Otter populations in the UK collapsed around the Sixties because of the deadly effects of chemical pollution in rivers and lakes – or so we thought. Our research took a closer have a look at what has happened to otters in Britain over the last 800 years and revealed a more complex picture.
As Eurasian otters () are at the top of the aquatic food chain in the UK, any contaminants consumed by their prey and prey animals accumulates in otters. Therefore, otters are especially prone to any toxic chemicals of their environment.
After many chemical pollutants were banned, otter populations began to recuperate and otters are actually present every county in the UK. Since 1977, national otter surveys have been conducted in Wales, Scotland and England, which have helped track population recovery.
However, we didn’t have a good idea of population size in the many years prior to this time. We only had anecdotal evidence that otter hunting was becoming less “successful” over time and that each sightings and signs of otters were becoming less frequent.
Otter population decline
Our research shows that between roughly 1950 and 1970 there was an extreme decline in population in the East of England and a strong decline in the South West of England. They were probably attributable to chemical contamination.
In Scotland, the otter population has shown a long-term but smaller decline, suggesting less chemical pollution. In Wales there was a smaller population decline starting around 1800, probably related to otter hunting and changes in the way people shaped and used the landscape.
Ondrej Chvatal/Shutterstock
While each cope with DNA, genetics focuses on individual genes and their roles, while genomics examines an organism’s entire set of DNA. Although genetic studies of otters have been carried out in the UK, our research is the first in the world to make use of genomics to check Eurasian otters.
Working with scientists from the Smithsonian Conservation Biology Institute and Wellcome Sanger’s Darwin Tree of Life project, we checked out the entire otter genome. The upgrade from genetics to genomics brought a few surprises.
First, a mitochondrial DNA sequence was present in eastern England that was very different from the sequence in the rest of the UK. Mitochondrial DNA is a DNA sequence present in a cell’s mitochondria that produces energy. Mitochondrial DNA is inherited exclusively from the mother, while the rest of the DNA is a mixture of maternal and paternal DNA.
Other recent research conducted by our research group in collaboration with colleagues in South Korea suggested a divergence between these two lineages at the least 80,000 years ago. Finding this mitochondrial lineage (which our data shows is restricted to Asia) in the UK was surprising.
Secondly, we found high levels of genetic diversity in eastern England. Typically, after extreme population declines like the one we have now identified on this area, genetic diversity declines. However, we saw much greater 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 now in the UK) were introduced to the UK from Thailand in the Sixties. Eurasian otter populations occur throughout Europe and Asia. Although they’re the same species, there are several genetically distinct subspecies, particularly in Asia.
It seems possible that these genetically different otters from Thailand interbred with otters from eastern England. In times of population decline, when the native population in Britain was at its lowest, even a few individuals introduced into the population could make a big difference. And they left unexpected traces in the genome.
We do not know of course if that is what happened, and we want to do more work to seek out out what effect this will have had on otters in East Anglia. High genetic diversity is frequently useful to a population or species. On the other hand, conservation often seeks to take care of genetic differences between populations, relatively than mixing distinct populations.
One technique to discover more can be to match the genome of the Eurasian otter from Thailand with the otters we see in eastern England. Unfortunately, it is not that easy. Since the Sixties, otters have grow to be increasingly rare in Thailand and throughout Asia. This is because of habitat loss, pollution and the illegal otter trade. So collecting samples for genome sequencing may be very difficult. It highlights the importance of protecting the species in Asia despite population recovery in Europe.
Our work shows the value of using modern genomic tools to take a look at the genetic diversity of an endangered species. The use of such tools can reveal surprising facts, even in the case of supposedly well-studied species.
