Native Plant Species Cover Research Articles

DYNAMICS OF FOREST ECOSYSTEM COMPONENTS IN THE BLACK SEA COAST OF THE CAUCASUS AGAINST THE BACKGROUND OF BOXWOOD DESTRUCTION AS A RESULT OF <i>CYDALIMA PERSPECTALIS</i> WALKER (LEPIDOPTERA, CRAMBIDAE) INVASION

The situation when the invasion of one species indirectly leads to ecosystem consequences of a larger scale is considered. The work was carried out in the relict ecosystem of Colchis, the flora and fauna of which was formed more than 15 million years ago. Disappearance of boxwood, the edifier of Colchian ecosystems, in the short term has not led to a significant change in soil characteristics. The succession of plant communities was expressed by an increase in vegetation cover of native plant species under the active introduction of alien species. The rates of seed regeneration of boxwood decreased, especially in the areas with low density of tree canopy. The study of the distribution of earthworms showed the general topical preferences of boxwood and most species of large soil oligochaetes that prefer shaded stations and moistened habitats.

Effects of the invasive common milkweed (Asclepias syriaca) on nematode communities in natural grasslands

Summary Thousands of plant species have been introduced, both deliberately and accidentally, to native ecosystems across the globe. The common milkweed, Asclepias syriaca, is an invasive fast-growing plant species that has invaded natural and managed areas in Europe in recent years. However, only a few studies have investigated its impacts on above-ground and below-ground native biota. Soil nematodes are considered to be the most abundant metazoans, comprising different trophic groups. We examined soil nematode populations on several independent sites invaded and non-invaded by A. syriaca in southern Slovak Republic. We hypothesised that alteration of above-ground nutrient sources caused by invasion would alter nematode abundance, species diversity and trophic structure. Our study revealed that the nematode communities were not negatively affected by A. syriaca invasion over a period of two vegetation seasons. The most surprising results were that common milkweed did not decrease the abundance of plant parasites, bacterivores or fungivores and did not affect species diversity. Moreover, basic physicochemical soil parameters were similar in both invaded and non-invaded sites, although there is evidence that invasive milkweed considerably decreases native plant species cover. It is thus reasonable to conclude that A. syriaca does not pose a threat to soil nematode communities but has a negative impact on native plants. As there is an absence of nematological studies from A. syriaca-invaded ecosystems and considering it has invaded a wide range of habitats, it is impossible to generalise our findings and further studies are required.

Twenty Years of Tallgrass Prairie Reconstruction and Restoration at Pawnee Prairie Natural Area, Missouri

In 1996 the Missouri Department of Conservation purchased Pawnee Prairie, a 190-ha mix of remnant tallgrass prairie and formerly row-cropped prairie with varying degrees of Festuca arundinacea invasion and past cattle grazing intensities on rolling terrain in the central dissected till plains ecological section. Management actions implemented over the following 20 y included prescribed fire, herbicide treatments of invasive nonnative species, and seeding of local ecotype prairie seed. Concurrently, four vegetation monitoring transects were sampled for plant species composition and cover five times between 1996 and 2017. Each of the transects increased significantly over time in the following per-quadrat means: % native plant species cover, plant species conservatism, and cover-weighted plant species conservatism. At the site level, native grasses increased by 22%, nonnative grasses declined by 76%, native forbs increased by 91%, nonnative forbs declined by 94%, and native sedges declined by 37%. In 1996 the top species in importance value across all transects included weedy native species (e.g., Dicanthelium lanuginosum) and nonnative species (e.g., Daucus carota). By 2017 the top species had transitioned to characteristic prairie species (e.g., Schizachyrium scoparium). Ordination results documented compositional trends across all transects toward greater native species richness, cover, and species conservatism values. At Pawnee Prairie, 20 y of sustained prairie reconstruction and restoration practices applied across an area of differing land use histories resulted in significant gains in the natural quality of the site's vegetation, including a greater abundance of prairie flora matrix species and some conservative species.

Solidago canadensis invasion in abandoned arable fields induces minor changes in soil properties and does not affect the performance of subsequent crops

AbstractThe global fast‐growing presence of invasive species requires urgent action to assess their ecological and economic impact. Although a number of recent works have shown that alien plant invasions can affect native plant communities, soil microbial communities, and soil physicochemical properties, rarely the effects of invaders on all these parameters have been examined at once. Moreover, no studies have focused on the effects of invasive plants on the ecosystem services after the removal of invaders. For this study, Solidago canadensis, one of the most successful global invasive species, was chosen. In the first stage, invasion‐induced changes in plant community and soil properties in abandoned arable fields were assessed. In the second stage, the effects of these changes on crops, Helianthus annuus and Zea mays, were assessed in a laboratory soil‐feedback experiment. The invader reduced substantially the richness and cover of native plant species. It had little impact on soil physicochemical and microbiological properties: Soil moisture was lower, whereas mycorrhizal parameters and urease activity were higher in invaded plots. The experiment revealed minor and inconsistent effects of the invader: It affected the mycorrhizal colonization and shoot mass of crop plants, but the direction of changes depended on the origin of the soil used. In conclusion, although S. canadensis invasion clearly displaces native vegetation, it has few and rather weak effects on soil properties that did not affect the performance of subsequent crops. The results are promising from the viewpoint of restoring the agricultural function to abandoned fields invaded by S. canadensis.

Expert allocation of primary growth form to the New South Wales flora underpins the biodiversity assessment method

ABSTRACTBiodiversity values under the New South Wales (NSW) Biodiversity Conservation Act 2016 are assessed in part according to the number and cover of native plant species within each of six growth form groups (trees, shrubs, grasses and grass-like, forbs, ferns, and others). Here we revise 19 growth form descriptions and use an independent expert process to allocate the most common (primary) growth form to the native terrestrial vascular plant flora of NSW. Independent allocations made by three botanists concurred for 6,153 taxa (84.7 per cent of the flora) and the remaining 1,112 taxa were resolved via a structured consensus making process. Allocation of each taxon to primary growth form has generated a single point of reference for the most common growth form for each native vascular plant species, expressed in its mature state across the extent of its range in NSW. The work presented here was undertaken to support transparent, repeatable and rigorous assessments of the richness and cover of growth form groups for the NSW Biodiversity Assessment Method. However, our approach and findings will be relevant to any government agency, industry group or researcher that uses plant growth forms to simplify ecological complexity or to assess the site-scale biodiversity values of terrestrial vegetation.

Native desert grassland plant species declines and accelerated erosion in the Paint Gap Hills of southwest Texas

Due to the hotter droughts occurring with climate change, grasslands of southwestern deserts and southern plains in the United States are predicted to increasingly lose cover of native plant species, be invaded by nonnative species, and have accelerated soil erosion. We evaluated these predictions for desert grasslands in the Paint Gap Hills of Big Bend National Park, Texas. We used data from 10 monitoring transects established in 1981 and surveyed vegetation composition, canopy cover, and soil surface elevations in 1981, 1983, 1995, and 2014. Four longer and hotter droughts occurred between 1985 and 2014. We found that by 2014 canopy covers of two dominant native warm-season perennial grasses, Bouteloua curtipendula and Bouteloua ramosa, were reduced to near zero, and the cover of many native shrubs and subshrubs had notably declined. By 2014 two nonnative perennial grasses, Eragrostis lehmanniana and Pennisetum ciliare, had invaded, and their expansion could have long-term ecological consequences. Soil surfaces changed from accumulating sediments at a rate of 0.7 mm/year for 1983–1995 to eroding at −1.6 mm/year for 1995–2014. These soil and vegetation changes support predictions of major declines in native desert grasslands and emphasize their vulnerability to climate change.

The unseen invaders: introduced earthworms as drivers of change in plant communities in North American forests (a meta-analysis).

Globally, biological invasions can have strong impacts on biodiversity as well as ecosystem functioning. While less conspicuous than introduced aboveground organisms, introduced belowground organisms may have similarly strong effects. Here, we synthesize for the first time the impacts of introduced earthworms on plant diversity and community composition in North American forests. We conducted a meta‐analysis using a total of 645 observations to quantify mean effect sizes of associations between introduced earthworm communities and plant diversity, cover of plant functional groups, and cover of native and non‐native plants. We found that plant diversity significantly declined with increasing richness of introduced earthworm ecological groups. While plant species richness or evenness did not change with earthworm invasion, our results indicate clear changes in plant community composition: cover of graminoids and non‐native plant species significantly increased, and cover of native plant species (of all functional groups) tended to decrease, with increasing earthworm biomass. Overall, these findings support the hypothesis that introduced earthworms facilitate particular plant species adapted to the abiotic conditions of earthworm‐invaded forests. Further, our study provides evidence that introduced earthworms are associated with declines in plant diversity in North American forests. Changing plant functional composition in these forests may have long‐lasting effects on ecosystem functioning.

A century of ecosystem change: human and seabird impacts on plant species extirpation and invasion on islands

We used 116 years of floral and faunal records from Mandarte Island, British Columbia, Canada, to estimate the indirect effects of humans on plant communities via their effects on the population size of a surface-nesting, colonial seabird, the Glaucous-winged gull (Larus glaucescens). Comparing current to historical records revealed 18 extirpations of native plant species (32% of species historically present), 31 exotic species introductions, and one case of exotic introduction followed by extirpation. Contemporary surveys indicated that native species cover declined dramatically from 1986 to 2006, coincident with the extirpation of ‘old-growth’ conifers. Because vegetation change co-occurred with an increasing gull population locally and regionally, we tested several predictions from the hypothesis that the presence and activities of seabirds help to explain those changes. Specifically, we predicted that on Mandarte and nearby islands with gull colonies, we should observe higher nutrient loading and exotic plant species richness and cover than on nearby islands without gull colonies, as a consequence of competitive dominance in species adapted to high soil nitrogen and trampling. As predicted, we found that native plant species cover and richness were lower, and exotic species cover and richness higher, on islands with versus without gull colonies. In addition, we found that soil carbon and nitrogen on islands with nesting gulls were positively related to soil depth and exotic species richness and cover across plots and islands. Our results support earlier suggestions that nesting seabirds can drive rapid change in insular plant communities by increasing nutrients and disturbing vegetation, and that human activities that affect seabird abundance may therefore indirectly affect plant community composition on islands with seabird colonies.

Twelve years of repeated wild hog activity promotes population maintenance of an invasive clonal plant in a coastal dune ecosystem.

Invasive animals can facilitate the success of invasive plant populations through disturbance. We examined the relationship between the repeated foraging disturbance of an invasive animal and the population maintenance of an invasive plant in a coastal dune ecosystem. We hypothesized that feral wild hog (Sus scrofa) populations repeatedly utilized tubers of the clonal perennial, yellow nutsedge (Cyperus esculentus) as a food source and evaluated whether hog activity promoted the long‐term maintenance of yellow nutsedge populations on St. Catherine's Island, Georgia, United States. Using generalized linear mixed models, we tested the effect of wild hog disturbance on permanent sites for yellow nutsedge culm density, tuber density, and percent cover of native plant species over a 12‐year period. We found that disturbance plots had a higher number of culms and tubers and a lower percentage of native live plant cover than undisturbed control plots. Wild hogs redisturbed the disturbed plots approximately every 5 years. Our research provides demographic evidence that repeated foraging disturbances by an invasive animal promote the long‐term population maintenance of an invasive clonal plant. Opportunistic facultative interactions such as we demonstrate in this study are likely to become more commonplace as greater numbers of introduced species are integrated into ecological communities around the world.

Loss of biodiversity and hydrologic function in seasonal wetlands persists over 10 years of livestock grazing removal

Ecological restoration provides a means to increase biodiversity in ecosystems degraded by natural and human‐induced changes. In some systems, disturbances such as grazing can be key factors in the successful restoration of biodiversity and ecological function, but few studies have addressed this experimentally, especially over long time periods and at landscape scales. In this study, we excluded livestock grazing from plots within a grassland landscape containing vernal pools in the Central Valley of California for 10 years and compared vernal pool hydrology and plant community composition with areas grazed under an historic regime. In all 10 years, the relative cover of native plant species remained between 5 and 20% higher in the grazed versus ungrazed plots. This effect was particularly prominent on the pool edges, though evidence of invasion into the pool basins was evident later in the study. Native species richness was lower in the ungrazed plots with 10–20% fewer native species found in ungrazed versus grazed plots in all years except the first year of treatment. Ungrazed pools held water for a shorter period of time than pools grazed under an historic regime. By the ninth year of the study, ungrazed pools took up to 2 weeks longer to fill and dried down 1–2 weeks sooner at the end of the rainy season compared to grazed pools. The results of this study confirm that livestock grazing plays a key role in maintaining biodiversity and ecosystem function in vernal pools.

Biology and Impacts of Pacific Island Invasive Species. 11.Cinchona pubescens(Red Quinine Tree) (Rubiaceae)

Cinchona pubescens Vahl (red quinine) is an evergreen tree ranging in height from 10 to 25 m with broad leaves and white or pink fragrant flowers arranged in clusters. Growing at altitudes between 130 and 3,300 m, it is one of 23 species in the genus Cinchona and has a natural distribution from Costa Rica to Bolivia. Cinchona pubescens has been cultivated in tropical regions (e.g., in South America, Africa, China, India, and Indonesia) for its quinine-containing bark and has become invasive in some regions. This is especially the case in the Pacific region, where C. pubescens has invaded humid highland areas of Galapagos, Hawai‘i, and Tahiti. It shades out and reduces cover of native plant species and adversely affects endemic birds. In addition, it changes microclimate and nutrient cycling in the soil, especially phosphorus, in Galapagos. Characteristics that make it such a successful invader include production of numerous, windborne seeds and vigorous vegetative reproduction by resprouting from underground stems and fallen trees. In Galapagos, C. pubescens is currently being manually controlled by uprooting the trees and by applying herbicides to cuts in the bark. However, this method requires continuous hand pulling of seedlings to be successful. Disturbance by control actions appears to facilitate establishment and invasion by other nonnative plant species, especially blackberry (Rubus niveus). Quinine and other alkaloids extracted from Cinchona bark are still being used for medicinal purposes today and the wood is increasingly used as construction material in Galapagos. Ironically, C. pubescens is now considered rare and endangered in its native range in Ecuador.

Manipulating livestock grazing to enhance native plant diversity and cover in native grasslands

Temperate perennial grasslands globally have been subject to extensive biodiversity loss. Identifying livestock grazing regimes that maintain and enhance the diversity and cover of native plant species in these ecosystems remains a key challenge. The responses of vegetation to different sheep grazing regimes were assessed over 3 years in grasslands of south-eastern Australia. An open communal experimental design was used to assess the effects of varying season and duration of exclusion of grazing by sheep, replicated at three locations. Manipulation of season and duration of exclusion of grazing led to few major changes in the cover of native perennial grasses or forbs, although seasonal variation was considerable. Exclusion of grazing in the spring did increase the likelihood of occurrence of grazing-sensitive native forb species but also lead to an increase in the cover of exotic annual species. However, cover of exotic annual species tended to decline with increasing duration of exclusion, while the abundance of native, grazing-sensitive forbs and the cover of perennial grasses increased. Small-scale richness of native perennial forb species increased with a 3-month period of exclusion of grazing, but declined with year-round exclusion of sheep. Total species richness also declined in response to year-round exclusion of sheep and rates of decline were correlated with the rate at which herbage mass accumulated. While strategic grazing did not result in major vegetation changes in the short term, it is suggested that some grazing exclusion may enhance the survival of infrequent species most sensitive to sheep grazing. Caution, however, should be taken when grazing regimes implemented benefit both desirable (native forbs) and undesirable (exotic annuals) species. Having a mosaic of flexible grazing management regimes across the landscape is likely to be beneficial for native plant diversity.

Evaluating Wildlife Response to Coastal Dune Habitat Restoration in San Francisco, California

The vast dune system that once dominated the entire western half of the San Francisco peninsula in California has been reduced to a few fragments that conserve locally threatened plant and animal species. We measured the effects of ongoing restoration efforts on wildlife abundance and diversity on one of the largest of these fragments, Fort Funston in the Golden Gate National Recreation Area. Efforts included removal of non-native species, active restoration of native dune vegetation, and restricted visitor use. We collected data regarding the composition and abundance of vegetation, birds, and ground-dwelling vertebrates on four treatments including an actively restored area with restricted visitor use, an unrestored area where visitor use had been restricted for ten years, an unrestored area where visitor use had been restricted for two years, and an unrestored area with unrestricted visitor use. Results indicated that the diversity and abundance of wildlife species, as well as the richness and cover of native plant species, were greater in the restored area than in all other sampled areas. Restricted visitor use alone had only modest positive effects on the abundance and diversity of wildlife and the richness and cover of native plant species.

Can pine forest restoration promote a diverse and abundant understory and simultaneously resist nonnative invasion?

An important goal of forest restoration is to increase native plant diversity and abundance. Thinning and burning treatments are a common method of reducing fire risk while simultaneously promoting understory production in ponderosa pine ( Pinus ponderosa) forests. In this study we examine the magnitude and direction of understory plant community recovery after thinning and burning restoration treatments in a ponderosa pine forest. Our objective was to determine if the post-treatment community was a diverse, abundant, and persistent assemblage of native species or if ecological restoration treatments resulted in nonnative species invasion. This project was initiated at the Grand Canyon-Parashant National Monument, Arizona, USA in 1997. We established four replicated blocks that spanned a gradient of soil types. Each block contained a control and a treated unit. Treated units were thinned to emulate pre-1870 forest stand conditions and prescribed-burned to reintroduce fire to a system that has not burned since ∼1870. We measured plant cover using the point-line intercept method and recorded species richness and composition on 0.05 ha belt transects. We examined the magnitude of treatment responses using Cohen's d effect size analysis. Changes in community composition were analyzed using nonmetric multidimensional scaling (NMS). Native plant species cover and richness increased in the thinned and burned areas compared to the controls. By the last year of the study, annual species comprised nearly 60% of the understory cover in the treatment units. Cheatgrass ( Bromus tectorum), a nonnative annual grass, spread into large areas of the treated units and became the dominant understory species on the study site. The ecological restoration treatments did promote a more diverse and abundant understory community in ponderosa pine forests. The disturbances generated by such treatments also promoted an invasion by an undesirable nonnative species. Our results demonstrate the need to minimize disturbances generated by restoration treatments and argue for the need to proactively facilitate the recovery of native species after treatment.

Contrasting effects of invasive plants in plant–pollinator networks

The structural organization of mutualism networks, typified by interspecific positive interactions, is important to maintain community diversity. However, there is little information available about the effect of introduced species on the structure of such networks. We compared uninvaded and invaded ecological communities, to examine how two species of invasive plants with large and showy flowers (Carpobrotus affine acinaciformis and Opuntia stricta) affect the structure of Mediterranean plant-pollinator networks. To attribute differences in pollination to the direct presence of the invasive species, areas were surveyed that contained similar native plant species cover, diversity and floral composition, with or without the invaders. Both invasive plant species received significantly more pollinator visits than any native species and invaders interacted strongly with pollinators. Overall, the pollinator community richness was similar in invaded and uninvaded plots, and only a few generalist pollinators visited invasive species exclusively. Invasive plants acted as pollination super generalists. The two species studied were visited by 43% and 31% of the total insect taxa in the community, respectively, suggesting they play a central role in the plant-pollinator networks. Carpobrotus and Opuntia had contrasting effects on pollinator visitation rates to native plants: Carpobrotus facilitated the visit of pollinators to native species, whereas Opuntia competed for pollinators with native species, increasing the nestedness of the plant-pollinator network. These results indicate that the introduction of a new species to a community can have important consequences for the structure of the plant-pollinator network.

Spread of the exotic croftonweed (Eupatorium adenophorum) across southwest China along roads and streams

Roads and streams are important conduits for the spread of invasive species, and croftonweed is an invasive plant in southwest China. We established six transects along one road and stream in Huili County, Sichuan Province to determine the pattern of invasion and identify major factors potentially influencing it. Results demonstrated that the invasion (cover, abundance, and number of clusters) of croftonweed declined significantly with distance from the road and stream. However, after compensation for environmental factors, only distance from the road was significantly negatively related to number of clusters of this plant. Instead, native plant species cover and richness were significantly negatively associated with croftonweed invasion along the road; elevation and native species cover were negatively correlated with the invasion along the stream. These findings indicated that roads and streams were main conduits for the spread of croftonweed in southwest China, and reduced native species cover could facilitate invasion by croftonweed in habitats along roads and streams.

Iterative Model Development for Natural Resource Managers: A Case Example in Utah's Grand Staircase-Escalante National Monument

Non-native plant species, which threaten native plant diversity, are a major concern to managers of Grand Staircase-Escalante National Monument in Utah. Predictive spatial maps with Inverse Distance Weighting provided an effective way to identify “hot spots” of occurrence for three cover types of interest: native species richness, cryptobiotic soil crust cover (lichen, moss, algae, and bacteria), and cover of non-native cheatgrass (Bromus tectorum). Maps based on regression tree analysis showed that B. tectorum was found throughout the Monument with cover usually <0.1%, but has heavily invaded mesic sites and areas of disturbance, (cover ranging from 3.4 to 17.8 %). The analysis also showed that B. tectorum cover could be predicted by positive correlations with percent soil nitrogen and phosphorous (ppm). We also found a significant inverse relationship between high native plant species cover and cryptobiotic soil crust cover. These methods provide managers with an effective way to concentrate mitigation and conservation programs.

Grazing effects on plant cover, soil and microclimate in fragmented woodlands in south‐western Australia: implications for restoration

AbstractThis study investigated the impacts of livestock grazing on native plant species cover, litter cover, soil surface condition, surface soil physical and chemical properties, surface soil hydrology, and near ground and soil microclimate in remnant Eucalyptus salmonophloia F. Muell woodlands. Vegetation and soil surveys were undertaken in three woodlands with a history of regular grazing and in three woodlands with a history of little or no grazing. Livestock grazing was associated with a decline in native perennial cover and an increase in exotic annual cover, reduced litter cover, reduced soil cryptogam cover, loss of surface soil microtopography, increased erosion, changes in the concentrations of soil nutrients, degradation of surface soil structure, reduced soil water infiltration rates and changes in near ground and soil microclimate. The results suggest that livestock grazing changes woodland conditions and disrupts the resource regulatory processes that maintain the natural biological array in E. salmonophloia woodlands. Consequently the conditions and resources in many remnant woodlands may be above or below critical thresholds for many species. The implications of these findings for restoration of plant species diversity and community structure are discussed. Simply removing livestock from degraded woodlands is unlikely to result in the restoration of plant species diversity and community structure. Restoration will require strategies that capture resources, increase their retention and improve microclimate.

EXOTIC PLANT SPECIES INVADE HOT SPOTS OF NATIVE PLANT DIVERSITY

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m2 subplots (20 1000-m2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m2 subplots (16 1000-m2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Exotic Plant Species Invade Hot Spots of Native Plant Diversity

Some theories and experimental studies suggest that areas of low plant species richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m2 subplots (20 1000-m2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m2 subplots (16 1000-m2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant invasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.