An Alternative Explanation for Leopold’s Kaibab Deer Herd Irruption of the 1920’s
The Kaibab Plateau: “Mountain lying down”/Buckskin Mountain • The Kaibab is a natural laboratory with specific geographic boundaries • It is a block plateau 45 miles wide x 60 miles north to south • To the South is GCNP • To the West is Kanab Canyon • To the East is desert and cliffs • To the North is open sagebrush flats to the canyons of Utah
The story begins: • Paiute Indians lived here for countless generations depending on deer for hides used as trade goods. • It can be postulated that some sort of dynamic equilibrium existed between the various animals and plants and the Native peoples who moved through them as hunter- gatherers. • Unprecedented change came to the Kaibab with settlers who began to run herds of cattle and sheep beginning in the late 1880’s. • No controls meant overgrazing and extensive damage and change to vegetation.
A Reserve is Established • Pres. Benjamin Harrison established Grand Canyon Forest Reserve, North in 1893. • Theodore Roosevelt set aside these lands for protection of game animals in 1906. Paiute hunting was ended. • In 1908, the area was renamed the Kaibab National Forest • In 1919, Grand Canyon National Park was created from 320,000 acres of the KNF.
Livestock and Predators • Along with cattle came the perceived need to control predators. • Predators present: Coyotes, Bobcats, Wolves, Mountain Lions • Predator hunting was done by stockmen until a concerted government program to eliminate all predators began in 1913. • Estimated predator kills 1907- 1923: Coyotes 3,000 Bobcats 120 Wolves 11 Mountain Lions 674 (Russo 1964)
Livestock Records • Earliest recorded use dates to 1885: 2,000 cattle. • 1887-1889: 20,000 cattle 200,000 sheep “in the surrounding desert country and on the Kaibab Mountain” (Mann and Locke 1931) • Records 1889-1906 not available. • 1906: 9,000 cattle 20,000 sheep • 1916: 15,000 cattle 5,000 sheep • Controls on livestock before 1934 were token gestures.
The Deer Problem Emerges • Deer may never have been numerous. Rasmussen (1941) estimates 4,000 in 1906. • For unknown reasons, deer began to multiply in the early 1900’s. • Range deterioration was noticed in 1918. • By 1924, estimates of deer numbers ranged from 50,000 - 100,000. • Mass starvation from 1925-1930 was accompanied by severe range deterioration.
The Kaibab Deer Population 1860 - 1940
Leopold’s Early Thoughts on Deer Population Dynamics • He was in the vanguard of a new scientific awareness of interactions in natural systems. • Deer populations were stable over time and could be easily managed by the manipulation of any limiting factor: food, water, cover, predation. Deer were influenced by their environment, but were not an influence upon it. • Predators could only harm deer. • Hunter demand was so high that there could never be too many deer. Flader 1974
Leopold Changes his Thinking • In 1936 on a visit to the Chihuahua sierra in Mexico, he observed for the first time deer and predator coexisting in an environment without human manipulation (Flader 1974) . • Wolves, mountain lions and deer were existing in equilibrium. • A pivotal experience. • Deer are not isolated items of management but are part of an interdependent biotic community. • The community has balance or natural health.
Leopold Changes his Thinking, cont. • Deer were beginning to overpopulate many areas of the country at this time (Leopold et al. 1947). • Predators were no longer the enemy but “precision instruments of control” (Flader 1974) and needed to be included in management plans. • This change in thinking is San Francisco Peaks passionately expressed in his essay “Thinking like a mountain” (1949).
Leopold and the Kaibab • Leopold, Aldo. 1943. Deer irruptions. Transactions of the Wisconsin Academy of Sciences, Arts and Letters 35:351-369. • The Kaibab was a finely balanced natural system • Deer were at the carrying capacity of the habitat. • Predation kept them from overpopulation and habitat destruction. • Removal of predators caused uncontrolled growth of the deer herd, an irruption. • A great story is born.
Irruption, cont. • No irruptions are recorded in North America or in Germany before predator removal. • There are only two irruptions known from Canada. • Disturbance of the system by human intervention, the removal of predators, paved the way for irruption. • By his own admission, the evidence is circumstantial. • It is an irresistible, easy to understand, intuitive explanation. • The Kaibab deer herd became a textbook example of the balance of nature.
Problems with the Paradigm • Challenges came first from an article by Graeme Caughley (1970) who characterized Leopold’s use of 100,000 deer as arbitrary. • Caughley stated that ungulate irruptions have usually been attributed to a change in food supply that allowed high rates of growth. • Leopold was an exception to all other known cases. • The Kaibab story quickly disappeared from ecology texts. • Colinvaux (1973) called his story a “fiction” and an exercise in “imaginations and artistry”.
What Do We Know? • Not much about deer, predator or livestock numbers before 1906. • Horses, dairy cattle and unlawful livestock were common. • Deer exceeded the carrying capacity of the Plateau and crashed in the late 1920’s. • The Kaibab deer herd is a story about human perceptions of the natural world and of how nature responds to human intervention, timely concepts indeed.
An Alternate Explanation for the Irruption • Livestock grazing, in particular overgrazing, caused a change in the composition and structure of the vegetation on the Kaibab that favored the deer herd and made irruption possible. • The roles of predators, fire and logging were were contributing factors but probably minimal. Top of Nankoweap Trail
Components of the Kaibab Irruption 1. Forage 2. Predator 3. Deer
1. The Forage Complex • Russo (1964): the only real change on the Plateau in the last 100 years that has any direct bearing on deer is vegetative and caused by livestock grazing. • At low density, cattle and deer do not compete, but they do at higher densities. • Deer do not compete with cattle for grass, but cattle compete with deer for forage. • Livestock do compete with deer for space
Summer Range • The Kaibab deer were most probably limited by summer range (Russo 1964, Mann 1941, Mackie et al. 1998). • Tender green forage is needed for lactation and fawn growth. • High elevation supports grassy parklands not frequented by deer until after cattle arrived. • Livestock stripped native grasses. Coleman Lake An invasion of forbs, annuals and shrubs followed. • “A weedy park in poor condition for cattle is better for deer than a grassy park in good condition for cattle” (Kimball and Watkins 1951).
Summer Range, cont. • Grazing by one animal can increase species favorable to another. • Increases can accumulate over time altering botanical composition and structure (Vallentine 2001) . • Effects are more pronounced when food preferences do not overlap. • Sinclair (1979) wrote at length of how different ungulates on the Serengeti facilitated each others food supply by differential grazing. • In general, across the West, heavy livestock pressure converted grasslands to shrub lands which favored increased deer productivity. This led to the widespread increases in deer populations of the 1920’s and 1930’s (Austin et al. 1986) . Little Springs • Did cattle facilitate deer on the Kaibab?
2. The Predator Felis concolor • The dominant carnivore. • 4,000 deer might be able to support 50 predators. • Hornocker (1970) reported a ratio of 1:114 and 14-20 kills/predator/year. • Can predation limit deer numbers in such a way as to regulate the population? • Regulation: equilibrium that depends on density dependent factors that reduce reproductive rates. • Do mountain lions have a direct numerical and functional response to increasing prey numbers?
The Predator, cont. Constraints • Territoriality: enforced boundaries and social avoidance behavior (Hornocker 1969 and Seidensticker et al. 1973). • Cougar density is probably determined by habitat quality (Logan et al. 1996). • Vegetation structure and physiography of habitat: broken country for hiding and stalking and for caching prey. • Nursery areas: security and water supply for raising kittens. • Other constraints: Prey distribution Prey defenses and behavior Prey quality
The Predator, cont. • The vegetation-topographic/predator numbers-prey vulnerability complex (Seidensticker et al. 1973). • Inter- and intraspecific relationships of both predator and prey species, combined with the influences of climate, topography and the distribution of vegetation form dynamic interactions at many spatial scales.
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