Seed dispersal by frugivorous birds. Design by Thejaswini Saravanan
In the documentary Our Planet, Sir David Attenborough argues that most forests cannot revive by themselves after a calamity and are helped by animals to recover. He refers to the dispersal community that succour trees to propagate. For example, certain fruit-eating animals consume fruits, thereby swallowing the seeds and later defecating or regurgitating them. Seeds thus find a new home. Fruit-eating animals, therefore, enable the escape of seeds away from the mother plant. Such ‘escape’ is critical because, under the parent tree, seeds accumulate in large numbers. Therefore, seeds face a multitude of threats from seed predators like fungi, insects, rats among others. If the seeds do manage to germinate, they are vulnerable to predation by herbivores and competition with siblings.
In a community, the shape and size of the beak of fruit-eating birds play an important role in deciding which birds will effectively disperse which seeds. For instance, if the seed is too large, the fruit-eating bird species may only peck on the pulp but not disperse the seed. Therefore, birds with large beaks effectively disperse bigger seeds while those with small beaks disperse smaller seeds. These interactions between different species of birds and plants result in the formation of an intricate network.
A typical wet forest can have many species of fruit-eating birds. In a few square kilometres of lowland forests in northeast India, there are as many as 50 species of birds that feed on fruits and disperse seeds! In such forests, typically, bulbuls and barbets are abundant and are the primary dispersers of smaller seeds. However, what if these smaller birds like barbets and bulbuls are absent in a forest? How are the small-seeded fruits dispersed and how does the system adjust itself around the absence of these small birds?
The answer to this question can be potentially found if one studies seed dispersal on islands! Islands have a much smaller assemblage of frugivores because they are isolated. Not many animals can cross the oceanic barrier and colonise the islands. Given the fewer number of species, islands allow us to understand the roles of certain bird groups which are present on the island. Islands, therefore, act as natural laboratories!
Map showing the location of Narcondam Island.
Narcondam Island. Photo by Wikimedia Commons
Narcondam Island in the Andaman Sea is one such unique island. An erupted volcano in the middle of an otherwise deep ocean ensured that the island was never connected to the mainland mass. Given its isolation, important fruit-eating birds like bulbuls and barbets have not managed to establish their populations on the island, offering a perfect opportunity to examine the role of fruit-eating birds like hornbills and Imperial-Pigeons. Dr Rohit Naniwadekar and his team thus decided to understand how a plant frugivore community thrives in the absence of bulbuls and barbets on the island of Narcondam. The team set out on their journey in December 2019 for two months on this remote island with this aim.
Researchers estimated the abundance of different fruit-eating birds on the island and recorded the different fruits eaten by them. They checked how frequently a bird species feed on a particular plant and also measured the sizes of fruits and their seeds. The size of the seed can determine whether the frugivore can swallow its seed or not. Small birds are unable to swallow large seeds and therefore may only peck at fruits and drop a lot of seeds under the parent plant, making them ineffective seed dispersers.
Researchers collecting data. Photo by Prasenjeet Yadav
The rigorous study involved the team walking on 20 different trails to estimate the population of different fruit-eating birds and their fruit diets. Data showed that a majestic frugivore, Narcondam Hornbill, only found on the Narcondam island, was a key disperser of seeds. Normally, the largest frugivores are less common than small-bodied frugivores. However, Narcondam Island is unique. The largest frugivore is the most common here! It is four times more abundant as compared to the second-highest frugivore, the Asian Koel. The density of this hornbill was as high as 151 hornbills / km2, the highest density of any hornbill recorded anywhere in the world! In addition, they estimated the density of figs to be around 2,700 fig trees per sq. km, which is also the highest recorded number worldwide. Narcondam hornbills fed the most on figs, resulting in a tight association between hornbills and figs. Apart from figs, this bird fed on 21 other fruit species, highlighting the central role it played in the community. They fed on several large-seeded plant species which none of the other frugivores was seen feeding on.
To better understand the nature of the plant-seed disperser community, the results were compared with a well-studied community on the mainland, Pakke tiger reserve (located in northeast India, 948 miles away from Narcondam). Pakke Tiger Reserve shares some frugivores and plants that are found in Narcondam. Here, bigger birds are considered important seed dispersers for large-seeded plants and smaller birds are critical mostly for small-seeded plants! However, on the island, it was seen that, in the absence of certain key frugivore groups, the larger birds like the Narcondam Hornbill fed on fruits of all sizes! Additionally, groups like the Imperial-Pigeons that were predominantly seen feeding on large-seeded plants on the mainland were seen exclusively feeding on small-seeded plants. The organisation of the plant-seed disperser community was thus very different from the mainland!
The organisation of plant-seed disperser community on a mainland vs island.
Figure: Naniwadekar et al.
Since the community on the island was organised differently, the role of certain bird groups changed on the island. Out of the six different species of avian seed-dispersers found on the island, the most abundant species after the Narcondam Hornbill were the Asian Koel, and the two Imperial Pigeons (Pied and Green), which are medium-sized. On the mainland site, these pigeons mostly ate medium and large-seeded fruits, and Asian Koels were rarely seen in tropical forests! However, on the island, perhaps due to the absence of small-sized frugivores, the pigeons were observed to feed on small-seeded fruits. It was interesting that the Asian Koel turned out to be the second most important seed disperser on the island, and was spotted feeding on as many as 11 different plant species in two months! This demonstrates how birds can shift their diets in the absence of competitors, and ensures that plants continue to receive seed dispersal services from frugivores even in the absence of key seed disperser groups!
Seed dispersers: Narcondam Hornbill, Asian Koel, Imperial Pigeon respectively.
Photo by Wikimedia Commons
It is quite evident from the study that the pristine island of Narcondam is a haven for birds. In fact, the migratory bird Daurian Starling was also reported for the first time. Minimal anthropogenic disturbance has allowed the maintenance of a high abundance of frugivores and plants. However, such studies, unfortunately, will give different results on other oceanic islands where the diversity is modified heavily due to human destructions. Islands are unique ecosystems, they give us exactly one chance to preserve them. Human intervention on the island can have drastic impacts on island biodiversity and the ecological processes that help maintain biodiversity.
Source: Naniwadekar, R., Gopal, A., Page, N., Ghuman, S., Ramachandran, V., & Joshi, (2021). Large frugivores matter more on an island: Insights from an island‐mainland comparison of plant–frugivore communities. Ecology and Evolution, 11(3), 1399-1412.
Link to the article: https://doi.org/10.1002/ece3.7151
Article written by: Anuja Vartak, edited by Devica Ranade A huge vote of thanks to Dr Rohit Naniwadekar for fact-checking the article and providing us with all the required media.