Introduced annual grass increases regional fire activity across the arid western USA (1980–2009)

Non‐native, invasive grasses have been linked to altered grass‐fire cycles worldwide. Although a few studies have quantified resulting changes in fire activity at local scales, and many have speculated about larger scales, regional alterations to fire regimes remain poorly documented. We assessed the influence of large‐scale Bromus tectorum (hereafter cheatgrass) invasion on fire size, duration, spread rate, and interannual variability in comparison to other prominent land cover classes across the Great Basin, USA. We compared regional land cover maps to burned area measured using the Moderate Resolution Imaging Spectroradiometer (MODIS) for 2000–2009 and to fire extents recorded by the USGS registry of fires from 1980 to 2009. Cheatgrass dominates at least 6% of the central Great Basin (650 000 km²). MODIS records show that 13% of these cheatgrass‐dominated lands burned, resulting in a fire return interval of 78 years for any given location within cheatgrass. This proportion was more than double the amount burned across all other vegetation types (range: 0.5–6% burned). During the 1990s, this difference was even more extreme, with cheatgrass burning nearly four times more frequently than any native vegetation type (16% of cheatgrass burned compared to 1–5% of native vegetation). Cheatgrass was also disproportionately represented in the largest fires, comprising 24% of the land area of the 50 largest fires recorded by MODIS during the 2000s. Furthermore, multi‐date fires that burned across multiple vegetation types were significantly more likely to have started in cheatgrass. Finally, cheatgrass fires showed a strong interannual response to wet years, a trend only weakly observed in native vegetation types. These results demonstrate that cheatgrass invasion has substantially altered the regional fire regime. Although this result has been suspected by managers for decades, this study is the first to document recent cheatgrass‐driven fire regimes at a regional scale.     

Exotic cheatgrass and loss of soil biota decrease the performance of a native grass

Soil disturbances can alter microbial communities including arbuscular mycorrhizal (AM) fungi, which may in turn, affect plant community structure and the abundance of exotic species. We hypothesized that altered soil microbial populations owing to disturbance would contribute to invasion by cheatgrass (Bromus tectorum), an exotic annual grass, at the expense of the native perennial grass, squirreltail (Elymus elymoides). Using a greenhouse experiment, we compared the responses of conspecific and heterospecific pairs of cheatgrass and squirreltail inoculated with soil (including live AM spores and other organisms) collected from fuel treatments with high, intermediate and no disturbance (pile burns, mastication, and intact woodlands) and a sterile control. Cheatgrass growth was unaffected by type of soil inoculum, whereas squirreltail growth, reproduction and nutrient uptake were higher in plants inoculated with soil from mastication and undisturbed treatments compared to pile burns and sterile controls. Squirreltail shoot biomass was positively correlated with AM colonization when inoculated with mastication and undisturbed soils, but not when inoculated with pile burn soils. In contrast, cheatgrass shoot biomass was negatively correlated with AM colonization, but this effect was less pronounced with pile burn inoculum. Cheatgrass had higher foliar N and P when grown with squirreltail compared to a conspecific, while squirreltail had lower foliar P, AM colonization and flower production when grown with cheatgrass. These results indicate that changes in AM communities resulting from high disturbance may favor exotic plant species that do not depend on mycorrhizal fungi, over native species that depend on particular taxa of AM fungi for growth and reproduction.     

Streetlights positively affect the presence of an invasive grass species

Anthropogenic disturbances associated with urban ecosystems can create favorable conditions for populations of some invasive plant species. Light pollution is one of these disturbances, but how it affects the growth and establishment of invasive plant populations is unknown. Cheatgrass (Bromus tectorum) is a problematic invasive species where it has displaced native grassland communities in the United States, but to our knowledge, there have been no studies of the ecological factors that affect cheatgrass presence in urban ecosystems. We conducted field surveys in urban alleys in Denver, Colorado, to compare the presence of cheatgrass at sites with and without artificial light at night (hereafter artificial light) from streetlights. These streetlights are mounted on utility poles, which cause ground disturbance when installed in alleys; we were able to test the independent effect of poles on cheatgrass establishment because not all poles have streetlights on them. We found that cheatgrass was positively associated with the presence of streetlights and to a lesser extent poles. In addition to cheatgrass, we also found that other plants were positively associated with the presence of both poles and streetlights. Our results suggest that artificial light may benefit the occurrence of cheatgrass and other plant species in urban settings. While invasive populations of cheatgrass in wild habitats attract the most attention from managers, we suggest more consideration for this grass in urban environments where its growth and establishment benefit from anthropogenic changes.     

Fire effects on the mobilization and uptake of nitrogen by cheatgrass (Bromus tectorum L.)

Cheatgrass (Bromus tectorum L.), an invasive annual grass, is displacing native species and causing increased fire frequency in the Great Basin of the southwestern United States. Growth and nitrogen uptake patterns by cheatgrass were examined in a greenhouse study using soils from sites with the same soil type but different fire histories: 1) an area that burned in 1999 that is now completely invaded with cheatgrass (CG); 2) an area that has not burned recently and is now dominated by Wyoming big sagebrush (Artemisia tridentatassp.wyomingensis Beetle and Young) and Sandberg’s bluegrass (Poa secunda J. Presl) (WBS); and 3) a Wyoming big sagebrush area that burned in August of 2008 just prior to soil collection (NB). Cheatgrass seedlings had higher leaf numbers, height and mass in the NB soil. Ammonium-N mobilized by fire in the NB soil had significantly enriched ¹⁵N than soils from CG or WBS sites and this pattern was reflected in the isotopic signatures of the plants. Fire-mobilized mineral N accounted for only 58% of N taken up by cheatgrass in the NB soil, suggesting fire enhanced the ability of cheatgrass to assimilate more recalcitrant soil organic N.     

The invasive annual cheatgrass releases more nitrogen than crested wheatgrass through root exudation and senescence

Plant-soil feedbacks are an important aspect of invasive species success. One type of feedback is alteration of soil nutrient cycling. Cheatgrass invasion in the western USA is associated with increases in plant-available nitrogen (N), but the mechanism for this has not been elucidated. We labeled cheatgrass and crested wheatgrass, a common perennial grass in western rangelands, with ¹⁵N-urea to determine if differences in root exudates and turnover could be a mechanism for increases in soil N. Mesocosms containing plants were either kept moist, or dried out during the final 10 days to determine the role of senescence in root N release. Soil N transformation rates were determined using ¹⁵N pool dilution. After 75 days of growth, cheatgrass accumulated 30 % more total soil N and organic carbon than crested wheatgrass. Cheatgrass roots released twice as much N as crested wheatgrass roots (0.11 vs. 0.05 mg N kg^?1 soil day^?1) in both soil moisture treatments. This occurred despite lower root abundance (7.0 vs. 17.3 g dry root kg^?1 soil) and N concentration (6.0 vs. 7.6 g N kg^?1 root) in cheatgrass vs. crested wheatgrass. We propose that increases in soil N pool sizes and transformation rates under cheatgrass are caused by higher rates of root exudation or release of organic matter containing relatively large amounts of labile N. Our results provide the first evidence for the underlying mechanism by which the invasive annual cheatgrass increases N availability and establishes positive plant-soil feedbacks that promote its success in western rangelands.     

Cheatgrass (<Emphasis Type="Italic">Bromus tectorum</Emphasis>) distribution in the intermountain Western United States and its relationship to fire frequency,seasonality, and ignitions

Cheatgrass (Bromus tectorum) is an invasive grass pervasive across the Intermountain Western US and linked to major increases in fire frequency. Despite widespread ecological impacts associated with cheatgrass, we lack a spatially extensive model of cheatgrass invasion in the Intermountain West. Here, we leverage satellite phenology predictors and thousands of field surveys of cheatgrass abundance to create regional models of cheatgrass distribution and percent cover. We compare cheatgrass presence to fire probability, fire seasonality and ignition source. Regional models of percent cover had low predictive power (34% of variance explained), but distribution models based on a threshold of 15% cover to differentiate high abundance from low abundance had an overall accuracy of 74%. Cheatgrass achieves ≥ 15% cover over 210,000 km² (31%) of the Intermountain West. These lands were twice as likely to burn as those with low abundance, and four times more likely to burn multiple times between 2000 and 2015. Fire probability increased rapidly at low cheatgrass cover (1–5%) but remained similar at higher cover, suggesting that even small amounts of cheatgrass in an ecosystem can increase fire risk. Abundant cheatgrass was also associated with a 10 days earlier fire seasonality and interacted strongly with anthropogenic ignitions. Fire in cheatgrass was particularly associated with human activity, suggesting that increased awareness of fire danger in invaded areas could reduce risk. This study suggests that cheatgrass is much more spatially extensive and abundant than previously documented and that invasion greatly increases fire frequency, even at low percent cover.     

Exploring invasibility with species distribution modeling: How does fire promote cheatgrass (Bromus tectorum) invasion within lower montane forests?

Aim

Cheatgrass (Bromus tectorum) is notorious for creating positive feedbacks that facilitate vegetation type conversion within sagebrush steppe ecosystems in the western United States. Similar dynamics may exist in adjacent lower montane forest. However, fire‐forest‐cheatgrass dynamics have not been examined. We used species distribution modeling to answer three questions about fire and invasibility in lower montane forests: (Q1) Does fire create more suitable habitat for cheatgrass? (Q2) If so, which site attributes are altered to increase site suitability? (Q3) Does fire increase connectivity among suitable habitat and enhance spread?

Location

Shoshone National Forest, Wyoming, USA.

Methods

We measured cheatgrass presence–absence in 93 plots within Interior Douglas‐fir (Pseudotsuga menziesii var. glauca) forests. Random Forests predicted cheatgrass distribution with and without fire using nine site attributes: elevation, slope, aspect, solar radiation, annual precipitation, maximum temperature in July, minimum temperature in January, forest canopy cover and distance to nearest trail or road. Additionally, invasion pathways and spread were mapped using Circuitscape.

Results

Cheatgrass distribution was controlled by topographic and climate variables in the absence of fire. In particular, cheatgrass was most likely to occur at low elevation along dry, south‐ and east‐facing slopes. High‐severity fire increased potential cheatgrass distribution when forest canopy cover was reduced to below 30%. This process created new invasion pathways, which enhanced cheatgrass spread when modelled in Circuitscape.

Main conclusions

Our study showed that in the absence of fire, drier south‐ and east‐facing slopes at low elevation are most susceptible to cheatgrass invasion. However, high‐severity fire increased the total area susceptible to invasion—allowing cheatgrass to expand into previously unsuitable sites within lower montane forests in the western United States. These results are important for present day management and reflect that integrating responses to disturbance in species distribution models can be critical for making predictions about dynamically changing systems.

     

Simulating cheatgrass (<Emphasis Type="Italic">Bromus tectorum</Emphasis>) invasion decreases access to food resources for small mammals in sagebrush steppe

Invasions by nonnative plants can alter the abundance of native animals, yet we know little about the mechanisms driving these changes. Shifts in vegetation characteristics resulting from nonnative plants can alter availability of food resources, predation risk, and foraging efficiency (both the access to and ability to find food), each providing a potential mechanism for documented changes in animal communities and populations in invaded systems. Cheatgrass (Bromus tectorum) is a nonnative grass that invades sagebrush steppe, resulting in declines in some small mammal populations. We examined whether changes in structural characteristics associated with cheatgrass invasion could alter foraging by small mammals, providing a potential mechanism for documented population declines. We quantified differences in vegetation structure between native and cheatgrass-invaded sagebrush steppe, then experimentally added artificial structure in native areas to simulate these differences. We placed grain at foraging stations and measured the amount removed by small mammals nightly. Adding litter at depths approximating invasion by cheatgrass reduced the average amount of grain removed in 2 of 3 study areas, but increasing stem density did not. Based on this experiment, the deeper litter created by cheatgrass invasion may increase costs to small mammals by decreasing foraging efficiency and access to existing food resources, which may explain population-level declines in small mammals documented in other studies. By isolating and identifying which structural attributes of cheatgrass invasion are most problematic for small mammals, land managers may be able to design treatments to efficiently mitigate impacts and restore invaded ecosystems.     

Biosolids Application and Long‐Term Noxious Weed Dominance in the Western United States

Vegetation characteristics were assessed on three sets of 10‐year‐old test plots and one set of 5‐year‐old plots that received 0, 34, 45, and 67 tons/ha (0, 15, 20, and 30 short tons/acre) of biosolids at a semiarid mine reclamation site in Utah. On average, noxious weed species such as Bromus tectorum L. (cheatgrass) provided two‐thirds of the cover on the biosolids test plots, but only one‐tenth of the cover on adjacent control plots that received no biosolids. Cheatgrass provided more than half of the total cover on every biosolids test plot. Seeded species provided about two times more cover at the control plots than at the biosolids plots. Surfaces treated with 45 tons/ha composted biosolids (one part biosolids and two parts wood chips) had a much lower percentage of noxious weed cover compared to biosolids alone. The relatively heavy initial nitrogen load associated with biosolids application may have promoted cheatgrass dominance. Although the available nitrogen eventually declines, once cheatgrass is established it may maintain its dominance indefinitely. Given the risk of weed invasion, heavy biosolids applications should be used with caution for reclamation projects in semiarid climates if perennial species establishment is desired. Consideration should instead be given to light applications (<45 tons/ha) of biosolids/wood chip compost or forgoing the use of biosolids entirely. The underapplication of nutrients may provide a slower, but ultimately more reliable, strategy for the establishment of a healthy, native perennial vegetation community.     

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.     

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.     

A novel plant-fungal mutualism associated with fire

Bromus tectorum, or cheatgrass, is native to Eurasia and widely invasive in western North America. By late spring, this annual plant has dispersed its seed and died; its aboveground biomass then becomes fine fuel that burns as frequently as once every 3-5 y in its invaded range. Cheatgrass has proven to be better adapted to fire there than many competing plants, but the contribution of its fungal symbionts to this adaptation had not previously been studied. In sampling cheatgrass endophytes, many fire-associated fungi were found, including Morchella in three western states (New Mexico, Idaho, and Washington). In greenhouse experiments, a New Mexico isolate of Morchella increased both the biomass and fecundity of its local cheatgrass population, thus simultaneously increasing both the probability of fire and survival of that event, via more fuel and a greater, belowground seed bank, respectively. Re-isolation efforts proved that Morchella could infect cheatgrass roots in a non-mycorrhizal manner and then grow up into aboveground tissues. The same Morchella isolate also increased survival of seed exposed to heat typical of that which develops in the seed bank during a cheatgrass fire. Phylogenetic analysis of Eurasian and North American Morchella revealed that this fire-associated mutualism was evolutionarily novel, in that cheatgrass isolates belonged to two phylogenetically distinct species, or phylotypes, designated Mel-6 and Mel-12 whose evolutionary origin appears to be within western North America. Mutualisms with fire-associated fungi may be contributing to the cheatgrass invasion of western North America.     

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.     

The Influence of Soil Inoculum and Nitrogen Availability on Restoration of High-Elevation Steppe Communities Invaded by Bromus tectorum

Cheatgrass ( Bromus tectorum L.) is an exotic annual grass that has invaded approximately 40,000,000 ha of rangelands in the United States, including montane ecosystems that are important habitats for wildlife and livestock. In addition to well-understood mechanisms by which Cheatgrass gains competitive advantage, recent studies have shown that Cheatgrass may also change the associated soil microbial community to impact native perennial plants and promote the persistence of Cheatgrass. Furthermore, reducing plant-available N represents a tool for initiating conditions that accelerate successional change from annual- to perennial-dominated communities. At a montane, mixed shrub–grassland Cheatgrass-dominated site in Colorado, we applied sucrose to reduce available N, and we added soil from a native plant community in order to reestablish the microbial community. This approach tested the idea that intact native soil microbial communities may enhance the beneficial effect of reducing soil N availability in a restoration setting. By the end of the experiment, reduced N availability decreased Cheatgrass by 9.8%, non-native annual/biennial plant cover by 15.0%, and increased relative perennial plant cover by 13.4%; soil inoculation reduced Cheatgrass by 7.6% and increased perennial abundance by 11.3%. Soil inoculum additions and reduced N availability both contributed toward restoring a perennial-dominated community and demonstrates that addition of native soil inoculum may be a useful tool for restoration efforts.     

When perception reflects reality: Non‐native grass invasion alters small mammal risk landscapes and survival

Modification of habitat structure due to invasive plants can alter the risk landscape for wildlife by, for example, changing the quality or availability of refuge habitat. Whether perceived risk corresponds with actual fitness outcomes, however, remains an important open question. We simultaneously measured how habitat changes due to a common invasive grass (cheatgrass, Bromus tectorum) affected the perceived risk, habitat selection, and apparent survival of a small mammal, enabling us to assess how well perceived risk influenced important behaviors and reflected actual risk. We measured perceived risk by nocturnal rodents using a giving‐up density foraging experiment with paired shrub (safe) and open (risky) foraging trays in cheatgrass and native habitats. We also evaluated microhabitat selection across a cheatgrass gradient as an additional assay of perceived risk and behavioral responses for deer mice (Peromyscus maniculatus) at two spatial scales of habitat availability. Finally, we used mark‐recapture analysis to quantify deer mouse apparent survival across a cheatgrass gradient while accounting for detection probability and other habitat features. In the foraging experiment, shrubs were more important as protective cover in cheatgrass‐dominated habitats, suggesting that cheatgrass increased perceived predation risk. Additionally, deer mice avoided cheatgrass and selected shrubs, and marginally avoided native grass, at two spatial scales. Deer mouse apparent survival varied with a cheatgrass–shrub interaction, corresponding with our foraging experiment results, and providing a rare example of a native plant mediating the effects of an invasive plant on wildlife. By synthesizing the results of three individual lines of evidence (foraging behavior, habitat selection, and apparent survival), we provide a rare example of linkage between behavioral responses of animals indicative of perceived predation risk and actual fitness outcomes. Moreover, our results suggest that exotic grass invasions can influence wildlife populations by altering risk landscapes and survival.     

Enemy release from the effects of generalist granivores can facilitate Bromus tectorum invasion in the Great Basin Desert

The enemy release hypothesis (ERH) of plant invasion asserts that natural enemies limit populations of invasive plants more strongly in native ranges than in non‐native ranges. Despite considerable empirical attention, few studies have directly tested this idea, especially with respect to generalist herbivores. This knowledge gap is important because escaping the effects of generalists is a critical aspect of the ERH that may help explain successful plant invasions. Here, we used consumer exclosures and seed addition experiments to contrast the effects of granivorous rodents (an important guild of generalists) on the establishment of cheatgrass (Bromus tectorum) in western Asia, where cheatgrass is native, versus the Great Basin Desert, USA, where cheatgrass is exotic and highly invasive. Consistent with the ERH, rodent foraging reduced cheatgrass establishment by nearly 60% in western Asia but had no effect in the Great Basin. This main result corresponded with a region‐specific foraging pattern: rodents in the Great Basin but not western Asia generally avoided seeds from cheatgrass relative to seeds from native competitors. Our results suggest that enemy release from the effects of an important guild of generalists may contribute to the explosive success of cheatgrass in the Great Basin. These findings corroborate classic theory on enemy release and expand our understanding of how generalists can influence the trajectory of exotic plant invasions.     

Structural changes in vegetation coincident with annual grass invasion negatively impacts sprint velocity of small vertebrates

Sagebrush ecosystems in the intermountain west of the United States are being threatened by conversion to the non-native grass, cheatgrass (Bromus tectorum). The dramatic shift in the physical structure of vegetation coincident with cheatgrass invasion likely has negative impacts on animal communities, yet these structural impacts have not been well-studied. In a previous study, dense cheatgrass stems reduced sprint velocity for the flattened, wide-bodied desert horned lizard (Phrynosoma platyrhinos). Here, we asked if a decrease in sprint velocity due to cheatgrass impediment can be generalized to the suite of small vertebrates inhabiting the sagebrush ecosystems of western Utah. We evaluated sprint performance of the common rodent (n = 3) and lizard (n = 4) species on two raceway types, cheatgrass and no-cheatgrass, and hypothesized that body size, body shape, and form of movement are important factors influencing sprint velocity through dense cheatgrass stems. All species showed significant reductions in speed on cheatgrass versus no-cheatgrass raceways, with percent reduction greatest for larger, wider, or hopping organisms compared to smaller, more slender, or running organisms. Of concern, surveys for rodents and lizards at our study areas support a common pattern: lower abundances of small vertebrates, as well as a loss of rodent species richness, in areas infested with cheatgrass compared to intact, native sagebrush communities. By extension, we expect a negative impact on animal communities in other semi-arid regions experiencing dramatic shifts in vegetation structure upon invasion by non-native grasses that are capable of forming dense stands in the interspaces of native desert plants [e.g., Sonoran desert invaded by buffelgrass (Pennisetum ciliare)].     

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.     

Increased Primary Production from an Exotic Invader Does Not Subsidize Native Rodents

Invasive plants have tremendous potential to enrich native food webs by subsidizing net primary productivity. Here, we explored how a potential food subsidy, seeds produced by the aggressive invader cheatgrass (Bromus tectorum), is utilized by an important guild of native consumers – granivorous small mammals – in the Great Basin Desert, USA. In a series of field experiments we examined 1) how cheatgrass invasion affects the density and biomass of seed rain at the ecosystem-level; 2) how seed resources from cheatgrass numerically affect granivorous small mammals; and 3) how the food preferences of native granivores might mediate the trophic integration of cheatgrass seeds. Relative to native productivity, cheatgrass invasion increased the density and biomass of seed rain by over 2000% (P < 0.01) and 3500% (P < 0.01), respectively. However, granivorous small mammals in native communities showed no positive response in abundance, richness, or diversity to experimental additions of cheatgrass seeds over one year. This lack of response correlated with a distinct preference for seeds from native grasses over seeds from cheatgrass. Our experiments demonstrate that increased primary productivity associated with exotic plant invasions may not necessarily subsidize consumers at higher trophic levels. In this context, cheatgrass invasion could disrupt native food webs by providing less-preferred resources that fail to enrich higher trophic levels.     

An Experimental Test of Resistance to Cheatgrass Invasion: Limiting Resources at Different Life Stages

Variable densities of an invasive species may represent variation in invasion resistance, due to variation in resource availability. This study determined whether low- and high-density cheatgrass (Bromus tectorum L.) patches within a shadscale-bunchgrass community of western Utah, USA, can be explained by variation in resource availability. It also explored the possible role of seed limitation and enemy pressure on invasion patterns. Two parallel field experiments were conducted:(1) increasing resources within low-density cheatgrass patches and, conversely (2) reducing resources within high-density cheatgrass patches. Treatments were applied at three life stages separately and across all stages. In low-density cheatgrass patches (assumed to represent high resistance), a disturbance that reduced soil compaction had the strongest positive effect, significantly increasing biomass by 250% and density by 104% in comparison to the control. The second strongest effect was reducing neighbors (native grasses), which significantly increased cheatgrass biomass and density. These results indicate that resources are present in low-density cheatgrass patches, but they are unavailable without disturbance and/or are exploited by competitors, and hence represent resistance to invasion. In high-density cheatgrass patches (assumed to represent low resistance), nitrogen availability was important in maintaining cheatgrass densities. Reducing nitrogen (via sucrose addition) significantly decreased density (by 37%) but not biomass. Life stages of cheatgrass were differentially affected by these resource manipulations. In addition, herbivore (primarily grasshoppers) and pathogen (head smut) pressures were documented to affect cheatgrass density, but did not explain resistance patterns. Instead, we found that differential resource availability explains the observed variation in cheatgrass density, and variation in natural resistance.