Mark-Recapture After Rain
On an ecology field trip to a restored tallgrass prairie, students estimate a flightless ground beetle population by mark-recapture. On Monday they capture 80 beetles in a 20 m by 20 m plot, mark each with fluorescent powder, and release them. On Tuesday, after heavy rain, they return to the same plot and capture 60 beetles; 12 have visible powder. A student calculates (80 × 60) ÷ 12 = 400 beetles and writes that the population is 400, so the restoration is supporting a healthy population. The teacher candidate notices that powder is faint on several beetles, that students searched only along the mowed path, and that marked beetles may be more visible to birds. The class has no prior estimate and only one recapture session. The teacher candidate must decide how to respond to both the number and the health claim.
1. Evaluate whether 400 is a valid estimate. Identify which Lincoln-Petersen assumptions are likely violated and the direction of each bias.
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Estimate: Lincoln-Petersen gives N = (80 × 60)/12 = 400. That is a point estimate, and with only 12 recaptures it has wide uncertainty.
Assumptions and likely violations: The method assumes a closed population, equal catchability, no mark loss, no trap response, and that marked animals mix randomly.
- Faint powder means some marked beetles are scored unmarked. That lowers R and inflates N.
- Predation on marked beetles also lowers R and inflates N.
- Searching only the mowed path may not represent the whole plot; the direction depends on where marked beetles were released and how they move, but it undermines equal catchability.
- Rain may change activity and mixing.
Conclusion: 400 is not a defensible absolute population size. It is a rough estimate that is probably biased upward by mark loss and possibly by predation. The candidate should present it as an estimate with assumptions, not as a census.
2. Propose a revised sampling plan that could produce a more defensible estimate within a 90-minute class period.
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Revised plan: Use three or four smaller plots or a stratified set of quadrats rather than one path. Capture for a fixed, equal time in each plot. Mark with a coded paint dot or bee tag and record mark type; keep a few marked beetles for 24 hours to check retention. Release marked beetles evenly across each plot, not only at the capture point. Recapture the next day at the same time and weather conditions, using the same effort. Have two observers independently score marks to reduce misclassification. Pool data across plots and use a Schnabel or Bailey estimator, or at least report the range from multiple plots. If time is tight, do a same-day recapture after two hours, but acknowledge that mixing may be incomplete. The goal is to reduce mark loss and unequal catchability, not to produce a single precise-looking number.
3. How should the teacher candidate respond to the student's claim that 400 beetles means the restoration is healthy?
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Response: The 400 is an estimate of abundance, not a measure of ecosystem health. A restored prairie could support 400 beetles and still be degraded, or support fewer beetles but higher diversity. Without a pre-restoration baseline, a reference site, or data on species diversity, vegetation structure, and habitat quality, the health claim is unsupported. The teacher candidate should have students separate two questions: How many beetles are here, and how uncertain is that number? and Is the community healthy? The first needs better sampling; the second needs multiple indicators and a comparison. A defensible sentence would be: Our rough estimate is about 400 beetles, but mark loss probably makes it too high, and we cannot judge restoration success from one species count.