“OF PREDATION AND LIFE”; this was the title of Paul L. Errington‘s (a pioneer in wildlife biology at Iowa State University) book on predator-prey relationships in natural ecosystems. Errington observed that predation is a dramatic and emotional event to observe and is frequently misinterpreted by people. I remember when I studied deer and coyotes on the Charles M. Russell National Wildlife Refuge in the 1970s it was hard for me to watch coyotes bring down a white-tailed deer and start feeding on it before it was dead. I had a biologist friend who once told me that he took his young son fishing at a small pond and they observed a garter snake catch and slowly swallow a frog. His son wanted to intervene on the behalf of the frog, but the father said “No, that’s the way it is in the world of snakes and frogs.”
Errington studied muskrats and mink – a classic predator-prey relationship. He concluded that the muskrats sustained the mink, and that the mink did not control the muskrat population. Another classic predator prey relationship is the northern goshawk and ruffed grouse in northern Minnesota. The grouse population is on an 11 year cycle and goshawk abundance mirrors the grouse cycle. The lynx and snowshoe hare are also locked into a similar relationship. In each case, it appears the prey sustains the predator (ok – it’s a little more complicated than that, but that’s the gist of it).
A few days before the ABC 20/20 crew came for their visit, I plowed a few paths through the pasture with our Zetor tractor so we would not get stuck in the snow while feeding the bison. Two hours later we returned to feed the bison and there was a freshly killed and eaten blue grouse (now offically known as the dusky grouse) on the snow berm kicked up by the tractor’s back blade. We examined the kill and kill site, and concluded that this was the work of a raptor. There were no tracks in the snow leading up to the kill, but instead there was a blood spot at the initial hit, signs of struggle, and the feeding site. The grouse was skinned out as it was eaten, and all that remained was one leg and the crop. The crop was full of Douglas fir needles (actually it was the terminal inch of twigs with needles attached). We assumed it was a northern goshawk that killed the grouse since this is the only diurnal raptor that winters on our ranch, and grouse are certainly on their list of acceptable prey. The dusky grouse is kind of a pet for us because they winter around the house, spending most of their time in our Douglas fir trees. If it were not for their droppings on the hay stack, I would not think to look up and see them in the fir trees overhanging the stack. So it was very dramatic tracing out the act of predation in the snow and holding the remains of the grouse, and I had to remember my friends words “That’s the way it is in the world of snakes and frogs.” – Craig