Primates in Peril

equire vast areas of contiguous rainforest. They are also
very restricted to lowland forests. Sumatran females
give birth to one infant every eight or nine years (Wich
et al. 2009) and the loss of as little as 1% of females each
year can place a population on an irreversible trajectory
to extinction (Marshall et al. 2009).
The largest Sumatran orangutan populations are
found in the Leuser Ecosystem, the only place in the
world where viable wild populations of the Sumatran
orangutan, Sumatran tiger, Sumatran rhinoceros and
Sumatran elephant co-exist. Approximately 78% of the
Sumatran Orangutan distribution lies within the Leuser
Ecosystem (Wich et al. 2008). Recognising its unique
biodiversity the Leuser Ecosystem is listed as one of the
“World’s Most Irreplaceable Protected Areas” (Le Saout
et al. 2013). It is a 26,000 km 2 protected area straddling
the border of Aceh with North Sumatra, established by
Presidential Decree in 1998 and since 2008 is also now a
National Strategic Area for its Environmental Function
whose protection is required under National Laws.
Within the Leuser Ecosystem lie the smaller 1,025 km 2
Singkil Swamps Wildlife Reserve and the 7,972 km 2
Gunung Leuser National Park, part of the UNESCO
Sumatran Rainforest World Heritage Site. The smaller
National Park is mostly high mountains, and as
Sumatran orangutans are rarely found above 1,500
m.a.s.l. most orangutans are outside of the National
Park, but within the Leuser Ecosystem.
The primary threat to Sumatran orangutans is habitat
destruction and fragmentation. Even within the Leuser
Ecosystem forests are still being cleared at a large scale,
primarily for conversion to oil palm plantations but also
for mining, settlement and agricultural encroachment.
Precise rates of forest loss are difficult to determine,
but primary lowland forests in Sumatra have been
devastated over the last 30 years. Wich et al. (2011)
report that 49.3% of all Sumatra’s forests were lost
between 1985 and 2007. In Aceh and North Sumatra
the figures were 22.7% and 43.4%, respectively. If only
the most important orangutan habitat is examined –
i.e. forests below 1,000 m – for the 1985-2007 period at
least 28% of Aceh’s forests were lost and 49% of North
Sumatra’s. When only the most species-rich forests
(below 500 m) are considered, forest loss between 1985
and 2007 was 36% for Aceh and 61% for North Sumatra.
For the carbon rich peat swamp forests, that harbour
the very highest densities of Sumatran orangutans, the
loss was 33% for Aceh and 78% for North Sumatra.
Numerous roads have also been cleared in orangutan
habitat in recent years, often in remote areas and
over unsuitable terrain. These roads are increasingly
fragmenting remaining orangutan populations and
opening new access for encroachment, settlement and
illegal wildlife poaching. Gaveau et al. 2009 concluded
deforestation rates could easily increase from 294 to 385
km 2 per year if all new roads scheduled for construction
at that time would be built. Forest cover present in
2006 would shrink by >25% (9226 km 2 ) and orangutan
habitat would reduce by 16% (1137 km 2 ), resulting in
the conservative loss of an estimated 1384 Sumatran
orangutans, or 25% of the global population at that
time, directly linked to road construction alone. These
losses would largely arise from extensive losses (56%) of
forest cover in lowland forests (