An Endemic Snail on a Limestone Cliff: Sampling and Managing a Tiny Population
On a limestone cliff in Gozo, an endemic snail species survives at three sites. A mark–recapture study gives 140, 45 and 20 individuals; a fourth site had 60 in 1998 but none now. Across 12 microsatellite loci, expected heterozygosity is 0.12, compared with 0.31 in a mainland population of the same genus. Two sites are separated by 800 m of exposed rock; the third is 4 km away. Rats are present at the largest site and snail shells show broken apertures. The environment agency proposes moving 30 snails from the largest site to the empty fourth site, and 10 from the smallest site to the largest to increase numbers. A consultant warns that mixing sites may reduce fitness. There is no captive colony and funding covers only one intervention.
1. Evaluate the sampling and the heterozygosity data: what can and cannot be concluded?
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Mark–recapture can estimate population size only if the population is closed, marks are not lost, and animals mix; with 20–140 individuals the confidence intervals are wide, and the smallest site may be too small for reliable estimation. The heterozygosity value (0.12) is lower than the mainland value (0.31), consistent with small population size, genetic drift and inbreeding, but heterozygosity alone does not measure inbreeding depression or fitness. Expected heterozygosity from 12 microsatellites is a reasonable diversity index, but sample sizes and possible null alleles can bias it. The 1998 site's loss shows local extinction, not necessarily genetic decline. Conclusion: the population is small and genetically less diverse, but whether this reduces fitness needs survival and reproduction data.
2. Assess the two proposed translocations, including genetic and ecological risks.
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Moving 30 snails to the empty fourth site could create a new population, but it would be founded by a subset of an already small gene pool, causing a founder effect and further loss of diversity. The empty site may lack the right food, moisture or shelter, and rats may still be present. Moving 10 from the smallest site to the largest could increase numbers and gene flow, but removing 10 of 20 is a major loss from the donor and could reduce its viability; mixing two populations that may be locally adapted risks outbreeding depression and introducing disease. The largest site is the only population large enough to donate without immediate harm, and rats there are an ongoing threat.
3. Recommend one intervention and justify it using population genetics.
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First control rats at the largest site and survey the empty site. Reducing adult mortality increases population size, which slows genetic drift and preserves the remaining diversity; it also protects the source population before any translocation. Translocation should follow only after habitat assessment and genetic screening, with a clear monitoring plan. If only one intervention is funded, rat control gives the best expected benefit per euro because it addresses the cause of decline and avoids the genetic risks of moving snails.