Sampling ecosystems with quadrats and transects
Estimate populations of plants and other stationary or slow-moving organisms using randomly located quadrats and area scaling. Use transects to study spatial changes in abundance, and explain what larger well-chosen samples can and cannot improve.
What a learner can do afterwards
- Scales a mean count per quadrat up to a whole field and states the units of the estimate.
- Explains why quadrat positions are chosen at random and what bias follows when they are not.
- Chooses a transect over random quadrats when the question is about change across a gradient.
1 · Read
A quadrat encloses a known area for counting plants or other stationary or slow-moving organisms reliably. For a whole-site estimate, divide a rectangular site into quadrat-sized cells. Use a random-number generator to pick a row and column, with each cell equally likely. Row 3, column 2 locates one cell. Place the frame gently there; throwing is not random selection. Count one species using a fixed rule, such as including plants whose centres lie inside.
Six equal-area quadrats contain 2, 4, 3, 5, 4 and 6 daisies. Include every count, even zero counts. Add these six counts to get 24, then divide by six quadrats. The mean is 4 daisies per quadrat. The bars show the observations. A mean of four does not mean every quadrat contains four daisies.
Estimated total = mean count per quadrat × study area / quadrat area. Use the same unit for both areas. A mean of 4 daisies per 0.5 m² quadrat in a 30 m² study area gives an area ratio of 30 / 0.5 = 60. The estimated total is 4 × 60 = 240 daisies in that area. Plant count differs from percentage cover, which describes the proportion of ground covered.
An environmental gradient is a change in conditions across a place, such as from sun into shade, not a graph's slope. A transect is a line across it. Place quadrats at recorded distances along the line and count consistently. Record light at the same positions to compare abundance with light. Quadrats can be used along a transect as well as at random positions. A pattern along one line does not by itself prove that light caused the change.
More independent, equal-area random samples usually reduce variation caused by chance placement. They do not guarantee accuracy or fix missed plants and biased choices. Sampling only dense flower patches can overestimate the whole-site mean. Report the species, study area, quadrat area, sample size and placement method. Give an estimated number of plants in the stated area, not an exact census.
Use consistent counts at random positions to estimate a whole site. Scale by the area ratio, state the study area, and use a transect to investigate spatial change.
Lesson revisions, questions, worked solutions and flashcards were written for this dataset.
The reading and flashcards were written for this dataset. Video credit: Estimating a population count from a random sample, Khan Academy. Video reuse terms have not been verified.
More to read
From sample counts to a field estimate
Define the study area and the species you will count. Use quadrats of a known, equal area. Agree on a boundary rule before starting, such as including plants whose centres lie inside the frame. Apply it consistently.
For a whole-site estimate, choose positions with random coordinates rather than looking for dense patches. Place the frame gently at each selected position. Record every count, including zeros. Add the counts and divide by the number of quadrats to find the mean.
Suppose 1 m² quadrats give a mean of 7 daisies and the study area is 80 m². The area ratio is 80 / 1 = 80. Multiplying 7 by 80 gives an estimated total of 560 daisies in that area. It remains an estimate, not a complete count.
A different question needs a different layout. To investigate change from a sunny edge into shade, sample at recorded distances along a transect. More well-chosen samples usually reduce chance variation, but cannot repair inconsistent counting or biased placement.
Explore more
2 · Watch
به انگلیسیTake it off screen
Where it sits
Where this leads
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.