Low levels of genetic exchange between isolated populations can become problematic.

The Southern Dunlin (Calidris alpina schinzii) is a loyal bird. Despite its long-distance migration to Africa, it nearly always returns to the same breeding grounds. This behavior – known as philopatry – can be a very clever strategy. Breeding grounds with stable conditions and plenty of resources increase the chances of successful reproduction. Why move when you have a nice home? But a recent study in the journal BMC Ecology and Evolution shows that strong philopatry can turn into an evolutionary trap, threatening the Southern Dunlin with extinction.

Isolation-by-distance

Nelli Rönkä and her colleagues collected blood samples of 344 Southern Dunlins across their distribution around the Baltic Sea. Genetic analyses – based on 12 microsatellites – revealed strong genetic differentiation between several populations. When the researchers calculated the degree of genetic relatedness between the populations, they found that “that the kinship coefficient slowly decreased with increasing distance between individuals, indicating an isolation-by-distance pattern.” In other words, distant populations are genetically more differentiated than close ones. This result is not particularly surprising, but it might have important consequences for the future of the Southern Dunlin.

Conservation Measures

The isolation-by-distance pattern suggests little genetic exchange between the different breeding populations. With increasing habitat fragmentation, this lack of gene flow might cause issues. Indeed, the researchers reported that “observed heterozygosity was lower than expected throughout the range.” A telltale sign of inbreeding. Putting it all together the authors conclude that:

The pronounced site fidelity in the declining and fragmented Baltic Southern Dunlin population results in inbreeding and may lead into an evolutionary trap.

This precarious situation calls for urgent conservation measures. Apart from safeguarding current breeding grounds and increasing habitat connectivity, it might even be necessary to translocate individual birds. Hopefully, conservationists will not need to implement this drastic measure.

References

Rönkä, N., Pakanen, V. M., Pauliny, A., Thomson, R. L., Nuotio, K., Pehlak, H., … & Kvist, L. (2021). Genetic differentiation in an endangered and strongly philopatric, migrant shorebird. BMC Ecology and Evolution, 21(1), 1-12.

Featured image: Dunlin (Calidris alpina) © Charles Homler | Wikimedia Commons