In the Jungle Book movie (2016), when a herd of elephants passes by, Bagheera instructs Mowgli to bow before them. “Show them respect,” he says. “The elephants created this jungle. They made all that belongs.” Bagheera was not exaggerating. A new study has highlighted this crucial role that elephants play – by dispersing the seeds of plants that might otherwise have a hard time staying around.
Organisms play specific roles in natural ecosystems. Plant-animal interactions are not always apparent; when they’re disturbed, they could precipitate a domino of misfortune through the ecosystem. Unravelling these interactions is not easy. “Naturalists need to become detectives to understand complex ecological processes, especially when a slew of very different species are involved,” Nitin Sekar, a conservation scientist working with World Wildlife Fund-India and who wasn’t involved in the study, told The Wire.
An international team of scientists from India, Malaysia and Thailand figured out how Platymitra macrocarpa, an uncommon plant in tropical rainforests, spreads its progeny. To this end, the researchers were interested in seeing just how important different animals were to the seed dispersal process. P. macrocarpa, a rainforest tree that belongs to the custard apple family, was a good representative for the study because it bears fruits three to five inches long, a size that ensures they’re sought by large animals. The scientists monitored four trees, 85 whole fruits, 66 partly-eaten fruits; scrunched up their noses over 91 piles of dung; and followed over 150 seeds to determine their fate over months – all of it inside the Mo Singto Forest Dynamics Plot, a permanent biodiversity research and monitoring area inside the Khao Yai National Park in Thailand
“Actually seeing whether seeds turn into seedlings is often one of the hardest parts of studying seed dispersal,” Sekar who has studied seed dispersal in North Bengal’s forests, said. The researchers used a combination of observational and experimental techniques.
“There are few studies reaching such level of detail,” said Pedro Jordano, a professor at the Spanish National Research Council in Sevilla who was not involved in the study. The researchers placed camera traps and supplemented it with direct observations to find out which animals were eating the fruit. They also sampled poop from sambar deer, gibbons and bears, and measured when and how these seeds were germinating into seedlings.
The study concluded that elephants were the most important cog in the P. marcocarpa seed dispersal wheel. These animals are opportunistic frugivores – they eat fruit when available – and poop the seeds out. Some of them may still germinate if they’ve survived the animal’s insides. The study found that though the elephants consumed only 3.2% of the fruits, they dispersed an impressive 37% of seedlings. To compare, the sambar deer consumed three times more fruit but dispersed only 17% of the seedlings. Beetles then ate the seeds that the sambar dropped, rendering the latter ineffective.
The study also found that gibbons, voracious frugivores themselves, played a crucial role in dispersing 21% of P. macrocarpa seeds. However, given that gibbons live in the canopy, their poop tends to scatter on the ground where it is susceptible to beetle attacks. This means that though gibbons are good dispersers, they cannot ensure the continued survival of P. macrocarpa. Only elephants can – and the secret is in their poop.
Elephant dung provides a safe haven as well as nutrients for the seeds until the wet season becomes warm enough and the conditions are right for germination. The trees also helps the elephants along: they have a long fruiting period over which they produce them at a steady click, making sure they’re stocked when the elephants pass by.
So although elephants rarely revisit the P. macrocarpa trees, they are rewarded when they do. “It made us appreciate the large scale at which elephants move and forage, and that they are potentially very important seed dispersers for some species, even if they are rare visitors to individual trees,” said Kim McConkey, of the University of Nottingham-Malaysia, Selangor, and the lead author of the study.
The study also found a lot (>40%) of uneaten P. macrocarpa fruit that rotted away under the parent tree. “The situation is suggestive of lower-than-ideal megaherbivore populations which would contribute to poor regeneration,” McConkey said.
It is possible that the forest is missing an unknown former disperser. For example, Thailand’s forests had another megaherbivore in the recent past: the rhinoceros. Could it have been a disperser? “Almost no information exists regarding the seed dispersal capacity of a forest rhinoceros, and their populations exist only as relics, having disappeared from almost all of their former ranges,” the authors wrote in their paper. “If we are to truly understand the roles of extant and extinct megafauna, we need to appreciate how these roles change as the communities and habitats changed,” McConkey said.
An other reason the P. macrocarpa fruit was wasting away could be a drop in elephant movements of late. The authors wrote that the Mo Singto Plot is near the park headquarters, whose development has blocked some elephant routes. This is a plausible explanation; the elephant population itself is considered healthy.
Yet another reason could be that some animal populations that disperse seeds might be so rare that they are essentially non-functional. “That is what we know as the ‘empty-forest syndrome’: the species might still be there, but their ecological functions are gone, the interactions are extinct,” said Jordano.
Beyond its more substantive findings, the study illustrates the fragility of plant-animal interactions. Although each species plays an important role in balancing ecosystems, some of them are first among equals. “If less-vulnerable large herbivores such as deer are unable to replicate the seed dispersal role of threatened mega-herbivores such as elephants, megafaunal fruit could suffer range contractions, affecting forest community composition and potentially forest carbon stocks,” McConkey elaborated.
Jordano calls elephants the ‘gardeners of the forest’. “Frugivores like the elephants guarantee the persistence of the park: if the interactions that McConkey and collaborators document would vanish, then the whole forest would collapse,” he said.
With so much at stake, no wonder Bagheera revered the elephants as much as he did.
The study was published in the journal PLOS ONE on July 18, 2018.
Rashmi Bhat is a wildlife researcher with an overarching interest in applied conservation biology. She is based in Bengaluru, India.