The classic image of the Porcupine Caribou Herd shows a huge congregation of animals spread across the arctic tundra. Far and away the largest caribou herd in the Yukon, the Porcupine herd numbered 129,000 animals at the last census, which sounds like a huge number of animals at first blush.

But that image -- and that number -- do not give the total picture of this group of animals. More is known about the Porcupine Caribou Herd than about any other migratory herd in North America. But when compared with other barrenground herds, the raw numbers alone can conjure up a troubling picture of this herd.

Don Russell, a biologist with the Canadian Wildlife Service, has been assembling and analyzing these numbers and other data for the last year, trying to help determine what the future might hold for this herd.

Russell has studied caribou in the North for almost three decades, which is about the length of time that there has been pressure to open up the herd's main calving grounds in the Arctic National Wildlife Refuge to energy development.

Research money has been available for work on the Porcupine Herd (PCH) because of the threat of development. Not as much is known about some of the other herds Russell looked at, but enough information was available to show that the PCH is not as overwhelming in size as some might think.

For example, the George River Herd in Labrador numbers 780,000 animals, the Qamanirjuaq Herd on the west coast of Hudson Bay included 500,000 caribou at last count, and the Western Arctic Herd in Alaska has about 430,000 animals.

"Of the migratory barrenground herds, the Porcupine Caribou Herd is one of the smallest," says Russell.

More importantly, the PCH did not increase in size as fast as the other migratory herds during the 1970s and 1980s. It grew at a rate of about 4.5 percent per year, the lowest rate of increase of all the herds.

Of the 12 herds that Russell looked at, the next slowest growing herd -- the Central Arctic Herd -- has increased at about twice that rate. Three other herds also use the Alaskan coastal plain during the calving and post-calving period, and some of them are now five times larger than they were in the 1970s.

"The Porcupine Herd's ability to increase seems lower than that of other herds. It's not a very productive herd," says Russell.

As well the PCH has been steadily declining from its 1989 peak of 178,000 animals. While other herds have also decreased in size, they started declining after the PCH.

The problem is not with the number of calves surviving their first month of life as that rate has actually gone up over time. Russell says a high adult mortality rate seems to set the PCH apart from other herds. Each year 16 percent of the cows in the PCH die, while the North American average is between seven and 13 percent.

Russell thinks that warmer spring temperatures could be contributing to the herd's decline. Analysis of weather records showed that the number of days when temperatures are above zero degrees Celsius has almost doubled since 1989 when then herd started to decline in size. This leads to harder travelling conditions for the caribou, and also forces them up onto the ridges where it is easier for wolves to kill them. (See yourYukon 183, "Spring the cruelest season?")

"So climate has played a role. I suspect it's the cause, but there is not a smoking gun," he says.

With climate change as a wild card, Russell has also taken a closer look at the Central Arctic Herd in Alaska, whose calving grounds have been affected by energy development in the Prudhoe Bay area. The lessons learned there can help predict how development in the Arctic National Wildlife Refuge could affect the Porcupine Caribou Herd.

Next week's column will cover this part of his presentation in more detail, but suffice it to say that Russell does not think that the Porcupine Caribou Herd is a particularly adaptable group of animals. There is already a lot of annual variation in factors that affect the herd, so it might not take much in the way of added pressure to put the herd into a constant state of decline.

"The bottom line is that this is a herd that is not very productive and not very resilient to change," says Russell.

More information on the Porcupine Caribou Herd can be found at www.taiga.net.