- The loss of 80 percent of all Grauer’s, or eastern lowland, gorillas in the past two decades has led to a severe reduction in the subspecies’ genetic diversity, new research has found.
- That slide could make it more difficult for the fewer than 4,000 remaining Grauer’s gorillas to adapt to changes in their environment.
- Scientists look for signs of hope in the animal’s sister subspecies, the mountain gorilla, which, studies suggest, has adapted to its own low levels of genetic diversity.
The recent plunge in numbers of Grauer’s gorillas in the eastern Democratic Republic of Congo (DRC) has lopped off one-fifth of the subspecies’ genetic diversity, leaving it potentially vulnerable to changes to its environment, a new study has found.
Recent surveys found that the count of the critically endangered Grauer’s, or sometimes referred to as the eastern lowland, gorilla (Gorilla beringei graueri), a sister subspecies to the mountain gorilla (Gorilla beringei beringei), is down by nearly 80 percent since the 1990s. That precipitous drop-off has few precedents, so when molecular ecologist Katerina Guschanski of Sweden’s Uppsala University and her colleagues set out to understand how the animal’s genome had changed, they were unsure of what they might find.
“We knew that these populations have had a very dynamic recent history,” Guschanski said in an interview, adding that this typically came with a slide in the variation found in an animal’s genome. But, she said, “To see a dramatic impact, as dramatic as we have seen for Grauer’s gorillas in such a short timeframe, is actually quite surprising.”
Deforestation and poaching have winnowed their numbers to fewer than 4,000 individuals. With that decline, the subspecies has lost 20 percent of its genetic diversity within a few generations, Guschanski and her team reported Dec. 27 in the journal Current Biology.
“If you don’t have this variation — this ‘heterozygosity’ — it means you’re stuck with whatever you’ve got,” Guschanski said. In the face of inevitable shifts in their environment, as the result of climate change, for example, or the ongoing armed conflict in the eastern DRC, they’ll be scrambling to adapt without a full genetic toolkit, she added.
The researchers sequenced the genomes of seven Grauer’s and four mountain gorillas that lived as many as 100 years ago, using DNA extracted from museum specimens. They then compared their results with previously published sequences of modern gorillas.
A century represents only four or five gorilla generations. In that short span, the Grauer’s gorilla genome homogenized, indicating that the gorillas that mate with each other today are more closely related to each another than they were in the past.
The team also discovered that, compounding the problem, harmful gene mutations are much more common in today’s gorillas. Some of these mutations could make it more difficult for gorillas to reproduce, Guschanski said. Others could leave animals with weakened immune systems, a handicap in their rapidly changing environment, she added.
As forests in the eastern DRC, the only place on Earth where Grauer’s gorillas live in the wild, are cut down for firewood and charcoal or to make way for farmland, gorillas and other animals must struggle to adapt to their altered habitat. Such shifts also create new spaces for diseases like malaria to lurk.
Genetic mutations that compromise the immune systems of individuals in an already-dwindling population are a recipe for disaster, University of Cambridge geneticist Aylwyn Scally told Mongabay.
“There’s no way around it,” said Scally, who was not involved in the current research. “If you’ve only got 200 individuals left, or even a thousand individuals, one bad disease or one nasty episode where a lot of them get poached or killed could wipe them out.”
This new study builds on work that he and his colleagues published in the journal Science in 2015, Scally said.
Genetic evidence suggests that mountain gorilla numbers have been declining for the past 100,000 years. From the evidence they gathered looking at the genome of today’s gorillas, Scally and his fellow researchers posited that while this gradual decline over millennia has led to low genetic diversity, it has also given the subspecies time to purge many harmful genetic mutations.
“Over a very long period, these kinds of problems get pruned out or filtered out of that population when it’s small,” Scally said.
Combined with the robust conservation effort mounted over the past few decades, these adaptations may have helped the mountain gorilla bounce back. From a low of 254 individuals in the 1980s, more than 1,000 mountain gorillas now live in DRC, Rwanda and Uganda, according to recent surveys. In 2018, the IUCN downgraded the severity of the animal’s threat status from critically endangered to endangered.
Now, the numbers of Grauer’s gorillas are ebbing too, but instead of taking place over thousands of years, it’s happened in just 20.
“They are experiencing this insane and very rapid and dramatic population reduction,” Guschanski said.
Still, she sees the mountain gorilla’s success story as a sign of hope for the Grauer’s gorilla, that if given enough time, they too will be able to adapt to a reduced genetic diversity.
“They’re not doomed at all,” she said, “and that’s something that we can actually see with the mountain gorillas.”
Guschanski acknowledged that pulling Grauer’s gorillas back from the brink means tackling the threats that still face them on a daily basis, be it poaching or habitat loss or the potential introduction of new diseases. These are tough challenges in the perpetually conflict-ridden eastern DRC. But if conservationists and local communities can work together, the gorillas have a chance.
“If we can stop the population decline in Grauer’s gorillas, or better even if we can reverse this process and allow the population to increase, we will absolutely be able to secure their long-term survival,” Guschanski said.
Banner image of Grauer’s gorillas by Amy Porter/Dian Fossey Gorilla Fund International.
John Cannon is a Mongabay staff writer based in the Middle East. Find him on Twitter: @johnccannon
Citations
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Plumptre, A., Nixon, S., Caillaud, D., Hall, J.S., Hart, J.A., Nishuli, R. & Williamson, E.A. (2016). Gorilla beringei ssp. graueri (errata version published in 2016). The IUCN Red List of Threatened Species 2016: e.T39995A102328430. Downloaded on 09 January 2019.
van der Valk, T., Díez-del-Molino, D., Marques-Bonet, T., Guschanski, K., & Dalén, L. (2018). Historical Genomes Reveal the Genomic Consequences of Recent Population Decline in Eastern Gorillas. Current Biology.
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