises, so more individuals compete, attract predators, and spread disease.
2. Density-dependent pressures intensify: food grows scarce, predation increases, and disease spreads faster.
3. The death rate rises or the birth rate falls, so the population declines.
4. Lower density eases the pressures, and the population settles near its carrying capacity.

Density-Independent Factors Act Regardless of Population Size

1. Density-dependent factors have a stronger effect as density rises, so they can regulate a population through feedback.
   1. Examples: competition, predation, disease, and the build-up of waste.
2. Density-independent factors affect the same proportion of a population whatever its size, so they do not regulate it.
   1. Examples: weather, fire, floods, and other natural disasters.

Negative Feedback Keeps Ecosystems Stable

1. It prevents overpopulation and the resource depletion that would follow.
2. It stops any single species from dominating and squeezing out others.
3. It keeps populations resilient as conditions shift over time.