Peter J. P. Gogan

The Northern Yellowstone Elk: Density Dependence and Climatic Conditions

We analyzed a time series of estimates of elk (Cervus elaphus) numbers on the northern Yellowstone winter range from 1964 to 1979 and 1986 to 1995 using a variety of discrete time stochastic population dynamic models. These models included adjustments for density, an increase in the area of winter range used by elk, lagged effects of the weather covariates of spring precipitation, snow depth and winter temperature, and the impacts of the 1988 drought and fires. An information-criteria-based model-selection process strongly supported evidence of density dependence. The best model, a Ricker model, distinguished between the 2 time periods. The bulk of the difference between the 2 periods is attributed to an increase in the amount of winter range used by elk. Inclusion of the covariates spring precipitation and spring precipitation squared greatly improved the model fit. We detected a short-lived increase in elk population growth rate following the 1988 drought and fires. Fertility and survivorship of adults appeared to have different density-dependent forms that together result in a biphasic relationship between population growth rate and density. This study confirms the presence of density-dependent regulation in the northern Yellowstone elk herd, and enhances our understanding of population dynamics of these ungulates.

What limits the Serengeti zebra population

The populations of the ecologically dominant ungulates in the Serengeti ecosystem (zebra, wildebeest and buffalo) have shown markedly different trends since the 1960s: the two ruminants both irrupted after the elimination of rinderpest in 1960, while the zebras have remained stable. The ruminants are resource limited (though parts of the buffalo population have been limited by poaching since the 1980s). The zebras’ resource acquisition tactics should allow them to outcompete the ruminants, but their greater spatial dispersion makes them more available to predators, and it has been suggested that this population is limited by predation. To investigate the mechanisms involved in the population dynamics of Serengeti zebra, we compared population dynamics among the three species using demographic models based on age-class-specific survival and fecundity. The only major difference between zebra and the two ruminants occurred in the first-year survival. We show that wildebeest have a higher reproductive potential than zebra (younger age at first breeding and shorter generation time). Nevertheless, these differences in reproduction cannot account for the observed differences in the population trends between the zebra and the ruminants. On the other hand, among-species differences in first-year survival are great enough to account for the constancy of zebra population size. We conclude that the very low first-year survival of zebra limits this population. We provide new data on predation in the Serengeti and show that, as in other ecosystems, predation rates on zebras are high, so predation could hold the population in a “predator pit”. However, lion and hyena feed principally on adult zebras, and further work is required to discover the process involved in the high mortality of foals.

A Simple Solar Radiation Index for Wildlife Habitat Studies

Abstract Solar radiation is a potentially important covariate in many wildlife habitat studies, but it is typically addressed only indirectly, using problematic surrogates like aspect or hillshade. We devised a simple solar radiation index (SRI) that combines readily available information about aspect, slope, and latitude. Our SRI is proportional to the amount of extraterrestrial solar radiation theoretically striking an arbitrarily oriented surface during the hour surrounding solar noon on the equinox. Because it derives from first geometric principles and is linearly distributed, SRI offers clear advantages over aspect-based surrogates. The SRI also is superior to hillshade, which we found to be sometimes imprecise and ill-behaved. To illustrate application of our SRI, we assessed niche separation among 3 ungulate species along a single environmental axis, solar radiation, on the northern Yellowstone winter range. We detected no difference between the niches occupied by bighorn sheep (Ovis canadensis) and elk (Cervus elaphus; P = 0.104), but found that mule deer (Odocoileus hemionus) tended to use areas receiving more solar radiation than either of the other species (P < 0.001). Overall, our SRI provides a useful metric that can reduce noise, improve interpretability, and increase parsimony in wildlife habitat models containing a solar radiation component.

Alternative Approaches to Aerial Censusing of Elk

When animals occur in groups, aerial counts usually miss more of the smaller groups. Previous studies produced corrections for this problem that were based on estimates of the probability of sighting various group sizes. In the present study, extensive ground relocations of groups containing radiotagged elk (Cerous elaphus) demonstrated that group size is very fluid in this species during winter. We thus explored estimation of total numbers by simulations using the Petersen method and by using field data collected on the Madison-Firehole Elk Herd of Yellowstone National Park (YNP). Comparisons with estimates based on a logistic regression of estimated sighting probabilities indicated the Petersen estimate may provide a suitable alternative method. The new approach avoids the need to repeatedly use the same correction factors.

Comparative dynamics of introduced tule elk populations

Growth rates in 2 re-established populations of tule elk (Cervus elaphus nannodes) were followed 6 and 7 years beyond the time of introduction. The elk population at Grizzly Island, California, grew at the maximum potential rate, whereas that at Point Reyes, California, grew at half the theoretical potential. The lower rate of growth at Point Reyes was attributed to an initial pulse of adult mortality, low calving rates, and high subadult mortality. J. WILDL. MANAGE. 51(1):20-27 Studies of introduced ungulate populations immediately following release are limited. The paucity of such information has led to experimental manipulation of populations to duplicate growth from low numbers (McCullough 1979, 1982). We report growth rates for 2 reintroductions of tule elk at Point Reyes and Grizzly Island, California. Both areas supported tule elk, at least seasonally, in pristine times but not since the 1860s (McCullough 1969). Reintroductions of elk began in 1977. Ten adult elk (2 males and 8 females) were transported in March 1978 from San Luis National Wildlife Refuge, Merced County, to a holding pen on a 1,030ha fenced portion of Point Reyes. Nine calves (1978 cohort) were born while the animals were confined within the pen (Ray 1981). Ten adults and 7 surviving calves (4 males and 3 females) were released to the entire range in September 1978. Three adult males from the Owens Valley, Inyo County, were reintroduced in December 1981. They were sighted occasionally in January and February 1982, but subsequently disappeared, and are not considered further. Seven adult elk (4 males and 3 females) were relocated to Grizzly Island from the Tule Elk State Reserve, Tupman, Kern County in February 1977. The animals were restricted to a holding pen until released, on site, in May. A yearling female, originally from Owens Valley, was released at Grizzly Island later that year. In May 1978, an adult female from Owens ValThis content downloaded from 153.90.6.19 on Wed, 25 Jun 2014 13:23:08 PM All use subject to JSTOR Terms and Conditions J. Wildl. Manage. 51(1):1987 INTRODUCED ELK POPULATIONS * Gogan and Barrett 21 ley joined the herd. One of the original 4 adult males was moved to another release site. An adult male was poached in November 1979, and a male seriously injured in the rut was sacrificed in September 1982. In 1978, both populations numbered 17 elk. We thank W. H. Sohrweide for sharing his observations of tule elk at Grizzly Island with us. Thanks are due J. J. Aho, Jr., T. S. Biller, W. J. Pierce, and R. T. Lesko of Point Reyes Natl. Seashore and D. A. Jessup and B. E. Curtis of the Calif. Dep. Fish and Game (CDFG) for assistance and encouragement during this study. We thank D. B. Houston, R. A. Lancia, and D. R. McCullough for reviewing earlier drafts of this manuscript, and L. M. Merkle for typing the final and earlier drafts. Research was funded by the Natl. Park Serv. under Contract CX 800-9-0029 and Calif. Agric. Exp. Stn. Proj. 3501-MS. The senior author was a Rob and Bessie Welder Wildl. Found. Fellow and this manuscript is Welder Wildl. Found. Contrib. 209.

Historic Distribution and Challenges to Bison Recovery in the Northern Chihuahuan Desert

Ecologists and conservationists have long assumed that large grazers, including bison (Bison bison), did not occur in post-Pleistocene southwestern North America. This perception has been influential in framing the debate over conservation and land use in the northern Chihuahuan Desert. The lack of an evolutionary history of large grazers is being used to challenge the validity of ranching as a conservation strategy and to limit the protection and reintroduction of bison as a significant component of desert grassland ecosystems. Archeological records and historical accounts from Mexican archives from AD 700 to the 19th century document that the historic range of the bison included northern Mexico and adjoining areas in the United States. The Janos-Hidalgo bison herd, one of the few free-ranging bison herds in North America, has moved between Chihuahua, Mexico, and New Mexico, United States, since at least the 1920s. The persistence of this cross-border bison herd in Chihuahuan Desert grasslands and shrublands demonstrates that the species can persist in desert landscapes. Additional lines of evidence include the existence of grazing-adapted grasslands and the results of experimental studies that document declines in vegetation density and diversity following the removal of large grazers. The Janos-Hidalgo herd was formed with animals from various sources at the turn of the 19th century. Yet the future of the herd is compromised by differing perceptions of the ecological and evolutionary role of bison in the Desert Grasslands of North America. In Mexico they are considered native and are protected by federal law, whereas in New Mexico, they are considered non-native livestock and therefore lack conservation status or federal protection. Evidence written in Spanish of the presence of bison south of the accepted range and evidence from the disciplines of archaeology and history illustrate how differences in language and academic disciplines, in addition to international boundaries, have acted as barriers in shaping comprehensive approaches to conservation. Bison recovery in the region depends on binational cooperation.

Spatial Population Structure of Yellowstone Bison

Abstract Increases in Yellowstone National Park, USA, bison (Bison bison) numbers and shifts in seasonal distribution have resulted in more frequent movements of bison beyond park boundaries and development of an interagency management plan for the Yellowstone bison population. Implementation of the plan under the adaptive management paradigm requires an understanding of the spatial and temporal structure of the population. We used polythetic agglomerative hierarchical cluster analysis of radiolocations obtained from free-ranging bison to investigate seasonal movements and aggregations. We classified radiolocations into 4 periods: annual, peak rut (15 Jul–15 Sep), extended rut (1 Jun–31 Oct), and winter (1 Nov–31 May). We documented spatial separation of Yellowstone bison into 2 segments, the northern and central herds, during all periods. The estimated year-round exchange rate (4.85–5.83%) of instrumented bison varied with the fusion strategy employed. We did not observe exchange between the 2 segments during the peak rut and it varied during the extended rut (2.15–3.23%). We estimated a winter exchange of 4.85–7.77%. The outcome and effectiveness of management actions directed at Yellowstone bison may be affected by spatial segregation and herd affinity within the population. Reductions based on total population size, but not applied to the entire population, may adversely affect one herd while having little effect on the other. Similarly, management actions targeting a segment of the population may benefit from the spatial segregation exhibited.

YELLOWSTONE BISON FETAL DEVELOPMENT AND PHENOLOGY OF PARTURITION

Abstract Knowledge of Yellowstone bison (Bison bison) parturition patterns allows managers to refine risk assessments and manage to reduce the potential for transmission of brucellosis between bison and cattle. We used historical (1941) and contemporary (1989–2002) weights and morphometric measurements of Yellowstone bison fetuses to describe fetal growth and to predict timing and synchrony of parturition. Our method was supported by agreement between our predicted parturition pattern and observed birth dates for bison that were taken in to captivity while pregnant. The distribution of parturition dates in Yellowstone bison is generally right-skewed with a majority of births in April and May and few births in the following months. Predicted timing of parturition was consistently earlier for bison of Yellowstones northern herd than central herd. The predicted median parturition date for northern herd bison in the historical period was 3 to 12 days earlier than for 2 years in the contemporary period, respectively. Median predicted birth dates and birthing synchrony differed within herds and years in the contemporary period. For a single year of paired data, the predicted median birth date for northern herd bison was 14 days earlier than for central herd bison. This difference is coincident with an earlier onset of spring plant growth on the northern range. Our findings permit refinement of the timing of separation between Yellowstone bison and cattle intended to reduce the probability of transmission of brucellosis from bison to cattle.

Copper deficiency in tule elk at Point Reyes, California

Tule elk (Cervus eiirphus uuuuos serum, x’ 2 1.4 ppm). These levels were consistent with documented coppsr defi- ciency in wild and domestfe ruminants. Copper serum levels increased in response to copper enriched dietary supplements and declined after the elk stopped eating the supplements. Analysis of plant and soil samples showed both are deficient in copper and normal in molybdenum and sulfur-sulfatss. Deficiency in plants and sops at Point Reyes are probably due to low copper levels in the underlyitq granitic parent material. Key Words: Corvus ebhus tumuodes, copper, molybdenum and sulfur levels in elk, vegetation and sops

Elk and deer diets in a coastal prairie-scrub Mosaic, California.

We examined the diets of reintroduced tule elk (Cervus elaphus nannodes Merriam) and resident Columbian black-tailed deer (Odocoileus hemionus columbianus Richardson) inhabiting the coastal prairie-scrub mosaic of Tomales Point, the northernmost portion of the Point Reyes Peninsula, Calif., during 1979-81. The elk diet differed between years whereas the deer diet did not. The pattern of seasonal quality of elk and deer diets, as measured by fecal nitrogen (FN) was similar between species and years. This was achieved although botanical composition differed between herbivores in some seasons. Dietary overlap was lowest in the wet winter months when fecal nitrogen was highest and vegetative standing crop was lowest. Conversely, dietary overlap was highest in the dry summer months when fecal nitrogen was lowest and vegetative standing crop highest. Both herbivore species showed selection and avoidance of certain plant species in June of both years. These findings are compared to other cervid-habitat systems.