Perognathus Parvus Research Articles

Expanding and invading plant species in sagebrush steppe affect multiple aspects of small‐mammal ecology

AbstractInvasion and expansion of non‐native and native plants have altered vegetation structure in many terrestrial ecosystems. Small mammals influence multiple ecosystem processes through their roles as ecosystem engineers, predators, and prey, and changes to vegetation structure can affect habitat use, community composition, and predator‐prey interactions for this assemblage of wildlife. In the sagebrush (Artemisia spp.) shrublands of the western United States, invasion by non‐native grasses and expansion of native conifer trees beyond their historical range has altered vegetation structure. These changes may potentially affect distributions and interactions of deer mice (Peromyscus maniculatus), which are generalist omnivores, and Columbia Plateau pocket mice (Perognathus parvus), more specialized granivores. To assess the extent to which altered habitat affects small‐mammal density, survival, and home‐range size, we examined these aspects of small‐mammal ecology along a gradient of cheatgrass (Bromus tectorum) invasion and western juniper (Juniperus occidentalis) establishment in sagebrush shrublands in southwestern Idaho, USA. From 2017–2019, we used a spatially explicit mark‐recapture design to examine attributes of small‐mammal ecology along an invasion gradient. We did not find support for an effect of cheatgrass cover on density or survival of either species. Home‐range size of deer mice was 2.3 times smaller in heavier cheatgrass cover (60%) compared to areas with little or no cheatgrass cover. Density of deer mice was highest (5 individuals/ha) in areas with 10% juniper cover and decreased with increasing juniper cover, whereas density of pocket mice was positively influenced by shrub cover. Survival of deer mice declined as juniper stem density increased. Conversely, survival of pocket mice increased with increasing juniper stem density. We found evidence for interspecific interactions between these 2 species, in the form of a density‐dependent effect of deer mice on pocket mouse home‐range size. Home‐range size for pocket mice was 2 times smaller in areas with the highest estimated density of deer mice compared to areas with low densities of deer mice. Our data provides unique information about how small mammals in the sagebrush steppe are affected by expanding and invasive plant species and potential ways that habitat restoration efforts, in the form of conifer removal, may influence small mammals. Understanding the response of small mammals to conifer expansion or removal may shed light on the demographic and numerical responses of other wildlife associated with the sagebrush biome, including predators.

Density and habitat selection of mice on cheatgrass-invaded sites in British Columbia, Canada

Globally, invasive species in grasslands and shrublands have substantially altered trophic interactions and ecosystem processes. In North America, cheatgrass (Bromus tectorum) has invaded much of the western sage-steppe, leading to substantial shifts in plant communities and fire regimes. The effects of this cheatgrass invasion on small mammals are inconsistent among species and regions, and the mechanisms behind these patterns may be trophic or non-trophic. Cheatgrass alters cover and the amount of bare ground, thus affecting movements and predation risk, but cheatgreass also provides food for some rodents. We examined the extent to which low-density cheatgrass invasion (<20% cover) affected population densities and habitat selection of mice in southern British Columbia, Canada. In this region, Great Basin pocket mice (Perognathus parvus) were far more common than western harvest mice (Reithrodontomys megalotis) and deermice (Peromyscus maniculatus). Density of Great Basin pocket mice increased with bare ground and annual grasses, but decreased with shrub cover; when pocket mice foraged, they selected habitats with cheatgrass and bare, open ground. Cheatgrass did not predict movements of deermice or harvest mice. These results indicate that low cheatgrass densities may not be negative for these three species of mice, at least in the northern distribution of sage-steppe.

What factors predict path tortuosity of Great Basin pocket mice in shrub-steppe habitat invaded by cheatgrass?

Abstract Foraging animals choose habitats based on characteristics that often cannot be satisfied simultaneously, such as easy mobility, abundant or high-quality foods, and safety from predators. Invasive plants may alter habitat structure and provide novel foods; thus, measuring how animals forage in invaded landscapes offers insights into these new ecological relationships. We examined the movements of Great Basin pocket mice (Perognathus parvus) in sage-steppe habitats invaded by cheatgreass (Bromus tectorum) in southcentral British Columbia, Canada. The pathway tortuosity (fractal D) of pocket mice increased with vegetative cover and population density and decreased with open habitat, but these variables explained little of the variation in tortuosity. The fractal dimension of movement pathways of pocket mice was consistent over spatial scales ranging from 0.5 m to two-thirds of the home range size, unlike in other species where fractal dimensions are not consistent over multiple spatial scales. Collectively, our results indicate that foraging movements of pocket mice were not affected by the low densities of cheatgrass in this system.

Detection of ‘Candidatus Ehrlichia khabarensis’ in rodents and ticks removed from rodents in British Columbia, Canada

‘Candidatus Ehrlichia khabarensis’ was first described from rodents and insectivores in the Far East territory of Khabarovsk on the Russian Pacific Coast. Here we report the detection of DNA from this microorganism in rodents and fed ticks collected from rodents in British Columbia, Canada in 2013–2014. ‘Candidatus Ehrlichia khabarensis’ was detected in (i) a female Ixodes angustus tick collected from a Peromyscus maniculatus; (ii) a female Dermacentor andersoni tick collected from a Perognathus parvus; (iii) a pool of 2 larval Ixodes pacificus ticks collected from a single P. maniculatus; and (iv) a pool of 3 nymphal I. pacificus ticks collected from a single P. maniculatus. Three of these four rodents (2 P. maniculatus and 1 P. parvus) with infected ticks also had evidence of ‘Candidatus Ehrlichia khabarensis’ in at least one tissue type. The infected P. maniculatus and Ixodes ticks came from the Vancouver area in western British Columbia and the P. parvus and Dermacentor tick from an inland site in central British Columbia. Although it remains to be determined whether ‘Candidatus Ehrlichia khabarensis’ has any negative impacts on wildlife, domestic animals or humans, we note that all three tick species found to contain the DNA of this microorganism are known to bite humans. Future detection of this microorganism either in ticks collected from rodents and allowed to molt to the next life stage prior to being tested, or from host-seeking ticks, is required to determine if it can survive the tick’s molt after being ingested via an infectious blood meal.

Small‐mammal isotope ecology tracks climate and vegetation gradients across western North America

Stable carbon, nitrogen, hydrogen and oxygen isotopes have been used to infer aspects of species ecology and environment in both modern ecosystems and the fossil record. Compared to large mammals, stable isotopic studies of small‐mammal ecology are limited; however, high species and ecological diversity within small mammals presents several advantages for quantifying resource use and organism–environment interactions using stable isotopes over various spatial and temporal scales. We analyzed the isotopic composition of hair from two heteromyid rodent species, Dipodomys ordii and Perognathus parvus, from localities across western North America in order to characterize dietary variation in relation to vegetation and climatic gradients. Significant correlations between the carbon isotopic composition (δ13C) of these species and several climatic variables imply that seasonal temperature and precipitation control the composition and distribution of dietary resources (grass seeds). Our results also suggest a moisture influence on the nitrogen isotopic composition (δ15N) of heteromyid diets. Population‐ and species‐level variation in δ13C and δ15N values record fine‐scale habitat heterogeneity and significant differences in resource use between species. Using classification and regression‐tree techniques, we modeled the geographic variation in heteromyid δ13Cdiet values based on 10 climatic variables and generated an isotope landscape model (‘isoscape’). The isoscape predictions for δ13Cdiet differ from expectations based on observed C4 distributions and instead indicate that D. ordii and P. parvus record seasonally abundant grass resources, with additional model deviations potentially attributed to geographic variation in dietary selection. The oxygen and hydrogen isotopic composition of D. ordii is enriched relative to local meteoric water and suggests that individuals rely on highly evaporated water sources, such as seed moisture. Based on the climatic influences on vegetation and diet documented in this study, the isotopic composition of small mammals has high potential for recording ecological responses to environmental changes over short and long time scales.

Surveillance for Borrelia burgdorferi in Ixodes Ticks and Small Rodents in British Columbia.

To determine the prevalence of Borrelia burgdorferi in British Columbian ticks, fieldwork was conducted over a 2-year period. In all, 893 ticks (Ixodes pacificus, I. angustus, I. soricis, Ixodes spp., and Dermacentor andersoni) of different life stages were retrieved from 483 small rodents (Peromyscus maniculatus, Perognathus parvus, and Reithrodontomys megalotis). B. burgdorferi DNA was detected in 5 out of 359 tick pools, and 41 out of 483 mice were serologically confirmed to have antibodies against B. burgdorferi. These results were consistent with previous studies, data from passive surveillance in British Columbia, and data from neighboring states in the Pacific Northwest, suggesting a continually low prevalence of B. burgdorferi in British Columbia ticks.

Cryptic divergence and revised species taxonomy within the Great Basin pocket mouse,Perognathus parvus(Peale, 1848), species group

The Great Basin pocket mouse, Perognathus parvus, inhabits temperate shrub-steppe and arid grassland biomes throughout the Columbia Plateau, Great Basin, and adjacent regions of western North America. We used both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) sequences to address phylogenetic and biogeographic structure within the P. parvus species group. Phylogenetic and biogeographic analyses divide haplotypes from the mtDNA cytochrome oxidase subunit 3 (COIII) gene into largely allopatric northern versus southern lineages (clades), with divergences of up to 18.8%. The southern mtDNA clade also includes at least 1 of the 2 recently extant populations of the white-eared pocket mouse, P. alticolus—an endangered species with a restricted range in southern California. The northern mtDNA clade is further subdivided into several additional lineages with divergences as high as 8.8%, whereas the southern clade has no divergence greater than 1.2%. The deeper mtDNA gene tree structure was recovered by use of nDNA exon sequences from the interphotoreceptor retinoid-binding protein (IRBP) and recombination activating gene 2 (RAG2) gene regions, supporting a “gene tree–species tree” congruence. We estimated through molecular clock analyses that the northern and southern clades likely diverged within a late Miocene time frame. The limited available karyological evidence is consistent with a genome-wide divergence between northern and southern clades—thus, is consistent with recognition of each as a separate species. Conversely, our morphometric analysis revealed a high level of morphological conservatism without detectable diagnostic differences, rendering them as “cryptic” species. We revised species-level taxonomy, designating 2 neotypes given apparent loss of original type specimens. We postulate that barriers associated with Columbia Plateau, Snake River Plain, and Great Basin physiographic evolution, during a Neogene time frame, resulted in persistent geographic isolation driving divergence between the northern and southern clades.

Rodent-Mediated Interactions Among Seed Species of Differing Quality in a Shrubsteppe Ecosystem

Interactions among seeds, mediated by granivorous rodents, are likely to play a strong role in shrubsteppe ecosystem restoration. Past studies typically consider only pairwise interactions between preferred and less preferred seed species, whereas rangeland seedings are likely to contain more than 2 seed species, potentially leading to complex interactions. We examined how the relative proportion of seeds in a 3-species polyculture changes rodent seed selectivity (i.e., removal) and indirect interactions among seeds. We presented 2 rodent species, Peromyscus maniculatus (deer mice) and Perognathus parvus (pocket mice), in arenas with 3-species seed mixtures that varied in the proportion of a highly preferred, moderately preferred, and least preferred seed species, based on preferences determined in this study. We then conducted a field experiment in a pocket mouse—dominated ecosystem with the same 3-species seed mixtures in both “treated” (reduced shrub and increased forb cover) and “untreated” shrubsteppe. In the arena experiment, we found that rodents removed more of the highly preferred seed when the proportions of all 3 seeds were equal. Moderately preferred seeds experienced increased removal when the least preferred seed was in highest proportion. Removal of the least preferred seed increased when the highly preferred seed was in highest proportion. In the field experiment, results were similar to those from the arena experiment and did not differ between treated and untreated shrubsteppe areas. Though our results suggest that 3-species mixtures induce complex interactions among seeds, managers applying these results to restoration efforts should carefully consider the rodent community present and the potential fate of removed seeds.

Cheatgrass (Bromus tectorum) Dominates Cheek Pouch Contents of the Great Basin Pocket Mouse (Perognathus Parvus)

Most of the native shrubsteppe habitat in the northern Columbia Basin of eastern Washington has been invaded by cheatgrass (Bromus tectorum) or converted to agricultural lands. Therefore, ecological patterns and dynamics on native shrubsteppe and reestablished grasslands are of high conservation interest. We analyzed the cheek pouch contents of Great Basin pocket mice (Perognathus parvus) from 48 study sites in this region to quantify seed collection by this species and to determine the influence of habitat type on cheek pouch seed contents. In all 3 habitat types-newly established Conservation Reserve Program (CRP) lands, older CRP lands, and shrubsteppe-B. tectorum constituted the majority of seeds collected from the cheek pouches. Mean generic richness of collected seeds was higher in shrubsteppe than in new CRP habitats. On average, females collected a greater number of seed genera than males did, and pocket mice collected more seed genera as the autumn season progressed. Exotic B. tectorum has become the most frequently collected autumn food resource for pocket mice in the northern Columbia Basin.

Dispersal of an herbaceous perennial, Paeonia brownii, by scatter-hoarding rodents

Most plants that are dispersed by seed-caching animals are large, woody trees that produce large, nutritious nuts. But a few species dispersed in this way are relatively small shrubs or perennial herbs. Wild peony (Paeonia brownii) is a perennial herb in western North America that is dispersed by seed-caching rodents such as chipmunks (Tamias sp.), deer mice (Peromyscus maniculatus), and pocket mice (Perognathus parvus). These rodents harvest seeds from the dehiscent, pendant pods and transport them short distances (most <20 m) and cache 1 or a few seeds from 0 to 15 mm deep in soil. Unrecovered seeds germinate in the spring. Unlike most nuts, peony seeds are not highly preferred food items; they are rich in carbohydrates and low in lipids and protein. Rodents remove peony seeds slowly compared to Jeffrey pine (Pinus jeffreyi) seeds, which are highly preferred by rodents and dispersed in the same manner. The low preference for peony seeds may benefit the plants: peony seeds are slow to be harvested and cached, but also slow to be removed from caches and eaten. Small herbaceous plants cannot produce large crops of large, attractive seeds to satiate potential seed dispersers, as do most nut-bearing trees, so producing low-preference food items probably helps these types of plants to ensure that some of the seeds survive to germinate.

Seed harvesting is influenced by associational effects in mixed seed neighbourhoods, not just by seed density

Summary Rodents frequently forage in a density‐dependent manner, increasing harvesting in patches with greater seed densities. Although seldom considered, seed harvesting may also depend on the species identities of other individuals in the seed neighbourhood. When the seed harvest of a focal species increases in association with another seed species, the focal species suffers from Associational Susceptibility. In contrast, if seeds of the focal species are harvested less when in association with a second species, the focal species benefits from Associational Resistance. To evaluate density dependence and associational effects among seeds in mixtures, we conducted seed removal experiments using a completely additive design patterned after a two‐species competition experiment using seeds of either Achnatherum hymenoides (Indian ricegrass), Leymus cinereus (basin wildrye) or Pseudoroegneria spicata (bluebunch wheatgrass), all native perennial grasses, combined with seeds of Bromus tectorum (cheatgrass), a non‐native annual grass. The experiment involved placing five fixed quantities of the native seeds mixed with five fixed quantities of B. tectorum seeds in a factorial design, resulting in 35 seed mixture combinations. The seed‐eating rodent community at our study sites, in order of abundance, is composed of Peromyscus maniculatus (North American deer mouse), Dipodomys ordii (Ord's kangaroo rat) and Perognathus parvus (Great Basin pocket mouse). Native seed harvesting was density dependent, with a greater proportion of seeds being harvested as density increased. In the mixed density model, the presence of B. tectorum did not affect harvest of any of the native species' seeds when analysed individually. However, when all three native species were analysed together, increasing quantities of B. tectorum resulted in reduced harvest of native seeds, demonstrating weak but significant Associational Resistance. In contrast, harvest of B. tectorum seeds increased when in combination with any of the native seed species individually, indicating relatively strong Associational Susceptibility. These results demonstrate that seed harvest is determined not just by seed density, but also by the local seed neighbourhood and suggest that associational effects between native seeds and B. tectorum can occur in field conditions. The ecological implications of seed selection and associational effects on plant populations in natural and managed systems are also discussed.

Abundance and diversity of small mammals in response to various linear habitats in semi-arid agricultural landscapes

We investigated the influence of linear habitats, three types of hedgerows and two types of field margins, on the small mammal community within a semi-arid agricultural landscape in south-central British Columbia, Canada. We examined whether abundance, species richness, and species diversity of small mammals in tree fruit orchards differed from linear, edge habitats along orchard margins. Our five edges spanned grassy to wooded habitats. Linear habitats were intensively sampled for vascular plants in 2003 and small mammal communities from 2003 to 2007. Vegetation and total abundance of small mammal species among three types of hedgerows were similar and population changes followed those within nearby apple orchards. Species richness and diversity of small mammals, however, were significantly higher in hedgerows than orchards. Fewer mammals occupied hedgerows with high volumes of herb and shrub biomass, but richness and diversity of mammals did increase with shrub volume. All seven species of small mammals were represented in the hedgerow communities, including two species at risk: the Great Basin pocket mouse Perognathus parvus and the western harvest mouse (Reithrodontomys megalotis). Two types of field margins (orchard-sagebrush and orchard-old field) along fencerows provided habitat for P. parvus. Field margins managed for herbaceous plant species along fencerows and other such linear configurations provided habitat for R. megalotis. Linear non-crop habitats should maintain the overall small mammal community, as well as the two species at risk, within this semi-arid agricultural landscape.

Effects of rodent species, seed species, and predator cues on seed fate

Seed selection, removal and subsequent management by granivorous animals is thought to be a complex interaction of factors including qualities of the seeds themselves (e.g., seed size, nutritional quality) and features of the local habitat (e.g. perceived predator risk). At the same time, differential seed selection and dispersal is thought to have profound effects on seed fate and potentially vegetation dynamics. In a feeding arena, we tested whether rodent species, seed species, and indirect and direct predation cues influence seed selection and handling behaviors (e.g., scatter hoarding versus larder hoarding) of two heteromyid rodents, Ord’s kangaroo rat ( Dipodomys ordii) and the Great Basin pocket mouse ( Perognathus parvus). The indirect cue was shrub cover, a feature of the environment. Direct cues, presented individually, were (1) control, (2) coyote ( Canis latrans) vocalization, (3) coyote scent, (4) red fox ( Vulpes vulpes) scent, or (5) short-eared owl ( Asio flammeus) vocalization. We offered seeds of three sizes: two native grasses, Indian ricegrass ( Achnatherum hymenoides) and bluebunch wheatgrass ( Pseudoroegneria spicata), and the non-native cereal rye ( Secale cereale), each in separate trays. Kangaroo rats preferentially harvested Indian ricegrass while pocket mice predominately harvested Indian ricegrass and cereal rye. Pocket mice were more likely to scatter hoard preferred seeds, whereas kangaroo rats mostly consumed and/or larder hoarded preferred seeds. No predator cue significantly affected seed preferences. However, both species altered seed handling behavior in response to direct predation cues by leaving more seeds available in the seed pool, though they responded to different predator cues. If these results translate to natural dynamics on the landscape, the two rodents are expected to have different impacts on seed survival and plant recruitment via their different seed selection and seed handling behaviors.

Seed dispersal by scatter‐hoarding rodents in arid environments

Summary 1. The seeds of many plants are dispersed by animals, but the nature of these plant–animal mutualisms is often moulded by the abiotic environment. Here, we show that desert peach (Prunus andersonii), with dry fruits and large seeds, relies on scatter‐hoarding rodents for dispersal and that this form of seed dispersal maintains the effectiveness of dispersal while reducing water expenditure. 2. The fruit pulp of desert peach dries and dehisces at maturity in early summer. No vertebrate frugivores consumed the fruits, but rodents quickly harvested the nuts. Nearly 75% of the nut crop was removed from plant canopies by scansorial rodents like white‐tailed antelope squirrels (Ammospermophilus leucurus), which scatter‐hoarded nuts in 1–2 seed caches 10–30 mm deep. Deer mice (Peromyscus maniculatus) also made many shallow one‐nut caches. 3. Heteromyid rodents (Great Basin pocket mouse, Perognathus parvus; Panamint kangaroo rat, Dipodomys panamintinus) primarily larder‐hoarded nuts (c. 60%) in burrows too deep for seedling emergence, but also scatter‐hoarded nuts in a few, large caches 10–40 mm deep. 4. Antelope squirrels were the most effective dispersers and deer mice and Panamint kangaroo rats were less effective. Abiotically dispersed nuts had virtually no recruitment (&lt;1% emergence), but nuts buried in soil to simulate rodent caches had 32% emergence and 5.6% survived after 1 year. Scatter‐hoarding rodents are responsible for virtually all recruitment in desert peach. 5. Synthesis. Desert peach arose from a fleshy‐fruited ancestor that was probably dispersed by endozoochorous frugivores. About 35 species of dry‐fruited Prunus occur in arid environments across North America and Eurasia. The most parsimonious explanation for the evolution of dry‐fruited Prunus is the loss of frugivory in a diplochorous ancestor (i.e. combined frugivore and scatter‐hoarding dispersal) to maintain the effectiveness of seed dispersal while reducing the demand for water in arid environments.

Habitat selection of rodents along a piñon–juniper woodland–savannah gradient

The biodiversity of pinon-juniper (P-J) woodlands of western North America has received increasing attention from conservationists because of concerns about woodland invasion into adjacent shrub-steppe communities. Small mammals contribute substantially to the faunal diversity of P-J woodlands, but the responses of small mammals to the inherent structural complexity of these woodlands have not been well described. We used occupancy modeling and ordination to describe habitat selection along a woodland-savannah gradient of 4 species of rodents, including 2 putative P-J woodland specialists, Peromyscus truei and Tamias dorsalis .W e accounted for the influences of season and prior conspecific capture on detectability. P. truei and T. dorsalis preferred old-growth woodland sites near rock outcrops with high canopy and bare-ground cover and low herb and shrub cover. Tamias minimus preferred savannah sites with high shrub and grass cover farther from rock outcrops. We found no evidence of resource selection along the gradient by Perognathus parvus. Our results provide insight into the role of habitat selection in facilitating coexistence among closely related species within P-J woodlands and suggest that management efforts that better discriminate between old, presettlement stands and young, invasive stands may enhance regional biodiversity. We demonstrate an effective application of occupancy modeling for describing habitat selection of cryptic rodents, which are particularly prone to imperfect detection. DOI: 10.1644/09-MAMM-A-060.1.

Dynamics of Peripheral Populations of Great Basin Pocket Mice, &lt;em&gt;Perognathus parvus&lt;/em&gt;, and Western Harvest Mice, &lt;em&gt;Reithrodontomys megalotis&lt;/em&gt;, in Southern British Columbia

The Great Basin Pocket Mouse (Perognathus parvus) and Western Harvest Mouse (Reithrodontomys megalotis) are two peripheral species occurring in the southern Okanagan Valley of British Columbia, Canada. Both species are listed as vulnerable to extirpation because of habitat loss, primarily due to conversion of natural habitat to agricultural uses and suburban expansion. Population dynamics of these two species were studied in three habitat types: old field, sagebrush, and pine forest. The Great Basin Pocket Mouse occurred at densities ranging from 12 to 28/ha in sagebrush habitats and at 2-8/ha in old fields and Ponderosa Pine forest. The Western Harvest Mouse occurred at variable densities up to 10/ha in old fields and up to 5/ha in sagebrush habitats. Mean number of lactating females for Great Basin Pocket Mice ranged from 4-8 in sagebrush, 1-5 in old fields and pine forests combined. Mean juvenile survival to adulthood ranged from 3.28 young Great Basin Pocket Mice per pregnant female in sagebrush, 4.67 in old field, and 1.82 in pine forest habitats. Mean juvenile survival to adulthood of Western Harvest Mice ranged from 1.46-1.72 young per female in old field and sagebrush habitats. Conservation of habitat features (high biomass and structural diversity of grasses and forbs) in linear habitats has the potential to maintain populations of Western Harvest Mice. The Great Basin Pocket Mouse needs features of sagebrush and old field habitats that need to be conserved as natural non-linear components in mosaics of natural and anthropogenic habitats. Both species could act as “indicators” of habitat integrity for a wide range of other vertebrate, invertebrate, and plant species in the Okanagan Valley.

HABITAT AFFINITIES OF RODENTS IN NORTHEASTERN NEVADA RANGELAND COMMUNITIES

Abstract From May 1978 through September 1980, baseline data for rodent populations were collected by live-trapping in the Saval Ranch area of northeastern Nevada. The objectives of this study were to determine species composition and relative abundance of rodents for the various range sites. The 3 most abundant and uniformly distributed species were deer mice (Peromyscus maniculatus), Great Basin pocket mice (Perognathus parvus), and least chipmunks (Neotamias minimus). Relative abundance indices and densities reported for most species were similar to those reported in other multiyear studies at similar sites. The composition and relative abundance/density of rodent species was variable among habitats and over time. Diverse rodent species within the sagebrush ecosystem have commensurately diverse habitat requirements that should be considered when vegetation management is planned. Annual population fluctuations and differences among similar habitats should be considered when researchers propose to test r...

MARK–RECAPTURE ESTIMATES OF POPULATION PARAMETERS FOR SELECTED SPECIES OF SMALL MAMMALS

We used program CAPTURE to analyze 1,535 capture–recapture data sets from 33 species of small mammals for sources of variation in capture probabilities and to characterize species-specific responses to mark–recapture trapping. Program CAPTURE contains 8 models to account for all possible combinations of heterogeneity (h), behavioral response (b), and time (t) effects on capture probabilities. Model Mo (equal capture probabilities) was selected most often by CAPTURE as the most appropriate model for small sample sizes, reflecting the difficulty of detecting unequal probabilities of capture in small data sets. Heterogeneity in capture probabilities among individuals was evident in larger data sets for all species of small mammals examined. Strong behavioral responses were detected in several species. Sylvilagus, Tamias amoenus, Glaucomys sabrinus, Perognathus longimembris, and Microtus ochrogaster were trap shy, whereas Tamias striatus, Tamias townsendii, Spermophilus richardsonii, Perognathus parvus, Reithrodontomys megalotis, Peromyscus maniculatus, Peromyscus truei, Sigmodon hispidus, Microtus californicus, and Microtus pennsylvanicus were trap prone. Time effects were not apparent for most species except where they were accompanied with heterogeneity. Murid species had the highest estimated probabilities of capture, followed by heteromyids, sciurids, and leporids in decreasing order. Estimates of abundance from model Mh were significantly greater than those from models Mb, Mo, and Mt, which likely reflects the negative bias of these latter estimators.

Effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla).

The effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla) was investigated in field enclosures. The rodent species were Panamint kangaroo rat (Dipodomys panamintinus), Great Basin pocket mouse (Perognathus parvus), pinyon mouse (Peromyscus truei), deer mouse (Peromyscus maniculatus), least chipmunk (Tamias minimus), and white-tailed antelope ground squirrel (Ammospermophilus leucurus). Five measures of seed harvesting and caching behavior were assessed and used to calculate the effect of species on the probability of effective dispersal (i.e., movement of seeds to potential establishment sites) of piñon pine seeds. These measures were proportion of seeds harvested, proportion of harvested seeds that were scatter hoarded, relative dispersion of scatter-hoarded seeds, seedling emergence and seedling establishment. To evaluate the effects of the last two variables, a 2-year study of seed germination and seedling establishment was conducted in the field. Seedling survival was 72-90% for seedlings emerging from 1-4 cm deep but was markedly lower from surface caches and for depths >4 cm. All seedlings in the open microsite perished before the end of the first year, whereas 38-97% of seedlings under shrubs survived. Cache size did not affect seedling survival over the 2 years, but large caches reduced the total number of potential establishment sites. All six species harvested over 90% of the piñon seeds presented and scatter hoarded from 31% (pocket mouse) to 66% (chipmunk) of the seeds they harvested. Piñon mice and deer mice distributed seeds to more cache sites (i.e., put fewer seeds in caches) than the other species and, consequently, created more potential establishment sites. All species scatter hoarded seeds at depths suitable for seedling emergence, but pocket mice and piñon mice cached under shrubs more than the other species, the microhabitat where seedlings had the greatest probability of establishing. Factors that had the greatest effect on the probability of effective dispersal were microhabitat selection and cache size.

INTERSPECIFIC VARIATION IN THE OLFACTORY ABILITIES OF GRANIVOROUS RODENTS

Olfaction plays an important role in the foraging behavior of rodents, but little is known about how this sense varies among taxa that evolved in different ecological settings. Field experiments were conducted in 10- by 10-m enclosures to test for interspecific differences in foraging success for ponderosa pine (Pinus ponderosa) or sunflower seeds, based on olfaction, of 4 rodent species: 2 heteromyid rodents (Panamint kangaroo rat, Dipodomys panamintinus, and Great Basin pocket mouse, Perognathus parvus) and 2 nonheteromyid rodents (yellow pine chipmunk, Tamias amoenus, and deer mouse, Peromyscus maniculatus). All species found large, shallow caches more frequently than small, deep caches. Chipmunks had the lowest foraging success for buried sunflower seeds under dry conditions. Deer mice did as well as the heteromyid rodents on large shallow caches but found relatively few small, deep caches. The heteromyids were the only species that found small, deep sunflower seed caches under dry conditions, and Panamint kangaroo rats always found more caches than did Great Basin pocket mice. These results confirm that olfaction plays an important role in foraging of rodents for buried seeds and support the notion that the olfactory sense of rodent species has evolved greater sensitivity to seeds in different ecological contexts.