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Difficulty: MediumPopulation Dynamics and Ecological Sampling

An ecology student investigated the population density of water hyacinth (*Eichhornia crassipes*) in a section of a freshwater creek in Bayelsa State. A quadrat frame measuring 0.5 m×0.5 m0.5\text{ m} \times 0.5\text{ m} was randomly thrown 2020 times across the sampling site. The cumulative count of water hyacinth plants recorded across all 2020 quadrat throws was 150150. What is the estimated population density of water hyacinth in plants/m2\text{plants/m}^2?

  1. A
    7.5 plants/m27.5\text{ plants/m}^2
  2. B
    15 plants/m215\text{ plants/m}^2
  3. 30 plants/m230\text{ plants/m}^2Answer
  4. D
    600 plants/m2600\text{ plants/m}^2

Answer

The population density of water hyacinth is 30 plants/m230\text{ plants/m}^2.
To find population density, the total number of organisms observed (150150) must be divided by the total area sampled. Since one 0.5 m×0.5 m0.5\text{ m} \times 0.5\text{ m} quadrat has an area of 0.25 m20.25\text{ m}^2, twenty throws cover a total area of 20×0.25 m2=5.0 m220 \times 0.25\text{ m}^2 = 5.0\text{ m}^2. Dividing 150150 plants by 5.0 m25.0\text{ m}^2 yields 30 plants/m230\text{ plants/m}^2.

Step-by-Step Solution

1
Calculate the surface area of a single quadrat frame
Area of one quadrat=0.5 m×0.5 m=0.25 m2\text{Area of one quadrat} = 0.5\text{ m} \times 0.5\text{ m} = 0.25\text{ m}^2
Population density must be expressed in units of area, so the quadrat dimensions must first be converted into area.
2
Calculate the total area sampled across all throws
Total area sampled=20 throws×0.25 m2=5.0 m2\text{Total area sampled} = 20 \text{ throws} \times 0.25\text{ m}^2 = 5.0\text{ m}^2
The cumulative plant count represents the total organisms found across the entire combined sampled space.
3
Calculate the population density per square metre
Population Density=Total organism countTotal area sampled=150 plants5.0 m2=30 plants/m2\text{Population Density} = \frac{\text{Total organism count}}{\text{Total area sampled}} = \frac{150\text{ plants}}{5.0\text{ m}^2} = 30\text{ plants/m}^2
Population density is defined as the total number of individuals of a species per unit area.

Key Concept

Quadrat Population Density Calculation
Estimated Time:1m 30s
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Topics

Population Dynamics and Ecological Sampling

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