A translocation decision for a fragmented lizard population
A grassland lizard occupies three habitat patches separated by a highway and cropland. Patch 1 holds about 240 adults, Patch 2 about 35, and Patch 3 about 12. Genetic sampling shows Patch 3 has very low heterozygosity and no evidence of recent immigration. A field team proposes moving 20 adults from Patch 1 to Patch 3 to boost numbers and genetic diversity. A conservation officer notes that Patch 3 is only 4 hectares and its vegetation is being invaded by a grass that reduces basking sites. Another officer argues that moving animals risks spreading a skin fungus detected in Patch 1 at low prevalence but not found in Patch 3.
1. Assess the risks and benefits of the proposed translocation and recommend whether to proceed now.
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Benefits. Patch 3 is small and genetically depauperate, so adding 20 adults could raise effective population size and heterozygosity and reduce inbreeding depression. Twenty animals is a meaningful fraction of a population of 12.
Risks.
- Pathogen transfer. Patch 1 carries a skin fungus that Patch 3 lacks. Moving animals could introduce a disease into a small, stressed population, which is a potentially irreversible harm.
- Habitat limitation. Patch 3 is 4 ha with declining basking sites. Adding animals to a declining habitat may increase competition without improving persistence.
- Genetic mismatch. Donors from Patch 1 may be adapted to different conditions, and outbreeding effects cannot be ruled out without understanding divergence.
- Source impact. Removing 20 from 240 is a small proportional cost but should still be modelled.
Recommendation. Do not proceed now. Pathogen screening and treatment of donors, habitat restoration at Patch 3, and a genetic assessment of divergence should come first. Proceeding before these steps trades a certain disease risk for an uncertain genetic benefit.
2. Explain how the concepts of effective population size and gene flow apply to Patch 3, and why census counts alone are misleading here.
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Census vs effective size. A count of 12 adults is a census size. Effective population size is usually much smaller because it accounts for unequal sex ratios, variance in reproductive success and inbreeding. In a tiny patch, a few successful breeders may contribute most offspring, so the effective size could be only a handful of individuals. Genetic drift then dominates, heterozygosity falls and deleterious alleles can rise in frequency, which matches the low heterozygosity observed.
Gene flow. Gene flow is the movement of alleles between populations. Patch 3 shows no evidence of recent immigration, so gene flow is effectively zero. Without gene flow, drift and inbreeding accumulate and the population loses adaptive potential. The highway and cropland act as barriers even if individual lizards occasionally cross.
Why counts mislead. Twelve animals may look viable, but if effective size is very low the population can be committed to decline despite stable numbers. Census counts also say nothing about habitat quality or disease. Management should target connectivity and habitat, not just headcounts.