Measure abundance without overlooking diversity
Compare edge and spread samples across a synthetic habitat gradient.
Learning outcome: Distinguish abundance density from detected richness and explain how spatial selection changes ecological evidence.
Starting recap
A quadrat is a defined sampling area. Richness counts taxa detected; abundance counts individuals. Neither alone fully describes biodiversity.
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Try
The first three quadrats contain only taxon A. Interior quadrats contain B and C as well. Similar total counts can conceal different community composition.
Open the experimentPredict
Can a sample have a reasonable mean density while missing taxa present elsewhere? Will two spread quadrats necessarily detect all three taxa?
Experiment
Compare three edge quadrats with three spread quadrats. Then compare two spread quadrats and a full twelve-quadrat census. Inspect the matrix, density and detected richness; capture a missed-taxon case.
Inspect spatial sampling and detected richness
Worked teaching model · synthetic values · Twelve fictional quadrats run from habitat edge to interior. Three counted taxa are labeled A, B and C. Edge selection takes the first n; spread selection takes evenly spaced indices. Neither is a probability sample. Detection is perfect only within selected quadrats.
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Inspect the spatial pattern and selected quadrats
| Quadrat | Selected? | Taxon A | Taxon B | Taxon C |
|---|---|---|---|---|
| 1 | Yes | 6 | 0 | 0 |
| 2 | Yes | 5 | 0 | 0 |
| 3 | Yes | 7 | 0 | 0 |
| 4 | No | 4 | 1 | 0 |
| 5 | No | 3 | 2 | 0 |
| 6 | No | 2 | 3 | 0 |
| 7 | No | 1 | 3 | 1 |
| 8 | No | 1 | 2 | 2 |
| 9 | No | 0 | 3 | 3 |
| 10 | No | 0 | 2 | 4 |
| 11 | No | 0 | 1 | 5 |
| 12 | No | 0 | 0 | 6 |
Selected counts: A 18, B 0, C 0. Detected richness: 1. Selection is marked with text as well as shading.
| Row | Quantity | Value (individuals/m²) |
|---|---|---|
| 1 | Quadrat 1 abundance density | 6 |
| 2 | Quadrat 2 abundance density | 5 |
| 3 | Quadrat 3 abundance density | 7 |
| 4 | Sample mean density | 6 |
| 5 | Full habitat mean density | 5.5833 |
Detected richness: 1 taxa; sample density: 6.000 individuals/m²; full habitat: 3 taxa and 5.583 individuals/m².
Richness counts detected taxa, whereas abundance counts individuals. A sample can estimate density reasonably while missing a taxon. Spatial coverage is inspectable here, but deterministic spread selection does not supply a probability-based confidence interval.
Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.
Understand
Apply
Public practice checks a specific claim. It does not establish independent mastery.
Propose a field sampling design with explicit spatial coverage, detection limits and repeated observations. Explain how you would evaluate biodiversity change after a disturbance without confusing detection with disappearance.
Continue
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