How do you combat dangerous primate hunters in a dense and secluded rainforest? Simple, you use the sounds of that rainforest against them.

After the completion of a road that runs through the heart of the Gran Caldera Scientific Reserve, Central Africa, primate populations began to drop. Consequently, this caused a ripple-effect that is currently being felt by the entire ecosystem. Now, researchers are using the sounds of the rainforest to measure the influence that hunters are having on the reserve’s biodiversity in the hope of developing a better understanding of ‘Empty Forest Syndrome’.


A secluded waterfall near Ureca, a town on Bioko Island, Equatorial Guinea. BiokoIslander@WikipediaCommons

In any ecosystem, the presence of one species has a direct influence on the presence of another. All the plants and animals are connected to the habitat which they share, each modifying how the entire system functions. This is more apparent with large mammals, who can have a significant effect on an area’s vegetation and fauna. When these animals are removed, the ecosystem’s function and community composition are altered. Subsequently, this can produce a domino-effect from one species to the next which can be positive for some, but negative for most.

When the removed species are primates, who have a sizeable ecological weight, this can lead to an environment void of large animals in what is known as Empty Forest Syndrome. A deterioration of the ecosystem to this level in a rainforest habitat can have the devastating consequences that are now being observed in Equatorial Guinea.

Located on the volcanic Bioko Island, the dense and pristine rainforest of the Gran Caldera Scientific Reserve is home to seven species of primate. This 196-square-mile reserve has previously been a refuge for wildlife, with access to the isolated forest posing a challenge, unless you possessed wings or the ability to swing through the treetop canopy.


A baby Drill, one of the species threatened by local hunters. Rufus46@WikipediaCommons

However, the completion of a road in 2014 that runs through the heart of the reserve has begun to relinquish rainforest access to a growing number of bushmeat hunters. These local people illegally hunt the seven species of primate and can now track the animals further into the previously-isolated forest from the main road. With the bushmeat trade growing annually and the number of primates who live on Bioko Island shrinking, researchers are now using bioacoustics and soundscape ecology to measure the dropping biodiversity more effectively, as well as tracking the hunter’s movements.

The study of bioacoustics, the sounds made by living organisms, was pioneered by studio musician turned natural sound recordist, Bernie Krause. He proposed the theory of ‘acoustic partitioning’ where each animal “carves out its own aural niche”, and that an entire ecosystem could be measured via the sounds produced by each species. This work formed the basis of modern soundscape ecology: using the ecosystems noise to measure its biodiversity. Michael Towsey from Queensland University of Technology has created what he describes to be an acoustic weather chart whereby sound recordings of an environment can be analysed to show changes over days, months and even years.


The Red-Eared Guenon of Bioko Island. LaetitiaC@WikipediaCommons

Recently, a team from the University of the West of England, led by Dr David Fernandez, has set up bioacoustic sensors around the Gran Caldera reserve, in both the isolated forest and locations accessible from the road. The first measure they investigated was primate density. Between 2011 and 2013 across a pre-road reserve, over 70 censuses were completed on primate populations via transects, ranging up to 105km in length. After the road’s construction, the team carried out a further 47 surveys on species’ abundance and recorded data regarding hunter activity.

The second measure was for acoustic complexity, being used as a proxy for biodiversity. Song-Meter SM4 Acoustic Sensors recorded one of every five minutes throughout the day at a sampling rate of 48kHz, enabling the scientists to analyse Entropy Index (H) and Number of Peak Index (NP).

Entropy Index is a method to analyse the acoustic evenness of a recording, i.e. the amount that the sounds mixed together. If there is low biodiversity in an area, then fewer species are producing sounds which means that on the recording there will be less sound mixing and therefore, a lower measure of entropy. Whilst Number of Peak Index calculates the number of frequency peaks, each peak correlates to a certain species, enabling this to be used as a proxy for the number of animals.


The pristine rainforest of the Gran Caldera that many primates call home. JamesShook@WikipediaCommons

They found that after the road’s construction primate hunting increased significantly, resulting in a reduced primate population. Along the road and in the accessible rainforest, the acoustics were less complex and contained fewer species; however, the isolated forest had high acoustic complexity and therefore, more biodiversity.

This evidence was further supported as the abundance of bird species did not change after the road’s completion. According to the researchers, this “indicates that changes in acoustic properties are related to the decrease in primate population which may have already altered community composition.” As a result, there is significant evidence that the Gran Caldera Scientific Reserve has begun to express Empty Forest Syndrome.

With more areas of the rainforest becoming accessible to hunters every year, the decreasing number of primates is already changing the community composition on Bioko Island. If the rate of bushmeat hunting continues, then the alterations being experienced by this wildlife community will produce a new ‘silent forest’ in the Gran Caldera Scientific Reserve.


Cronin, D., Sesink Clee, P. R., Matthew W, M., Demetrio, B. Meñe, Fernández, D., Cirilo, R., Fero Meñe, M., Jose Manuel, E. E., Gail W, H. and Mary Katherine, G. (2017). Conservation strategies for understanding and combating the primate bushmeat trade on Bioko Island, Equatorial Guinea. American Journal of primatology. 79 (11)


Discover the story behind the research through the scientist’s eyes, subscribe to Biosphere digital magazine for access to in-depth articles that bring the natural world to life.