Explain how limiting factors and carrying capacity control population size, and interpret population growth curves, distinguishing exponential from logistic growth (MA STE HS-LS2-1, HS-LS2-2, stability and change).

What this topic is asking

The Massachusetts STE framework (HS-LS2-1 and HS-LS2-2) asks you to use evidence and mathematics to explain what controls the size of a population within the limits of an ecosystem. On the High School Biology MCAS, this is tested with population growth graphs: you identify the carrying capacity, name the limiting factors, and explain why a population stops growing. The crosscutting concept is stability and change: populations grow, then settle around a stable level set by their environment.

What controls population size

A population grows when the birth rate is higher than the death rate. But growth cannot continue indefinitely, because resources are finite. As a population gets larger, limiting factors kick in: food and space run short, waste builds up, and predators and disease spread more easily. These factors raise the death rate (and may lower the birth rate) until growth slows and stops. The population then settles around the carrying capacity, the level the environment can sustain.

Limiting factors come in two broad kinds: density-dependent factors that get stronger as the population grows (food shortage, disease, predation) and density-independent factors that act regardless of size (a drought, a fire, a cold winter). The MCAS mostly asks you to name limiting factors and link them to the leveling off.

Reading population growth curves

Two curve shapes appear again and again, and you should be able to tell them apart:

• Exponential growth (J-shaped). When resources are plentiful and limiting factors are weak (for example, a few organisms newly arriving in a rich, empty habitat), the population grows faster and faster, producing a J-shaped curve. This cannot last.
• Logistic growth (S-shaped). More realistically, rapid early growth slows as limiting factors increase, and the population levels off at the carrying capacity, producing an S-shaped curve. The flat top of the S marks the carrying capacity.

On a graph, the carrying capacity is the level at which the curve flattens out, and the population fluctuates slightly around it afterward. Identifying the carrying capacity on a graph is a very common MCAS task.

Why populations level off

The key explanation the MCAS wants is the balance of births and deaths. As a population approaches carrying capacity:

• resources such as food, water, and space become limited,
• competition increases, and predators and disease take a larger toll,
• so the death rate rises (and the birth rate may fall) until it matches the birth rate.

When births and deaths balance, the population stops growing and stabilizes at the carrying capacity. If the population overshoots, deaths exceed births and it falls back. This self-limiting behavior keeps populations within the limits of their ecosystem, an example of stability arising from interacting factors.

Try this

Q1. Define carrying capacity. [1]

• Cue. The maximum population size an environment can support over the long term.

Q2. Name two limiting factors that could cause a population to stop growing. [2]

• Cue. Any two of: food supply, water, space, predators, disease.