Do you know how does a forest re-generate naturally? Do you know the role played by frugivorous (fruit-eating) birds by way of bird-seeding in forest regeneration?
Let us tell you a story about a rich cluster known for its remarkable biodiversity and contiguous tracts of lowland tropical evergreen forests from eastern Assam in northeast India, especially the Upper Brahmaputra Valley.
But that was about half a century ago. Now, for multitude of reasons, including agriculture, conversion to tea plantations and extraction of natural resources such as oil and timber, these rich forests have undergone severe fragmentation.
Scientists tell us that as much as up to 90 per cent of plants in the tropical forests could be dependent on birds and animals for the dispersal of their seeds.
For this reason, frugivorous birds are particularly referred to as the gardeners of the forests. And the relation is mutual. While the birds benefit from the nutrition they get from the fruit pulp, plants benefit, as birds carry their seeds away from the mother plant before dispersing them. In fact, for some species, the treatment that seeds receive in the guts of animals enhances their ability to germinate.
Such mutually benefiting actions — called the ecological interactions — between plants and frugivorous birds facilitate the regeneration of plants, maintain biodiversity, and ensure good health of the forests.
These are the forests that harbour a rich diversity of mammals such as the Asian elephant, Western hoolock gibbon (the only non-human ape species in India), Capped langur, Clouded leopard, Indian giant squirrel, and Malayan sun bear, to name a few.
And unfortunately, these tropical lowland forests in northeast India experience among the highest rates of forest loss and habitat fragmentation in the world.
A recent study published in the journal Biotropica has now revealed how India’s last remaining rainforest fragments harbour crucial ecological interactions and is of high conservation value.
“The scale at which forests are shrinking, it is crucial to find out how ecological interactions like the ones between plants and seed-dispersing birds are altered because of fragmentation. Our study looked at the differences in the plant and frugivorous bird communities between forest fragments and contiguous forest and found that fragments harbour diverse sets of interactions, said Abir Jain, the lead author of the study.
Titled ‘A comparison of plant-seed disperser communities between fragments and contiguous forest in northeast India’, the study was carried out by a team of scientists from the Wildlife Institute of India (WII), Dehradun, and the Nature Conservation Foundation (NCF), Bengaluru.
The researchers collected field data between January and April 2019 by observing the feeding activity of frugivorous birds inside the forests and documented the diversity of fruit species they fed upon. They picked up sampling from two forest fragments experiencing degradation pressures — Doom Dooma Reserve Forest and Kakojan Reserve Forest.
Samplings were also picked up from a less disturbed, better protected contiguous forest patch – Dehing Patkai National Park.
The observations showed that the habitat fragmentation resulted in a reduction in the diversity of interactions between plants and frugivorous birds as well as brought changes in their composition.
However, despite this, forest fragments continue to harbour distinct sets of interactions between plants and frugivorous birds. The authors documented 238 interactions between 63 fruiting plant and 44 frugivorous bird species across all sites.
The study also reported that small-sized frugivorous birds (bulbuls and barbets) fed upon the highest number of fruit species at all sites, highlighting their crucial role in seed dispersal in contiguous and fragmented forests.
And that is not all. There were related interesting findings too. Habitat degradation and associated gaps in the tree canopy in forest fragments could promote the fast growth of shrubs and climbers, it was found. Interestingly, fruits of certain climber species and fig trees were found to be important resources for frugivores, particularly in the fragments.
“Different species and individuals of figs fruit at different times of the year and serve as a vital source of food all year round including the lean season. This underscores the importance of figs and the need to conserve them,” said co-author of the study, Navendu Page, a plant ecologist from the Wildlife Institute of India (WII).
Interestingly, the near-threatened White-throated Brown Hornbills were also found to be important seed-dispersers of large-seeded plants in the fragmented forests. Besides, the authors also documented their breeding in these rainforest fragments.
“This study highlights the value of the remnant forest patches even for large-sized birds, like the Brown Hornbill, that plays an important role in the dispersal of large seeds which not many other birds can disperse. The range of Brown Hornbill is shrinking in north-east India due to habitat loss and hunting, thus conserving these remnant forest patches for the conservation of these birds is vital,” said Rohit Naniwadekar from NCF, the co-author of the paper.