Rabbit abundance relative to rainfall and plant production in northern Chihuahuan Desert grassland and shrubland habitats

Rabbit abundance patterns in the North American Southwest are poorly known, particularly relative to the potentially important influences of rainfall and plant production. We conducted a 10-year study on the comparative densities of black-tailed jackrabbits ( Lepus californicus ) and desert cottontail rabbits ( Sylvilagus audubonii ) in relation to annual rainfall and plant production in both black grama ( Bouteloua eriopoda ) grassland and creosotebush ( Larrea tridentata ) shrubland habitats in the northern Chihuahuan Desert. We found that both species of rabbits were more abundant in black grama grassland than in creosotebush shrubland and that plant production was positively correlated with rainfall at both habitats. However, rabbit densities were not positively correlated with rainfall or plant production in either habitat. In fact, both species of rabbits exhibited gradual, multiyear changes in abundance rather than the patterns of high annual variation observed in desert rodent communities. Our findings of no significant relationship between plant production and rabbit densities indicate that rabbit abundance in the northern Chihuahuan Desert may not be strongly regulated by short-term, bottom-up effects of annual variation in plant production. Long-term trends indicate that other factors such as disease or predation may be equally or more important. We suggest that future research evaluate both top-down and bottom-up influences to elucidate the temporal dynamics of Chihuahuan Desert rabbit abundance. Se conoce poco sobre los patrones de abundancia de conejos desérticos en el suroeste de Norte America, particularmente en relación con las influencias potencialmente importantes de la precipitación y la producción de plantas. Llevamos a cabo una investigación de diez años sobre la densidad relativa de la liebre de California ( Lepus californicus ) y el conejo cola de algodón ( Sylvilagus audubonii ) en los hábitats tanto de pradera de grama negra ( Bouteloua eriopoda ) como de matorral de gobernadora ( Larrea tridentata ) en el norte del Desierto Chihuahuense, en relación con la precipitación y la producción anuales. Encontramos que ambas especies de conejo fueron más abundantes en la pradera de grama negra que en el matorral de gobernadora, y que la producción de plantas estuvo correlacionada positivamente con la precipitación en ambos hábitats. Sin embargo, las densidades de conejos no estuvieron correlacionadas positivamente con la precipitación o con la producción de plantas en ninguno de los dos hábitats. De hecho, en cuanto a la fluctuación de su abundancia, las dos especies de conejo exhibieron tendencias graduales a lo largo de varios años, en comparación con los patrones de gran variación anual que se han observado en comunidades de roedores desérticos. La falta de una relación significativa entre la producción de plantas y la abundancia de conejos, indica que la abundancia de conejos en el norte del Desierto Chihuahuense no está fuertemente regulada por los efectos ascendentes a corto plazo de la variación anual en la producción de plantas. Los cambios reflejados en las tendencias a largo plazo indican que otros factores como las enfermedades o la depredación pueden ser de igual o mayor importancia, y sugerimos que investigaciones futuras deberán evaluar tanto las influencias ascendentes como las descendentes para entender las dinámicas temporales en la abundancia de conejos del Desierto Chihuahuense.     

Holocene bison from Arches National Park, southeastern Utah

An artifact constructed from a bison ( Bison bison ) hornsheath was recovered from the surface of a sandstone shelter also containing skeletal remains of bison and bighorn ( Ovis Canadensis ). Radiocarbon dating of bison, bighorn, and the artifact indicate that all co-occurred on the central Colorado Plateau, southeastern Utah, either between A. D. 1405 and A. D. 1420, or between A. D. 1535 and A. D. 1605. Skeletal remains of the bison imply that the animal was of the local faunal community during the transition between Protohistoric and Historic time.     

Breeding biology of Mountain Plovers ( Charadrius montanus ) in the Uinta Basin

The known Mountain Plover population breeding on the Myton Bench, Duchesne County, Utah, is small, composed roughly of 30 adults and young after each breeding season. Currently, its location is peripheral to the species main range. This shrub-steppe breeding habitat differs from the shortgrass prairie habitat with which this bird is historically associated. Between 1996 and 1998 we made observations at nesting sites located consistently in 2 concentrated areas surrounded by large tracts of similar habitat. Activity may be focused in these specific areas because of breeding-site fidelity; this behavior is common among most shorebirds and has been documented for the Mountain Plover in Colorado. Also, Mountain Plovers are social and tend to choose nest sites near others. Most nests in Utah were located within close proximity of mounds of white-tailed prairie dogs ( Cynomys leucurus ), and all were situated near roadways or oil well pads. Mountain Plovers were often observed with broods on these bare areas at night. We conclude that Mountain Plovers on the Myton Bench are distributed in clumped breeding colonies within large areas of apparently favorable habitat.     

Habitat use and subspecific status of Merlins, Falco columbarius , wintering in central Utah

Most ornithological literature for Utah reports the status of the Merlin ( Falco columbarius ) as rare or scarce. Only the most recently published checklists (Behle and Perry 1975, Behle et al. 1985) record it as uncommon. Likewise, a 1998 printed checklist (Utah Ornithological Society 1998) list it as uncommon. Also misrepresented in the literature are the status and distribution of the 3 rather distinct subspecies, each of which occupies dissimilar habitats during the breeding season. We made random observations of Merlins in northern Utah beginning in the 1950s and then studied them continuously between 1992 and 1997 in Utah County, Utah. During the 1992-1997 period, we were able to clearly allocate 95 males and 76 females to subspecies, including within our count 58 falcons trapped (some fitted with radio telemetry). The winter habitat in which they occurred was categorized as either urban (mainly dense residential area) or rural (agricultural lands, dairy farms, or scattered homes). There was a statistically significant ( X 2 F. c. columbarius frequenting urban areas 68% of the time, and prairie parkland-breeding F. c. richardsonii frequenting rural habitats 78% of the time.     

Centric diatoms of Lake Tahoe

An understanding of the mechanisms of phytoplankton species interaction is dependent on a precise knowledge of what species exist within the community. The centric diatoms of Lake Tahoe, California-Nevada, which are often the dominant component of the phytoplankton community, are presented in both light and scanning electron microscopy (SEM) photographs. Specific attention has been given to initial cell forms of Cyclotella stelligera Cleve and Grunow and C. comta (Ehrenberg) Kützing through the aid of the SEM.     

Catastrophic wildfire and number of populations as factors influencing risk of extinction for Gila trout ( Oncorhynchus gilae )

We used the computer program RAMAS to explore the sensitivity of an extinction-risk model for the Gila trout ( Oncorhynchus gilae ) to management of wildfires and number of populations of the species. The Gila trout is an endangered salmonid presently restricted to very few headwaters of the Gila and San Francisco river tributaries in southwestern New Mexico. Life history data for 10 extant populations were used to examine sensitivity of the species viability to changes in a variety of factors including population size, fecundity, life stage structure, number of populations, severity and probability of forest fires, and a regulated fishery. The probability and severity of forest fires and number of populations had the greatest effect on viability. Results indicate that successful conservation of Gila trout requires establishment of additional populations and reduction of the severity of forest fires through a program incorporating more frequent, but less severe, fires.     

Understory patterns in cut western juniper ( Juniperus occidentalis spp. occidentalis Hook.) woodlands

Western juniper ( Juniperus occidentalis spp. occidentalis ) has rapidly expanded into shrub steppe communities in the intermountain Northwest during the past 120 yr. Cutting juniper is a management tool used to restore shrub steppe communities. Response of the understory after cutting is strongly influenced by plant species composition existing prior to treatment. This study assessed distribution patterns of understory plants over 2 growing seasons after tree cutting in a western juniper woodland. Cover, density, and diversity of understory species were compared among 3 locations: interspaces, duff zones (previously under tree canopies), and debris zones (beneath cut trees). Plant cover density increased in all zones following tree cutting. Understory vegetation in cut woodlands exhibited strong zonal distribution. Cover and density of Poa sandbergii and Sitanion hystrix and canopy cover of annual forbs were greatest in duff zones ( P P < 0.05). Debris zones tended to have the lowest overall understory cover and plant density values. Under juniper debris many species common to interspaces were reduced in density, although plants that survived or established beneath debris grew larger than their counterparts in interspaces. Species increased in density and cover under debris were plants characteristic of duff zones and whose seeds are typically wind dispersed.     

Use of interstate highway overpasses and billboards for nesting by the common raven ( Corvus corax )

Common ravens are usually rather shy birds around their nests and avoid nesting too near human activity. We here report the use of overpasses along heavily traveled Interstate Highway 84 in Idaho and Utah for nest placement. Nests are within 6 m of passing vehicles.