Gardeners of the rainforest
Iconic inhabitants of tropical forests across Asia, hornbills play a key role in dispersing seeds of crucial trees in the region thereby keeping the jungles healthy—a significant ecological function that is gravely hindered by a range of man-made factors, from habitat loss to hunting for the pet trade.
Love at first sight — is not how I would describe my relationship with the hornbills of Arunachal. That happened only after two years of painstakingly watching and studying them. It was a chance encounter at the Namdapha Tiger Reserve in November 2008 that made me see these birds in a new light. My assistant was yet again showing symptoms of malaria and I was accompanying him back on the trail from Bulbulia to Deban. We were both walking quietly, engrossed in our own thought when suddenly a noise similar to that of a small aircraft taking off rooted us to the spot. We watched open-mouthed as hornbill after hornbill noisily flew out of the tree above us in every direction, flapping those great wings of theirs with a whooshing sound that I can never forget. We had, in our reverie, accidentally surprised a large flock by walking right up to the fig tree they were feeding on. Although flocks feeding on fig trees are common, I had never seen so many and from such close quarters. The beauty of these ancient, enigmatic birds as experienced through this encounter was like an epiphany – I suddenly knew what I had to do with my life.
In huge numbers
I had started my research in Namdapha after a five-month survey on hornbills in 2009, wherein I travelled nearly 10,000 km through Arunachal Pradesh and walked more than 300 km over its forested hills. I chose Namdapha because of the sheer beauty of the place, and also because of its people and wildlife. The park was relatively unexplored, and in many ways it was a frontier for wildlife research in India. Back then, there was no information available on hornbill densities from anywhere in the northeast, so my initial efforts were to that end.
Over the next four years, my fabulous team—all members of the Lisu tribe from northeast Arunachal Pradesh—and I, trudged more than 800 km of transects across Namdapha, counting hornbills. We saw five species and were able to estimate the densities of all but one of them (the Oriental Pied Hornbill). Ironically, in spite of the considerable research done on hornbills in Asia, these were the first density estimates of the Rufous-necked Hornbill from India, and of the Near Threatened Brown Hornbill from anywhere across its global range.
Our results corroborated what we had felt all along —in the winter months of November and December, the overall densities on Hornbill Plateau (where I had seen the large flock) were as high as 100 birds per sq km, with Wreathed Hornbill densities being as high as 68 birds per sq km. These are amongst the highest reported densities of hornbills, not only in India, but in the whole of Asia.
This initial study gave rise to the obvious question—how were the different hornbill species co-occurring in such large numbers? When the ranges of two very similar species overlap geographically, the competition either drives one of them out or they occupy different niches within the same environment. This is usually done by utilising different, mutually exclusive resources and thus reducing competition.
My advisor Aparajita Datta, and other scientists had already studied the feeding ecology of hornbills. Taking cues from their work, we began to follow the hornbills to document the fruits they were eating. Hornbill’s food is broadly classified into figs and non-fig fruits. These fruits differ in their abundance and nutritional content. As suspected, we found that the three large species – Great, Rufous-necked and Wreathed Hornbills, differed in terms of the proportion of figs and non-fig fruits in their preferred diet. While the Great Hornbills ate more figs, the Wreathed Hornbill showed a clear preference for non-fig fruits, while the Rufous-necked Hornbills fed on almost equal proportions of the two.
In addition to this, we also observed fruiting trees for hours at a stretch. We soon realised that hornbills were the most frequent visitors on large-seeded, non-fig fruiting plants in Namdapha, more than all other frugivorous birds and even more than primates like macaques and hoolock gibbons. Being the largest avian frugivores in tropical forests, hornbills thus not only eat more large-seeded fruits, they are also better at handling them. Small frugivores, like cochoas and barbets, drop a large portion of the fruits while eating them and end up depositing most of the seeds below the mother plant itself. Hornbills, on the other hand, easily swallow the whole fruit and then regurgitate the undamaged seeds after about an hour, by which time they have flown far away from the mother plant.
This has great relevance to the ecology of the fruiting plant. The pulp or the aril around the seed is the plant’s investment to induce efficient dispersal; in other words, it is the plant’s gift to the frugivore for taking the seed away. Seeds dispersed away from the mother plant generally have a better chance of survival and germination, since the lower density not only decreases the chances of fungal infection and predation by insects and rodents, but it also reduces the competition for space, light and nutrients with sibling seeds.
The more we understood how hornbills interact with their environment, the more complex our question became. Seeing that hornbills in such high densities were good dispersers of seeds, the logical step was to explore the impact that they had on their forest environment.
We found that hornbill food plants are patchily distributed, with some areas of high density which are almost like orchards. These areas understandably have more hornbills and as a consequence we found more seeds dispersed in these ‘orchards’. We estimated that hornbills potentially disperse as many as 3,000 seeds per day per sq. km. This overwhelming shower of seeds on the forest floor is referred to as seed rain, and it illustrates the role that the hornbills play for several tropical plant species in Namdapha.
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Given that they perform such an important ecological role, we became concerned about what would happen if hornbills were to disappear from tropical forests. To find an answer we had to find a comparably disturbed site. We selected one just outside Namdapha, in Miao Reserve Forest, that experiences a lot of hunting and logging pressures. Several important hornbill food plants, like Canarium strictum and Phoebe, are logged for their timber here and hornbills are hunted. On comparison, it was clear that Miao had lower profusion of hornbill food plants that are targeted by commercial logging. The density of figs and other non-fig food plants that are not targeted for logging was also similar. As a result, hornbill density was 22 times lower in Miao as compared to Namdapha. This, in turn, affected the dispersal of large seeds, which was found to be seven times lower. Fewer adult trees, along with far fewer dispersers, resulted in extremely poor recruitment of new trees in Miao, as gauged by the number of seedlings and saplings we counted. These results indicate a change in the future plant species composition of Miao, a change that is extremely difficult to reverse.
Although no further subspecies of the Wreathed Hornbill (Rhyticeros undulatus) are recognized, there is considerable geographic variation in its appearance. | Illustrations: Sartaj Ghuman
Save the hornbills, save the forests
With this understanding of the ecological role of hornbills, the results of the state-wide survey that I had initially undertaken were cause of even greater concern. The study had indicated that while hornbills, especially the Rufous-necked Hornbill, are higher inside Protected Areas, vast areas of non-protected forests continue to harbour hornbills, albeit in lower densities. Given that 81% of the forested area in the state is outside Protected Area network, it is bound to harbour a larger population of hornbills overall. This highlights the importance of conserving these regions. Further, interviews with hunters to gather historical data regarding hornbill presence indicated that there were places even within Protected Areas from where hornbills have probably disappeared over the last five years.
This is really worrying, because we know now that the hornbills and their forest ecosystems are tightly interdependent. With the loss of hornbills, not only do we lose these beautiful, intelligent creatures, whose calls resonate through the forest, but we also lose an important link that is essential for a functional and healthy ecosystem. When the hornbills go, the forests will lose these caretakers who have tended to them for over millions of years.
Over all the years that I’ve spent walking through forests in different Protected Areas across northeast India, I have never again felt the way I did on that cool November morning in Namdapha. At that time, their huge, magnificent gathering had instilled a sense of awe within me that continues to resonate through all the work that I do. But now, in retrospect, I have also come to realise that encounters like that, which I took for granted during my six-year stay in Namdapha, are in fact, rare marvels of a world that we’re losing far too rapidly. It is a heart-rending reality that the ecosystems we are just barely beginning to understand, are already under such tremendous pressure and may be lost forever, unless we act now.
Simply put, seed dispersal is the journey undertaken by a seed from the plant on which it is produced to the place where it germinates and grows into a plant. Plants, being largely immobile themselves, employ various agents to get this work done for them. This includes abiotic agent like wind and water, as well as biotic agents such as insects, birds, mammals, etc. Dispersing seeds over a greater distance has several advantages for a plant species. Seeds which travel greater distances have a higher survival rate and also end up not competing with the parent plant for resources. Seed dispersal also helps plants in colonising new areas, thus ensuring their survival in the long run. Plants which make use of wind or water to disperse their seeds, invest a lot of energy in producing large number of seeds, as a lot of seeds end up landing in unfavourable conditions. Plants which use animals as their delivery boys spend energy elsewhere. Some sincere plants will pay the dispersers in the form of fleshy fruits, nectar or even grooming accessories for the services employed. But not all plants are honest in this respect. Some develop adhesive mucous or a variety of hooks and spikes on their seeds, which hitchhike on an animal, or even humans, and travel long distances. Dispersal of seeds plays an important role in ecology and evolution of plants. In case of flowering plants, dispersal of seeds is not only important for the survival of the species, but it also plays a role in enabling migration of entire forests!
Cover Photo: Great Hornbill (Buceros bicornis) is a predominantly frugivorous bird, but will opportunistically feed on small mammals, bird, reptiles and amphibians as well.
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