Explain how limiting factors and carrying capacity regulate population size in an ecosystem (North Carolina Standard Course of Study, Biology, LS.Bio.5).

What this topic is asking

North Carolina LS.Bio.5 asks how limiting factors and carrying capacity regulate population size. For the Biology EOC you need to know what carrying capacity is, the difference between density-dependent and density-independent limiting factors, the shapes of exponential and logistic growth, and how to read a population-growth graph. Items often show a growth curve and ask you to interpret it.

Carrying capacity

No population can grow forever, because resources are limited. As a population approaches the carrying capacity, growth slows; if it overshoots, deaths exceed births and the population falls back toward the carrying capacity. So carrying capacity acts as a ceiling that the population fluctuates around. The EOC often shows a growth curve leveling off at a value and asks you to name that value (the carrying capacity).

Limiting factors: density-dependent and density-independent

The distinction is a common EOC item. A quick test: does the factor get worse when individuals are crowded (density-dependent, like disease) or does it strike no matter how many there are (density-independent, like a flood)?

Exponential and logistic growth

Population growth typically follows one of two patterns, both common as graphs.

• Exponential growth produces a J-shaped curve: when resources are unlimited, the population grows faster and faster. This happens early, or when a species enters a new area with abundant resources, but it cannot continue indefinitely.
• Logistic growth produces an S-shaped curve: rapid growth at first, then slowing as limiting factors take effect, until the population levels off at the carrying capacity. The leveling-off is where births and deaths roughly balance.

Real populations usually follow logistic growth and then fluctuate around the carrying capacity as conditions change. The shift from a J-shape to an S-shape happens because limiting factors (food running short, predators increasing, disease spreading) kick in as the population grows.

Try this

Q1. Define carrying capacity. [1]

• Cue. The maximum population size an environment can support over the long term, given its resources.

Q2. State one density-dependent and one density-independent limiting factor. [2]

• Cue. Density-dependent, for example food, predators, or disease; density-independent, for example a natural disaster, weather, or human activity.