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Wolf History, Conservation, Ecology and Behavior

Wolfology Item # 1306
v83 n3 (August 2002)

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Dietary Overlap Between Wolves and Coyotes in Northwestern Montana
Wendy M. Arjo, Daniel H. Pletscher, Robert R. Ream.
Arjo et al studied effects of recolonizing wolves in the North Fork of the Flathead area of northwestern Montana on the diets of coyotes from 1994 to 1997. Wolf and coyote diets differed in frequency of occurrence of prey species during three of the four summers and winters during the study.
Distribution and abundance of carnivores in western United States has changed drastically over the last century. Although originally sympatric, gray wolves (Canis lupus) and coyotes (C. latrans) did not coexist in Montana for 50 years, as a result of campaigns to exterminate predators in the early part of the 20th century. Wolves began to recolonize the Flathead area in northwestern Montana just north of Glacier National Park in 1982. Competition may limit coyotes as wolf populations increase.
Coexistence is usually facilitated by morphological differences, such as size, between sympatric species. Differences in body size are often related to the prey consumed. Larger predators can kill and consume both large and small prey and thereby increase diversity in their diets. Wolves usually feed on large ungulate species but can kill most vertebrate species within their home range. Coyotes usually rely on small mammals. Coyotes are opportunistic and generalist predators that can feed on ungulates. Much of the ungulate remains in the coyote scats, however, may be the result of feeding on carrion.
....The competitive exclusion theory implies that coexistence of closely related species depends on resource partitioning and the degree to which shared resources are limited....[L]imited resources may cause a shift in niche use or, in extreme cases, competitive exclusion of the subordinate species....
Voigt and Berg (1987) noted that where prey is limited in the winter sympatric species might have overlapping diets, especially when ungulates are the primary food resource....We examined whether resource partitioning allowed for coexistence of coyotes with wolves in an area recently recolonized by wolves. We made three predictions that coyotes would avoid competing with wolves for food resources by using smaller prey items, that coyotes would have a greater diversity in their diet, and that coyotes within established wolf territories would scavenge on large mammals more than coyotes outside wolf territories.
Study area.-We conducted this study in northwestern Montana along the North Fork of the Flathead River drainage, from the Canadian border south to the Apgar Mountains. The North Fork River divides Glacier National Park on the east side from the Flathead National Forest, Coal Creek State Forest and from private lands on the west side of the 3,000 square kilometer study area....The Whitefish Range borders the North Fork Valley on the west and the Continental Divide borders the valley on the east.
Habitat was diverse, ranging from dry, forested sites to meadows and relatively moist valley bottoms....Snow usually remained on the ground from mid-November through mid-April, and total snowfall varied widely....
The North Fork area in northwestern Montana is one of the last ecosystems in the contiguous United States still containing a full complement of predators that were present during pre-Columbian America. Not only does the North Fork area contain the highest density of grizzly bears (Ursus arctos) in the lower 48 states but it is also the first area in the western United States where gray wolves successfully recolonized. Numbers of wolves in the area increased steadily over the years, reaching a plateau of approximately 32 wolves in 1993. Black bears (U. americanus), cougars (Puma concolor), coyotes, and wolverines (Gulo gulo) comprise the rest of the large mammalian predators in the area. The coyote population declined in the study area between 1991 and 1997. Midsized mammalian predators in the area included bobcats (Lynx rufus), lynx (L. canadensis), and fisher (Martes pennanti). Ungulate prey species in the Flathead Valley included elk (Cervus elaphus), moose (Alces alces), white-tailed deer (Odocoileus virginianus), and mule deer (0. hemionus). Smaller mammalian prey species included beaver (Castor canadensis), snowshoe hare, mountain cottontail (Sylvilagus nuttalli), Columbian ground squirrel (Spermophilus columbianus), red squirrel (Tamiasciurus hudsonicus), and various species of murids.
Food habits from carcasses.-Food habits for coyotes and wolves were determined by locating kills in the winter and by analyzing scats. We backtracked wolves and coyotes during the winters of 1994-1995, 1995-1996, and 1996-1997 (November through March) to locate kills....Each carcass was examined for cause of death, age, and species of prey killed. When tracks were not distinguishable for determining species of predator, we examined carcasses for evidence indicating methods of killing, which could be used to identify the predator involved. We determined frequency of coyote scavenging on wolf and cougar kills from tracks present at carcasses and...test[ed] whether frequency of coyote scavenging was similar across prey species (elk, deer, and moose).
Food habits from scats.-We determined food habits from 617 wolf scats and 279 coyote scats collected from 1994 to 1997. Scats were collected along roads, trails, den sites, and at rendezvous sites. We relied on presence of tracks or other predator sign in the area to distinguish wolf from coyote scats....The majority of the wolf scats collected in summer, when confusion between wolf pups and coyotes could potentially occur, were collected at wolf dens and rendezvous sites. Each scat was categorized by season....
We estimated frequency of occurrence (100 times the number of occurrences of a food item divided by the total number of occurrences of all food items) for both canid species for each year and season. Coyote scats collected inside the core home range of wolf packs were classified as inside wolf territories; others were considered outside wolf territories....
Age and size of prey.-....Prey in the smallest category included mustelids, rodents, and leporids. Average weights for newborn fawns, elk calves, and moose calves placed them in the middle category. Beavers and coyotes were also grouped in the middle category. Adult ungulates, and moose and elk calves after July, comprised the heaviest category....
Food Habits from Carcasses
We found 152 wolf kills....Deer comprised the largest proportion (74%) of wolf kills during winters. Coyotes scavenged wolf kills more frequently than they scavenged cougar kills. Wolf-killed elk and wolf-killed moose were scavenged more frequently than wolf-killed deer. Coyotes also scavenged more in the milder winters of 1994-1995 and 1995-1996 than in the deep-snow winters of 1993-1994 and 1996-1997.
We did not find many coyote kills, and those found were usually large prey (no remains were usually left when a coyote fed on small mammals). Ten deer (including one fawn) comprised the coyote-killed ungulates for 1994-1997. Two red squirrels, a snowshoe hare, two grouse, and a mallard made up the remaining kills discovered.
Food Diets from Scat Analysis
Coyote diets inside and outside wolf territories.-Summer food habits on the basis of scats of coyotes inside wolf territories were similar to those of coyotes outside wolf territories; however, in winter, food habits of coyotes inside and outside wolf territories were different. Coyotes inside wolf territories used a greater proportion of deer and lagomorphs than did coyotes outside wolf territories. Coyote scats collected outside wolf territories contained a greater proportion of elk.
Comparison of coyote and wolf diets.-- We divided food items into 8 categories to compare coyote and wolf diets for 1994-1997: deer, elk, moose, murids, squirrels, other rodents, lagomorphs, and miscellaneous food items. The most common species in the other-rodent category was beaver; however, northern pocket gophers (Thomomys talpoides) and yellow pine chipmunks (Tamias amoenus) also appeared in wolf diets. Miscellaneous food items included insects, vegetation, coyotes, avian remains, two instances of domestic cow (in the coyote diet), a marten (Martes americana, in the coyote diet), and a long-tailed weasel (Mustela frenata, in the wolf diet)....
Coyote diets from 1994 to 1997 differed from year to year during winter and summer. Coyotes depended on rodents (murids and squirrels) in the winters of 1994 and 1995 and more heavily on ungulates during the winters of 1996 and 1997. Murids (mostly Microtus) comprised the largest portion of the coyote summer diet in 1994 and 1995; ungulate use increased in 1996 and 1997. Wolf diets also differed yearly in winter and summer.
Scavenging can be a reliable method for obtaining food at a reasonably low level of energetic expenditure. Nonetheless, it can incur considerable costs such as aggressive encounters with larger predators. In some areas coyotes obtain the majority of their diet from scavenging. Coyotes scavenged both wolf and cougar kills and chose the larger ungulates, moose and elk, over deer. Boyd et al. (1994) found that wolf-killed ungulates were more completely consumed in mild winters than in severe winters, and deer were consumed more than elk. To compensate for the risk of scavenging, coyotes increase their energy benefits by scavenging the larger ungulates (elk and moose) that usually have more carrion left on them than wolf-killed deer. In addition, coyotes before wolf colonization were found to be mostly single animals or pairs. After wolf recolonization the majority of the coyotes were found as pairs or small packs. We believe that the addition of a large food source, wolf-killed elk and moose, has allowed for the increased size in coyote social groups. In addition, scavenging was greater in mild winters of 1994-1995 and 1995-1996. Coyotes are more successful in killing ungulates in deep snow than in shallow snow and may have scavenged more on wolf kills during mild winters to compensate for their decreased ability to capture large prey. Coyotes in our study area before wolf recolonization relied on lagomorphs and small mammals. Ungulates, especially elk and moose, were relatively unavailable to coyotes then, and winter mortality was not a significant factor for the ungulate population. Coyotes consumed ungulates much more frequently during our study than before wolf recolonization. This added food source is advantageous to the coyotes in that it allows for another source of prey previously unattainable. Before wolf colonization coyotes relied mainly upon lagomorphs. Although wolves relied only slightly on this prey species, the availability of lagomorphs to coyotes has decreased from historical times. The loss of lagomorphs as an important coyote prey item may now be offset by the addition of ungulates to the coyote diet.
Leopold and Krausman (1986) and Litvaitis and Harrison (1989) documented changes in predator diets caused by the presence of a competing predator. Coyote diets in our study area were different within wolf territories from those outside wolf territories, during winter. Deer were more prevalent in coyote diets within wolf territories, probably as a function of scavenging by coyotes even though we found that elk and moose, when available, were preferred by coyotes. During the severe winter of 1996-1997, ungulates became a major food source for coyotes both inside and outside wolf territories. The deep snows impeded ungulate movement and increased winter mortalities in both areas. Coyote home ranges were usually located along the edge of wolf territories; however, during winter, overlap of home ranges increased. The difference in coyote diets inside wolf territories compared with that outside wolf territories exemplifies the opportunistic foraging behavior that allows coyotes to coexist with wolves.
....Although dietary overlap between coyotes and wolves was high, diversity of diets was usually different.... High dietary overlap may contribute partially to the decline observed in coyote densities in the North Fork area because there is an increased demand on all prey species with the increase in wolf population. In addition, the ungulate populations declined during the study period. Dietary diversity, however, may help to negate this overlap.
....Coyote and wolf diets, for both summer and winter, were different every year, except in 1997 when use of ungulates increased in the coyote diets. Ungulates were the primary source of food in coyote winter diets in 1996 (deer) and 1997 (deer and elk). Occurrence of elk in coyote scats was likely influenced by the severe winter of 1997 and by increased mortality of ungulates. During summer, coyotes could rely on both small mammals and fawns, but in winter, diets of the [two] canid species became more similar because coyotes relied more on ungulates in the absence of available microtines during winters of deep snow. Bowen (1981), Hamilton (1974), and Meinzer et al. (1975) found that diversity in coyote diets usually increases in summer when smaller mammal species are more available. The addition of murids and squirrels to the coyote diet during summer may have allowed overlap in ungulate prey species and continued coexistence of the two species in our study.
Competitors may use different prey sizes to decrease competition for food resources. Meleshko (1986), in Riding Mountain National Park...found that wolves and coyotes partition prey by size. Coyotes and wolves in the North Fork area used different size of prey, with coyotes focusing on prey items [less than] 2 kg in body weight and wolves on large ungulates. White et al. (1995) hypothesized that endangered San Joaquin kit fox (V. macrotis mutica) are able to coexist with coyotes even with high dietary overlap because they partition food by size. Similar results were seen in central Kentucky where coyotes kill and consume larger prey than do red foxes (Crossett 1990).
Another method of partitioning prey resources is by differential use of prey age classes. Craig (1986) and Koehler and Hornocker (1991) found that competition among carnivores was offset by use of similar prey items in different proportions. Although deer were usually the dominant food item in wolf diets and most dominant food item (or at least the second most dominant) in the coyote diets in this study, proportional use between the two canids was almost always significantly different (deer were used in lower proportion in the coyote diet than in the wolf diet). In addition, coyotes obtained most of their ungulate diet from scavenging, a mechanism similar to that which allows extensive spatial overlap and coexistence between coyotes and wolves in Riding Mountain National Park. Differential use by partitioning prey by age can also decrease competition. Mills (1984) showed that lions (Panthera leo) and spotted hyenas in the southern Kalahari partition similar prey, the gemsbok (Oryx gazella), by age; lions kill adults and hyenas kill juveniles. Differential use of similar ungulate prey was seen in coyotes and wolves in the North Fork area. Coyotes used proportionally more juvenile elk and deer, whereas wolves used more adults.
Paquet (1992) suggests that coexistence between coyotes and wolves would be low in areas where deer are preferred because of the potential loss of scavenging by coyotes when entire carcasses are consumed. Deer were the preferred prey of wolves in the North Fork area, and deer populations declined throughout our study. We documented interference competition in the study area inside wolf core areas (at least three coyotes were killed by wolves-Arjo 1998), and in some years wolves left the coyotes little to scavenge. We believe that coyotes and wolves in the North Fork area are able to coexist and feed on similar prey items using several partitioning methods. Coyotes and wolves used different age and size classes of prey, coyotes exploited alternative prey (small mammals) during summer, and coyotes scavenged during winter. In addition, when coyotes and wolves converge on ungulate species during winter, coyotes can....avoid larger predators. Coyote populations did, however, coexist at lower levels than they did before wolf recolonization, probably because of increased competition and demand on available prey resources and interference competition. Before wolf reintroduction, coyotes in Yellowstone National Park relied heavily on winter-killed elk as a food resource....We believe that coyotes and wolves in Yellowstone National Park will also be able to coexist as a result of the ability of coyotes to exploit other prey resources such as small mammals.
Table 1:Frequency of occurrence of food items in coyote scats from 1994 to 1997 in northwestern Montana.
Table 2: Frequency of occurrence of food items in wolf scats from 1994 to 1997 in northwestern Montana.
References: 62