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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.

 Fig 1 from Zafir et al, showing Sumatran rhino distribution.
1 Bukit Barisan Selatan National Park

2 Way Kambas National Park
3 Danum Valley Conservation Area
4 Tabin Wildlife Reserve
5 Royal Belum State Park
6 Taman Negara National Park
7 Endau Rompin National Park
8 Gunung Leuser National Park
9 Kerinci Seblat National Park.

 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. In the case of the Sumatran rhino, these ‘stochastic’ (random) events would also be exacerbated by their long gestation and dependency period of calves – rhino usually only have one calf every three years. Another example is that as a population decreases, it gets more difficult to find a mate and to reproduce. In addition, inbreeding and lack of genetic diversity can also have an enlarged detrimental effect in small populations. These combined factors can result in birth rates too low to replace the population, and so the population will gradually diminish to extinction.

 

The good news is that Zafir et al think the Sumatran rhino can be saved, and they outline the strategies that need to be concentrated on to do so. These include moving (‘translocating’) individuals in isolated populations (that are unlikely to be large enough to be self-sustaining) into a semi captive sanctuary in Sumatra. Here the rhinos can be monitored closely and males and females can be introduced at the best time for reproduction, hopefully maximising reproduction rates. A similar sanctuary has been established on Borneo. The second main strategy is to increase anti-poaching measures for those reserves that do have good rhino populations, as well as greater judicial enforcement when poachers are caught. We need to keep those rhino of breeding age alive to reproduce and build up the populations. These methods will entail considerable funding, although as the authors point out, no more money than was recently paid for an auctioned comic book;  the alternative is yet another species vanishing forever as we watch from the sidelines.

 

Reference

AWA Zafir et al 2011. Now or never: what will it take to save the Sumatran rhinoceros Dicerorhinus sumatrensis from extinction? Oryx 45(2): 225-233

 Further Info
International Rhino Foundation
The Rhino Resource Centre
Animal Info
Ultimate Ungulate 

Allee Effect:
The Encyclopedia of Earth
Endless Forms

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