William S. Longland

Direct Observations of Owls and Heteromyid Rodents: Can Predation Risk Explain Microhabitat Use?

Coexisting heteromyid rodent species of North American deserts differ in habitat use and in locomotory morphology. Quadrupedal species forage primarily in struc- turally complex microhabitats, such as under bush canopies, while bipedal species forage in open spaces. A common explanation for this morphology-microhabitat association is that species differing in morphology also differ in vulnerability to predators, that micro- habitat structure affects predation risk, and that animals preferentially forage in the safest microhabitats. We tested this for two bipedal and two quadrupedal heteromyid species (matched by body size), and one cricetid species, by quantifying effects of habitat and illumination on activity and on risk of predation by Great Homed Owls. Capture frequencies were lower for all heteromyid species than for the cricetid species, Peromyscus maniculatus. Heteromyid activity was lower in open habitat and under bright illumination. Illumination had no significant effect on risk, perhaps because rodents changed activity patterns under full moon to compensate for a potential increment in risk. Habitat, however, did affect risk: all species were attacked and captured more frequently in the open. Bipedal species were attacked relatively more in the open than were quadrupeds. If these results apply to all predators, they indicate that predation alone cannot account for the divergent microhabitat associations of bipedal and quadrupedal species. Bipedal het- eromyids, however, escaped owl attacks more frequently than did quadrupeds of equivalent size. It is therefore conceivable that they experience lower overall risk in nature, where owls may preferentially attack more easily captured prey species when given a choice. Under these circumstances, owl predation could reinforce divergent microhabitat special- izations based on some other factor, such as foraging economics, by restricting quadrupeds more strongly than bipeds to the safety of bushes.

Sex and Age Affect Vulnerability of Desert Rodents to Owl Predation

Pelvic morphology was examined to distinguish sexes of three prey species eaten by great horned owls, and age classes of one of these species. Comparisons of sex ratios in prey populations with frequencies of males and females in owl diets indicated that male Dipodomys ordii and female Peromyscus maniculatus were more vulnerable to owl predation than the opposite sexes of these species. Furthermore, juvenile Microtus montanus were more vulnerable than adults and, within the juveniles, females were more vulnerable than males. We examined morphological (body size) and behavioral (degree of activity) attributes of these prey species, and concluded that either morphology or behavior may facilitate differential vulnerability in Dipodomys , whereas behavioral differences between sex and age classes are more likely to account for differential vulnerability in Microtus . For Peromyscus there are no body size or activity differences between sexes that are consistent with differential vulnerability, but body weights of female Peromyscus are more variable than weights of males, which could promote female-biased vulnerability if predators select large prey.

PATTERNS OF ECTOMYCORRHIZAL-FUNGI CONSUMPTION BY SMALL MAMMALS IN REMNANT OLD-GROWTH FORESTS OF THE SIERRA NEVADA

Abstract We investigated fungal consumption and resulting patterns of potential spore dispersal by 6 small mammals in old-growth habitat in the Sierra Nevada of the United States, a region in which this ecological interrelationship is poorly understood. Small mammals consumed a wide array of food items, although only feces of northern flying squirrels (Glaucomys sabrinus), golden-mantled ground squirrels (Spermophilus lateralis), and Douglas squirrels (Tamiasciurus douglassi) frequently contained spores of hypogeous fungi, whereas those of lodgepole chipmunks (Tamias speciosus), long-eared chipmunks (T. quadrimaculatus), and deer mice (Peromyscus maniculatus) did so rarely. Feces from the 3 squirrel species also contained a greater number of hypogeous fungal genera per sample than did samples from the latter 3 species. Flying squirrels potentially dispersed the greatest variety of ectomycorrhizal fungi (16 genera): 8 in spring and 15 in autumn. Frequency of occurrence of 9 genera in the feces of flying squirrels differed between spring and autumn. Interspecific differences in patterns of fungal consumption, coupled with differences among these small mammals in habitat use, mobility, and digestive physiology, suggest that these small mammals may disperse fungi in ecologically nonredundant ways and that the integrity of entire small-mammal communities may be important to the maintenance of ectomycorrhizal diversity in coniferous forests.

Identification of source-sink dynamics in mountain lions of the Great Basin

Natural and anthropogenic boundaries have been shown to affect population dynamics and population structure for many species with movement patterns at the landscape level. Understanding population boundaries and movement rates in the field for species that are cryptic and occur at low densities is often extremely difficult and logistically prohibitive; however genetic techniques may offer insights that have previously been unattainable. We analysed thirteen microsatellite loci for 739 mountain lions (Puma concolor) using muscle tissue samples from individuals in the Great Basin throughout Nevada and the Sierra Nevada mountain range to test the hypothesis that heterogeneous hunting pressure results in source‐sink dynamics at the landscape scale. We used a combination of non‐spatial and spatial model‐based Bayesian clustering methods to identify genetic populations. We then used a recently developed Bayesian multilocus genotyping method to estimate asymmetrical rates of contemporary movement between those subpopulations and to identify source and sink populations. We identified two populations at the highest level of genetic structuring with a total of five subpopulations in the Great Basin of Nevada and the Sierra Nevada range. Our results suggest that source‐sink dynamics occur at landscape scales for wide‐ranging species, such as mountain lions, and that source populations may be those that are under relatively less hunting pressure and that occupy refugia.

Use of Fluorescent Pigments in Studies of Seed Caching by Rodents

We describe a new technique for locating seeds cached by rodents that offers practical and logistical advantages over previous methods. The new technique uses ultraviolet illumination and seeds marked with powdered fluorescent pigments, which transfer readily between surfaces upon contact and have been applied successfully for other research on small mammals. We used the technique at a field site in the Great Basin Desert of western Nevada to test and verify the prediction that seed-caching, heteromyid rodents would transport a preferred seed type longer distances to make caches in a habitat where this seed is rare relative to an adjacent habitat. We also tested two variations of our seed-tracking technique. The first uses only marked seeds and allows an investigator to locate rodent scatterhoards by searching systematically for pigment spots on the ground surface. The second technique requires rodents to harvest pigmented seeds from a bait station that also is laced with pigments, so that the animals leave pigmented footprints when transporting the seeds. Significantly more seed caches were located, and the caches were located at significantly longer distances from the seed source by following footprints with the second technique. Moreover, the footprint trails provided a detailed record of paths traversed by rodents during seed transport, they revealed locations of rodent burrows where seeds are stored in larders, and they made it easier to find scatterboards.

Effects of Cattle Grazing on Salt Desert Rodent Communities

Abstract Cattle grazing has been shown to alter various features of desert communities that may impact microhabitats required by various species of desert rodents, with unknown implications for desert rodent communities. We conducted a series of studies at heavily and lightly grazed sites to investigate effects of cattle grazing on desert rodent relative abundances, home range sizes and microhabitat use in salt desert shrub communities of the western Great Basin Desert. Monitoring of rodent populations with repeated live trapping showed that different levels of grazing were associated with differences in relative abundances of some species of rodents. Specifically, Dipodomys merriami was significantly more abundant in heavily grazed areas, and Perognathus longimembris was significantly more abundant in lightly grazed areas. Our studies showed that cattle, by preferentially feeding on certain plants, can create conditions that are more suitable for some species of rodents, while reducing important microhabitat for other species.

EFFECTS OF SEED DENSITY ON GERMINATION AND ESTABLISHMENT OF A NATIVE AND AN INTRODUCED GRASS SPECIES DISPERSED BY GRANIVOROUS RODENTS

-Dense aggregations of Oryzopsis hymenoides (Indian ricegrass) and Bromus tectorum (cheatgrass) seeds occur in rodent caches in the Great Basin. Rodent caching behavior may influence establishment and persistence of these two desert grasses. Seed caches of rodents clearly introduce exaggerated seedling competition in these species. Greenhouse experiments were used to determine if establishment and persistence of these species were affected differently by densities of seeds in caches. Germination and establishment of Indian ricegrass, a native perennial grass, were less affected by high seed densities than germination and establishment of cheatgrass, an introduced annual weed. The different natural histories of these species, as well as data presented here, suggest that the high seed densities introduced by caching behavior of desert rodents may be beneficial to Indian ricegrass and harmful to cheatgrass.

Rattlesnake Predation on Desert Rodents: Microhabitat and Species-Specific Effects on Risk

Risk of rattlesnake predation was assessed in laboratory experiments for two bipedal and two quadrupedal species of desert rodents. Rodents were left overnight with Great Basin rattlesnakes ( Crotalus viridis lutosus ) in experimental arenas that had either 37% sagebrushcanopy cover or no cover. Predation rates did not differ significantly between arena types for any one rodent species or for all four species grouped. The lack of a shrub-cover effect on risk suggests that rattlesnake predation cannot account for microhabitat associations found in desert rodents. Risk did not differ between bipedal and quadrupedal species, but differed significantly among individual species. Dipodomys merriami and Peromyscus maniculatus seldom were caught by rattlesnakes, but Microdipodops megacephalus and Perognathus parvus often were caught. Such species differences in risk may affect foraging behavior and distribution patterns of some species of desert rodents.

Competitive Environment Affects Food-Hoarding Behavior of Merriam's Kangaroo Rats (Dipodomys merriami)

Abstract North American desert rodents in the family Heteromyidae live in an unpredictable environment characterized by extremes in temperature and food availability; therefore, the ability to hoard food is a vital adaptation. Although much laboratory research has investigated food-hoarding tactics of heteromyid rodents, data from natural systems are scarce. We used a combination of fluorescently labeled seeds and observations of focal individuals to evaluate food-hoarding behavior in wild Merriams kangaroo rats (Dipodomys merriami) living in different competitive environments. There was considerable individual variation within populations in the tendency to larderhoard seeds in a burrow versus scatterhoard seeds in widely dispersed locations. However, Merriams kangaroo rats living in a system where competitors were predominately conspecific scatterhoarded more than those living in a system where conspecifics were less abundant and heterospecific competitors also were present. Also, pilferage was more common between Merriams kangaroo rats than across species. Comparisons of food hoarding between communities with different species composition indicate that intraspecific variation in behavior may be associated with variation in competitive environments.

Seeding Indian ricegrass in an arid environment in the Great Basin.

Indian ricegrass [Oryzopsis hymenoides (R. & S.) Rickerl is a valuable forage species adapted to arid rangelands in temperate deserts. The purpose of this study was to test the influence of seeding date, depth, and rate on Indian ricegrass emergence and seedling establishment of acid scarified and intact caryopses (seeds). The seeding experiments were conducted on a wind eroding sand sheet of Lahontan age in western Nevada. During the initial year of planting, seeds of the cultivars Nezpar and Paloma Indian ricegrass were successfully established without pretreatment by acid scarification. Acid scarified seeds did not result in the established seedling stands in the field. Initial seedings were done in a season with prolonged moisture events with total precipitation about twice the average. Seedling stands of crested wheatgrass [Agropyron desertorum (Fisch.) ex Link Schult] as well as other exotic and native herbaceous and woody species were established during the first year. During the next 4 years crested wheatgrass seedlings were never again established. Indian ricegrass seedlings were established in 3 of the 4 subsequent years of seeding trials using a seeding rate of 0.8 seeds/cm of row and a seeding depth of 1 cm. Indian ricegrass seedling emergence was increased by either increasing the planting depth to 5 cm or by reducing the seeding rate to 0.03 seeds/cm of row. The ultra-low seeding rate resulted in a significant saving in seed cost.