Carrying capacity & population growth
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Question
Define carrying capacity.
Answer
The **maximum population size** of a species that a habitat can support over a long period, given its resources.
Question
What shape is a population growth curve?
Answer
A **sigmoid (S-shaped) curve**: a slow lag start, a rapid exponential rise, then a plateau at the carrying capacity.
Question
Name the phases of the sigmoid growth curve in order.
Answer
**Lag → exponential → transitional → plateau.**
Question
Why is growth so fast in the exponential phase?
Answer
There are **plenty of resources and few limiting factors**, so nearly all individuals survive and reproduce — the population grows by ever-larger amounts.
Question
Why does a population level off at the plateau?
Answer
**Limiting factors** (shortage of food, water, space; disease; predation) raise deaths until **births ≈ deaths**, so growth stops at the carrying capacity.
Question
What is happening to births and deaths at the carrying capacity?
Answer
**Births ≈ deaths** — they are roughly equal, so the population stays about the same size.
Question
Define a limiting factor.
Answer
Any factor that **slows or stops** a population growing — e.g. shortage of food, water or space, disease, or predation.
Question
What is a density-DEPENDENT limiting factor? Give an example.
Answer
One whose effect gets **stronger as the population becomes more crowded** — e.g. competition, disease or predation.
Question
What is a density-INDEPENDENT limiting factor? Give an example.
Answer
One that acts the **same regardless of population density** — e.g. drought, fire, flood or extreme cold.
Question
If the flat top of a growth curve (region X) is labelled, what factor causes it?
Answer
A **limiting factor** such as competition for food / limited space, as the population reaches its carrying capacity.
Question
How does temperature influence the population growth of a plant like duckweed?
Answer
There is an **optimum temperature**; temperature sets the rate of enzyme reactions (e.g. photosynthesis), so growth is fastest at the optimum, slow when too cold, and reduced when too hot (enzymes denature).
Question
Why can't a population grow exponentially forever?
Answer
Resources (food, water, space) are **limited**, so as numbers rise, limiting factors take effect and growth slows to the carrying capacity.
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Topic 3.8 hub
Populations and communities
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