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traits from a two-month field study (Sumatra), local ecological knowledge (Peninsular Malaysia), and published and unpublished accounts.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.