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Biodiversity

Far-Flung Relatives: A seabird’s story of population fragmentation

Cristofari, R., Plaza, P., Fernández, C. E., Trucchi, E., Gouin, N., Le Bohec, C., … Luna-Jorquera, G. (2019). Unexpected population fragmentation in an endangered seabird: the case of the Peruvian diving-petrel. Scientific Reports, 9(1), 2021. doi:10.1038/s41598-019-38682-9

Yunco (Pelecanoides garnotii) by Pablo Caceres Contreras. (2008). https://www.flickr.com/

The Pacific coast of South America is a unique location on our planet; thanks to nutrient-rich waters rocketing upwards from the deep ocean, the Humboldt Current System (HCS) supports a staggering diversity of marine life. This biodiversity is increasingly being stressed by climate change and coastal development, though, so scientists are constantly on the lookout for ecosystem issues. Seabirds are often bioindicator species, which means any problems related to their health or overall populations are likely a result of problems with the ecosystem as a whole—sort of like the canaries in the coal mines.

One of the local species, the Peruvian diving petrel (Pelecanoides garnotii), has recently gotten attention due to the population’s stark decline in the last century. In the early 1900s, these petrels could be found along 4,000 km (~2,500 miles) of Peruvian and Chilean coastline, but impacts (e.g. poaching by humans, guano extraction, natural competition over fish stocks, etc.) nearly drove the population to extinction. Only seven breeding sites remain active today, clustered into two main areas and separated by 1,300 km (~800 miles).

While the global population tanked, some surveys suggested local populations around these breeding sites were starting to recover; scientists were skeptical, though, given how little data had been collected about the species before the population crash. Without much information on the petrel’s life history, its distribution at-sea, or patterns within an individual’s genes (aka its genetic structure), an international team of Scandinavian and South American scientists set out to look for clues hidden in DNA. Specifically, Robin Cristofari and Guillermo Luna-Jorquera’s group hoped to find clues that would reveal breeding habits and shed light on whether or not the species was more resilient than the few surveys to date would suggest.

Fig. 1: Cristofari et al., 2019. Depicts the breeding sites of the Peruvian diving petrel. The grey area is the historical range of the species. Box A shows the two Peruvian breeding sites while Box B shows the remaining five Chilean sites. Isla Grande (orange marker) is one of the newest breeding sites, having only been noted as of 2000. Isla Chañaral (white marker) was not active at the time of survey, and Isla Pajáros (grey marker) was not sampled.

The group sampled DNA from 109 petrels from across five breeding sites before beginning the process of searching within the genome for clear mitochondrial markers and abnormalities (e.g. single-nucleotide polymorphisms, or SNPs). Geographic ranges of a species can be figured out through mitochondrial DNA, since those genetic sequences are only passed down through the mother’s line and begin rapidly changing the more isolated a species is. Single-nucleotide polymorphisms (SNPs), on the other hand, are already fairly common but can be used like genetic fingerprints to identify individuals and their close relatives. Using these different pieces of information from the genome, the team examined if different subpopulations were cropping up within the larger population, and also tried estimating gene flow (genetic mixing) between the separate breeding sites.

The results revealed a bleak reality for the petrel. Examining the mitochondrial DNA showed that petrels tended to stay with their breeding colony; while some gene flow was seen between breeding sites closer to one another, the greater the distance the less mixing occurred. SNP data supported what the mitochondrial analysis had shown. One exception to this was in data collected from petrels on Isla Grande de Atacama (a Chilean breeding site [see Fig. 1]); specimens from this site had genetic markers also found in individuals from other breeding sites. Cristofari and Luna-Jorquera’s team suggested migrating petrels from nearby populations could have recolonized this site. Considering this colony was only discovered in the year 2000, it’s likely any petrels living there in the 20thcentury died out and these founders have come in to start a new population.

Fig. 2: Cristofari et al., 2019. (Article’s Fig. 3.) Sections A and B are of particular interest here. A. A comparison of four breeding sites on two Chilean islands, Isla Choros (red) and Isla Pan de Azúcar (tan). The closer together clusters of shapes and colors, the more those groups have in common. There is a split between islands, meaning individuals are more similar to others on the same island. B. A NeighborNet visualizing genetic relationships between individuals on separate islands. There is a difference between Choros and Pan individuals, stemming from a single, similar point. C. Model using SNP data to see if either island population shared certain genetic patterns or quirks. D. Visualizing the relationships between individuals across sampling sites using mitochondrial DNA markers.

By now you must be wondering what the big deal is, right? These birds have their home turf and don’t seem inclined to fly beyond certain borders to breed. How does this all play into conservation then? In simplest terms, these petrels are unlike other seabirds, which usually have a lot of genetic mixing happening between breeding sites. Mixing is thought to make a species hardier against environmental stress. With the low gene flow between sites, the petrels fall on the ‘Very Vulnerable’ side of the conservation spectrum. Efforts to increase their population will have to be tailored to each breeding site, with extensive and regular surveys to make sure the colonies are stable. And if you thought it was difficult to jumpstart a monitoring program in one large colony, try getting seven started at once for one species.

The fate of the Peruvian diving petrel is still uncertain; this recent study has uncovered a bit of data we were lacking before and has suggested one course of action. If conservation efforts go well, a few more colonies may be able to start on their own as individuals gradually spread back out along the coastline, and with regular, intensive monitoring, the future could brighten again for this little bird. This isn’t the first species struggling with a fragmented population, but the more we understand about an organism’s needs, the better we’ll be able to manage our impacts and increase their chances of survival.

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