by Otter populations in Britain collapsed around the 1960s due to the deadly effects of chemical pollution in rivers and lakes – or so we thought. Our research looked at what has happened to otters in Britain over the past 800 years and has revealed a more complex picture.
Since Eurasian otters (Lutra lutra(b) are at the top of the aquatic food chain in Britain, any contamination that eats their prey, and their prey, accumulates in otters. Otters are therefore particularly susceptible to any toxic chemicals in their environment.
After many chemical pollutants were banned, otter populations began to recover, and we now have otters in every county in Britain. National water dog surveys have been carried out in Wales, Scotland and England since 1977 and have helped to track population recovery.
However, we did not have a good understanding of the population sizes in the years before this time. We only had anecdotal evidence that otter hunting was becoming less “successful” over time, and otter sightings and signs were rarer.
Reduction of the water dog population
Our research shows that between 1950 and 1970, around 1950 and 1970, there was a major population decline in the east of England, and a strong decline in the south-west of England. They were probably caused by chemical pollution.
In Scotland, otter populations have shown long-term, but smaller, declines, suggesting less chemical pollution. Wales experienced a smaller decline in population, which began around 1800, possibly linked to otter hunting and changes in the way people shaped and used the landscape.
While both deal with DNA, genetics focuses on individual genes and their roles, while genomics examines the entire set of an organism’s DNA. Although genetic studies have been carried out on otters in Britain, our research was the first time that genomics has been used to study Eurasian otters anywhere in the world.
Working with scientists from the Smithsonian Conservation Biology Institute and Wellcome Sanger’s Darwin Tree of Life project, we looked at the whole water dog genome. The upgrade from genetics to genomics threw up some surprises.
First, a mitochondrial DNA sequence was found in the east of England, which was very different from the sequences in the rest of Britain. Mitochondrial DNA is a DNA sequence found in the mitochondria of a cell, which generates energy. Mitochondrial DNA is only obtained from the mother, and the rest of the DNA is a mixture of both maternal and paternal DNA.
Another recent study by our research group, in collaboration with colleagues in South Korea, suggested that these two lineages diverged at least 80,000 years ago. This mitochondrial lineage (which, based on our data, is otherwise restricted to Asia) was surprisingly found in the UK.
Secondly, we found high levels of genetic diversity in the east of England. Typically, after a severe population decline like the one we identified in this area, genetic diversity is reduced. However we saw much more variation here than in the population in Scotland, where there was no clear evidence of such a decline.
Thai otters
With a bit of detective work, we discovered that a pair of Eurasian otters (the same species we have in the UK), were brought to Britain from Thailand in the 1960s. Eurasian otter populations range across Europe and Asia. Although they are the same species, there are several genetically distinct subspecies, particularly in Asia.
It seems that these genetically different water dogs from Thailand may have bred with otters in eastern England. When the population declined, when the native population of the UK was at its lowest, even a few people introduced to the population could have made a big difference. And they left unexpected marks on the genome.
We don’t know for sure if this is what happened, and we need to do more work to find out what effect this may have had on the water dog in eastern England. High genetic diversity is usually good for a population or species. But on the other hand, conservation often tries to maintain genetic differences between populations, rather than mixing distinct populations.
One way to find out more is to compare the genome of a Eurasian otter from Thailand with the otters we see in eastern England. Unfortunately, it’s not that easy. Since the 1960s, otters in Thailand and throughout Asia have become increasingly rare. This is due to habitat loss, pollution and the illegal trade of the water dog. So it is very difficult to get samples for genome sequencing. It underlines the importance of conserving the species in Asia, despite population recovery in Europe.
Our work demonstrates the value of using modern genomics tools to look at the genetic diversity of threatened species. Application of such tools can reveal surprising facts, even in well-studied species.
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This article from The Conversation is republished under a Creative Commons license. Read the original article.
Frank Hailer receives funding from NERC and Dŵr Cymru Welsh Water.
Elizabeth Chadwick receives funding from the Natural Environment Research Council in the UK and the Environment Agency
Sarah du Plessis receives funding from the UK’s Natural Environment Research Council and the Global Wales International Mobility Fund.
