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A central question of invasion biology is how an exotic species invades new habitats following its initial establishment. Three hypotheses to explain this expansion are: (1) the existence of ‘general purpose’ genotypes, (2) the in situ evolution of novel genotypes, and (3) the dispersal of existing specialized genotypes into habitats for which they are pre-adapted. Bromus tectorum is a selfing exotic winter annual grass that has achieved widespread dominance in semiarid western North America and that is actively invading salt desert habitats. We examined mechanisms driving this invasion in three complementary studies. In reciprocal seeding experiments with ten populations from saline playa, salt desert shrubland,...
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Historically, ecosystems in the southwestern United States derived much of their nitrogen (N) from N-fixation in biological soil crusts. Today, these regions have highly reduced crust cover, and atmospheric deposition may be the dominant source of N. This study investigates the effects of increased nitrogen deposition on nitrogen uptake, photosynthesis, and growth of the two main forage grasses on the Colorado Plateau, galleta (Hilaria jamesii [Torr.] Benth.) and Indian ricegrass (Oryzopsis hymenoides, [Roemer & J.S. Schultes] Ricker ex Piper). Plots were fertilized for 2 years with 0, 10, 20, and 40 kg nitrogen ha?1 annually, up to 4� the estimated current annual deposition rate, in 2 applications per year (spring...
Biological invasions are one of the greatest threats to native species in natural ecological systems. One of the most successful invasive species is Bromus tectorum L. (cheatgrass), which is having marked impacts on native plant communities and ecosystem processes. However, we know little about the effects of this invasion on native animal species in the Intermountain West. Because ants have been used to detect ecological change associated with anthropogenic land use, they seem well suited for a preliminary evaluation of the consequences of cheatgrass-driven habitat conversion. In our study, we used pitfall traps to assess ant community assemblages in intact sagebrush and nearby cheatgrass-dominated vegetation....
Bromus tectorum L. is a non-native, annual grass that has invaded western North America. In SE Utah, B. tectorum generally occurs in grasslands dominated by the native perennial grass, Hilaria jamesii (Torr.) Benth. and rarely where the natives Stipa hymenoides Roem. and Schult. and S. comata Trin. & Rupr. are dominant. This patchy invasion is likely due to differences in soil chemistry. Previous laboratory experiments investigated using soil amendments that would allow B. tectorum to germinate but would reduce B. tectorum emergence without affecting H. jamesii. For this study we selected the most successful treatments (CaCl2, MgCl2, NaCl and zeolite) from a previous laboratory study and applied them in the field...
The primary hypothesis for the astonishing success of many exotics as community invaders relative to their importance in their native communities is that they have escaped the natural enemies that control their population growth ? the `natural enemies hypothesis'. However, the frequent failure of introduced biocontrols, weak consumer effects on the growth and reproduction of some invaders, and the lack of consistent strong top-down regulation in many natural ecological systems indicate that other mechanisms must be involved in the success of some exotic plants. One mechanism may be the release by the invader of chemical compounds that have harmful effects on the members of the recipient plant community (i.e., allelopathy)....
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Current invasion ecology theory predicts that disturbance will stimulate invasion by exotic plant species. Cheatgrass or Downy brome (Bromus tectorum) was surveyed in three sites near Florence, Colorado, U.S.A., immediately following Tamarisk or Saltcedar (Tamarix spp.) control and restoration activities that caused disturbance. Despite predictions to the contrary, neither mowing with heavy machinery nor tilling for seedbed preparation stimulated invasion, with a trend for the opposite pattern such that highest percent cover of B. tectorum was observed in the least disturbed transects. Aerial application of imazapyr for Tamarix spp. control caused mortality of nearly all B. tectorum and other understory plant species...
Biological invasions can be substantially influenced by the genetic sampling associated with a species? introduction. As a result, we assessed the genetic and evolutionary consequences of the entry and spread of the invasive grass Bromus tectorum (cheatgrass) across the United States midcontinent through an analysis of 54 populations, using enzyme electrophoresis. On average, these populations display 1.04 alleles per locus (A), 4.1% percent polymorphic loci per population (%P) and an expected mean heterozygosity (Hexp) value of 0.009. Heterozygotes, which have been rarely reported for B. tectorum in North America, occur in three populations in the midcontinent and are likely novel multilocus genotypes that arose...
This model was constructed to model the risk of invasion by exotic plant species. Roads may directly influence exotic plant dispersal via disturbance during road construction or via alterations in soil regimes. For example, in Californian serpentine soil ecosystems, exotic plant species can be found up to 1km from the nearest road and Russian thistle (Salsola kali), an exotic forb growing along roads, is wind-dispersed over distances greater than 4km. Roads may also indirectly facilitate the dispersal of exotic grasses, such as crested wheatgrass (Agropyron cristatum), via human seeding along road verges or in burned areas near roads as a management strategy to curb the establishment of less desirable exotic grass...
Dams create barriers to fish migration and dispersal in drainage basins, and the removal of dams is often viewed as a means of increasing habitat availability and restoring migratory routes of several fish species. However, these barriers can also isolate and protect native taxa from aggressive downstream invaders.We examined fish community composition two years prior to and two years after the removal of a pair of low-head dams from Boulder Creek,Wisconsin, U.S.A. in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder...
Cheatgrass (Bromus tectorum L.) is the most widespread invasive weed in sagebrush ecosystems of North America. Restoration of perennial vegetation is difficult and land managers have often used introduced bunchgrasses to restore degraded sagebrush communities. Our objective was to evaluate the potential of ‘Vavilov’ Siberian wheatgrass (Agropyron fragile [Roth] P. Candargy) to establish on cheatgrass-dominated sites. We examined Vavilov establishment in response to different levels of soil nitrogen availability by adding sucrose to the soil to promote nitrogen (N) immobilization and examined cheatgrass competition by seeding different levels of cheatgrass. We used a blocked split-split plot design with two sucrose...
Aim: To test the hypothesis that anthropogenic alteration of stream-flow regimes is a key driver of compositional shifts from native to introduced riparian plant species. Location: The arid south-western United States; 24 river reaches in the Gila and Lower Colorado drainage basins of Arizona. Methods: We compared the abundance of three dominant woody riparian taxa (native Populus fremontii and Salix gooddingii, and introduced Tamarix) between river reaches that varied in stream-flow permanence (perennial vs. intermittent), presence or absence of an upstream flow-regulating dam, and presence or absence of municipal effluent as a stream water source. Results: Populus and Salix were the dominant pioneer trees along...
Questions: Exotic plant invasion may be aided by facilitation and broad tolerance of environmental conditions, yet these processes are poorly understood in species-rich ecosystems such as riparian zones. In the southwestern United States (US) two plant species have invaded riparian zones: tamarisk (Tamarix ramosissima, T. chinensis, and their hybrids) and Russian olive (Elaeagnus angustifolia). We addressed the following questions: (1) is Russian olive able to tolerate drier and shadier conditions than cottonwood and tamarisk? (2) Can tamarisk and cottonwood facilitate Russian olive invasion? Location: Arid riparian zones, southwestern US. Methods: We analyzed riparian tree seedling requirements in a controlled...
The United States National Park Service was created to protect and make accessible to the public the nation?s most precious natural resources and cultural features for present and future generations. However, this heritage is threatened by the invasion of non-native plants, animals, and pathogens. To evaluate the scope of invasions, the USNPS has inventoried non-native plant species in the 216 parks that have significant natural resources, documenting the identity of non-native species. We investigated relationships among non-native plant species richness, the number of threatened and endangered plant species, native species richness, latitude, elevation, park area and park corridors and vectors. Parks with many...
    map background search result map search result map Effects of Nitrogen Deposition on an Arid Grassland in the Colorado Plateau Cold Desert First-Year Responses of Cheatgrass Following Tamarix spp. Control and Restoration-Related Disturbances Risk model of invasion of native shrublands by Juniper and Pinyon Pine for the Western USA Exotic Plant Invasion Risk in the Western United States Risk of Pinyon-Juniper Invasion into Shrubland Habitats Effects of Nitrogen Deposition on an Arid Grassland in the Colorado Plateau Cold Desert First-Year Responses of Cheatgrass Following Tamarix spp. Control and Restoration-Related Disturbances Exotic Plant Invasion Risk in the Western United States Risk of Pinyon-Juniper Invasion into Shrubland Habitats Risk model of invasion of native shrublands by Juniper and Pinyon Pine for the Western USA