What controls the size of a population, and what is meant by carrying capacity?
Explain how populations grow and how limiting factors and carrying capacity control population size, interpreting population-growth graphs (NYSSLS LS2, stability and change; analyzing data).
A NYSSLS-level answer on population dynamics for the New York Life Science: Biology Regents: how populations grow, the limiting factors that control them, carrying capacity, and how to interpret population-growth graphs.
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What this topic is asking
NYSSLS LS2 wants you to explain what controls population size and the idea of carrying capacity. On the Life Science: Biology Regents this is tested almost entirely through population-growth graphs: you identify the carrying capacity, name limiting factors, and explain why a population levels off. The crosscutting concepts are stability and change and the practice of analyzing data.
How populations grow
A population changes size through births, deaths, and movement in and out. When resources are abundant and limiting factors are weak, a population can grow rapidly. On a graph, this shows as a steeply rising curve. But this rapid growth does not continue forever, because the environment cannot supply unlimited resources.
Limiting factors
As a population grows, these factors bite harder: food and space per individual fall, competition rises, and predators and disease spread more easily. This increases the death rate and can lower the birth rate, slowing growth. Limiting factors are the brake on population growth.
Carrying capacity
On a typical population-growth graph, the curve rises steeply, then levels off and wobbles around a steady value: that value is the carrying capacity. If a population temporarily overshoots the carrying capacity, resources become too scarce, deaths exceed births, and the population falls back toward what the environment can support. This is a clear example of stability and change: the population is held near a sustainable level by feedback between its size and its resources.
Reading population graphs
The exam expects you to read these graphs confidently:
- A steep rise means rapid growth while resources are plentiful.
- A levelling-off means the population has reached the carrying capacity, as limiting factors balance growth.
- Fluctuations around the steady value are normal, as the population adjusts to changing conditions.
- A sharp fall after a peak means the population overshot the carrying capacity, or a limiting factor (such as disease or loss of food) became severe.
Try this
Q1. Define carrying capacity. [2]
- Cue. The maximum population size that an environment can support over the long term, given its resources.
Q2. State three limiting factors that could control the size of an animal population. [2]
- Cue. Any three of: food supply, water, space, predators, disease, competition.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Life Science sample, 2024)3 marksA graph shows a population of rabbits growing rapidly at first, then levelling off and fluctuating around a steady value. (a) State the term for the steady value the population levels off at. (b) State two factors that could limit the rabbit population. (c) Explain why the population stops growing and levels off.Show worked answer →
A 3-point constructed-response item assessing analyzing data and stability and change.
(a) 1 point: the carrying capacity.
(b) 1 point: any two limiting factors, for example food supply, space, water, predators, or disease.
(c) 1 point: as the population grows, resources such as food and space become limited and predators and disease increase, so the death rate rises (and birth rate falls) until births balance deaths and the population stops growing, levelling off at the carrying capacity.
Markers reward "carrying capacity", two valid limiting factors, and resources running short so the population stabilizes.
Regents (Life Science CR, 2025)2 marksA population of deer grows beyond the carrying capacity of its habitat one year. (a) Predict what is likely to happen to the population. (b) Explain why.Show worked answer →
A 2-point item on carrying capacity.
(a) 1 point: the population is likely to fall (decline) back toward the carrying capacity.
(b) 1 point: above the carrying capacity there are not enough resources (food, space) for all, so competition increases and more deer die (or fewer reproduce), reducing the population until it matches what the habitat can support.
Markers reward the predicted decline and the shortage of resources above the carrying capacity.
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Sources & how we know this
- New York State P-12 Science Learning Standards (Life Science) — New York State Education Department (2016)
- Educator Guide to the Regents Examination in Life Science: Biology — New York State Education Department (2025)