Lost mutualisms: seed dispersal by Sumatran rhinos, the world’s most threatened megafauna

Diverse assemblages of seed-dispersing megafauna once existed in Asian
rainforests, but are now almost solely represented by elephants. Asia’s
rhinos persist in remnant, ecologically-extinct populations and the most
threatened of these is the Sumatran rhino, Dicerorhinus sumatrensis. To
understand the seed dispersal role of Sumatran rhinos, we consolidated
information on fruit consumption, seed dispersal and fruit traits from a
two-month field study (Sumatra), local ecological knowledge (Peninsular
Malaysia), and published and unpublished accounts. We evaluated
differences between the taxa and traits of fruits dispersed by rhinos and
elephants, and identified other dispersers of megafaunal-syndrome fruits
that were rhino-dispersed. At least 79 plant species were dispersed by
rhinos: overstorey plants (trees and climbers; 78% of species) had large,
usually “mammal-coloured”, fruits and seeds, and were mainly drupes and
berries; 61% of these were megafaunal-syndrome fruits (>4 cm wide).
Understorey plants (herbs, shrubs, small trees) had small, often capsular,
fruits and seeds that are potentially dispersed following the
“foliage-is-the-fruit” hypothesis. Rhinos were the only known disperser
for 35% of the megafaunal-fruit genera. The highest dispersal overlap
shown was with elephants: fruits dispersed by rhinos tended to be capsular
and were smaller than fruits dispersed by both elephants and rhinos. Given
these findings and the different foraging and ranging behaviour of
Sumatran rhinos and elephants, we suggest these megafauna had important
differences in their seed dispersal roles. Asian rainforests have,
therefore, lost an important seed dispersal mutualist. Conservation
efforts should aim to protect and restore the ecological function of these
unique creatures.