Biologists in B.C. say a dramatic drop in the number of endangered mountain caribou has them concerned about the species' survival.

The South Selkirk herd, which ranges from the Kootenay region of southeastern B.C. and into the northern United States, is considered endangered in both countries. Over the last decade, the herd has dropped from a high of 46 animals to a low of 27. 

'It is a very big concern because those low numbers are not sustainable for the long-term," said Nelson biologist Leo DeGroot.

"This is the most southerly of the mountain caribou population, so if we lost the South Selkirk caribou population, the total range would contract significantly."

DeGroot says biologists do not know why the herd's numbers are dropping so quickly. Logging has been banned in critical caribou habitat, and snowmobiling has been restricted.

"For a few years we can get by as long as they starting increasing fairly soon. But definitely over the long-term it is perilously low," said DeGroot.

Biologists will continue their research and do another count of the herd next month.

With files from CBC's Bob Keating