A Supplemental HCVF Assessment on the Sumatran Tiger ...

FINAL REPORT Tiger <strong>HCVF</strong> <strong>Supplemental</strong> <strong>Assessment</strong> – Serapung FMU
individuals were identified to be living outside the protected area system, thus
exposed to an immediate risk of local extermination. The largest contiguous
subpopulations were identified within the four major national parks in Sumatra,
ranging from 110 individuals in Gunung Leuser to 20 individuals in Way Kambas.
Using the VORTEX model even the Gunung Leuser population was estimated to
have a probability of extinction of 20% over 100 years, while in Way Kambas the
population of 20 tigers was estimated to have a probability of extinction over 100
years of between 49-94% even when no artificial extraction of individuals (e.g.
poaching) was assumed to occur. The remaining 22 areas were considered to
contain small, isolated and fragmented tiger subpopulations, at high risk of
extinction from the interaction of random and deterministic processes (Tanaka
2000), such as skewed sex ratio, disease, genetic drift, inbreeding depression,
reduction in prey-base, poaching and environmental catastrophes (Fahrig &
Merriam 1994, Crnocrak & Roff 1999, Karanth & Stith 1999, Franklin 2002).
In the absence of more recent Sumatra-wide assessments, this 1992 PHVA
estimation of 500 wild tigers continues to represent Indonesia’s official population
estimate for the Sumatran tiger.
2.4.2 Minimum viable population for Sumatran tigers
From the results of the 1992 PHVA for the Sumatran tiger (Tilson et al. 1994)
using VORTEX (summarised in Table 3), a population with a carrying capacity of
50 animals (ignoring the potential effects of inbreeding, poaching, and
catastrophes) has a very low probability of extinction (Pe = 0.042), but for a
carrying capacity of 25 animals there is a significant risk of extinction (Pe =
0.404). The probability of extinction in this case is purely due to random events,
such as the number and sex of cubs per litter, and fluctuations in survival rates of
the different age classes. The effect of inbreeding was also modelled, recognising
that for populations with less than 100 individuals there is a greatly increased risk
of extinction due to these genetic factors.
Further modelling showed that the size of the starting population does not appear
to have a major effect on extinction risk, primarily due to the high reproductive
rate of tigers allowing populations to rapidly expand towards carrying capacity
potential wherever opportunities arise. However the effects of even relatively
small levels of poaching (2 individuals a year), on a population with a carrying
capacity of 50 or less, significantly increased the overall probability of population
extinction (Pe).
A summary of results from this PHVA indicate that a population with a carrying
capacity of 100 individuals is resilient to stochastic variations in demography and
to the detrimental effects of inbreeding, and is less sensitive to low levels of
poaching. A population with a carrying capacity of 50 individuals results in an
acceptable probability of population persistence providing that periodic genetic
supplementation is ensured and that the population is carefully protected against
poaching. An optimal habitat for the Sumatran tiger should therefore possess a
carrying capacity of 100 tigers, while the minimum carrying capacity of an
independent tiger habitat (albeit requiring interventive genetic management and
protection against poaching) should be sufficient to support 50 tiger individuals.
Sumatran Tiger Conservation Program Pg 16/68