Genetic analysis of Bromus tectorum (Poaceae) in the Mediterranean region: biogeographical pattern of native populations.

Genetic diversity within and among 42 native populations of Bromus tectorum (cheatgrass) was characterized within two regions, the eastern Mediterranean and the western Mediterranean. Two hypotheses were tested for the genetic diversity of these populations: (1) populations from the eastern Mediterranean are more genetically diverse compared with populations to the west, a potential consequence of the species' westward dispersal with the spread of agriculture, and (2) populations across the Mediterranean contain comparable genetic diversity but display high genetic differentiation, a potential consequence of both regions having served as refugia during glacial advances in the late Quaternary Period. Populations in the eastern Mediterranean possess 16 polymorphic loci and 37 multilocus genotypes. In contrast, populations from the western Mediterranean include a subset of these polymorphic loci (9) and fewer multilocus genotypes (19), consistent with the dispersal of B. tectorum with the east-west Holocene spread of agriculture. Among the 19 multilocus genotypes identified in populations from the western Mediterranean, 13 are undetected among eastern Mediterranean populations. Average genetic diversity within populations from the eastern Mediterranean is nonetheless comparable to the genetic diversity in populations from the Iberian Peninsula, whereas diversity is the lowest in the populations from southern France. Our results suggest a prominent role for agriculture in the grass's western spread, although glacial history and environmental heterogeneity also could have influenced the grass's genetic diversity. The exceptionally high level of self-pollination (>99%) in B. tectorum has contributed to preserving the genetic signature associated with the species' biogeographical history across the Mediterranean region.

Potential role of masting by introduced bamboos in deer mice (Peromyscus maniculatus) population irruptions holds public health consequences.

We hypothesized that the ongoing naturalization of frost/shade tolerant Asian bamboos in North America could cause environmental consequences involving introduced bamboos, native rodents and ultimately humans. More specifically, we asked whether the eventual masting by an abundant leptomorphic ("running") bamboo within Pacific Northwest coniferous forests could produce a temporary surfeit of food capable of driving a population irruption of a common native seed predator, the deer mouse (Peromyscus maniculatus), a hantavirus carrier. Single-choice and cafeteria-style feeding trials were conducted for deer mice with seeds of two bamboo species (Bambusa distegia and Yushania brevipaniculata), wheat, Pinus ponderosa, and native mixed diets compared to rodent laboratory feed. Adult deer mice consumed bamboo seeds as readily as they consumed native seeds. In the cafeteria-style feeding trials, Y. brevipaniculata seeds were consumed at the same rate as native seeds but more frequently than wheat seeds or rodent laboratory feed. Females produced a median litter of 4 pups on a bamboo diet. Given the ability of deer mice to reproduce frequently whenever food is abundant, we employed our feeding trial results in a modified Rosenzweig-MacArthur consumer-resource model to project the population-level response of deer mice to a suddenly available/rapidly depleted supply of bamboo seeds. The simulations predict rodent population irruptions and declines similar to reported cycles involving Asian and South American rodents but unprecedented in deer mice. Following depletion of a mast seed supply, the incidence of Sin Nombre Virus (SNV) transmission to humans could subsequently rise with dispersal of the peridomestic deer mice into nearby human settlements seeking food.

Naturalization of plant populations: the role of cultivation and population size and density.

Field experimentation is required to assess the effects of environmental stochasticity on small immigrant plant populations-a widely understood but largely unexplored aspect of predicting any species' likelihood of naturalization and potential invasion. Cultivation can mitigate this stochasticity, although the outcome for a population under cultivation nevertheless varies enormously from extinction to persistence. Using factorial experiments, we investigated the effects of population size, density, and cultivation (irrigation) on the fate of founder populations for four alien species with different life history characteristics (Echinochloa frumentacea, Fagopyrum esculentum, Helianthus annuus, and Trifolium incarnatum) in eastern Washington, USA. The fate of founder populations was highly variable within and among the 3 years of experimentation and illustrates the often precarious environment encountered by plant immigrants. Larger founder populations produced more seeds (P < 0.001); the role of founder population size, however, differed among years. Irrigation resulted in higher percent survival (P < 0.001) and correspondingly larger net reproductive rate (R(0); P < 0.001). But the minimum level of irrigation for establishment, R(0) > 1, differed among years and species. Sowing density did not affect the likelihood of establishment for any species. Our results underscore the importance of environmental stochasticity in determining the fate of founder populations and the potential of cultivation and large population size in countering the long odds against naturalization. Any implementation of often proposed post-immigration field trials to assess the risk of an alien species becoming naturalized, a requisite step toward invasion, will need to assess different sizes of founder populations and the extent and character of cultivation (intentional or unintentional) that the immigrants might receive.

Environmental constraints on the invasion of Triadica sebifera in the eastern United States: an experimental field assessment.

Identifying the environmental constraints that affect the distribution of an invasive species is fundamental to its effective control. Triadica sebifera (Chinese tallow tree) has invaded the southeastern United States, but its potential for further range and habitat extension has been unresolved. We explored experimentally environmental factors in macro- and microhabitats that affect its persistence at five widely separated sites along the Atlantic seaboard of the United States and at two sites inland; three sites occur well beyond the tree's current range. At each site, seeds and young vegetative plants (0.5-0.65 m tall) of T. sebifera were placed in four microhabitats (closed-canopy upland, closed-canopy lowland, open-canopy upland, and open-canopy lowland). Plant growth, leaf CO(2) assimilation rates, leaf N concentrations and delta(13)C ratios, and stem water potential were measured for two growing seasons. Percent seed germination was consistently higher in open-canopy microhabitats and lowest at northern and inland sites. T. sebifera grew in all open-canopy microhabitats, even 300-500 km beyond its current distribution. Plant growth in closed-canopy habitats was lower, attributable to lower carbon gain per unit leaf area in shaded compared with open-canopy environments, especially at northern and inland sites. Neither competition, other than canopy shade, nor grazing was a key constraint on distribution at any scale. Our results demonstrate that T. sebifera is dispersal limited at landscape scales but limited locally by dispersal and overstory shade; it has yet to occupy the full extent of its new range in North America. Quantifying environmental factors both within and well beyond a species' current range can effectively highlight the limits on its distribution.

Bromus tectorum (Poaceae) in midcontinental United States: Population genetic analysis of an ongoing invasion.

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 (H(exp)) 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 postimmigration. The midcontinent distribution of multilocus genotypes suggests that plant immigrants came directly from either the native range or the eastern United States, or both. Continued dispersal of preadapted genotypes and the assembly of populations that are genetic admixtures may enhance this invasion by increasing both the genetic diversity within populations and the selection of novel genotypes arising from occasional outcrossing. The potential for postimmigration evolution in most species points to the largely unrecognized need to block the introduction of new, potentially aggressive genotypes of an alien species already in the United States.

Introduction history and population genetics of the invasive grass Bromus tectorum (Poaceae) in Canada.

The invasive annual Bromus tectorum (cheatgrass) is distributed in Canada primarily south of 52° N latitude in two diffuse ranges separated by the extensive coniferous forest in western Ontario. The grass was likely introduced independently to eastern and western Canada post-1880. We detected regional variation in the grass's genetic diversity using starch gel electrophoresis to analyze genetic diversity at 25 allozyme loci in 60 populations collected across Canada. The Pgm-1a & Pgm-2a multilocus genotype, which occurs in the grass's native range in Eastern Europe, is prevalent in eastern Canada but occurs at low frequency in western Canada. In contrast, the Got-4c multilocus genotype, found in the native range in Central Europe, is widespread in populations from western Canada. Overall genetic diversity of B. tectorum is much higher in eastern Canada than in the eastern U.S., while the genetic diversity in populations in western North America is similar between Canada and the U.S. The distribution of genetic diversity across Canada strongly suggests multiple introduction events. Heterozygous individuals, which are exceedingly rare in B. tectorum, were detected in three Canadian populations. Formation of novel genotypes through occasional outcrossing events could spark adaptive evolution and further range expansion across Canada of this exceedingly damaging grass.

Biological invasions: recommendations for U.S. policy and management.

The Ecological Society of America has evaluated current U.S. national policies and practices on biological invasions in light of current scientific knowledge. Invasions by harmful nonnative species are increasing in number and area affected; the damages to ecosystems, economic activity, and human welfare are accumulating. Without improved strategies based on recent scientific advances and increased investments to counter invasions, harm from invasive species is likely to accelerate. Federal leadership, with the cooperation of state and local governments, is required to increase the effectiveness of prevention of invasions, detect and respond quickly to new potentially harmful invasions, control and slow the spread of existing invasions, and provide a national center to ensure that these efforts are coordinated and cost effective. Specifically, the Ecological Society of America recommends that the federal government take the following six actions: (1) Use new information and practices to better manage commercial and other pathways to reduce the transport and release of potentially harmful species; (2) Adopt more quantitative procedures for risk analysis and apply them to every species proposed for importation into the country; (3) Use new cost-effective diagnostic technologies to increase active surveillance and sharing of information about invasive species so that responses to new invasions can be more rapid and effective; (4) Create new legal authority and provide emergency funding to support rapid responses to emerging invasions; (5) Provide funding and incentives for cost-effective programs to slow the spread of existing invasive species in order to protect still uninvaded ecosystems, social and industrial infrastructure, and human welfare; and (6) Establish a National Center for Invasive Species Management (under the existing National Invasive Species Council) to coordinate and lead improvements in federal, state, and international policies on invasive species. Recent scientific and technical advances provide a sound basis for more cost-effective national responses to invasive species. Greater investments in improved technology and management practices would be more than repaid by reduced damages from current and future invasive species. The Ecological Society of America is committed to assist all levels of government and provide scientific advice to improve all aspects of invasive-species management.

Genetic variation in Bromus tectorum (Poaceae): differentiation in the eastern United States.

Bromus tectorum, a devastating plant invader in western North America, had entered Pennsylvania by 1790. Although rare, or extirpated, in the east until the 1850s, it was collected with increasing frequency after 1859 from Vermont to Virginia. Using enzyme electrophoresis, we analyzed 38 populations of this grass in the eastern U.S. to determine their genetic variation and structure as well as assess their relatedness to populations in the west. Genetic variation among eastern U.S. populations is low: mean number of alleles per locus (A), percent polymorphic loci per population (%P), and expected heterozygosity (H(exp)) are 1.01, 1.05%, and 0.002, respectively. No heterozygotes were detected. The eastern populations are genetically similar: mean genetic identity for all populations was 0.990 with values among population pairs ranged from 0.913 to 1.000. Thirteen populations in eastern and western North America shared Pgm-1a and Pgm-2a, while eight populations shared Mdh-2b and Mdh-3b. Other alleles detected in western North America (Got-4c, Got-4d, and Pgi-2b) were not, however, found in eastern U.S. populations. The invasion of North America by B. tectorum occurred through multiple introductions on both coasts; results from historical and genetic evidence suggest that eastern populations stem from a minimum of two introductions. The 19th century westward spread of B. tectorum from the East appears to be plausible.

Ecological genetics of Bromus tectorum : I. A hierarchical analysis of phenotypic variation.

An understanding of how genetic differentiation and phenotypic plasticity may interact to promote the spread of an introduced species requires information on the hierarchical distribution of genetic variation within the species in its new range. For example, a lack of genetic variation within marginal populations of an introduced species may slow its rate of spread into new habitats. In a glasshouse study, we examined the phenotypic variation among populations, among families, within families, and the homogeneity of variances within families for morphological and life history characters of an alien, self-pollinating annual grass, Bromus tectorum. The populations of B. tectorum studied were collected from both large, central populations in steppe habitats and small, peripheral populations in forest sites along a broad temperature-moisture gradient in western North America. Most variation in average flowering time was due to differences among seed source populations; among family variation contributed less than 20% to the total variance for flowering time. Populations from arid steppe habitats were the earliest flowering while the population from the most mesic forest habitat was last to flower. In contrast, the within-family variance was a major contributor to the total variance for plant dry weight, seed number per plant, total seed weight, and individual seed weight. The amount of total variation explained by among-family differences ranged between 18% for average seed weight to 30% for total plant dry weight. There was no consistent difference in within-population genetic variability between large, central populations in steppe habitats and smaller, potentially more isolated populations in forest habitats. Significant heterogeneity in within-family variance in some of the source populations suggest that families differ in the capacity for phenotypic response to environmental variation. Considered independently from source population, there was no consistent trend linking a particular trait to increased heterogeneity of within-family variances.

Ecological genetics of Bromus tectorum : II. Intraspecific variation in phenotypic plasticity.

For Bromus tectorum, an alien annual grass now widespread in western North America, we assessed the sensitivity of the phenotypic expression of populational differences in phenology and demography to variation in plant density. Plants were grown in an unheated glasshouse from seeds collected from six habitat types located along a moisture-temperature gradient. Survival to flowering was high with a mean overall survival of 91%. Survival was highest among plants from the coolest, most mesic site. Time to first flowering decreased at the higher sowing density and was dependent on seed source. Plant dry weight and average seed production was lowest within high density plots; the magnitude of this reduction was significantly dependent on seed source. Individual seed weight was also dependent on the interaction of seed source and sowing density. Seed weight did not vary as much as seed number or plant weight in response to density. Differences among sources in hierarchies of plant size and seed production were detected at low sowing densities. The degree of inequality in the distributions of plant size and seed number increased at high density.

Ecological genetics of Bromus tectorum : III. The demography of reciprocally sown populations.

By incorporating demographic analyses of fitness components (e.g., survival and reproduction) within a reciprocal sowing design, we tested for 3 consecutive years whether local adaptation has occurred in the alien grass Bromus tectorum (cheatgrass) within 7 habitats along an environmental gradient from arid steppe to subalpine forest in the Intermontain Region of western North America. Patterns of emergence and survival were strongly influenced by the local environment. In terms of survival, expression of significant local adaptation in Tsuga heterophylla habitat varied among years. In contrast, relative differences in flowering time among seed sources were stable across sites and years. Populations from the arid steppe were the earliest to flower; flowering was latest in populations from the mesic Tsuga heterophylla habitat. In terms of net reproductive rate, evidence for local adaptation in B. tectorum was obtained in populations from habitats representing environmental extremes: an arid, saline site dominated by the shrub Sarcobatus vermiculatus and clearings within the cool, mesic Tsuga heterophylla forest habitat. Unlike the plants introduced from other sites, members of the resident population at the Sarcobatus site flowered and produced seeds before soil water became limiting. In contrast, net reproductive rates in other habitats were sometimes the lowest for populations in their home site. This lack of an advantage for local populations within more environmentally moderate sites suggests that limited dispersal may restrict the rate at which superior genotypes are introduced into a particular site.

The population biology of Bromus tectorum in forests: distinguishing the opportunity for dispersal from environmental restriction.

With increasing elevation and corresponding changes in the macroclimate, forest zones in the Intermountain Region of western North America are often dominated in turn by Pinus ponderosa, Pseudotsuga menziesii, Abies grandis, an Thuja plicata. Bromus tectorum (cheatgrass), and introduced annual grass now abundant in the Region's steppe, is uncommon in mature stands representative of these forest zones. In order to determine whether B. tectorum is largely excluded from these forests by insufficient seed dispersal or environmental restriction(s), the grass's demography was compared in each of four years among populations experimentally-introduced into mature forests. The number of recruits did not differ among the Pinus, Pseudotsuga, and Abies sites; recruitment was however significantly lower on the coolest site dominated by Thuja. Emergence in both the low elevation Pinus and Pseudotsuga sites was about the same in autumn, winter, and spring. In the cooler, moister Abies and Thuja sites, emergence was limited to autumn and early winter. Survival in these forest sites ranged between 0 and 87%. The percentage of the total population to survive until harvest was highest in the Pseudotsuga site, intermediate in the Pinus and Abies sites, and lowest in the Thuja site. Compared with B. tectorum in the steppe, the surviving plants were small, and few produced seeds. All parents were members of either the autumn or winter cohort, and most parents produced only one seed. No seeds were produced at the Thuja site. Although phenotypic plasticity apparently contributes to the wide ecological amplitude of this grass, its growing season on these sites in most years is too short for it to reproduce. Consequently, these forest zones broadly define the current environmental limits to the distribution of cheatgrass in this portion of its new geographic range.

The population biology of Bromus tectorum in forests: effect of disturbance, grazing, and litter on seedling establishment and reproduction.

The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.

The effect of shading on photosynthesis, growth, and regrowth following defoliation for Bromus tectorum.

The effect of full sunlight, 60%, or 90% attenuated light on photosynthetic rate, growth, leaf morphology, dry weight allocation patterns, phenology, and tolerance to clipping was examined in the glasshouse for steppe populations of the introduced grass, Bromus tectorum. The net photosynthetic response to light for plants grown in shade was comparable to responses for plants grown in full sunlight. Plants grown in full sunlight produced more biomass, tillers and leaves, and allocated a larger proportion of their total production to roots than plants grown in shade. The accumulation of root and shoot biomass over the first two months of seedling growth was primarily responsible for the larger size at harvest of plants grown in full sunlight. Plants grown under 60% and 90% shade flowered an average of 2 and 6 weeks later, respectively, than plants grown in full sunlight. Regrowth after clipping was greater for plants grown in full sunlight compared to those grown in shade. Even a one-time clipping delayed flowering and seed maturation; the older the individual when leaf area was removed, the greater the delay in its phenology. Repeated removal of leaf area was more frequently fatal for plants in shade than in full sunlight. For plants originally grown in full sunlight, regrowth in the dark was greater than for shaded plants and was more closely correlated to non-flowering tiller number than to plant size. This correlation suggests that etiolated regrowth is more likely regulated by the number of functional meristems than by differences in the size of carbohydrate pools. Thus, shading reduces the rate of growth, number of tillers, and ability to replace leaf area lost to herbivory for B. tectorum. These responses, in turn, intensify the effect of competition and defoliation for this grass in forests. B. tectorum is largely restricted to forest gaps at least in part because of its inability to acclimate photosynthetically, the influence of shade on resource allocation, and the role of herbivory in exacerbating these effects.

Competition between Bromus tectorum L. and Poa pratensis L.: the role of light.

Bromus tectorum L. dominates sites of large-scale disturbance, while Poa pratensis L. dominates the sites of small-scale disturbance in the Festuca/Symphoricarpos habitat type in eastern Washington (USA). The role of incident irradiation in influencing these distributions was examined using field and glasshouse experiments. Glasshouse grown swards of B. tectorum growing beneath an established canopy of P. pratensis displayed larger biomass and higher survival when exposed to supplemental light versus controls. Neither mean plant height nor the skewness in the individual biomass distribution were significantly different between the supplemental light and control plots. Maximum net photosynthesis for P. pratensis and B. tectorum was 11.5 and 14.87 mg CO2·dm-2·h-1, respectively. For Poa, light utilization efficiency was greater, although light compensation point was lower than that exhibited by Bromus. These photosynthetic characteristics along with the seasonal pattern of light transmission through the meadow steppe canopy may largely account for the successful establishment of P. pratensis in small sites of disturbance with little light, while restricting the establishment of B. tectorum to patches with more irradiance.