Limpets, Reserves and a Confounded Comparison
Two sixth-form students compare limpets on a rocky shore inside a marine reserve and on a public beach 8 km away. At each site they run one 30 m tape from the high-water mark to the low-water mark and place a 0.25 m² quadrat every 3 m, counting every limpet in each quadrat. Reserve: mean 34.2 limpets per quadrat, values ranged from 12 to 61. Public beach: mean 8.6, values ranged from 0 to 19. Their teacher points out that the reserve shore faces the open sea and is broken by deep crevices, while the public beach is sheltered and smoother, and that the reserve has been closed to collecting for 12 years. A local fisher says limpets have 'always been scarce' on the public beach. The students want to present the conclusion 'the reserve is working, so every beach should be closed to collecting' to a community meeting.
1. Calculate the students' sampling effort and assess the reliability of their comparison. What is the main weakness in the design, and how does the data hint at it?
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Effort: a 30 m tape with a quadrat every 3 m gives 10 quadrats per site, and 10 × 0.25 m² = 2.5 m² sampled at each shore. That is a small sample of one shore of each type.
Reliability: the spread of counts is large - 12 to 61 in the reserve and 0 to 19 on the public beach. With only 10 quadrats, a mean of 34.2 could shift by several limpets if the tape happened to cross a crevice-rich patch or a bare stretch. There was one transect per site, no repeats at other shores of the same type, no statement that quadrats were placed randomly, one observer, and no record of tide state or time of day. The wide ranges are exactly what you would expect from a small, single-transect sample, and they warn that the means are imprecise.
The main weakness is that the two shores differ in more than protection. One reserve shore and one public beach cannot act as replicates of a treatment, so the difference is confounded with shore type: exposure, rock texture and crevice density all vary along with protection status. The comparison is suggestive but not a fair test.
2. Identify two confounding variables and explain how each could produce the observed difference without any effect of protection. Describe how you would redesign the survey to control for them.
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- Wave exposure and rock texture. The reserve shore faces the open sea and has deep crevices; limpets need firm, damp rock and use crevices as refuge from drying out and from predators. A smooth, sheltered beach may simply offer fewer places to live, so the difference could be habitat rather than protection.
- Human activity other than collecting. The public beach is likely to have more trampling, litter, bait digging, dog walking and pollution, any of which can reduce limpets whether or not anyone collects them.
Redesign: choose at least three reserves and three comparable control shores, matched for exposure, rock type, shore slope and height on the shore. Place quadrats randomly within defined shore zones rather than along a single tape, keep the same observer (or check agreement between observers), and sample at the same tide state and season. Record limpet size as well as number, since a population with many small limpets on the public beach suggests recent removal, whereas natural scarcity would give a similar size structure at both sites. Add direct evidence on collecting, such as counts along access paths, interviews, or permit records.
3. How should the fisher's comment affect the students' conclusion? What additional evidence would make the case for closing beaches stronger, and what conclusion would you actually present?
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The fisher's observation is only an anecdote, but it is relevant. If limpets were always scarce there, the present low count may reflect the shore's natural state rather than recent collecting, and the 'reserve effect' would be overstated. It also points to the need for a baseline: without data from before the reserve was closed, or from comparable unfished shores, the students cannot show that numbers changed.
Evidence that would strengthen the case: repeated surveys over several years at both shores (a before/after, control/impact design); matching results from other reserves; size-frequency data showing larger limpets where collecting is banned; and records of how much collecting actually takes place. Socioeconomic information on who depends on collecting also matters for any management decision.
Conclusion to present: 'Our survey found about four times as many limpets inside the reserve as on the public beach, but the two shores also differ in exposure and rock type, and each was sampled only once. The pattern is consistent with protection but does not prove it. We recommend a matched, replicated survey before any decision to close beaches, because closures affect people's livelihoods.'