Some species exhibit a greater capacity to respond in this way than others, most notably furbearers. Humans acting as predators can trap a densely populated muskrat marsh and induce the muskrats to breed two or three times a year instead of only once. Similarly, though less dramatically, moose can increase their rate of twinning and calve at a younger age in response to better nutrition. Species that physiologically cannot produce twins or rarely do, such as caribou, are more restricted in their ability to offset the effects of predation in this way.
Another mechanism, compensatory recruitment, leads to better survival of young to breeding age. It also is a response to improved nutrition. Normally in wildlife populations, especially in dense ones, losses of young are high and often related to poor nutrition.
But most importantly, Errington focused on compensatory mortality, the idea that if one environmental factor does not kill members of a prey population, another will. Compensatory mortality works most commonly in a prey population near its carrying capacity for food. In this situation, if predation does not lower prey numbers, more animals will simply starve — so predation is of little consequence. This potential for interplay between mortality factors introduces considerable complexity into deciphering the importance of any single factor.
Besides these compensations is the idea that if breeding opportunities limit the size of a prey population, any non-breeders beyond that limit represent a doomed surplus. Predation on them will not reduce the number of breeders. Hole-nesting birds may be limited by the number of trees with suitable holes, making members of the population expendable beyond those using all the holes.
Finally, there is the deflating idea for males that in a polygamous species, a few males can do all the breeding. As long as enough males are present to breed with all the females, the rest are superfluous. Numbers may drop accordingly but not the continuing productivity of the prey.
When these principles are applied to wolves, it appears that as long as they are living off capital that will either be replaced annually or die anyway, the effect of wolf predation is small. Or, if wolf numbers stay low, their effect may be only to slow the growth of a prey population. Doug Pimlott estimated from his Algonquin work in the early 1960s that when a deer population exceeds seven or eight per square kilometre, a wolf population cannot hold its numbers down. At this point the prey has escaped from the “predator pit,” envisioned as a low place on a graph of its numbers. Bill Gasaway and colleagues in Alaska concluded that moose escape the effects of wolves at approximately thirty per wolf, whereas below twenty per wolf, predation may be limiting. In multipleprey systems, a predator:prey ratio means little because the effect of predation may be spread out and thus minimize its consequences on each species.
Ultimately, available energy, lost in the metabolic costs of life and siphoned off between trophic levels, sets the maximum density a predator population can reach. But below that upper threshold, there is still plenty of room for wolves to have either a big or little influence on the numbers of their prey. Doug Pimlott concluded that over evolutionary time, wolves, perhaps in consort with other predators, have limited their prey commonly enough to be the norm. David Gauthier and I concluded in a 1985 review that in slightly more than half the reported studies, wolves acted as an important limiting influence on their prey populations.
But bedevilling complexity still surrounds the interpretation of interacting environmental factors and compensatory mechanisms. Nobody has discovered any universal set of conditions that, in anything but the short run, determines the role of wolf predation. Until that is uncovered, if such a generalization exists, there will be more to learn.
A WOLF PACK is a superb hunting device, coordinated, tactical, efficient. It fans out through the forest, swarms over hillocks, braids across lakes. With senses fine-tuned, it is ready instantly to turn on speed, dispatch flankers, cut off, corner, and outmanoeuvre its prey. Wolves instinctively know the rudiments of the hunt, and through additional training they learn how to kill. Time-tested, they have survived where more than ten thousand years ago the larger, stronger dire wolf and the smaller, weaker edwardian wolf died out.
A moose is a formidable prey. Its strong legs and hooves are lethal weapons. One of few surviving large herbivores from the Pleistocene Epoch, it managed to live through environmental change and technical advances in human hunting to which other species succumbed. Past successful strategies, such as calving on ridges or on islands where visibility is good, programmed survival into its genes. Long legged, snow adapted, moose have been shaped by eons of tough winters. They have also been shaped by wolves.
Wolf and moose: they first met in Eurasia or ice-locked Beringia (now Alaska and central Yukon). Dispersing south in an interglacial period, they have lived together in various places in North America for the past eighty thousand years. Across the broad, shifting boreal forest on both continents, moose sustain wolves. Despite the vicissitudes of deep or shallow snow, sickness or health, good or poor nutrition that at times favour one over the other, they have co-existed in tandem over the millennia in a remarkable, tenuous balance, like every other predator-prey combination that persists.
In Algonquin Park, moose are one of three major species of prey. Wolves think about deer and beaver too as they search their land. A particular focus of Graham’s Ph.D. work was to understand wolf-moose relationships in this multiprey system.
In the deep snow and penetrating cold of an Algonquin winter, almost everything just seems to be hanging on. Not wolves, however; they do well. We have recorded no winter deaths by starvation. Life looks good when you see them from the air curled up in the sun on a south-facing lakeshore where snow is shallow or the ground is bare.
One March day the Cessna landed to pick up Mary, Graham, and me on the ice of Grand Lake. Graham handled the telemetry in the front seat. I, as usual, was barely keeping my stomach down in the back. Soon we were circling over the Grand Lake pack, the signals from the two collared wolves loud on the right side of the banked plane. What I thought at first were red pine stumps on the sunny edge of a pond suddenly stood up, stretched, and gazed at the plane. When we circled again they were all lying down. Likely they had fed on a moose carcass nearby.
Life may be relatively good for moose too, whose ratio of surface to body core alone favours heat retention. Living a negative-energy balance in winter, nonetheless, they know how to conserve what they need to stay alive. They know to seek out the dense conifers on cold nights to take advantage of any vestiges of trapped daytime warmth away from the heat sink of open sky. They know to head for heavy cover when the deepening snows make travel difficult.
Rumour had it, when we began our study, that the small Algonquin wolf was unable to kill moose. That incapacity was thought to be an important reason moose numbers had increased to roughly one per two square kilometres, high by eastern North American standards (although in Newfoundland they may reach three per square kilometre). Yet our analysis of hair in wolf scats showed that roughly one-third of both winter and summer diet was moose. We wanted to find out why.
Our routine for getting to moose carcasses in winter was for Graham to fly early-morning telemetry flights and Mary and I to be waiting at the Pem Air hanger when he landed. We would cross-examine him in his motion-sickness-fatigued state to find out what wolves were accessible. On bad days, Graham returned to Achray to sleep off the effects of the flight; on good days, when he was more coherent, he would join us. We drove the ploughed logging roads, or used a combination of snowmobile and snowshoes to reach whatever radio-collared wolves we could. Sometimes the wolves were on the move so when we got there we found only their tracks. Other times we located their bedding sites, suddenly abandoned. Often enough they were at a carcass.
A kill is a place of biological success and failure. Here is the most immediate moment of an ancient evolutionary drama — natural selection at work. This is the moment when genes succeed or fail. There is something solemn about such a place. We talk qui
etly and keep our visits short, staying just long enough to collect mandibles and femurs, take some measurements, and make a few notes.
The first moose provided by the Grand Lake pack died only a few kilometres from Achray under dense spruces at the side of a bog. A chickadee watched our approach from its perch on a chewed rib. A pair of gray jays flew up from inside the abdomen. The collared wolves had vanished into the trees.
The moose lay on its brisket as if it had just collapsed. The wolves had not dismembered it, rather they had tunnelled into its carcass from the abdomen, then up into the thoracic cavity where they had consumed heart, liver, lungs, and other delicacies housed there. The rumen lay to one side, its outer lining eaten off but otherwise intact, a football-sized frozen ball of half-digested browse. All the evidence added up to one thing. The wolves had not killed this moose but had found it already dead and waiting for them at the frozen-food counter.
It was the same for the first moose we found belonging to the Travers pack. Late one afternoon we snowshoed towards them, pushing through conifers that too often released their burden of snow down our necks. When we thought we were close, I gave one short howl to find their exact location. The whole pack answered from the trees just ahead. We stood still, expecting wolves to come pouring out into the open, but nothing happened — only silence as if we had imagined their howls. When we advanced again, the radio signal changed direction as the wolves suddenly left. We found the same scene: moose on its brisket, legs still attached. Again, the wolves had tunnelled in from its abdomen.
The calls of squabbling ravens help guide us to many winter carcasses. Ravens keep an eye on travelling wolves, flying loose aerial patrols over them. Food is scarce, and wolves undoubtedly are a principal means of support. If the wolves detect us and leave before we have pinpointed their carcass, we are faced with the choice of following the signal coming from one direction or the ravens raising hell from another. We have learned to choose the ravens; they typically stay put on a carcass even after the wolves have left. Only when we are in sight do they lift off in a rush of wings, a black avian shroud unveiling the carcass below. They fly up into the trees or circle overhead waiting for us to perform our post-mortem ritual and leave.
Native people have speculated in legend about the special relationship between wolves and ravens. Bernt Heinrich’s research in New England has shown that ravens can consume a prodigious amount of meat. Yet, apparently not resenting what goes down each other’s throats, wolves and ravens feed side by side, sharing the banquet. Never have we found as much as a single raven feather at a carcass to suggest hostilities. Some speculate that ravens “deliberately” call wolves in to a carcass because they need wolf teeth to tear it open. Heinrich concluded, however, that ravens call at carcasses for a complex variety of reasons that includes, for example, recruiting a mob of juvenile ravens to swamp out a defending pair of adults.
Our data gave us even more to speculate about. Do ravens rather than wolves find these frozen moose carcasses first? Frozen meat is considerably less odorous than fresh meat, making its discovery difficult for the wolves. Although these moose were typically dying under heavy conifers, maybe ravens flying just above the treetops, as they normally do, could still spot them. If the ravens find carcasses first, are their calls a dinner gong for the wolves?
Evidence of what was killing these moose lay abundantly around each carcass. Ticks — thousands of them, each engorged with moose blood to the size of a nickel. If blood loss approximates one cubic centimetre for each ten ticks, then ten thousand ticks remove a thousand cubic centimetres, or one litre, of blood. Twenty thousand remove two litres. That is about one-fifth of a moose’s blood volume, enough to make it anemic. However, the cause of death normally is attributed to hair loss rather than anemia. Each of these tick-infected moose displayed significant patches of skin — entire shoulders, rump, and flanks — where it had tried to rub the ticks off. Hair provides vital insulation; if a moose is only partially clothed when temperatures plummet to −30°C, hypothermia results.
Hypothermia is just the final blow. Depleted fat in the bone marrow of these moose showed that they had suffered for weeks before they collapsed. As they became increasingly temperature-stressed, they utilized the fat stored in their bone marrow, turning it from pasty, off-white to a crimson jelly. Bone marrow is a fat reserve of last resort. Hair and blood loss slowly drained them of energy until an especially cold night finished them off.
Winter tick is a classic parasite whose population rises with its prey population as transmission becomes easier. In that way, the probability that the tick may curb the size of the moose population increases with the density of moose. Characteristic density dependence such as this is the basic condition for population regulation of most species.
But nothing in ecosystems is ever simple. If winter snows stay late, engorged ticks fall off moose onto snow and perish instead of laying eggs to complete their life cycle. So, variability in weather superimposes its lack of pattern onto what otherwise would be a more cyclical moose-tick relationship. In the end, moose die of tick infections brought on by both their own density and snow conditions the previous spring.
Winter ticks took their share of moose in our study and may have helped prevent the moose population from becoming larger, but they were never prevalent enough to cause a moose decline. They can do that and did on Isle Royale in the winter of 1995 and 1996, according to Rolf Peterson. In 1992, Graham and I published “Importance of Scavenging on Moose in Algonquin Park, Ontario” in the journal Alces. At fully 83 per cent of the carcasses we examined, wolves were scavengers not predators — the moose had already died. In subsequent years we found slightly less tick effect, and that figure fell to 71 per cent. Despite this drop, MNR biologist Mike Wilton continued to document some heavy tick years, but for some reason, tick killing occurred in late March and throughout April rather than in February and early March. The reasons are shrouded in the tick’s still poorly understood ecology.
Wolves and ravens are not the only species to benefit from the table set down in the conifers by winter ticks. So do foxes, martens, fishers, gray jays, blue jays, and chickadees, placing these tiny invertebrates in a pivotal position in the ecological circuitry. Without winter ticks, little “invertebrate agitators” of big ecological systems, fewer birds and mammals could live in the winter forest.
Not all the moose we examined had died of ticks. Algonquin wolves are not too small to make their own kills. Moose put up a good enough fight to leave obvious signs. There were plenty of signs where a moose was killed by the Grand Lake pack at Clemow Lake. It took us three attempts to reach it. Our battered and ageing blue pickup by then had developed terminal electrical complications; not always did the engine deliver sparks to the cylinders. Worse were the snowmobiles. We tried to get to Clemow first using graduate student Lee Swanson’s ancient Alouette borrowed from her father, but the very first hill stopped us. Every time we throttled, the snowmobile ground its way down instead of forward. We pushed and pulled until exhausted, then reloaded it on the trailer and returned to Achray.
We made the second attempt directly up Grand Lake from Achray, setting off from the garage. The snowmobile died just past the dock. We pulled on the rope for a while, then manhandled the machine back. Tom Stephenson looked it over, pulled on the rope once, and it started. Anyone familiar with snowmobiles knows that the first place to check when it stalls is at the kill switch on the handlebars — which we had inadvertently knocked down.
Our third try began back at the road with the hill, but this time Tom had broken a trail part of the way with his smaller, lighter machine. Our truck died two kilometres short of the snowmobile road, so we unloaded there. The day was bright and sunny with wind sweeping up wisps of ground snow, but not enough to cover the tracks of three wolves on Tom’s trail made the night before. Down beside a creek, three more wolves joined up, confirming Graham’s aerial observation of six.
The snowmobile seemed to be
running well. After a while I glanced back expecting to see Mary standing on the sled runners, but she was gone. This was no trivial matter, because it forced me to get off and jerk the heavy machine around. I found her a couple of kilometres back, floundering along in the deep snow. On the second turnaround, the snowmobile stalled, but Tom had shown us the trick of pouring some ether-based engine starter into the carburetor, and that worked.
Five kilometres farther on, the machine died again and this time refused to be whipped alive, so we put on our snowshoes. Soon we heard Grand Lake 2’s signal from the east end of Clemow Lake. As we walked along, the number of tracks increased until the road was plastered, especially where it swung into the trees near the lake. The wolves had not heard us coming and one ambled across the road into the alders ahead. About twenty metres farther on, we came upon their moose, splayed out on the open snowy road.
Unlike tick-killed moose, this one was on its side and partly dismembered. Wolf teeth had punctured its rumen before it had frozen, and its semifluid contents had poured out onto the snow. We reconstructed what had happened. The moose had been browsing red maples and balsam fir, judging by the clipped twigs nearby and the needles stuck between his teeth, when the pack jumped him. He may have stood his ground for a while. Plenty of moose tracks pummelled the snow, but none showed him running. However, his days were over, wolves or not. He was an ancient moose who had experienced nineteen winters, making him the oldest we have found. His premolars and molars were worn flat to the gum line. When I split his femur with my hatchet, his red bone marrow flowed out. This time the wolves were merely overeager scavengers.
We scouted around looking for more evidence and flushed collared Grand 2 from the trees in a flurry of snow. While we were at the kill, the wolves had stayed within fifty metres, undoubtedly anxious to resume their feast. So we packed up our carcass kit and left.
Wolf Country Page 12