March 23, 2000
Seeley Swan Pathfinder
Seeley Lake, Montana
by Mike Thompson,
Montana Fish, Wildlife & Parks
"Game Range Ramblings" column Seeley Swan Pathfinder
by Mike Thompson
What would you blame as the cause of declining calf survival in the Blackfoot-Clearwater elk herd?
First, check the data. The graph on this page shows ratios of calves per hundred cows counted each January or February by helicopter. Throw out the oddball highs (1989 and 1993) and the harsh after-effects of the 1996-97 winter (1998), and you're left with a gradual, mostly declining trend in normal calf:cow ratios through the 1990s.
The search for causes of this decline leads us back in time. Farther back than you might think. We're going back 16 months, and two Septembers, to the rut that produced the calves we count in January.
For now, I see no reason to worry whether cows are being bred. Bull:cow ratios have been steady through the 1990s, averaging about 20 bulls per 100 cows after hunting season. This past January, we observed 23 bulls per 100 cows in our helicopter survey, which is far above any threshold for concern. All the same, it would be wise for us to capture a sample of cows next winter to double-check pregnancy rates. If all's well, we'll expect to see pregnancy rates of 90% or better among cows over 2 years old.
But, it takes two to tango. Maybe more Game Range cows are too young, or too old, to maintain good calf:cow ratios. Actually, a higher proportion of our cow herd was of prime breeding age (2-8 years) from 1995-1998 than from 1988-1992, according to ages of harvested elk collected at the Game Range check station. So, the age structure of our cow herd doesn't appear to be a problem.
This brings us to the long, Montana winter, a critical time in the life of a developing elk fetus. Pregnant cows need to enter the winter fat from a summer of consuming lush forage, and they need high quality winter habitat without frequent bouts of wind-driven cold and snow. If habitat and weather conditions are unfavorable, more calves tend to be born light and late, which results in poor survival. We saw this very thing in the January 1998 survey (check the graph for yourself), following the severe winter of 1996-97.
I find no evidence for problems with habitat or unfavorable weather that would account for declining calf:cow ratios in the Blackfoot-Clearwater herd. Take 1997-1999 as an example. You couldn't hope for more lush forage in the summer of 1997. Hunter-killed elk were very fat that fall, so you would also expect that pregnant cows entered winter in excellent condition. The winter of 1997-98 was mild, and forage was abundant on the winter range. The summer of 1998 was another good one for nursing newborn calves and keeping them alive. The stage was set for a high calf:cow ratio in January 1999.
We really couldn't have had better habitat and weather conditions for growing elk during that period, but conditions were almost as good for the next year's calf cropthe one we counted this past January. Once again, calf:cow ratios were down.
In desperation, I reviewed my old notes from college. One of the fundamental principles of wildlife management is that productivity declines at high population levels. Scientists don't understand all the factors involved here. Simply, we can imagine 50 elk grazing in a large, fenced lot. If we add 50 more elk and don't add more food, there will be less food for all and lower rates of reproduction. I think it's a little more complicated than that when you're talking about a wild elk population ranging over half a million acres, but you get the idea.
On the surface, concerns about too many elk don't seem to explain the declining calf:cow ratios we see. Elk numbers on the Game Range have gradually declined over the same period, from 1,135 in 1989 to lows of 762 in 1995 and 758 in 1998. Although elk numbers in the Game Range herd are lower today than they were in the late 1980s, there still may be too many elk to trigger a positive response in calf:cow ratios.
And, the main reason I'm not willing to recommend any further decline in the Game Range elk populationeven on an experimental basisis predation. It is important to maintain strength in numbers for prey populations of elk and deer that seemingly face increased pressure from predators today, compared with the 1980s. Although I can't rule out other factors, I suspect that increased predation rates on elk calves by mountain lions, grizzly and/or black bears, wolves and possibly coyotes account for much of the change we see in calf:cow ratios.
But, haven't predators been part of the system all along? Yes, they have. Today, numbers of black bears may be at or even below 1980s levels and wolf predation on our elk herd has probably been sporadic and light through the period displayed on the graph. But, in the absence of indisputable data, there is reason to believe that lion numbers have been higher in the mid and late 1990s than they were in the 1980s. And, it seems we see more grizzly bears between the Game Range and the Wilderness than we did only a few years ago. When you add them all up, and sprinkle in an opportunistic coyote or two, a case can be made for increasing predation pressure that coincides with declining calf:cow ratios.
Fortunately, I've run out of space before I have to present a solution. Truthfully, there is no easy answer. First, we need to learn more about the timing of calf mortality. Is it all happening in the first few weeks after birth (which might suggest predation by bears), or are calves lost throughout summer and fall (which sounds more like lions)? How many more calves do we lose later in winter, after our January surveys? In the meantime, we need management strategies in place that address the most likely concerns.
Let's leave something to talk about in a future column.