Conversion of sagebrush shrublands to exotic annual grasslands negatively impacts small mammal communities

Aim The exotic annual cheatgrass (Bromus tectorum) is fast replacing sagebrush (Artemisia tridentata) communities throughout the Great Basin Desert and nearby regions in the Western United States, impacting native plant communities and altering fire regimes, which contributes to the long‐term persistence of this weedy species. The effect of this conversion on native faunal communities remains largely unexamined. We assess the impact of conversion from native perennial to exotic annual plant communities on desert rodent communities. Location Wyoming big sagebrush shrublands and nearby sites previously converted to cheatgrass‐dominated annual grasslands in the Great Basin Desert, Utah, USA. Methods At two sites in Tooele County, Utah, USA, we investigated with Sherman live trapping whether intact sagebrush vegetation and nearby converted Bromus tectorum‐dominated vegetation differed in rodent abundance, diversity and community composition. Results Rodent abundance and species richness were considerably greater in sagebrush plots than in cheatgrass‐dominated plots. Nine species were captured in sagebrush plots; five of these were also trapped in cheatgrass plots, all at lower abundances than in the sagebrush. In contrast, cheatgrass‐dominated plots had no species that were not found in sagebrush. In addition, the site that had been converted to cheatgrass longer had lower abundances of rodents than the site more recently converted to cheatgrass‐dominated plots. Despite large differences in abundances and species richness, Simpson’s D diversity and Shannon‐Wiener diversity and Brillouin evenness indices did not differ between sagebrush and cheatgrass‐dominated plots. Main conclusions This survey of rodent communities in native sagebrush and in converted cheatgrass‐dominated vegetation suggests that the abundances and community composition of rodents may be shifting, potentially at the larger spatial scale of the entire Great Basin, where cheatgrass continues to invade and dominate more landscape at a rapid rate.     

Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.     

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

Invasion 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.     

Exotic plants increase and native plants decrease with loss of foundation species in sagebrush steppe

Dominant plant species, or foundation species, are recognized to have a disproportionate control over resources in ecosystems, but few studies have evaluated their relationship to exotic invasions. Loss of foundation species could increase resource availability to the benefit of exotic plants, and could thereby facilitate invasion. The success of exotic plant invasions in sagebrush steppe was hypothesized to benefit from increased available soil water following removal of sagebrush (Artemisia tridentata), a foundation species. We examined the effects of sagebrush removal, with and without the extra soil water made available by exclusion of sagebrush, on abundance of exotic and native plants in the shrub steppe of southern Idaho, USA. We compared plant responses in three treatments: undisturbed sagebrush steppe; sagebrush removed; and sagebrush removed plus plots covered with “rainout” shelters that blocked winter-spring recharge of soil water. The third treatment allowed us to examine effects of sagebrush removal alone, without the associated increase in deep-soil water that is expected to accompany removal of sagebrush. Overall, exotic herbs (the grass Bromus tectorum and four forbs) were 3–4 times more abundant in shrub-removal and 2 times more abundant in shrub-removal + rainout-shelter treatments than in the control treatment, where sagebrush was undisturbed. Conversely, native forbs were only about half as abundant in shrub removal compared to control plots. These results indicate that removal of sagebrush facilitates invasion of exotic plants, and that increased soil water is one of the causes. Our findings suggest that sagebrush plays an important role in reducing invasions by exotic plants and maintaining native plant communities, in the cold desert we evaluated.     

Distribution of ecosystem C and N within contrasting vegetation types in a semiarid rangeland in the Great Basin,USA

Semiarid sagebrush ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions, but the effects on ecosystem C and N dynamics are poorly understood. We compared ecosystem C and N pools to 1 m depth among historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities, to examine whether the quantity and quality of plant inputs to soil differs among vegetation types. Natural abundance δ¹⁵N isotope ratios were used to examine differences in ecosystem N balance. Sagebrush-dominated sites had greater C and N storage in plant biomass compared to perennial or annual grass systems, but this was predominantly due to woody biomass accumulation. Plant C and N inputs to soil were greatest for cheatgrass compared to sagebrush and crested wheatgrass systems, largely because of slower root turnover in perennial plants. The organic matter quality of roots and leaf litter (as C:N ratios) was similar among vegetation types, but lignin:N ratios were greater for sagebrush than grasses. While cheatgrass invasion has been predicted to result in net C loss and ecosystem degradation, we observed that surface soil organic C and N pools were greater in cheatgrass and crested wheatgrass than sagebrush-dominated sites. Greater biomass turnover in cheatgrass and crested wheatgrass versus sagebrush stands may result in faster rates of soil C and N cycling, with redistribution of actively cycled N towards the soil surface. Plant biomass and surface soil δ¹⁵N ratios were enriched in cheatgrass and crested wheatgrass relative to sagebrush-dominated sites. Source pools of plant available N could become ¹⁵N enriched if faster soil N cycling rates lead to greater N trace gas losses. In the absence of wildfire, if cheatgrass invasion does lead to degradation of ecosystem function, this may be due to faster nutrient cycling and greater nutrient losses, rather than reduced organic matter inputs.     

Declines in plant species richness and endemic plant species in longleaf pine savannas invaded by <Emphasis Type="Italic">Imperata cylindrica</Emphasis>

Despite the widespread perception that non-native species threaten biodiversity, there are few documented cases of non-native species displacing rare or specialized native species. Here, I examined changes in plant species composition over 5 years during patch expansion of a non-native grass, Imperata cylindrica, in longleaf pine flatwoods in Mississippi, USA. I used a multivariate approach to quantify the degree of habitat specialization and geographic range of all species encountered. I examined losses of species collectively as a function of plant height (controlling for initial frequency) and then the relationship between height and the degree of association with longleaf pine flatwoods, disturbed habitats, and the outer Gulf Coastal Plain of the southeastern USA. Patch expansion resulted in dramatic declines in species richness and increases in ground-level shade at both sites in just 3 years. Most tall saplings, shrubs, and vines were not endemic to longleaf pine communities and were less likely to be displaced than short herbs, most of which were indicative of longleaf pine communities. These results suggest that invasion of longleaf pine communities by I. cylindrica will likely cause significant losses of short, habitat-specialists and reduce the distinctiveness of the native flora of these threatened ecosystems.     

Relative Abundance of and Composition within Fungal Orders Differ between Cheatgrass (Bromus tectorum) and Sagebrush (Artemisia tridentata)-Associated Soils

Nonnative Bromus tectorum (cheatgrass) is decimating sagebrush steppe, one of the largest ecosystems in the Western United States, and is causing regional-scale shifts in the predominant plant-fungal interactions. Sagebrush, a native perennial, hosts arbuscular mycorrhizal fungi (AMF), whereas cheatgrass, a winter annual, is a relatively poor host of AMF. This shift is likely intertwined with decreased carbon (C)-sequestration in cheatgrass-invaded soils and alterations in overall soil fungal community composition and structure, but the latter remain unresolved. We examined soil fungal communities using high throughput amplicon sequencing (ribosomal large subunit gene) in the 0–4 cm and 4–8 cm depth intervals of six cores from cheatgrass- and six cores from sagebrush-dominated soils. Sagebrush core surfaces (0–4 cm) contained higher nitrogen and total C than cheatgrass core surfaces; these differences mirrored the presence of glomalin related soil proteins (GRSP), which has been associated with AMF activity and increased C-sequestration. Fungal richness was not significantly affected by vegetation type, depth or an interaction of the two factors. However, the relative abundance of seven taxonomic orders was significantly affected by vegetation type or the interaction between vegetation type and depth. Teloschistales, Spizellomycetales, Pezizales and Cantharellales were more abundant in sagebrush libraries and contain mycorrhizal, lichenized and basal lineages of fungi. Only two orders (Coniochaetales and Sordariales), which contain numerous economically important pathogens and opportunistic saprotrophs, were more abundant in cheatgrass libraries. Pleosporales, Agaricales, Helotiales and Hypocreales were most abundant across all libraries, but the number of genera detected within these orders was as much as 29 times lower in cheatgrass relative to sagebrush libraries. These compositional differences between fungal communities associated with cheatgrass- and sagebrush-dominated soils warrant future research to examine soil fungal community composition across more sites and time points as well as in association with native grass species that also occupy cheatgrass- invaded ecosystems.     

Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species

Sporadic summer rainfall in semi-arid ecosystems can provide enough soil moisture to drastically increase CO₂ efflux and rates of soil N cycling. The magnitudes of C and N pulses are highly variable, however, and the factors regulating these pulses are poorly understood. We examined changes in soil respiration, bacterial, fungal and microfaunal populations, and gross rates of N mineralization, nitrification, and NH₄⁺ and NO₃^– immobilization during the 10 days following wetting of dry soils collected from stands of big sagebrush (Artemisia tridentata) and cheatgrass (Bromus tectorum) in central Utah. Soil CO₂ production increased more than tenfold during the 17 h immediately following wetting. The labile organic C pool released by wetting was almost completely respired within 2–3 days, and was nearly three times as large in sagebrush soil as in cheatgrass. In spite of larger labile C pools beneath sagebrush, microbial and microfaunal populations were nearly equal in the two soils. Bacterial and fungal growth coincided with depletion of labile C, and populations peaked in both soils 2 days after wetting. Protozoan populations, whose biomass was nearly 3,000-fold lower than bacteria and fungi, peaked after 2–4 days. Gross N mineralization and nitrification rates were both faster in cheatgrass soil than in sagebrush, and caused greater nitrate accumulation in cheatgrass soil. Grazing of bacteria and fungi by protozoans and nematodes could explain neither temporal trends in N mineralization rates nor differences between soil types. However, a mass balance model indicated that the initial N pulse was associated with degradation of microbial substrates that were rich in N (C:N <8.3), and that microbes had shifted to substrates with lower N contents (C:N =15–25) by day 7 of the incubation. The model also suggested that the labile organic matter in cheatgrass soil had a lower C:N ratio than in sagebrush, and this promoted faster N cycling rates and greater N availability. This study provides evidence that the high N availability often associated with wetting of cheatgrass soils is a result of cheatgrass supplying substrates to microbes that are of high decomposability and N content.     

Invasion resistance and persistence: established plants win,even with disturbance and high propagule pressure

Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 × 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plant-available soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.     

Do invasive plants structure microbial communities to accelerate decomposition in intermountain grasslands?

Invasive plants are often associated with greater productivity and soil nutrient availabilities, but whether invasive plants with dissimilar traits change decomposer communities and decomposition rates in consistent ways is little known. We compared decomposition rates and the fungal and bacterial communities associated with the litter of three problematic invaders in intermountain grasslands; cheatgrass (Bromus tectorum), spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula), as well as the native bluebunch wheatgrass (Pseudoroegneria spicata). Shoot and root litter from each plant was placed in cheatgrass, spotted knapweed, and leafy spurge invasions as well as remnant native communities in a fully reciprocal design for 6 months to see whether decomposer communities were species‐specific, and whether litter decomposed fastest when placed in a community composed of its own species (referred to hereafter as home‐field advantage–HFA). Overall, litter from the two invasive forbs, spotted knapweed and leafy spurge, decomposed faster than the native and invasive grasses, regardless of the plant community of incubation. Thus, we found no evidence of HFA. T‐RFLP profiles indicated that both fungal and bacterial communities differed between roots and shoots and among plant species, and that fungal communities also differed among plant community types. Synthesis. These results show that litter from three common invaders to intermountain grasslands decomposes at different rates and cultures microbial communities that are species‐specific, widespread, and persistent through the dramatic shifts in plant communities associated with invasions.     

Seed harvesting by a generalist consumer is context‐dependent: Interactive effects across multiple spatial scales

Granivore foraging decisions affect consumer success and determine the quantity and spatial pattern of seed survival. These decisions are influenced by environmental variation at spatial scales ranging from landscapes to local foraging patches. In a field experiment, the effects of seed patch variation across three spatial scales on seed removal by western harvester ants Pogonomyrmex occidentalis were evaluated. At the largest scale we assessed harvesting in different plant communities, at the intermediate scale we assessed harvesting at different distances from ant mounds, and at the smallest scale we assessed the effects of interactions among seed species in local seed neighborhoods on seed harvesting (i.e. resource–consumer interface). Selected seed species were presented alone (monospecific treatment) and in mixture with Bromus tectorum (cheatgrass; mixture treatment) at four distances from P. occidentalis mounds in adjacent intact sagebrush and non‐native cheatgrass‐dominated communities in the Great Basin, Utah, USA. Seed species differed in harvest, with B. tectorum being least preferred. Large and intermediate scale variation influenced harvest. More seeds were harvested in sagebrush than in cheatgrass‐dominated communities (largest scale), and the quantity of seed harvested varied with distance from mounds (intermediate‐scale), although the form of the distance effect differed between plant communities. At the smallest scale, seed neighborhood affected harvest, but the patterns differed among seed species considered. Ants harvested fewer seeds from mixed‐seed neighborhoods than from monospecific neighborhoods, suggesting context dependence and potential associational resistance. Further, the effects of plant community and distance from mound on seed harvest in mixtures differed from their effects in monospecific treatments. Beyond the local seed neighborhood, selection of seed resources is better understood by simultaneously evaluating removal at multiple scales. Associational effects provide a useful theoretical basis for better understanding harvester ant foraging decisions. These results demonstrate the importance of ecological context for seed removal, which has implications for seed pools, plant populations and communities.     

The potential importance of unburned islands as refugia for the persistence of wildlife species in fire‐prone ecosystems

相似文献   

Effects of grazing and invasive grasses on desert vertebrates in California

Much of California's San Joaquin Valley is a desert and, like portions of other North American deserts, is experiencing an ecological shift from being dominated by ephemeral native forbs, with widely spaced shrubs, to fire-prone non-native annual grasses. Small terrestrial vertebrates, many of which are adapted to open desert habitats, are declining. One hypothesis is that the invasive plants contribute to the decline by altering vegetative structure. Although cattle may have originally been a factor in the establishment of these non-native plants, their grazing may benefit the terrestrial vertebrates by maintaining an open structure, especially during average or wet winters when the exotic grasses grow especially dense. We experimentally tested the effect of cattle grazing on invasive plants and a community of small vertebrates at a site in the southwestern San Joaquin Desert. We established and monitored 4 treatment (grazed) and 4 control (ungrazed) plots from 1997 to 2006, and assessed the abundances of blunt-nosed leopard lizards (Gambelia sila), giant kangaroo rats (Dipodomys ingens), short-nosed kangaroo rats (Dipodomys nitratoides nitratoides), and San Joaquin antelope squirrels (Ammospermophilus nelsoni), all of which are listed as threatened or endangered by state or federal agencies. We also recorded abundances of the non-protected western whiptail lizards (Aspidoscelis tigris), side-blotched lizards (Uta stansburiana), San Joaquin pocket mice (Perognathus inornatus inornatus), and Heermann's kangaroo rats (Dipdomys heermanni). Based on repeated measures analysis of variance (ANOVA) and a 0.05 alpha level, only Heermann's kangaroo rats showed a treatment effect; they were more abundant on the control plots. However, this effect could be accounted for by the natural re-establishment of saltbush (Atriplex spp.) on part of the study site. Saltbush return also favored western whiptail lizards and San Joaquin antelope squirrels. A regression analysis indicated that populations of blunt-nosed leopard lizard and giant kangaroo rat increased significantly faster in grazed plots than the ungrazed controls, and abundances of 6 of 8 species were negatively correlated with increased residual dry matter. With relaxed alpha values to decrease Type II error, populations of blunt-nosed leopard lizards (500% greater), San Joaquin antelope squirrels (85% greater), and short-nosed kangaroo rats (73% greater) increased significantly on grazed plots over the course of the study compared to ungrazed plots. We did not find grazing to negatively affect abundance of any species we studied. When herbaceous cover is low during years of below average rainfall in deserts and other arid areas, grazing may not be necessary to maintain populations of small vertebrates. However, if cattle grazing is closely monitored in space and time to minimize adverse effects on the habitat, it could be an effective tool to control dense stands of non-native grasses and benefit native wildlife. © 2011 The Wildlife Society.     

Rapid evolution in response to introduced predators I: rates and patterns of morphological and life-history trait divergence

Background  

Introduced species can have profound effects on native species, communities, and ecosystems, and have caused extinctions or declines in native species globally. We examined the evolutionary response of native zooplankton populations to the introduction of non-native salmonids in alpine lakes in the Sierra Nevada of California, USA. We compared morphological and life-history traits in populations of Daphnia with a known history of introduced salmonids and populations that have no history of salmonid introductions.     

Native granivores reduce the establishment of native grasses but not invasive <Emphasis Type="Italic">Bromus tectorum</Emphasis>

Seed predation can structure plant communities by imposing strong population controls on some species but not others. In this context, studies from various ecosystems report that native granivores selectively forage for seeds from native species over seeds from exotic invaders, which could disproportionately favor the establishment of invaders and facilitate their dominance in communities. However, few studies have connected selective foraging for native seeds to differential patterns of establishment among native and invasive species. Thus, the extent to which preferential foraging for native seeds favors the establishment of invasive plants is unclear. Here, we used experimental seed additions and exclosure treatments at five field sites distributed across?≈?80,000 km² of the Great Basin Desert, USA to compare the effects of rodent foraging on the establishment of less-preferred cheatgrass (Bromus tectorum—an annual species native to Eurasia that is exotic and highly invasive across the Great Basin) and four species of more-preferred native grasses that commonly co-occur with cheatgrass. Rodent foraging reduced the establishment of each native species by at least 80% but had no effect on the establishment of cheatgrass, and this finding was consistent across study sites. Our results suggest that selective foraging for native species may favor the establishment of cheatgrass over native grasses, potentially exacerbating one of the most extensive plant invasions in North America.     

Adapting management to a changing world: Warm temperatures,dry soil,and interannual variability limit restoration success of a dominant woody shrub in temperate drylands

Restoration and rehabilitation of native vegetation in dryland ecosystems, which encompass over 40% of terrestrial ecosystems, is a common challenge that continues to grow as wildfire and biological invasions transform dryland plant communities. The difficulty in part stems from low and variable precipitation, combined with limited understanding about how weather conditions influence restoration outcomes, and increasing recognition that one‐time seeding approaches can fail if they do not occur during appropriate plant establishment conditions. The sagebrush biome, which once covered over 620,000 km² of western North America, is a prime example of a pressing dryland restoration challenge for which restoration success has been variable. We analyzed field data on Artemisia tridentata (big sagebrush) restoration collected at 771 plots in 177 wildfire sites across its western range, and used process‐based ecohydrological modeling to identify factors leading to its establishment. Our results indicate big sagebrush occurrence is most strongly associated with relatively cool temperatures and wet soils in the first spring after seeding. In particular, the amount of winter snowpack, but not total precipitation, helped explain the availability of spring soil moisture and restoration success. We also find considerable interannual variability in the probability of sagebrush establishment. Adaptive management strategies that target seeding during cool, wet years or mitigate effects of variability through repeated seeding may improve the likelihood of successful restoration in dryland ecosystems. Given consistent projections of increasing temperatures, declining snowpack, and increasing weather variability throughout midlatitude drylands, weather‐centric adaptive management approaches to restoration will be increasingly important for dryland restoration success.     

Negative Effects of an Exotic Grass Invasion on Small-Mammal Communities

Exotic invasive species can directly and indirectly influence natural ecological communities. Cheatgrass (Bromus tectorum) is non-native to the western United States and has invaded large areas of the Great Basin. Changes to the structure and composition of plant communities invaded by cheatgrass likely have effects at higher trophic levels. As a keystone guild in North American deserts, granivorous small mammals drive and maintain plant diversity. Our objective was to assess potential effects of invasion by cheatgrass on small-mammal communities. We sampled small-mammal and plant communities at 70 sites (Great Basin, Utah). We assessed abundance and diversity of the small-mammal community, diversity of the plant community, and the percentage of cheatgrass cover and shrub species. Abundance and diversity of the small-mammal community decreased with increasing abundance of cheatgrass. Similarly, cover of cheatgrass remained a significant predictor of small-mammal abundance even after accounting for the loss of the shrub layer and plant diversity, suggesting that there are direct and indirect effects of cheatgrass. The change in the small-mammal communities associated with invasion of cheatgrass likely has effects through higher and lower trophic levels and has the potential to cause major changes in ecosystem structure and function.     

Responses of native and non-native Mojave Desert winter annuals to soil disturbance and water additions

Habitat modification (i.e., disturbance) and resource availability have been identified as possible mechanisms that may influence the invasibility of plant communities. In the Mojave Desert, habitat disturbance has increased dramatically over the last 50 years due to increased human activities. Additionally, water availability is considered to be a main limiting resource for plant production. To elucidate the effects of soil disturbance and water availability on plant invasions, we created experimental patches where we varied the levels of soil disturbance and water availability in a fully crossed factorial experiment at five replicated field sites, and documented responses of native and non-native winter annuals. The treatments did not significantly affect the density (seedlings m⁻²) of the non-native forb, Brassica tournefortii. However, the relationship between silique production and plant height differed among treatments, with greater silique production in disturbed plots. In contrast to Brassica, density of the non-native Schismus spp. increased in soil disturbed and watered plots, and was greatest in disturbed plots during 2009 (the second year of the study). Species composition of the native annual community was not affected by treatments in 2008 but was influenced by treatments in 2009. The native forb Eriophyllum sp. was most dense on water-addition plots, while density of Chaenactis freemontii was highest in disturbed plots. Results illustrate that habitat invasibility in arid systems can be influenced by dynamics in disturbance regimes and water availability, and suggest that invasiveness can differ between non-native annual species and among native annuals in habitats undergoing changing disturbance and precipitation regimes. Understanding the mechanistic relationships between water availability and non-native plant responses will be important for understanding the effects of shifting precipitation and vegetation patterns under predicted climate change in arid ecosystems.     

Impact of grasshoppers and an invasive grass on establishment and initial growth of restoration plant species

Exotic plant invasion can have dramatic impacts on native plants making restoration of native vegetation at invaded sites challenging. Though invasives may be superior competitors, it is possible their dominance could be enhanced by insect herbivores if native plants are preferred food sources. Insect herbivory can regulate plant populations, but little is known of its effects in restoration settings. There is a need to better understand relationships between insect herbivores and invasive plants with regard to their combined potential for impacting native plant establishment and restoration success. The objective of this study was to assess impacts of grasshopper herbivory and the invasive grass Bromus tectorum (cheatgrass) on mortality and growth of 17 native plant species used in restoration of critical sagebrush steppe ecosystems. Field and greenhouse experiments were conducted using moderate densities of a common, generalist pest grasshopper (Melanoplus bivittatus). Grasshoppers had stronger and more consistent impacts on native restoration plants in field and greenhouse studies than cheatgrass. After 6 weeks in the greenhouse, grasshoppers were associated with 36% mortality over all native restoration species compared to 2% when grasshoppers were absent. Herbivory was also associated with an approximately 50% decrease in native plant biomass. However, effects varied among species. Artemisia tridentata, Chrysothamnus viscidiflorus, and Coreopsis tinctoria were among the most negatively impacted, while Oenothera pallida, Pascopyrum smithii, and Leymus cinerus were unaffected. These findings suggest restoration species could be selected to more effectively establish and persist within cheatgrass infestations, particularly when grasshopper populations are forecasted to be high.     

不同干扰下阿拉善荒漠啮齿动物优势种对气候变化的响应

气候变化已对物种分布范围和丰富度产生了极大的影响。荒漠生态系统对气候变化的反应可能更加敏感。作为荒漠生态系统的重要组成者,了解荒漠啮齿动物特别是优势鼠种将对气候变化如何响应,对于荒漠地区生物多样性的维持将具有重要意义。2002—2010年,采用标志重捕法对阿拉善荒漠4种不同生境下啮齿动物优势种群进行了研究,分别利用Spearman相关分析以及典范对应分析(Canonical Correspondence analysis,CCA)对啮齿动物优势种群动态与年平均温度和年降水量的相关性进行了分析。结果表明,不同啮齿动物优势种对温度和降雨的响应不同,尤其以子午沙鼠表现最为显著。跳鼠对温度的适宜性要高于仓鼠科的子午沙鼠和黑线仓鼠,而仓鼠科啮齿动物对降雨的适宜性高于跳鼠。较小尺度上的人为干扰更可能从改变食性和生境的途径上加剧或缓冲降雨对荒漠啮齿动物优势种的影响,而不是改变温度对啮齿动物的作用。