Sunlight streams through the canopy of a verdant rainforest… Photo: tauntingpanda/Flickr, CC BY 2.0.
The thundering rain lashes down. Flashes of lightning illuminate the dark, dripping jungle. Leeches snake up my trousers, leaving itchy, bloody trails. Somewhere close by is a herd of wild elephants. But here I am hemmed in by steep red cliffs, making escape impossible. This is Pakke tiger reserve in the monsoon. A time when ominous purple-black clouds throttle the first rays of dawn to dapple the mountains of Arunachal Pradesh. When rivers run deep, and the smallest stream transforms into a mighty torrent. When my only way of accessing the forests of Pakke is atop an elephant’s back as it half swims, half wades across the raging Pakke river. The river in a fit had swept away the connecting bridge just the year before.
The monsoon is doubtless the time of year when no researcher willingly ventures into the forests – especially in search of answers to weighty ecological questions. But it is the season when hornbills breed. When the males pass fruity morsels and occasional protein-laden insects and other fare, to their consorts walled into the cavities of fluted trees. It is also the time of year when the rainforest is flush with fruit. When several tree species are dotted with red, brown and orange capsules that look for all the world like apples, oranges and cheekoos. These capsules cocoon the lipid-encased large seeds that hornbills covet. And so here am I, cursing my fate as I spend 12 long, rain-soaked hours every day watching the story of hornbills and large seeds unfold in the canopy above.
It was serendipity that had brought me here. Years before the idea of doing a PhD had taken root, someone at the Wildlife Institute of India pointed out Aparajita Datta to me. I looked at her in awe – this woman studying hornbills in the magical lowland rainforests of Arunachal Pradesh. Little did I know then that years later, a hastily hatched plan of studying birds and the seeds they disperse would take me to the northeast as well, to understand how hornbill hunting affects the dispersal and recruitment of large seeds.
I wanted to study the impact of fragmentation on seed dispersal by birds. Fragmentation dismembers continuous tracts of forest into uneven patches, with many negative effects. Studies have found that fragmentation leads to cascades of extinctions and population decline. As one species dwindles, it often impacts others that depend on it — and this sometimes unravels the entire, fragile forest web.
But I was in for a rude shock. When I discussed this idea with my PhD advisor, Henry Howe, the ‘guru’ of seed dispersal, he simply said, “You have no idea what you want to work on for your PhD, do you?” Yes, it had continued to elude me. The hypothesis that each graduate student must track down, even to the ends of the earth.
The slight got me thinking, however. What if nature is more resilient than we think? What if some species are redundant, and can be replaced by others? And what functional traits would render some organisms more or less important than others?
Slowly my plans began to coalesce, and I had a kernel of an idea to work with. Seeds need to move away from the trees that bear them, just as a child leaves home to establish a new life. Rodents, insects and pathogens feast on the carpet of seeds and seedlings under a parent tree. Even siblings are forced to compete for light in the darkness of the forest. So most seeds eventually die of what ecologists call density-dependent mortality – the sheer weight of numbers. Instead, seeds that leave their parent’s side find it easier to survive and grow. Because they are then able to escape their enemies, or land in particularly favourable sites rich in nutrients, or colonise new areas.
The forest’s deals
Seeds move in many ways – borne on gusts of wind, flowing water or through the weight of gravity. But in tropical forests, animals disperse the seeds of up to 90% of tree species. The mechanical and chemical churning through their guts sometimes even enhances germination. Think of civet coffee – the prized kopi-luwak of Indonesia consisting of digested coffee beans that the Asian palm civets poop. The journey through the gut is supposed to enhance the coffee’s flavour. I haven’t tasted it yet.
Animals and tree species are often involved in a mutually beneficial relationship called mutualism. If the net benefits outweigh the costs, then trees and animals forge a pact to look out for each other. Animals derive nutrients from the fruit pulp or seed arils, and in return they carry the tree’s seeds away unharmed. As long as each member of the mutualism doesn’t cheat – by dropping or harming the seeds or failing to provide nutrition – the deal endures. Seeds then depend on birds, mammals and even insects to ferry them to safety. Which is why an empty forest without fruit-ingesting creatures, or frugivores, is a tomb. A silent testimony to a once-living forest.
My first sight of a great pied hornbill in Pakke, glistening in the monsoonal sun, takes my breath away. Wind roaring through arcing jet-black and white wings shot with gold, it glides above the blue-green canopy. Orange-yellow casque held proudly aloft, its black-striped, white tail fans out behind. In that instant, in a flight of fantasy, I realise this is no ordinary bird but a winged creature of magic. The gods have wrought it to nurture and rejuvenate forests. A feeling reinforced, as I watch a hornbill carefully prise out a glistening black covered seed from a saffron capsule before delicately quaffing it. Or tilt its head to peer at me with binocular vision before winging away, mocking my feeble attempts at camouflage.
Pakke has so many of these wonders, including the great, the wreathed and the Oriental pied hornbills. And even rufous-necked hornbills, erubescent birds of stealth, that pluck fruit with their corrugated bills in the higher reaches of the jungle. But the local tribes use the white and black tail streamers of the great pied hornbill and the casques to decorate their headgear. They also use the meat of all species for food and medicine. And so, these prized birds are hunted relentlessly, declining precipitously where they remain unprotected. But traditional wise-use practices born of myth, folklore and reverence for nature do shield them. Take for instance the bans on hunting during the breeding season. The true value of all these traditional taboos in protecting the ecological function of forests is only now becoming apparent to me.
Bird bills and seed sizes
Hornbills with their large gapes can swallow larger seeds than most other birds. They regurgitate them intact, ready to sprout deep in the forest. Tree species that bear these large seeds characterise old growth, mature forests. Their dispersal and regeneration to the overstory is critical to maintaining a vibrant, functioning rainforest. Perhaps the only other birds in Pakke that can swallow large seeds are the Ducula, or imperial pigeons.
Pakke has two species of these birds – the mountain and the green; the former is far more common. Ducula‘s green pigeon cousins destroy seeds, grinding them down in their gizzards. But the Ducula fly long distances, regurgitating and pooping intact seeds en route, in much the same way as hornbills. But when I set out to study trees, seeds and their assemblages of dispersers in 2005, few seemed to think Ducula pigeons could be important dispersers of large seeds. Such is the charisma of hornbills that pigeons – even imperial ones – pale in comparison.
Some tree species in the forest depend solely on hornbills to transport their seeds. This is known as obligate mutualism. These include tree species of the Meliaceae family, such as Chisocheton cumingianus, Dysoxylum binectariferum and Aglaia spectabilis, with large seeds encased in colourful red, orange and brown globules. These trees have long fruiting seasons that coincide with the hornbills’ breeding period. Primates or other mammals don’t disperse their fruits. They produce small fruit crops. Only a few capsules ripen to reveal the nutrient laden, aril clad seeds, at any one time. All this suggests these trees depend on only one to few reliable dispersers. Without these dispersers, the seeds will likely fall below the tree in large numbers. They then fail to disperse and sprout, perhaps even disappearing from the forest.
Many small, fruit-eating birds in the forest have narrower gapes like bulbuls, fairy bluebirds and barbets. They can easily swallow small- to medium-sized seeds. Tree species with small or medium seeds might not mind then if the hornbills vanish. Other birds could potentially compensate for hornbill declines, increasing in number or eating more fruits. Tree species with these small seeds might rely only opportunistically on hornbills to move them. When hornbills disappear, they shift their allegiances to other birds.
My final hypothesis was deceptively simple. Where hornbills are declining or are being hunted out, the large seeds of capsular species like Chisocheton and Dysoxylum will fail to disperse. Large clumps of their undispersed seeds, seedlings and juveniles will accumulate below the parents in hunted sites. But almost none of their seeds will escape to regenerate away from parent trees in hunted sites. In contrast, the abundant fruit crops of small-seeded trees will attract whatever frugivores are present in the forest. This will also hold true for Polyalthia, a tree with medium sized jamun-like drupes. Consequently, their seedlings and juveniles will likely follow similar recruitment patterns in both hunted and protected sites.
But nature often defies simplistic interpretation. And while doing a PhD, expect the unexpected – especially whatever can turn your hypothesis on its head. Some of my results fit my hypothesis exceedingly well. Overall, disturbed forests had 77% fewer seedling and 69% fewer juvenile recruitment. There was clear evidence that few seeds of the large-seeded Chisocheton were being dispersed in hunted sites, where hornbill numbers are dropping. The medium-seeded Polyalthia, as expected, had many disperser friends and was not limited by the absence of hornbills. But the large-seeded Dysoxylum didn’t appear to miss the hornbills, defying our general expectations for large seeds.
What was happening in the rainforests of Pakke and the surrounding areas? Why did one capsular species need its hornbill dispersers but not another? Further investigation suggested that the mountain imperial pigeon, or MIP, was compensating for the loss of hornbills to Dysoxylum in hunted forests.
In disturbed sites, the MIP accounted for 92% of all visits to Dysoxylum trees and removed 70% of seeds. Moreover, MIP abundances were elevated in disturbed forests, although only marginally. Perhaps, in the absence of their hornbill competitors, MIP were now free to visit Dysoxylum trees more frequently, and/or remove more seeds? I had noticed some MIPs in protected forests, quietly preening some distance away from hornbills. Only when the hornbill left would the pigeon start feeding on the large seeds, perhaps recalling the ghosts of competitions past.
The results supported my hypothesis of resilience and redundancy, but not quite in the way I had anticipated. I thought nothing could ever quite replace the premium dispersal services hornbills offer large-seeded trees. But Ducula pigeons appeared to be doing just that, at least for Dysoxylum.
As expected, trees with medium and small seeds such as Polyalthia and Cinnamomum bejolghota relied on their popularity with other dispersal agents, faring quite well without the hornbills. In fact, three bird species of secondary forests or forest edges visited the Cinnamomum trees in hunted but not in protected forests. But as if to prove that nature couldn’t follow a ‘one-size fits all’ rule, both hornbills and Ducula pigeons were more important than smaller birds to the dispersal of small-seeded Litsea monopetala – perhaps because this fruits during the dry season, when fruit is scarce in forests.
Imperial pigeons appear to be more important dispersal agents in a forest than previously suspected. They fly long distances, handle fruits twice the size of their gape width, and spit or poop seeds out intact.
But a niggling doubt also prompted the question whether hornbills are really effective dispersers when they breed. Seeds landing in large numbers below hornbill roosts or nest trees suffer immense density-dependent mortality. Since males ferried seeds to their holed-in partners, females would eject most seeds beneath the nest tree rather than beyond. And presumably, a male also flew shorter distances in search of food during the breeding season, selecting from the abundant options available nearby. It would be a bit of a waste then from the tree’s perspective to invest so much in a fickle mutualist that dropped most seeds to their death below the nest tree. Or on a breeding male that dropped seeds closer on average to a parent tree.
This made me wonder if it is actually the proportion of non-breeding hornbills that are beneficial to a fruiting tree during the breeding season – although non-breeding hornbills are wasteful as well, dropping many seeds below the trees where they roost.
Generations of researchers have unraveled tangled webs, picking apart connected strands over time. This has helped us view more of the story with each passing year. I had leaned on those who went before me into the forests of Pakke, and I was to pick up a clue from a 2019 paper by Rohit Naniwadekar and others on how far Asian hornbills dispersed seeds.
Naniwadekar and his team had shown that the median or middle distance to which breeding and non-breeding birds disperse seeds is similar. However, non-breeding great hornbills do move seeds much farther from parent plants – the ‘tail’ of a seed distribution, up to 13 km! The seed tail of breeding hornbills reaches a maximum of only 2.5 km.
Long-distance dispersal is an important indicator of how effective a disperser is. Non-breeding hornbills may be particularly important in carrying seeds across long distances. Large-seeded species would do well then to invest in hornbills and Ducula pigeons, which probably carry enough seeds to count away from the shadow of death near a parent, roost or nest tree.
But how does the ancient wisdom of tribal communities (when still followed) protect rainforests? By not hunting hornbills during the breeding season, they protect the next generation of birds, and ensure the survival of the species. They also contribute to the continued existence of a forest, with all its tangled interconnections. Some large-seeded tree species that fruit during the hornbill’s breeding season and others too are able to escape their enemies. They move to distant areas, seeding new areas of forest and starting life anew. The tribal practices safeguard the birds, the trees that depend on them and myriad other interactions we know nothing about.
Much remains unknown in the rainforest. Do the seeds that travel long distances sprout and grow to adulthood? Why do imperial pigeons prefer the black-clad seeds of Dysoxylum but reject those of Chisocheton? What are the other tribal stories or folklore that inspire conservation, and do they protect other ecological interactions? These answers and more, I’m sure, will emerge from the stories of the next generations of ecological detectives.
My PhD felt as much like art as it did science, so I asked the artist Meena Subramaniam to bring it alive for me. In her painting, entitled the ‘Fruit Crowd’, hornbills hang out near large-seeded brown Aglaia and orange Dysoxylum capsules, while imperial pigeons wait patiently below, awaiting the moment when they too can feast on the large seeds. Other dispersers of smaller fruits in the forest, like fairy bluebirds, abound.
Pia Sethi has a PhD in ecology from the University of Illinois. She now freelances and consults.