COMPARATIVE ECOLOGY OF DESERT BIGHORN SHEEP AND FERAL BURROS
Charles L. Douglas
USGS/BRD Cooperative Research Unit,
Department of Biological Sciences, University of Nevada
Las Vegas 89154-4004
The comparative ecology of desert bighorn sheep and feral burros demonstrate how competitive interactions and system disturbances occur when an exotic herbivore is introduced into the habitat of a native herbivore. Desert bighorn sheep evolved their adaptation strategies within desert ecosystems in the United States while feral burros evolved in deserts of northern Africa. A comparison of these adaptation strategies and their consequences comprises the message of this presentation.
Desert bighorn sheep and feral burros are medium-sized ungulates; feral burros weigh about 1.5 to 2 times as much as the largest desert bighorn ram, and 3 to 4 times as much as a bighorn ewe. Bighorn sheep and feral burros are among the largest animals in the Mojave Desert system, and both have large home ranges. During summer months both species restrict movements to remain close (1-2mi.) to permanent water sources, but range farther from water when not under water stress. Both species have life expectancies of 10-15 years in the wild, and longer in captivity.
Space use is directly related to body weight, and space requirements can be estimated based on scalar relationships to weight. Large herbivores use relatively large amounts of space in their daily and seasonal movements; this causes home ranges of larger herbivores to overlap with more individuals of their own species than is the case with small mammals. Large herbivores also tend to be more social than small animals, probably as an adaptation to these spatial realities.
The amount of food an animal requires is another important scalar relationship. Small herbivores, such as rodents, rabbits, squirrels, and hares, have high metabolic rates compared to large herbivores, and require much higher intakes of nutrients and protein. For example, 500kg of small herbivores, such as mice, would expend about 10 times the amount of energy that 500kg of large herbivores would expend to maintain a constant biomass. Thus, a given energy supply will support a much smaller biomass of mice than of burros or bighorn. As a consequence, any degradation of the available plant resources (i.e. energy supply) will impact the smallest herbivores in the system long before the larger herbivores are affected. We found twice the number and twice the biomass of rodents in a lightly used burro area compared to a heavily used burro area. Similar findings have been reported for burro use areas in the Grand Canyon by Carothers et al.
Bighorn sheep are ruminants; they have a 4-chambered stomach and are able to extract high percentages of nutrients from their forage species. The down side of being a ruminant is that forage must be processed into fine particulate size before it can be broken down by the rumen micro-organisms and passed through the system. Thus bighorn must select the most highly nutritious parts of plants because processing time is longer than for a non-ruminant. Conversely, burros have a monogastic digestive system that is not limited by particle size nor as much by processing time. Burros have a large caecum in which fibrous plant materials are broken down, but this arrangement is not as efficient as that of ruminants. Burros, however, can vary their rate of gut clearance and can process more forage by eating more. They can thereby subsist on poorer quality forage that bighorn can. Our studies demonstrated that in Death Valley burro and bighorn diets had a 65% overlap of selected forage species. Overgrazing by burros in the Panamint Mountains of Death Valley has caused changes in the vegetative composition of some plant communities, with favored browse species being reduced in size and number and some species being removed.
Both bighorn and burros have conservative reproductive strategies, and have one young per birth. Generally both species give birth in their third year; however, if forage conditions are good, yearlings may breed and reproduce at two years of age. Gestation periods are about 6 months for bighorn and 12 for burros. Breeding in bighorn is confined to autumn months with births in the spring. Burros breed mostly during summer months when they are congregated near water sources, then give birth the next summer. However, burros are polyestrous and some young may be seen throughout the year.
Lamb survival is a boom or bust phenomenon in the desert. In years having ample precipitation to generate new vegetative growth, and growth of annual plant species, ewes are able to obtain the nutrition needed to produce enough milk to wean healthy lambs. In years having poor precipitation ewes may not conceive, or may have to terminate nursing before the lambs are able to obtain adequate nutrition from forage species. If lambs must be weaned early, they have a slim chance of survival because their nutrition requirements are higher than any other animals in the population including lactating ewes. In the River Mountain herd in Lake Mead National Recreation Area, lamb survival in October-November has averaged about 40 lambs/100 ewes and has varied from 8 to 76 lambs/100 ewes over the past 20 years. Our experience with feral burros is that very few foals die before they are a year old. This agrees with the findings of other investigators in the western United States, and in Australia. This has a profound effect on potential population growth. Bighorn sheep must have about 25 lambs/100 ewes surviving to yearling status in order to maintain stable population numbers. Because burros can survive on poor quality forage and lose very few foals they may have population growth of as much as 20% per year. While bighorn populations frequently struggle to maintain their numbers, burro populations may double in size within 3.5 years.
Burro impacts to bighorn habitat include soil compaction of trails, dust bath areas, and areas around springs. Compaction of burro trails can equal the compaction of long-established dirt roads, which creates an unsuitable site for plant growth. Burros eat many of the same plant species as native herbivores, and have been shown to negatively affect the composition of plant communities by removing native bunch grasses, churning the soil surface and thereby reducing soil crusts and soil moisture and promoting growth of weeds. Small mammals decrease in numbers and diversity in areas used by high densities of burros. Because of their larger body size and digestive system, burros eat more than bighorn sheep, and can not only survive on forage with low nutrient content by adjusting their intake rate, but they can successfully nurse foals to weaning under poor forage conditions. Burros require significantly more water per unit body weight than bighorn sheep because their urine is more dilute than that of bighorn, and their feces has higher water content. Burros also congregate and remain for extended periods of time at water sources. Monitoring with time-lapse cameras and camcorders showed that desert bighorn did not approach water sources when 3 or more burros were in the immediate vicinity.
Carothers, S.W., M.E. Stitt, and R.R. Johnson. 1976. Feral asses on public lands: an analysis of biotic impact, legal considerations and management alternatives. Trans. North Am. Wildl. and Nat. Resour. Conf. 48:396-406.
Douglas, C. L. and D. M. Leslie, Jr. 1996. Feral Animals on Rangelands. Chap.17 pp.281-292 in Rangeland Wildlife, Paul R. Krausman editor, Society for Range Management, Denver, Colorado.