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Difficulty: HardPopulation Dynamics and Ecological Sampling

An ecological researcher investigated the population of freshwater snails (*Bulinus globosus*) in a stream marsh measuring 250 m2250\text{ m}^2 near Oguta Lake, Imo State. Using a quadrat frame of size 0.5 m20.5\text{ m}^2, the researcher randomly threw the quadrat 1010 times across the sampling site and recorded snail counts of 4,6,3,5,7,2,8,4,5,4, 6, 3, 5, 7, 2, 8, 4, 5, and 66. Based on these sample measurements, what is the estimated total population size of *Bulinus globosus* in the entire 250 m2250\text{ m}^2 marsh area?

  1. A
    1,250 snails1,250\text{ snails}
  2. 2,500 snails2,500\text{ snails}Answer
  3. C
    12,500 snails12,500\text{ snails}
  4. D
    25,000 snails25,000\text{ snails}

Answer

The estimated total population size of Bulinus globosus in the marsh plot is 2,500 snails.
To estimate total population size using quadrat sampling, calculate the total area sampled (10×0.5 m2=5 m210 \times 0.5\text{ m}^2 = 5\text{ m}^2). Divide the total count of organisms (50 snails50\text{ snails}) by the total sampled area (5 m25\text{ m}^2) to obtain a population density of 10 snails/m210\text{ snails/m}^2. Multiplying this density by the total marsh plot area (250 m2250\text{ m}^2) yields the correct total population estimate of 2,500 snails2,500\text{ snails}.

Step-by-Step Solution

1
Calculate the total number of organisms counted across all sampled quadrats
Sum of counts = 4+6+3+5+7+2+8+4+5+6=50 snails4 + 6 + 3 + 5 + 7 + 2 + 8 + 4 + 5 + 6 = 50\text{ snails}
Determines the total sample count obtained from field sampling.
2
Calculate the total surface area sampled by the quadrats
Total sampled area = Number of quadrats×Area of one quadrat=10×0.5 m2=5 m2\text{Number of quadrats} \times \text{Area of one quadrat} = 10 \times 0.5\text{ m}^2 = 5\text{ m}^2
Required to compute the population density per square metre.
3
Calculate the mean population density per unit area
Density = Total organism countTotal sampled area=50 snails5 m2=10 snails/m2\frac{\text{Total organism count}}{\text{Total sampled area}} = \frac{50\text{ snails}}{5\text{ m}^2} = 10\text{ snails/m}^2
Provides the average concentration of organisms per square metre.
4
Extrapolate the density to estimate the total population in the study area
Total population = Population density×Total study area=10 snails/m2×250 m2=2,500 snails\text{Population density} \times \text{Total study area} = 10\text{ snails/m}^2 \times 250\text{ m}^2 = 2,500\text{ snails}
Scales the sample density to the full size of the habitat plot.

Key Concept

Population Density and Quadrat Sampling Calculation
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Topics

Population Dynamics and Ecological Sampling

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