Camera-trapping could augur for a more reliable method Of tiger census
WHEN Joseph Niepce invented photography in 1816, little did he realise that
among the millions of uses his technology would be put to, counting tigers
would be one of them! That is precisely
what Ullas Karanth has begun to do at
Nagarhole National Park near Mysore
in Karnataka. (Biological Conservation,
No 71, 1995).
In a 15 kilometre area, Karanth set
up automatic cameras at 15 sites during
nine different periods in 1991-92, for a
cumulative period of 387 camera trap-
nights. The cameras were so designed as
to take pictures when any large object
moved in front of them. After exposing
15 rolls of film, he obtained 31 usable
photographs of 10 unique tigers.
Remarkable as it may seem, Karanth has
been able to use this data to estimate
that Nagarhole has 13.3 to 14.7 tigers
per 10,000 kms. This estimate fits
impressively well with an independent
estimate of 15.1 tigers per 10,000 kms
that Nagarhole can be expected to support, based on the availability of prey in
the area.
Now how does Karanth get such an
apparently precise and accurate (accurate because of the close fit to the independent estimate of tigers that can be
supported at Nagarhole) estimate? That
is where the power of statistics comes in.
Mark-recapture or capture-recapture is
a standard technique for estimating the
density of mobile animal populations.
For example, of Karanth's 10 tigers, two
were captured (by the camera of course)
only once each, five were captured twice
each, two were captured five times each
and one tiger was captured six times.
Karanth was able to individually identify the tigers by studying the pattern of
stripes on their bodies.
Reliable methods are
available to fit a statistical distribution to the observed
and/or expected distribution
of individuals captured once,
twice, thrice, four times and
so on. Once this is done, the
number of individuals captured zero times, such that
those not captured at all can
be estimated. From the number not captured and the
number captured, the total
population can be estimated.
Karanth's effort at using
camera trap data and application of
capture- recapture models:for estimating population densities is the first such
attempt for tigers. If used widely, it
promises to yield an independent and
more reliable estimate of tiger populations in India, compared to the presently available, rather unreliable estimates,
based as they are on the traditional
method of pug-mark census.
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