Invasive plants in the riparian zone can negatively affect the characteristics and quality of a watershed. To support the development of a watershed management plan and foster public appreciation of the value of the riparian zone, Mohonk Preserve established a volunteer monitoring program surveying sites for invasive species. Between 2017 and 2019, citizen scientists repeatedly surveyed 20 sites in the Hudson River Valley in New York for 10 invasive plant species: purple loosestrife (Lythrum salicaria L.), common reed [Phragmites australis (Cav.) Trin. ex Steud], multiflora rose (Rosa multiflora Thunb.), garlic mustard [Alliaria petiolata (M. Bieb.) Cavara & Grande], dame’s rocket (Hesperis matronalis L.), Japanese knotweed (Polygonum cuspidatum Siebold & Zucc.), wineberry (Rubus phoenicolasius Maxim.), barberry (Berberis spp.), Japanese stiltgrass [Microstegium vimineum (Trin.) A. Camus], and Asiatic bittersweet (oriental bittersweet, Celastrus orbiculatus Thunb.). We found that the number of target species detected was higher on sites closer to paved roads and with increasing drainage area size, while lower with higher percentages of forested land in the basin. Our analysis results highlight variation in the presence of target invasive species across the Hudson River Valley region, highlighting sites and areas to monitor for future introductions and take action to prevent species’ invasions. Our results highlight differences in the most relevant abiotic factors for hydrophytes and non-hydrophyte species, underscoring the importance of considering species’ life-history traits before the development of management plans for invasive plant species in the riparian zone. Our case study of community-collected data in the Hudson River Valley region using a relatively simple monitoring protocol can provide a road map for other regions fostering volunteer engagement with invasive plants.

Understanding the ecological characteristics of areas invaded and not invaded by exotic plants is a priority for invasive plant science and management. Buffelgrass is an invasive perennial species that managers view as a major threat to indigenous ecosystems of conservation lands in Australia, Mexico, the United States, and other locations where the species is not native. At 14 sites in Saguaro National Park in the Arizona Uplands of the Sonoran Desert, we compared the soil, vegetation, and soil seed bank of patches invaded and not invaded by buffelgrass. Abiotic variables, such as slope aspect and soil texture, did not differ between buffelgrass patches and patches without buffelgrass. In contrast, variables under primarily biotic control differed between patch types. Soil nutrients, such as organic C and NO3–N, were approximately twofold greater in buffelgrass compared with nonbuffelgrass patches. Average native species richness was identical (14 species 100 m−2) between patch types, but native plant cover was 43% lower in buffelgrass patches. Unexpectedly, native seed-bank densities did not differ significantly between patch types and were 40% greater than buffelgrass seed density below buffelgrass canopies. Results suggest that (1) soil nutrient status should not be unfavorable for native plant colonization at buffelgrass sites if buffelgrass is treated; (2) at least in the early stages of buffelgrass patch formation (studied patches were about 10 yr old), native vegetation species were not excluded, but rather, their cover was reduced; and (3) native soil seed banks were not reduced in buffelgrass patches.

Habitat suitability and disturbance can shape the early stages of biological invasions in important ways. Much that we know about habitat suitability and invasion originates from point-in-time studies, which characterize invasive plant abundance and associated site characteristics. In our study, we tested the influence of habitat suitability by creating small-scale invasions in a range of environments. Seeds of the invasive annual grass Japanese stiltgrass [Microstegium vimineum (Trin.) A. Camus] were planted into six environments in a deciduous forest in central Pennsylvania, and patch growth was monitored for 4 yr. Each of the 30 sites included a subplot subjected to litter disturbance at time of planting. This litter disturbance led to increased seedling recruitment only in the first 2 yr. Although patches were generally larger in wetland and roadside habitats, site influence was highly variable. Environmental variables (soil moisture, ammonium–N, pH, and plant species richness) measured in each plot were better predictors of population success than broad habitat categories. We conclude that risk assessment for species such as M. vimineum should focus not on habitat types but on areas likely to experience the physical changes that release M. vimineum populations.

The sterile hybrid, giant miscanthus, has emerged as a promising cellulosic bioenergy crop because of its rapid growth rate, high biomass yields, and tolerance to poor growing conditions; these are traits that are desirable for cultivation, but also have caused concern for their contribution to invasiveness. New seed-bearing lines of giant miscanthus would decrease establishment costs for growers, yet this previously unresearched propagule source increases fears of escape from cultivation. To evaluate the consequences of seed escape, we compared seedling establishment among seven habitats: no-till agricultural field, agricultural field edge, forest understory, forest edge, riparian, pasture and roadside; these were replicated in Virginia (Blacksburg and Virginia Beach) and Georgia (Tifton), USA. We use a novel head-to-head comparison of giant miscanthus against five invasive and three noninvasive species, thus generating relative comparisons. Overall seed germination was low, with no single species achieving germination rates >37%, in all habitats and geographies. However, habitats with available bare ground and low resident plant competition, such as the agricultural field and forest understory, were more invasible by all species. Giant miscanthus seeds emerged in the roadside and forest edge habitats at all sites. Early in the growing season, we observed significantly more seedlings of giant miscanthus than the invasive and noninvasive species in the agricultural field. Interestingly, overall seedling mortality of giant miscanthus was 99.9%, with only a single 4 cm (1.58 in) tall giant miscanthus seedling surviving at the conclusion of the 6-mo study. The ability to make relative comparisons, by using multiple control species, was necessary for our conclusions in which both giant miscanthus and the noninvasive control species survival (≤1%) contrasted with that of our well-documented invasive species (≤10%). Considering the low overall emergence, increased propagule pressure may be necessary to increase the possibility of giant miscanthus escape. Knowledge gained from our results may help prepare for widespread commercialization, while helping to identify susceptible habitats to seedling establishment and aiding in the development of management protocols.

Cogongrass invades forests through rhizomatous growth and wind-dispersed seeds. Increased density and abundance of woody vegetation along forest edges may strengthen biotic resistance to invasion by creating a vegetative barrier to dispersal, growth, or establishment of cogongrass. We evaluated differences in dispersal of cogongrass spikelets experimentally released from road edges into tallgrass-dominated and shrub-encroached longleaf pine forests (Pinus palustris). Average maximum dispersal distances were greater in the pine–tallgrass forest (17.3 m) compared to the pine–shrub forest association (9.4 m). Spikelets were more likely to be intercepted by vegetation in pine–shrub forests compared to pine–tallgrass forests. Results suggest that dense woody vegetation along forest edges will slow spread from wind-dispersed cogongrass seeds.

An ecosystem's invasibility is influenced by changes in biotic and abiotic resistances, which often occur due to shifts in the prevailing disturbance regime. The susceptibility of a community to intrusion by nonnative species may interact with propagule pressure to determine the extent of a biological invasion. We examined how propagule pressure, forest community structure, and disturbance interact to influence the invasibility of temperate Pacific Northwest forests by the newly invasive grass, perennial false-brome (Brachypodium sylvaticum). Our goal was to identify factors enabling shifts from establishment to population growth in B. sylvaticum populations at the edge of its expanding range. Ecological sampling methods were used to identify patterns in B. sylvaticum habitat. An inverse relationship between the amount of B. sylvaticum and all perennial vegetation types and soil litter depth was found, suggesting that disturbance might play a role in B. sylvaticum population establishment or growth. An experimental study was then performed to test the effects of disturbance, propagule pressure, and habitat on B. sylvaticum seedling establishment in sites where B. sylvaticum was already naturalized. We found evidence that disturbance of the soil and vegetation led to increased B. sylvaticum seedling recruitment within naturalized sites, especially where conditions of high propagule pressure and deciduous forest canopy existed. In contrast, B. sylvaticum populations dominated by coniferous forest canopy were much more invasible than deciduous forests and did not show increased seedling recruitment in response to our disturbance treatments. Our study shows how propagule pressure and plant community dynamics interact to alter the invasibility of Pacific Northwest forests allowing B. sylvaticum to transition from establishment to population growth thus allowing this weed to cause greater negative impacts on the ecosystem.

The negative effect of invasive species on native species, communities, and ecosystems is widely recognized, and the economic effects in the United States are estimated to be billions of dollars annually. Studies often examine traits of nonnative species or examine what makes a particular habitat invasible. To better understand the factors governing invasions, we used the flora of Nebraska to characterize and compare native and nonnative plant occurrences throughout the state. In addition, we assessed four critical landscape predictors of nonnative plant richness: human population size and three land cover attributes that included percentage of grassland, percentage of agriculture, and percentage of public lands. Results indicated that individual plant species richness has increased by about 35% through invasions (primarily of annuals from the family Poaceae). In addition, human population density, percentage of agriculture, and percentage of public lands all show a positive association with nonnative plant richness. Successful plant invasions may change the composition of species communities, basic ecological functions, and the delivery of ecosystem services. Thus, identifying the factors that influence such variation in distribution patterns can be fundamental to recognizing the present and potential future extent of nonnative plant infestations and, in turn, developing appropriate management programs.

Because of its ability to transform ecosystems by increasing the prevalence of fire, the invasive annual red brome is a priority exotic species for management in arid lands of the southwestern United States. By sampling red brome presence and 97 environmental (climatic, topographic, and soil) and native vegetation (e.g., perennial species richness) variables on 126 sites, we assessed biophysical correlates with red brome distribution on a 755,000-ha (1.9 million ac) Mojave Desert landscape. Brome occupied 55 of 126 (44%) 0.09-ha plots. The simplest models (i.e., those containing the fewest or most easily obtained variables) in multivariate (classification trees and nonparametric multiplicative regression) and univariate (χ2) models often portrayed red brome distribution as well, or nearly as well, as more complicated models containing more variables harder to obtain. The models varied, however, in their abilities for describing brome presence compared with absence. For example, a simple classification tree using only elevation, soil great group, parent material, and vegetation type improved estimates of brome presence for 55% of sites, absences for 87%, and overall for 73% of sites compared with a naïve model containing the observed frequency of brome in the data. Conversely, a more complicated model, including soil boron and sulfur, performed better for presences (96%) than for absences (73%; 83% overall). Results also showed variable support for two general postulates in invasive species science. Red brome distribution was not correlated with soil N, which is inconsistent with the supposition that nutrient-rich soils are more prone to invasion. Brome was correlated with native perennial species richness to support the postulate that exotic species abundance is correlated with species-rich habitats, but the correlation was weak (r = 0.38) and similar in strength to correlations with many other environmental variables. On this relatively low-elevation landscape, the areas currently most invaded by red brome include the higher elevations (> 777 m [2,549 ft]), limestone–sandstone soils, and burrobush and mixed perennial communities. Areas least inhabited by brome are the lowest elevations (< 491 m), gypsum soils, and creosotebush and saltbush communities.

The interactions between native and exotic species occur on a continuum from facilitative to competitive. A growing thrust in invasive species science is differentiating where particular native species occur along this continuum, with practical implications for identifying species that might reduce the invasibility of ecosystems. We used a greenhouse experiment to develop a competitive hierarchy of 27 native species with red brome, an invasive annual grass in the arid lands of the southwestern United States, and a field study to assess in situ responses of brome to native perennial species in the Mojave Desert. Native species most competitive with brome in the competition experiment included the annuals Esteve's pincushion and western fiddleneck and the perennials eastern Mojave buckwheat, sweetbush, and brittlebush, which reduced brome biomass to 49 to 70% of its grown-alone amount. There was no clear difference in competitive abilities with brome between annual and perennial natives, and competiveness was not strongly correlated (r = 0.15) with the biomass of the native species. In the field, sweetbush and brittlebush supported among the least cover of brome, suggesting congruence of the strong early competitive abilities of these species with in situ patterns of brome distribution. At the other extreme, brome attained its highest average cover (19%) below littleleaf ratany, significantly greater than all but 3 of the 16 species evaluated. Cover by brome was only weakly related (r = 0.19) to the area of the perennial canopy, suggesting that factors other than the sizes of perennial plants were linked to differences in brome cover among species. Results suggest that (1) interactions with brome differ substantially among native species, (2) these interactions are not as closely linked to biomass production as in more temperate regions, and (3) there is potential for identifying native species that can reduce invasion of desert ecosystems.

An important challenge of invasion biology is to understand how interactions between species traits and ecosystem properties enable alien species to become invasive at particular locations. We investigated how gap dynamics in a tropical rain forest on the island of Réunion affected the invasiveness of alien plants. In the 12 000-m2 study area, alien plants occupied 24.9% of the area of gaps, which represented 5.62% of the forest area, but only 0.8% of the understorey area. The most abundant invasive species was Rubus alceifolius, which formed dense, monospecific stands in the largest gaps (> 25 m2). Although plants could persist in the shade, a germination experiment revealed that canopy openings were essential for seedling establishment. A cyclone that struck the study area in 2002 caused a temporary thinning of the canopy, increasing light levels to above the threshold needed for germination of R. alceifolius and also stimulating the growth of established plants. We conclude that the ability of this and other alien species to colonize intact lowland tropical rain forest is strongly influenced by the prevailing gap dynamics. Because gaps are also essential for the regeneration of many native trees in our study area, there is a real danger of the forest being progressively degraded by alien plants. There are no simple solutions to controlling species such as Rubus alceifolius, but efforts should be focused mainly upon the larger gaps where the species are most invasive.

It has traditionally been considered that areas with high natural species richness are likely to be more resistant to non-indigenous species than those with lower numbers of species. However, this theory has been the subject of a debate over the last decade, since some studies have shown the opposite trend. In the present study, a macroalgal survey was carried out at 24 localities in Northern Ireland and southern England, using a quadrat approach in the lower littoral. The two opposing hypotheses were tested (negative versus positive relationship between native and non-indigenous species richness) in this marine environment. The effect of the presence of ‘impacts’, potential sources of disturbance and species introduction (e.g. marina, harbour or aquaculture), was also tested. A positive relationship was found between the number of non-indigenous species and the native species richness at the three different scales tested (quadrats, sites and localities). At no scale did a richer native assemblage appear to restrict the establishment of introduced species. The analyses revealed greater species richness and different community composition, as well as more non-indigenous species, in southern England relative to Northern Ireland. The presence of the considered impacts had an effect on the community composition and species richness in southern England but not in Northern Ireland. Such impacts had no effect on the non-indigenous species richness in either area.

Many introduced plant species rely on mutualisms in their new habitats to overcome barriers to establishment and to become naturalized and, in some cases, invasive. Mutualisms involving animal-mediated pollination and seed dispersal, and symbioses between plant roots and microbiota often facilitate invasions. The spread of many alien plants, particularly woody ones, depends on pollinator mutualisms. Most alien plants are well served by generalist pollinators (insects and birds), and pollinator limitation does not appear to be a major barrier for the spread of introduced plants (special conditions relating to Ficus and orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly birds and mammals. Our review supports the view that tightly coevolved, plant-vertebrate seed dispersal systems are extremely rare. Vertebrate-dispersed plants are generally not limited reproductively by the lack of dispersers. Most mycorrhizal plants form associations with arbuscular mycorrhizal fungi which, because of their low specificity, do not seem to play a major role in facilitating or hindering plant invasions (except possibly on remote islands such as the Galapagos which are poor in arbuscular mycorrhizal fungi). The lack of symbionts has, however, been a major barrier for many ectomycorrhizal plants, notably for Pinus spp. in parts of the southern hemisphere. The roles of nitrogen-fixing associations between legumes and rhizobia and between actinorhizal plants and Frankia spp. in promoting or hindering invasions have been virtually ignored in the invasions literature. Symbionts required to induce nitrogen fixation in many plants are extremely widespread, but intentional introductions of symbionts have altered the invasibility of many, if not most, systems. Some of the world's worst invasive alien species only invaded after the introduction of symbionts. Mutualisms in the new environment sometimes re-unite the same species that form partnerships in the native range of the plant. Very often, however, different species are involved, emphasizing the diffuse nature of many (most) mutualisms. Mutualisms in new habitats usually duplicate functions or strategies that exist in the natural range of the plant. Occasionally, mutualisms forge totally novel combinations, with profound implications for the behaviour of the introduced plant in the new environment (examples are seed dispersal mutualisms involving wind-dispersed pines and cockatoos in Australia; and mycorrhizal associations involving plant roots and fungi). Many ecosystems are becoming more susceptible to invasion by introduced plants because: (a) they contain an increasing array of potential mutualistic partners (e.g. generalist frugivores and pollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across genera); and (b) conditions conducive for the establishment of various alien/alien synergisms are becoming more abundant. Incorporating perspectives on mutualisms in screening protocols will improve (but not perfect) our ability to predict whether a given plant species could invade a particular habitat.