Persistence of Different‐sized Populations: An Empirical Assessment of Rapid Extinctions in Bighorn Sheep

Abstract: Theory and simulation models suggest that small populations are more susceptible to extinction than large populations, yet assessment of this idea has been hampered by lack of an empirical base. I address the problem by asking how long different-sized populations persist and present demographic and weather data spanning up to 70 years for 122 bighorn sheep (Ovis canadensis) populations in southwestern North America Analyses reveal that: (1) 100 percent of the populations with fewer than 50 individuals went extinct within 50 years; (2) populations with greater than 100 individuals persisted for up to 70 years; and (3) the rapid loss of populations was not likely to be caused by food shortages, severe weather, predation, or interspecific competition These data suggest that population size is a marker of persistence trajectories and they indicate that local extinction cannot be overcome because 50 individuals, even in the short term, are not a minimum viable population size for bighorn sheep.

[1]  E. Williams,et al.  Disease and Endangered Species: The Black‐footed Ferret as a Recent Example , 1988, Conservation Biology.

[2]  M. Hornocker,et al.  Analysis of mountain lion predation upon mule deer and elk in the Idaho Primitive area , 1967 .

[3]  M. Soulé,et al.  Viable Populations for Conservation: Introduction , 1987 .

[4]  Stuart L. Pimm,et al.  On the Risk of Extinction , 1988, The American Naturalist.

[5]  T. A. Hanley,et al.  Food resource partitioning by sympatric ungulates on Great Basin rangeland. , 1982 .

[6]  W. B. Davis,et al.  The wolves of Mount McKinley , 1944 .

[7]  S. Rawluk,et al.  Susceptibility of Rocky Mountain bighorn sheep and domestic sheep to pneumonia induced by bighorn and domestic livestock strains of Pasteurella haemolytica. , 1988, Canadian journal of veterinary research = Revue canadienne de recherche veterinaire.

[8]  J. Diamond,et al.  Subdivision of nature reserves and the maintenance of species diversity , 1980, Nature.

[9]  D. Goodman.  How Do Any Species Persist? Lessons for Conservation Biology , 1987 .

[10]  R. Lande.  Genetics and demography in biological conservation. , 1988, Science.

[11]  W. B. Davis,et al.  The Bighorn Sheep of Texas , 1939 .

[12]  Mark L. Shaffer,et al.  Population Size and Extinction: A Note on Determining Critical Population Sizes , 1985, The American Naturalist.

[13]  V. Bleich,et al.  Genetics and the conservation of mountain sheep ovis canadensis nelsoni , 1986 .

[14]  M. Soulé,et al.  Benign neglect: A model of faunal collapse in the game reserves of East Africa , 1979 .

[15]  C. L. Douglas,et al.  Influence of weather and density on lamb survival of desert mountain sheep , 1986 .

[16]  H. K. Buechner.  The bighorn sheep in the United States, its past, present and future. , 1960 .

[17]  W. Newmark.  A land-bridge island perspective on mammalian extinctions in western North American parks , 1987, Nature.

[18]  Dennis D. Murphy,et al.  Conservation Strategy: The Effects of Fragmentation on Extinction , 1985, The American Naturalist.

[19]  T. Clark.  Black‐Footed Ferret Recovery: A Progress Report , 1987 .

[20]  Gary E. Belovsky,et al.  Viable Populations for Conservation: Extinction models and mammalian persistence , 1987 .

[21]  Leslie A. Wakelyn.  Changing habitat conditions on bighorn sheep ranges in Colorado , 1987 .

[22]  James H. Brown.  Mammals on Mountaintops: Nonequilibrium Insular Biogeography , 1971, The American Naturalist.

[23]  J. Wehausen,et al.  Desert-dwelling Mountain Sheep: Conservation Implications of a Naturally Fragmented Distribution , 1990 .

[24]  D. Onderka,et al.  EXPERIMENTAL CONTACT TRANSMISSION OF PASTEURELLA HAEMOLYTICA FROM CLINICALLY NORMAL DOMESTIC SHEEP CAUSING PNEUMONIA IN ROCKY MOUNTAIN BIGHORN SHEEP , 1988, Journal of wildlife diseases.

[25]  J. Wehausen,et al.  Recruitment Dynamics in a Southern California Mountain Sheep Population , 1987 .

[26]  M. Sunquist,et al.  Ecological Constraints on Predation by Large Felids , 1989 .

[27]  F. Jones,et al.  A Survey of the Sierra Nevada Bighorn , 1951 .

[28]  Lynn A. Maguire,et al.  Sample Sizes for Minimum Viable Population Estimation , 1987 .

[29]  Andrew T. Smith.  THE DISTRIBUTION AND DISPERSAL OF PIKAS: CONSEQUENCES OF INSULAR POPULATION STRUCTURE' , 1974 .

[30]  Karen A. Sausman.  Survival of captive‐born Ovis canadensis in North American zoos , 1984 .

[31]  M. Rosenzweig.  Net Primary Productivity of Terrestrial Communities: Prediction from Climatological Data , 1968, The American Naturalist.

[32]  J. Berger,et al.  Pregnancy incentives, predation constraints and habitat shifts: experimental and field evidence for wild bighorn sheep , 1991, Animal Behaviour.

[33]  D. Jessup,et al.  FATAL PNEUMONIA OF BIGHORN SHEEP FOLLOWING ASSOCIATION WITH DOMESTIC SHEEP , 1982, Journal of wildlife diseases.