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biology 0610printed 273 · PDF 273
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printed 273 · PDF 273
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273
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An ecosystem is a unit containing the community of organisms and their
environment, interacting together.
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Most populations remain the same size over a period of time. They may
fluctuate but the average population is unlikely to change.
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The population growth rate depends on a number of factors including food
supply, competition, predation and disease. These in turn influence death rate and
birth rate, which will decide whether the population grows or decreases.
Sigmoid curve of population growth
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To understand our own population better we can put in organisms such as
bacteria and yeast with a container of nutrient broth and count the number every few
hours.
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The normal graph that forms when a small amount of organisms is provided
with a fixed amount of food, to show population growth, is called a sigmoid growth
curve.
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There are four phases of this graph.
1.
Lag Phase: In the first hour(or multiple hours depending on the population)
the population needs time to adjust to the new conditions and there are not many
cells to reproduce so the rate of population growth is very slow or lagged.
2.
Log(Exponential) Phase: In the next hour the cells reproduce rapidly and
make more of themselves because they have adapted to their surroundings and have
sufficient food supplies.