On the Indonesian islands of Sumatra and Borneo, fewer than 80 Sumatran rhinos are all that remains of this critically endangered species. Huge conservation efforts strive to prevent these charismatic mammals from being lost forever. But with the species on the brink of extinction, new research suggests its absence from much of its former range has left a troubling void in its role as seed-dispersing eco-engineers.

Drawing on empirical research, published literature and Indigenous knowledge, a study in Biotropica reports that Sumatran rhinos (Dicerorhinus sumatrensis) once dispersed the seeds of 79 plant species. Some of these plants are still propagated by Sumatran elephants (Elephas maximus sumatranus), the only other megafauna disperser remaining in the region. Evidence suggests, however, that neither elephants nor smaller animals can completely replace the Sumatran rhino as a seed disperser – and the consequences of this loss are playing out in real time.

Absent: Southeast Asian megafauna

The decline of the Sumatran rhino is relatively recent, according to the study’s lead author, Kim McConkey, from the School of Environmental Sciences at the University of Nottingham in Malaysia. A hundred years ago, these massive animals roamed much of Southeast Asia in temperate and tropical forests and grasslands. Since then, however, habitat loss and relentless poaching pressure have devastated populations of this slow-breeding mammal.

For Rudi Putra, an ecologist from the Leuser ecosystem in North Sumatra, the decline in rhino numbers has visibly impacted the forest during his 22 years of working with Sumatran rhinos. “In places where there are no rhinos, you see very dense forests, difficult to cross and dominated by… thorns. I think the typical rhino as a navigator really helps to make the forest more healthy, by influencing the species composition on the forest floor.

In tropical forests, seed dispersal, the process by which the offspring of a plant move away from its parent, is often facilitated by animals. The rainforests of the Southeast Asian archipelago are no exception. Plants in this region evolved alongside Sumatran rhinos and other megafauna; their large, fleshy fruits attract animals, which in turn spread the seeds to new places through their droppings.

Megafauna can swallow large fruits whole, carrying seeds in their guts for miles before depositing them intact in a pile of nutrient-rich manure. When disperser extinction disrupts this plant-animal partnership, a tree’s offspring can become stranded and must compete with their parents and siblings to survive.

“I suspect…that the rhinos are much more important to the plants they disperse than the fruits of those plants are to the rhinos,” says study co-author Ahimsa Campos-Arceiz, a researcher at Southeast Asia Biodiversity Research from the Chinese Academy of Sciences. Institute.

The fruits that rhinos ate

Because opportunities to study Sumatran rhinos in the wild are rare, McConkey and his colleagues turned to local indigenous ecological knowledge to get a clearer picture of rhino diets and seed dispersal potential.

“The [Indigenous] the knowledge of the forest is just amazing,” says McConkey. And, like the Sumatran rhinoceros, it risks being lost forever.

Indigenous peoples of the Belum Rainforest in Peninsular Malaysia lived alongside the Sumatran rhino until it disappeared from the area in the early 2000s. To gather their insights, the study co-author , Lisa Ong, also from the University of Nottingham in Malaysia, designed a survey that showed participants forest fruits and asked which animals ate them. This survey, combined with conventional approaches – a field survey and a review of published literature – gave the research team a list of 79 species presumed to be dispersed by rhinos.

Then they wanted to know if the role of the rhinoceros in the dispersal of these species is irreplaceable, or if elephants and small animals replace the extinct rhinos. Almost everything we know today about the dispersal of megafauna in Southeast Asia focuses on the Sumatran elephant – a species that is currently doing better than rhinos but faces similar threats to its health. long-term survival.

Of the 79 species of rhinos dispersed, elephants only disperse two-thirds. “For me, that’s the take-home message,” McConkey told Mongabay. “[Rhinos] do something different from the elephant, so we lost a role even though there might still be elephants there.

Elephants and rhinos also have distinct behavior. Elephants travel long distances, sticking to known and well-used routes. In contrast, rhinos have much smaller home ranges, but move more randomly over steep slopes and rough terrain.

Small animals can also move large seeds. However, McConkey says they “are not good for the fruit.” At the height of the fruiting season, small rodents and birds are “completely inundated with fruit and they just can’t handle it. So it’s clear that this fruit has evolved into something much bigger.

An uncertain future

The future of Sumatran rhinos is uncertain. Either way, evidence collected by McConkey and his colleagues indicates that Sumatran rhinos are unwitting and irreplaceable ecological engineers who shape forests simply by existing.

“We are still in the early stages of understanding the effect [rhino loss] would have on the forest and its diversity,” says McConkey.

Banner image: A young Sumatran rhino at the Sumatran Rhino Sanctuary in Way Kambas National Park, by Rhett A. Butler/Mongabay.

Quotes:

• McConkey, KR, Aldy, F., Ong, L., Sutisna, DJ and Campos-Arceiz, A. (2022). Lost mutualisms: Seed dispersal by Sumatran rhinos, the world’s most endangered megafauna. Biotropica. do I:10.1111/btp.13056
• Ong, L., Campos-Arceiz, A., Loke, VP, Pura, PB, Tunil, CM, Din, HS, … McConkey, KR (2021). Building ecological networks with local ecological knowledge in hyper-diverse and logistically challenging ecosystems. Methods in ecology and evolution, 12(10), 2042-2053. do I:10.1111/2041-210x.13685

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