By Sam Vermaak
A recent review paper by Zafir et al on the status of the Sumatran rhinoceros, Dicerorhinus sumatrensis, makes for rather depressing reading. The Sumatran rhino is currently listed as Critically Endangered by the IUCN (click here for an explanation of what this means), and Zafir’s review of current research suggests that there may be just 216 adult rhino left in the wild.
The Sumatran rhino is the smallest rhino species and is rather more hairy than its African cousins. It has two horns and lives in tropical rainforest, eating leaves. Although previously found throughout South-East Asia, its population is now restricted to reserves in Peninsular Malaysia, Sumatra and Borneo and it is the most endangered rhino species on our planet. Zafir’s review shows there have been large drops in the rhino populations in many reserves over the last 20 years. In Sumatra’s Kerinci-Seblat National Park, for example, there were an estimated 28 rhinos in 1995, but by 2007 the Sumatran rhino was deemed extinct in this reserve, while in 1995 Taman Negara National Park in Malaysia was a stronghold with 44 rhino, but extensive photo-trapping in 2004 didn’t produce any rhino photographs and track encounter rates were low, suggesting this population has also dramatically decreased.
Poaching remains one of the main threats to the survival of the Sumatran rhino, with 1kg of horn selling for $45,000. A similarly large threat, however, is the effect of small population size – unfortunately, once a population is reduced it can enter a phenomenon known as the ‘Allee effect’, essentially a vicious cycle where the fact that the population is small increases its chances of getting smaller and going extinct through chance events.
For example, entirely due to chance there may be a larger number of male rhinos born one year, say a 2:1 skew instead of the normal 1:1 ratio. In a large population this skew would have little effect – if the offspring are 20,000:10,000 then there are still plenty of females around. In a small population though this could be catastrophic – if only three offspring are born that year we now have two males and only one female, and this can affect the long-term prospects of the population. More....