Thank you gorillas!

The work of Barbara Haurez, a researcher at the Forestry Laboratory of Tropical and Subtropical Regions of Gembloux Agro-Bio Tech (University of Liege), shows that western lowland gorilla populations, which might be resilient to the practice of selective deforestation common in Central Africa, play a vital role in seed dispersal both from a quantitative and qualitative point of view. By dropping the majority of ingested seeds in its nest site, the gorilla contributes to the regeneration of certain plant species: liana, herbaceous plants, trees etc. This can only be beneficial for the forest and all the different species that dwell in it (including humans!).

The mountain gorilla (Gorilla beringei beringei) is probably one of the most emblematic animals in the Congo Basin, particularly in Virunga National Park in the Democratic Republic of Congo. Less known to the wider public but nonetheless spectacular, the western lowland gorilla (Gorilla gorilla gorilla) frequents the more western part of Central Africa. Even though its name does not suggest it, the western lowland gorilla likes the forest habitat, particularly dense humid forest typical of Gabon, Cameroon, the Central African Republic and Equatorial Guinea etc. Despite its relatively low population (estimates vary between 100,000 and 200,000 individuals), this species has been classed as being “in critical danger of extinction” by the International Union for the Conservation of Nature (UICN). In barely twenty years, this species with low fertility levels and delayed sexual maturity has seen its numbers fall by nearly 50%. The different factors threatening the survival of this species (deforestation, poaching…) have led the UICN to predict that the population of this species will fall by 80% over the next forty years, a very worrying statistic.

Like its mountain cousin, the lowlands gorilla lives in groups dominated by an adult male (or “silverback”). However, unlike the mountain gorilla, it is essentially frugivorous. It is also diurnal and generally covers up to two or three kilometres per day in search of fruit, seeds, leaves, tree bark and occasionally insects. Being able to observe the animal in its natural environment which is dense and impenetrable is a daunting prospect. The animal is also shy and will retreat at the slightest hint of an unusual odour or suspicious noise. Every evening, the gorillas chose a nesting site, under a somewhat open canopy, where they build nests (mostly on the ground) made of grass stems, lianas or tree branches. There they spend the night, the females share their nest with unweaned youngsters.

Does the combination of the gorilla’s diet and daily movements play a role in the dynamics of the dense tropical rainforest? This question is central to the work conducted by Barbara Haurez who was recently awarded a doctorate in Agronomic Sciences and Biological Engineering at Gembloux Agro-Bio Tech (ULg). “For twenty years, research teams have been interested in the role played by various animals in seed dispersal (elephants, monkeys etc.) by means of their excrement. In particular, they have demonstrated the phenomenon of “directed dispersal: certain “dispersing” animals drop the seeds in places that are particularly favourable for the germination of seeds and the growth of seedlings. This is the case with the gorilla which, by dropping most of the seeds in the nest site, contributes to the regeneration of certain plant species: lianas, herbaceous plants and trees”, she explains.

Tracking the gorillas’ nests

It is still necessary to distinguish the beneficial species according to the region in question. For example, the question arises as to whether this phenomenon of seed dispersal could benefit certain species of trees or plants that are intended for human use in the form of food, pharmacopeia products or the timber industry. In the context of the exploitation of central African forests for timber production, it is also interesting to examine the impact of timber harvesting on gorilla populations and, conversely, the impact of the gorillas themselves on forest regeneration following the activities of lumberjacks and loggers.

For four years, during missions that were spread over several months in the forest, the young researcher set about finding gorilla nests, harvesting their faeces (taking advantage of the abundant amount of fecal matter in the nest sites), in order to examine the seeds contained in the faeces and identifying and testing their germination conditions in various types of substrates and environments. This type of work, which is both laborious and somewhat adventurous, is facilitated by the partnership approach developed by the Forestry Laboratory of Tropical and Subtropical Regions of Gembloux, which is today an integral part of the Forestry Management Axis of Gembloux Agro-Bio Tech: in exchange for specialised scientific support (particularly for management plans designed in the context of certification frameworks like the “Forest Stewardship Council”, FSC), the researchers and doctoral students from the university benefit from logistical facilities within vast field study areas which are relatively well-preserved. In the present case, Barbara Haurez was able to complete her study in a forestry concession of some 600,000 hectares, managed by the Swiss company Precious Woods.  

“I became familiar with the tropical forest in Cameroon while doing a course to prepare my Master thesis which was on the subject of seed dispersal by the lowland gorillas of Gabon. In Cameroon, I had accompanied research teams in monitoring gorilla trails between nesting sites and had participated in the creation of a tree nursery. In Gabon, the main difficulty I faced at the outset was the same as the one I experienced in Cameroon: finding a sufficient quantity of excrement. It was like looking for a needle in a haystack! I found it helpful to use the traditional method of linear transects”. These grid systems (arranged in straight lines) covering vast territories are put in place by forestry prospectors in order to establish an inventory of tree species used in the timber industry. These systems enabled the young researcher to methodically cover an area of nearly 13,500 hectares, within which she counted slightly fewer than 150 nesting sites built by the gorillas. “I was able to calculate the density of nests using a computer software programme and to convert this into the density of animals using conversion factors”.

Washing dung in clear water

Once the dung were collected (making up about one kilo per unit and carried on the back…) they were washed in river water using sieves that are normally used for cooking. The purpose of this was to gather the seeds and begin the work of identifying them, mostly visually, with the help of a botanist. “During the twenty months spent gathering the seeds, I counted 59 plant species dispersed by the gorillas. One third of these species were of economic interest because of their use in the timber industry or non-timber forest products (Omvong - Dialium pachyphyllum; Ossabel - Dacroydes normandii; Adjouba - Dacroydes klaineana; etc.). The dung contained several dozen or even several hundred intact seeds (up to 500). But on average, they contained two different species. The most common were the Ebo (Santiria trimera), which is a small tree (ten to twenty metres in height) varying in colour from grey to a shade of yellow and whose fruit, is also eaten by humans, the Longhi (Chrysophyllum lacourtianum) and the Omvong (Dialium pachyphyllum), a larger species (up to 30 metres) whose larger individuals are generally indicative of older forests. The fruit of the Longhi and the pulp surrounding the seed of the Omvong are enjoyed by gorillas and humans alike. I also found a large quantity of Aframomum sp. or wild ginger. The fruit of this herbaceous plant is consists in a softish red shell containing seeds in a slightly sweet and spongy tissue”.  The dispersion of these four types of seeds by the gorillas is good for the local communities because, apart from crops such as the banana or the cassava, the villagers stock up on forest fruits both for food and traditional medicine. The gorillas clearly participate in the regeneration of species that supply fruit, roots and wood to human populations.

The second step involved studying the impact of the passage of the seeds through the digestive tract. For methodological reasons, two species were chosen:  the Ebo and the red Longhi (whose wood is occasionally used for industrial timber). “For these two species, I compared the rate of germination of four categories of seeds: those harvested in fruits and left as they were, those harvested in fruits but with the pulp removed, those harvested in excrement and cleaned, and finally, those harvested in excrement but covered in fecal matter. All of these were grown in the nursery. It became evident that the seeds that passed through the digestive system of the animal systematically showed an improved rate of germination. I also observed that the fecal matter contributed to the development of the seedling”. Barbara Haurez considers that this positive conclusion is probably valid for seeds other than those of the Ebo and the Longhi but this remains to be proven. The physico-chemical processes at work in the digestive tract have not yet been identified. It is clear, however, that passage through the digestive tract of the gorilla nullifies the negative impact of a fungus (still in the process of being identified) on the germination of Longhi seeds.

Open canopy, a sign of success

Should we be grateful to the gorillas? It would seem natural to think so because without the existence of these and other animals and their contribution to the effective dispersal of seeds fertilized by their fecal matter (thereby supplying minerals and humidity), these seeds would have less chance of developing into healthy and vigorous plants and therefore into plants that can be beneficial for humans as well. Another observation corroborates this assessment: the seedlings develop much better under an open canopy than in the forest where the light cannot penetrate. “The positive effect of dropping seeds in open habitats has already been mentioned, but this is the first time that the role of light has been directly demonstrated. In order to quantify this, I used hemispherical photographs of the canopy (at 360°C) taken vertically from the ground. Using softwares, I was able to transform the pictures into contrasting black and white images revealing the percentage of the canopy that was open. It emerged that the canopy around the gorilla nest sites showed a 7% to 10% level of openness. Apart from these sites, where the canopy was closed, the level of openness was only around 2% or 3%. The difference may not seem much but it is a determining factor: the seedlings develop two to ten times more rapidly in the open sites. Luminosity in tropical forests is a fundamentally important factor”.

The gorillas therefore drop the ingested seeds where these have a better chance of developing: in the light. They are effective agents of directed seed dispersal. This discovery is of fundamental importance: it seems to show that the western lowland gorilla populations, which are resilient to forest expoitation as it is often practiced in Central Africa (selectively), play a vital role in seed dispersal both from a qualitative and quantitative point of view.

The privilege of certified forests

As of now, it would certainly be a step too far to draw the conclusion that the gorillas show a preference for artificial felling light gaps and therefore contribute directly to the conditions necessary for regeneration of human-exploited forests. “Over the three years, I compiled three nest-site inventories: one of these was done just before the loggers had passed through the area, a second just after they had passed followed by a third 12 months later. I wanted to observe whether the gorillas showed a preference for areas that were open by logging activities. This would have given credence to the idea that when forest exploitation is practiced in a sustainable way, it is beneficial to the gorillas because it creates new habitats for them. This was not seen to be the case but this was probably because it was too early: the herbaceous plants had not yet had time to regenerate sufficiently. In my opinion, it is just a matter of time”.

If it is verified, this observation still has one major drawback. The work of Barbara Haurez was conducted in a certified forest concession. This means that the logging company has put in place a full range of measures for conservation of the flora and fauna, particularly in terms of the fight against poaching. In exchange for this, it is rewarded with a label (in this case from the FSC) certifying that it is engaged in sustainable activities. Therefore, in order to dissuade the villagers and forestry workers living in the camps from excessively hunting wild animals, they are supplied with sources of animal protein such as chicken or frozen fish. In addition, during the exploitation of designated sections of forest, surveillance teams regularly patrol the trails used by the logging trucks to prevent any poaching from taking place. Also, after use, (each parcel of the forest is exploited every twenty-five or thirty years), the pathways are closed to vehicles and so on.

It is only in conditions such as these (which are ideal on paper but, in reality, constantly being adapted -  Read : Forestry: between a rock and a hard place), that the survival of the gorilla and forestry exploitation can be declared perfectly compatible. Unfortunately, at the moment, only a minority of African Forest Concessions are certified...