The Aspinall Foundation’s reintroduction of western lowland gorillas to areas of Africa where they have been hunted to extinction appears to be working, according to a scientific study published today in the international conservation journal Oryx. Western lowland gorillas are classified by the World Conservation Union as Critically Endangered, based on a projected 80% decline in the wild over just three generations, ranking them alongside the most threatened species on the planet. Reintroduction of gorillas to protected areas from where they have previously been exterminated is still considered controversial, but a pioneering, long-term programme to do just that is starting to show it may be possible after all.

Two gorilla populations are currently in the process of being re-established in the neighbouring African republics of Congo and Gabon, by the UK-based charity The Aspinall Foundation in collaboration with the respective governments. Fifty-one gorillas were released between 1996 and 2006, 25 in the Lesio-Louna Reserve in Congo, and 26 in the Batéké Plateau National Park in Gabon. Most of the released gorillas are rehabilitated orphans of the illegal bush-meat trade, taken as young babies from their slaughtered mothers by opportunistic hunters. The majority of orphaned gorillas die of depression and mistreatment, but a few survive long-enough to be confiscated and handed over to long-term rehabilitation programmes.

In the Gabon project, in addition to the wild-born orphans the released gorillas also include seven captive-borns, sent back to Africa from The Aspinall Foundation’s successful captive-breeding population at Howletts and Port Lympne Wild Animal Parks in the UK.

Dedicated field staff have been monitoring the released gorillas for over ten years at both reintroduction sites. A previous analysis, published in 2012 in the International Journal of Primatology, illustrated that the reintroduction programme had been successful in terms of post-release survival, birth rates and dispersal, all of which were comparable with wild populations.

The new study goes a step further, using this information to develop a computer simulation model of the growth of the two reintroduced gorilla populations over a 200-year period.

Lead author of the new study, The Aspinall Foundation’s Conservation and Reintroduction Co-ordinator Tony King, explained, “We have seen with our own eyes the remarkable ways in which the released gorillas adapt to their new homes, and have celebrated numerous successful births to orphaned gorillas who never had the chance of a normal upbringing in a gorilla family – but this is the first time that we have put all this together to help predict the future success of the reintroductions.”

The results of the study suggest that the reintroduced gorilla populations have a good chance of sustaining themselves for 200 years and more, but illustrated that reinforcement of the populations by further releases could significantly improve probabilities of population persistence and retention of genetic diversity. Damian Aspinall, chairman of The Aspinall Foundation, said, “This is incredibly useful information. Only last week three more gorillas were released in Gabon, and we are currently preparing an entire family group for imminent release.”

Developing the model was a challenge. “Gorillas can live for over forty years, usually don’t reproduce until they are at least 10 years old, and females produce one surviving off-spring only every five years or so,” added co-author Christelle Chamberlan, who has worked with both reintroduced lowland gorilla populations and the wild mountain gorillas of Rwanda. “Even after a decade of monitoring our released gorillas, there are still many aspects of their life-history patterns that we don’t know. We tested our model to see which factors were most significant in changing the predicted success of the reintroduction.

Relatively small changes to annual birth rates or to female survival rates made big changes to the predicted long-term growth of the populations. Good numbers of healthy, reproducing female gorillas are therefore critical to population persistence.”

“It is definitely an ambitious project,” King concluded. “Results so far have exceeded most expectations. The gorillas are still living on a knife-edge though. Small reintroduced populations are always susceptible to crashes due to random changes in any number of factors. We plan to release more gorillas at both sites, which will increase the chances that the populations will survive. In reality we are still only just beginning.”