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第二篇

Don't Count on Dung

Conservationists（自然保护主义者）may be miscalculating the numbers of the
threatened animals such as elephants,say African and American researchers.The error
occurs because of a flaw in the way they estimate animal numbers from the piles of dung
（粪）the creatures leave behind.
The mistake could lead researchers to think that there are twice as many elephants as
there really are in some regions,according to Andrew Plumptre of the Wildlife Conservation
Society(WCS)in New York.
Biologist Katy Payne of Cornell University in Ithaca,New York,agrees.“We really
need to know elephant numbers and the evidence that we have is quite indirect,"says
Payne,who electronically tracks elephants.
Counting elephants from planes is impossible in the vast rainforests of Central Africa.
So researchers often estimate elephant numbers by counting dung piles in a given area.
They also need to know the rate at which dung decays.Because it's extremely difficult to
determine these rates,however,researchers counting elephants in one region tend to rely
on standard decay rates established elsewhere.
But researchers at the WCS have found that this decay rate varies from region to region
depending on the climate and environment.Using the wrong values can lead the census
astray（离开正道）,says Plumptre.
He and his colleague Anthony Chifu Nchanji studied decaying elephant dung in the
forests of Cameroon.They found that the dung decayed between 55 and 65 per cent more
slowly than the dung in the rainforests of neighbouring Gabon.If researchers use decay
rates from Gabon to count elephants in Cameroon,they would probably find more elephants
than are actually around.
This could mean estimates in Cameroon are at least twice as high as those derived from
decay rates calculated locally,says Plumptre."However accurate your dung density
estimate might be,the decay rate can severely affect the result.''
Plumptre also says that the dung-pile census should be carried out over a region similar
in size to an elephant's natural range.The usual technique of monitoring only small,
protected areas distorts numbers because elephants move in and out of these regions,he
says."If the elephant population increases within the protected area,you can not determine
whether it is a real increase or whether it is due to elephants moving in because they are
being poached（入侵偷猎）outside."
Plumptre says that similar problems may also affect other animal census studies that
rely on indirect evidence such as nests,tracks or burrows（地洞）.

According to Plumptre,the region over which a dung-pile census is carried out should be
A:small enough.
B:well protected.
C:carefully monitored.
D:large enough.