Invasive Plant Populations Research Articles

Differences in cold hardiness between introduced populations of an invasive tree

The potential for populations of exotic invasive plants to differ in their response to stressful environmental conditions is an underexplored issue in determining invasive species’ range limits. Introduced genotypes may differ in response to climatic, edaphic, or biotic factors within their introduced range leading to differences in potential ranges among populations. We examined differences in cold hardiness (resistance and tolerance to winter conditions and freeze events) among Chinese tallow tree (Triadica sebifera (L.) Small) seeds and seedlings from two genetically distinct populations in the northernmost portion of its introduced U.S. range (North Carolina and South Carolina). Seed germination from these two sources was compared between fall plantings (mimicking natural dispersal timing) and spring plantings (occurring post-frost) as well as among areas within and inland of Chinese tallow’s core coastal distribution in South Carolina. Overwinter seedling survival and damage were also assessed among seedlings planted in the piedmont of South Carolina and following artificial freeze events in the lab. Seeds and seedlings from South Carolina sources showed greater reductions in germination success by inland winters, greater winter damage in field plantings in the piedmont, and lower survival after prolonged freezes than those from North Carolina. These results indicate that differences in cold hardiness exist among introduced populations of Chinese tallow and suggest that genotypes from North Carolina possess greater potential for expansion into areas with more severe winters. Differences among introduced populations should be considered when evaluating the potential range expansion of Chinese tallow and other invasive species.

Trees, Deer and Non-Native Vines: Two Decades of Northern Piedmont Forest Restoration

The presence of white-tailed deer ( Odocoileus virginianus ) and non-native invasive plants at high densities can significantly delay or even completely stall successional processes in urbanized northern Piedmont old-fields and floodplains. Such areas may require intervention in the form of active tree planting to renew forest succession. I report on the creation and subsequent stewardship of 2 forest restoration plantings established by the Pennypack Ecological Restoration Trust 2 decades ago in the Pennypack Preserve, a private natural area in the northern Piedmont Uplands northeast of Philadelphia, Pennsylvania, USA. While these plantings now appear to be on a trajectory toward becoming forest, achieving that status required a post-planting stewardship strategy that developed over many years and that the Trust has adapted for the management of its newer reforestation projects. My experiences at Pennypack Preserve suggest that woodlands developing from such plantings must grow for at least 25 yr before the trees achieve a stature resistant to deer damage and their canopies cast shade deep enough to suppress the growth of sun-loving invasive plants. Routine maintenance is required for this entire period. Restorationists must recognize that re-establishing forest ecosystems in areas with dense deer and invasive plant populations is a difficult, complex, and expensive undertaking; success is unlikely without a long-term commitment to ongoing stewardship.

Ecosystem Services: The Significance of Contributions by Invasive Plant Species

Across vast expanses of rangelands, prairies, woodlands, and forests and throughout many wetlands and riparian corridors, invasive plant species have established and continue to threaten ecosystems. The number of invasive plant species on record in the United States is greater than 4,000 (Westbrooks 2004). The harm done by the establishment of invasive plant species includes reduced flow in waterways, disrupted fire regimes, and the loss of biodiversity in many natural areas. In the United States, it has been estimated that the cost for controlling invasive plant species is in excess of $120 billion per year (Pejchar and Mooney 2009). Ecosystem services are the inherent and tangible provisions supplied by and for natural systems that have direct benefits to humans (MEA 2005). The value of ecosystem services include: direct consumptive use values such as food, fiber, fuel, and pharmaceuticals; indirect use values such as nutrient cycles, disease resistance, or resilience to disturbances; direct nonconsumptive use values such as waste decomposition and air purification; and pure existence use values such as educational value, cultural importance, or aesthetic beauty. The means by which ecosystems provide these values go beyond simple primary productivity to include the maintenance of stable food webs and soil fertility, structural components such as soil and community composition, and biodiversity itself, which all can be affected by invasive plant species. In quantifying ecosystem services, two objectives of the U.S. Environmental Protection Agency (USEPA 2007) stand out in relation to invasive plant species: (1) identify knowledge gaps in the processes underlying ecosystem services and (2) evaluate benefits of ecosystem services and trade-offs among management actions that affect these services. Although the number of hectares of invasive plant species continues to increase, in some cases rapidly (Duncan et al. 2004), information on their contribution to ecosystem services is severely lacking. Once the value of ecosystem services is determined, the value of managing invasive plant species can be fully realized, and better decisions can be made regarding the implementation of policies. It is widely recognized that invasive plant species can establish in diverse environments, and with the increase in human mobility, they are no longer restricted to isolated pockets in remote parts of the world. Less emphasis is given to the fact that invasive plant species contribute to ecosystem services similar to those of other plant types, including native or nonnative desirable species. The significance of their contribution(s) is largely dependent on plant community composition and environmental conditions, which can change rapidly in a short period. For example, cheatgrass (Bromus tectorum L.) in rangelands, purple loosestrife (Lythrum salicaria L.) in wetlands, and tamarisk (Tamarix spp.) in riparian areas were introduced into the United States before the 19th century, yet their effects on economies and environments have only been felt more recently (Duncan et al. 2004). Invasive plant species’ contribution to ecosystem services is primarily viewed as negative because of their ability to out-compete native plant species and alter entire ecosystems, although in a few cases, services are enhanced (see Hershner and Havens 2008). In many states, laws have been established that enforce the removal of state and federal noxious plants and other invasive plants with monetary fines and penalties for noncompliance. By no means does this special feature or the symposium related to it advocated an alternative view. The main focus is to acknowledge existing invasive plant populations, determine their contributions to ecosystems, and better understand the entirety of the invasive plant species situation in the United States and around the world. With these concepts in mind, a symposium was organized to investigate the area of invasive plant species and ecosystem services and further our understanding of the contributions that invasive plants are making to DOI: 10.1614/IPSM-D-11-00004.1 * Assistant Professor, Department of Agronomy & Horticulture, University of Nebraska-Lincoln, 402 West State Farm Road, North Platte, NE 69101. Corresponding author’s E-mail: syoung4@unl.edu Invasive Plant Science and Management 2012 5:106–107

Nonnative invasive plants in the Penobscot Experimental Forest in Maine, USA: Influence of site, silviculture, and land use history1,2

Abstract We investigated the occurrence of nonnative invasive plants on approximately 175 ha comprising a long-term, 60-year-old U.S. Forest Service silvicultural experiment and old-field stands in the Penobscot Experimental Forest (PEF) in central Maine. Stands in the silvicultural experiment were never cleared for agriculture, but have been repeatedly partially cut. Our objectives were to determine the extent of nonnative invasive plant populations in the PEF, and to relate invasive plant abundance and distribution to management history and environmental factors (overstory composition and basal area, canopy openness, and soil characteristics). We found ten invasive plant species in the study area. Very few occurrences of these were in the silvicultural experiment; where present, invasive plants there appear to be associated with proximity to seed source, and a greater degree of recreational or silvicultural disturbance. Ordination showed that the environmental variables which were associated with invasi...

Maintenance of High Genetic Diversity during Invasion ofRhododendron ponticum

Adaptive processes in invasive plant populations may explain exotic range expansions and ecotypic shifts between native and invasive populations of the same species. Sufficient genetic variation in invasive populations occurring within the new range is a precondition for selection. While genetic diversity can be considerably reduced during invasions following founder effects and genetic drift, it can also be maintained via mechanisms of effective gene flow. Rhododendron ponticum L., an aggressive invader in the British Isles, is an appropriate test species for investigating changes in genetic diversity and differentiation. Six native populations in Georgia and Spain each and six invasive populations in Ireland were analyzed with amplified fragment length polymorphism markers to calculate genetic distances, genetic diversity, and differentiation within and among populations. While the highest level of genetic diversity was found within native Georgian populations, thus indicating a refugium, native Spanish...

A comparison of univariate and multivariate methods for analyzing clinal variation in an invasive species

The evolution of clinal variation has become a topic widely studied for invasive species. Most studies of this kind have found significant correlations between latitude and various plant traits, usually using univariate analytic methods. However, plants are composed of multiple, interacting traits, and it is this correlation among traits that can affect how quickly or even whether the populations of invasive plants adapt to their local climatic conditions. We used data from a common garden experiment to determine the possible formation of latitudinal clines in invasive North American populations of Lythrum salicaria L. (purple loosestrife) from the central portion of its invasive range. Analyses were conducted using the more common univariate approach (nested and oneway ANOVAs; linear regression) on individual plant traits (e.g., time to flowering, plant height, various mass measures, and growth rate) and then a multivariate approach (principle components analysis followed by redundancy analysis). Significant among-population differences (P < 0.01) were noted when using both the nested and oneway ANOVAs, and multivariate techniques. However, there were no significant relationships between individual plants traits to latitude when using linear regressions, most likely as a result of the small number of populations used in the study (n = 4). On the contrary, the multivariate analyses showed a significant effect of latitude (P < 0.001) on the invasive populations, but this explained only 4% of the variance; latitude explained 8% of the variance when both invasive and native populations were analyzed. Because of the integrated nature of plant phenotypes, a multivariate approach should provide a clearer and deeper understanding of population responses to changing conditions than univariate techniques.

Climate change and weed adaptation: can evolution of invasive plants lead to greater range expansion than forecasted?

ClementsDR &amp; DitommasoA (2011). Climate change and weed adaptation: can evolution of invasive plants lead to greater range expansion than forecasted?Weed Research51, 227–240.SummaryInvasive plants are frequently viewed as harbingers of climate change owing to their potential to cause economic and ecological damage in the process of expanding their ranges. Models are being developed to help predict the range expansion of these plants, based on known tolerance ranges. Success of weeds has often been attributed to an ‘all‐purpose genotype’, implying a high level of phenotypic plasticity. However, recent work has shown that many species are capable of relatively rapid genetic change as well, enhancing their ability to invade new areas in response to anthropogenic ecosystem modification. We thus predict that range expansion by many invasive species will exceed that predicted by modelling approaches that do not consider potential evolutionary change. We highlight a number of cases where weeds have expanded their latitudinal ranges or are predicted to do so in response to climatic selection pressures. We also list ten traits as likely targets for natural selection under climate change. The lag phase commonly observed for invasive species may frequently be a result of the time needed for the invader to evolve to fit the new habitat. During this present period of climate change, many invasive plant populations are likely to be in the process of developing adaptations that could lead to exponential population growth in the near future. Thus, assessment of the risk of invasive species owing to changing climate must incorporate evolutionary potential.

Are We “Missing the Boat” on Preventing the Spread of Invasive Plants in Rangelands?

AbstractInvasive plants are negatively affecting the ecological and economic production of rangelands by reducing resource productivity, decreasing biodiversity, displacing native vegetation, and altering ecosystem processes and functions. However, despite these well-known negative effects, once invasive plants are regionally established, limited effort is directed at preventing their continued spread across rangelands. Most efforts are directed at restoration at specific locations while additional rangelands are invaded. Restoring native plant communities invaded by exotic plants is frequently unsuccessful, especially in more arid environments, and is often too costly to apply at the scale required to make meaningful progress in reducing invasive plant populations relative to their expansion. Of the few prevention efforts being implemented, most are a second priority to control and restoration efforts. Integrating strategies to prevent new infestations and restrict the expansion of existing populations in invasive plant management programs is critical to limiting the negative effects of invasive plants in rangelands. However, we are “missing the boat” on this issue by not providing sufficiently developed and validated management actions. Limited information is available for developing management strategies to prevent the spread of invasive plants, although it has been suggested that land managers need to increase biotic resistance of desired plant communities, decrease invasive plant propagule pressure, and eradicate small incipient infestations to prevent the continued expansion of invasive plants. Thus, instead of scientifically validated methods developed to limit the spread of invasive plants, managers are often left with vague suggestions for preventing the continued spread of invasive plants. We suggest that if prevention is going to be successful, researchers are going to need to conduct more applied research to provide land managers with specific prevention strategies and quantify the benefits of various prevention strategies.

Effects of Rhinoncomimus latipes on the growth and reproduction of Persicaria perfoliata, an invasive plant in North America

Native to Asia, mile-a-minute Persicaria perfoliata, is an invasive weed in North America, and the weevil Rhinoncomimus latipes is a host-specific insect agent which occurs widely in China. We conducted a common garden experiment to compare P. perfoliata plant responses of native and invasive populations to herbivory by the weevils from different origins. We found weevils from Hunan, Hubei and Heilongjiang Provinces had strong, moderate and weak ability to suppress host plant, respectively. Weevils from Hunan and Hubei Provinces had stronger impact on the growth of both native and invasive plant populations than the weevil from Heilongjiang Province. The losses in seed output of invasive plants were also significantly greater than natives in the weevil treatments. Our results suggested that the weevil population from Hunan Province may be the most suitable for the control of mile-a-minute, while the population from Heilongjiang Province may be the least suitable due to climate matching.

WHIPPET: A novel tool for prioritizing invasive plant populations for regional eradication

Large geographic areas can have numerous incipient invasive plant populations that necessitate eradication. However, resources are often deficient to address every infestation. Within the United States, weed lists (either state-level or smaller unit) generally guide the prioritization of eradication of each listed species uniformly across the focus region. This strategy has several limitations that can compromise overall effectiveness, which include spending limited resources on 1) low impact populations, 2) difficult to access populations, or 3) missing high impact populations of low priority species. Therefore, we developed a novel science-based, transparent, analytical ranking tool to prioritize weed populations, instead of species, for eradication and tested it on a group of noxious weeds in California. For outreach purposes, we named the tool WHIPPET (Weed Heuristics: Invasive Population Prioritization for Eradication Tool). Using the Analytic Hierarchy Process that included expert opinion, we developed three major criteria, four sub-criteria, and four sub-sub-criteria, taking into account both species and population characteristics. Subject matter experts weighted and scored these criteria to assess the relative impact, potential spread, and feasibility of eradication (major criteria) for 100 total populations of 19 species. Species-wide population scores indicated that conspecific populations do not necessarily group together in the final ranked output. Thus, priority lists based solely on species-level characteristics are less effective compared to a blended prioritization based on both species attributes and individual population and site parameters. WHIPPET should facilitate a more efficacious decision-making process allocating limited resources to target invasive plant infestations with the greatest predicted impacts to the region under consideration.

Resource allocation to defence and growth are driven by different responses to generalist and specialist herbivory in an invasive plant

Summary1. Invasive plants often have novel biotic interactions in their introduced ranges. These interactions, including less frequent herbivore attacks, may convey a competitive advantage over native plants. Invasive plants may vary in defence strategies (resistance vs. tolerance) or in response to the type of herbivore (generalists vs. specialists), but no study to date has examined this broad set of traits simultaneously.2. Here, we examined resistance and tolerance of Chinese tallow (Triadica sebifera) populations from the introduced and native ranges to generalist (Cnidocampa flavescens) and specialist herbivores (Gadirtha inexacta) in the native range.3. In a field common‐garden test of resistance, caterpillars of each species were raised on plants from native and invasive populations. We found the specialist grew larger on and consumed more mass of invasive plant populations than native populations, while the generalist showed the same performance between them. The results were consistent with our laboratory bioassay using excised leaves. Chemical analyses showed that the invasive plants had lower tannin content and higher ratio of carbohydrate to protein than those of their native counterparts, suggesting that plants from invasive populations have altered chemistry that has a larger impact on specialist than on generalist resistance.4. To test for differences in herbivore tolerance, plants were first defoliated by specialist or generalist herbivory and then allowed to regrow for 100 days in a field common garden. We found that plants from invasive populations had greater herbivore tolerance than native populations, especially for tolerance to generalists. They also grew more rapidly than native counterparts in the absence of herbivory.5. Synthesis.The results of these experiments indicate that differences in selective pressures between ranges have caused dramatic reductions in resistance to specialist herbivores and those changes in plant secondary chemistry likely underlie these differences. The greater tolerance of invasive populations to herbivory appears to at least partly reflect an increase in growth rate in the introduced range. The greater tolerance to generalist herbivores suggests the intriguing possibility of selection for traits that allow plants to tolerate generalist herbivores more than specialist herbivores.

Coevolution between native and invasive plant competitors: implications for invasive species management.

Invasive species may establish in communities because they are better competitors than natives, but in order to remain community dominants, the competitive advantage of invasive species must be persistent. Native species that are not extirpated when highly invasive species are introduced are likely to compete with invaders. When population sizes and genetic diversity of native species are large enough, natives may be able to evolve traits that allow them to co-occur with invasive species. Native species may also evolve to become significant competitors with invasive species, and thus affect the fitness of invaders. Invasive species may respond in turn, creating either transient or continuing coevolution between competing species. In addition to demographic factors such as population size and growth rates, a number of factors including gene flow, genetic drift, the number of selection agents, encounter rates, and genetic diversity may affect the ability of native and invasive species to evolve competitive ability against one another. We discuss how these factors may differ between populations of native and invasive plants, and how this might affect their ability to respond to selection. Management actions that maintain genetic diversity in native species while reducing population sizes and genetic diversity in invasive species could promote the ability of natives to evolve improved competitive ability.

The small-scale spatiotemporal pattern of the seedbank and vegetation of a highly invasive weed,Centaurea solstitialis: strength in numbers

The dynamics of invasive plant populations are intriguing and informative of the importance of population and community-level processes. A dominant approach to understanding and describing invasion has been the development of unique hypotheses to explain invasion. However, here we directly explore the relevance of the small-scale, spatiotemporal pattern in seedbanks and plants of the highly invasive weed, Centaurea solstitialis, to determine whether pattern can be used to contrast predictions associated with the simple ecological hypotheses of seed versus microsite limitations. At three invaded grasslands in California, highly invaded (> 20 adult plants present), invaded (< 10 adults), and uninvaded (no C. solstitialis plants) sites were selected. The spatial pattern of the seedbank was assessed using fine-scale, 2 cm diameter contiguous cores and geostatistical statistics, and the number of C. solstitialis seeds in the seedbank was recorded in addition to the total community seedbank density. Three of the four critical predictions associated with the seed limitation hypothesis were clearly supported as an explanation for the patterns of C. solstitialis invasion observed in the field. The density of C. solstitialis seeds decreased from high to low extents of invasion, there was no relationship between the community seedbank and C. solstitialis seeds, and the distances between C. solstitialis plants was inversely related to the density of C. solstitialis seeds. However, both the persistent and transient seedbanks of C. solstitialis were spatially aggregated with autocorrelation up to 12 cm 2 which suggests that aggregation is a consistent attribute of this species in the seedbank regardless of extent of invasion. This basic pattern-based approach clearly detected an ecological signal of invasive seedbank dynamics and is thus a useful tool for subsequent studies of invasions in grasslands.

Plant–soil feedback induces shifts in biomass allocation in the invasive plant Chromolaena odorata

Summary 1. Soil communities and their interactions with plants may play a major role in determining the success of invasive species. However, rigorous investigations of this idea using cross‐continental comparisons, including native and invasive plant populations, are still scarce. 2. We investigated if interactions with the soil community affect the growth and biomass allocation of the (sub)tropical invasive shrub Chromolaena odorata. We performed a cross‐continental comparison with both native and non‐native‐range soil and native and non‐native‐range plant populations in two glasshouse experiments. 3. Results are interpreted in the light of three prominent hypotheses that explain the dominance of invasive plants in the non‐native range: the enemy release hypothesis, the evolution of increased competitive ability hypothesis and the accumulation of local pathogens hypothesis. 4. Our results show that C. odorata performed significantly better when grown in soil pre‐cultured by a plant species other than C. odorata. Soil communities from the native and non‐native ranges did not differ in their effect on C. odorata performance. However, soil origin had a significant effect on plant allocation responses. 5. Non‐native C. odorata plants increased relative allocation to stem biomass and height growth when confronted with soil communities from the non‐native range. This is a plastic response that may allow species to be more successful when competing for light. This response differed between native and non‐native‐range populations, suggesting that selection may have taken place during the process of invasion. Whether this plastic response to soil organisms will indeed select for increased competitive ability needs further study. 6. The native grass Panicum maximum did not perform worse when grown in soil pre‐cultured by C. odorata. Therefore, our results did not support the accumulation of local pathogens hypothesis. 7. Synthesis. Non‐native C. odorata did not show release from soil‐borne enemies compared to its native range. However, non‐native plants responded to soil biota from the non‐native range by enhanced allocation in stem biomass and height growth. This response can affect the competitive balance between native and invasive species. The evolutionary potential of this soil biota‐induced change in plant biomass allocation needs further study.

Complex population dynamics and control of the invasive biennial Alliaria petiolata (garlic mustard)

Controlling species invasions is a leading problem for applied ecology. While controlling populations expanding linearly or exponentially is straightforward, intervention in systems with complex dynamics can have complicated, and sometimes counterintuitive, consequences. Most invasive plant populations are stage-structured and density-dependent--a recipe for complex dynamics--and yet few population models have been created to explore the effects of control efforts on such species. We examined the demography of the invasive biennial plant Alliaria petiolata (garlic mustard) on the front of its spread into a natural area and found evidence of strong density dependence in vital rates of first-year rosette and second-year adult stage classes. We parameterized a density-dependent, stage-structured projection model using field-collected data. This model produces two-point cycles with alternating years in which adults vs. rosettes are more prevalent. Such population dynamics match observations in natural populations, suggesting that these complicated population dynamics may result from deterministic rules. We used this model to evaluate simulated management strategies, including herbicide treatment of rosettes and clipping or pulling of adult plants. Management of A. petiolata by inducing mortality of either rosettes or adults will not be effective at reducing population density unless the induced mortality is very high (>95% for rosettes and >85% for adults) and repeated every year. Indeed, induced mortality of rosettes can be counterproductive, causing increases in the stationary distribution of A. petiolata density. This species is typical of many invasive plants (stage-structured, short-lived, high fertility) and exhibits common forms of density dependence. Thus, the management implications of our study should apply broadly to other species with similar life histories. We suggest that management should focus on managing adults rather than rosettes, and on creating efficient control in targeted areas of the population, rather than spreading less efficient efforts widely.

Invasion, Disturbance, and Competition: Modeling the Fate of Coastal Plant Populations

Wetland habitats are besieged by biotic and abiotic disturbances such as invasive species, hurricanes, habitat fragmentation, and salinization. Predicting how these factors will alter local population dynamics and community structure is a monumental challenge. By examining ecologically similar congeners, such as Iris hexagona and I. pseudacorus (which reproduce clonally and sexually and tolerate a wide range of environmental conditions), one can identify life-history traits that are most influential to population growth and viability. We combined empirical data and stage-structured matrix models to investigate the demographic responses of native (I. hexagona) and invasive (I. pseudacorus) plant populations to hurricanes and salinity stress in freshwater and brackish wetlands. In our models I. hexagona and I. pseudacorus responded differently to salinity stress, and species coexistence was rare. In 82% of computer simulations of freshwater marsh, invasive iris populations excluded the native species within 50 years, whereas native populations excluded the invasive species in 99% of the simulations in brackish marsh. The occurrence of hurricanes allowed the species to coexist, and species persistence was determined by the length of time it took the ecosystem to recover. Rapid recovery (2 years) favored the invasive species, whereas gradual recovery (30 years) favored the native species. Little is known about the effects of hurricanes on competitive interactions between native and invasive plant species in marsh ecosystems. Our models contribute new insight into the relationship between environmental disturbance and invasion and demonstrate how influential abiotic factors such as climate change will be in determining interspecific interactions.

The Rationale for Monitoring Invasive Plant Populations as a Crucial Step for Management

AbstractMany land managers are faced with trying to optimize management of invasive plant species based on budget constraints and lack of knowledge of the true potential of the species. Generally, “early detection rapid response” (EDRR) is the assumed best management strategy and tends to drive management regardless of the invasion stage or possible variation in the invasion potential of the population. We created a simulation model to evaluate the optimal management strategy to reduce the rate of invasion of nonindigenous plant species. The strategies were specifically chosen to assess the value of information from monitoring populations. We compared four management strategies and a no-management control over a 20-yr period in the context of a management area: (1) managing a fixed number of populations at random each year (EDRR random), (2) managing an equivalent number of populations along a road each year (EDRR road), (3) managing half of the fixed populations that were determined by monitoring to be sources of new populations (monitoring every year), and (4) managing an equivalent set of source populations only on even years, leaving the odd years for monitoring (monitoring every other year). EDRR random location without regard to population invasion potential, and monitoring every year targeting management on populations determined to be invasive (sources for new populations), were the most successful strategies for reducing the increase in total number of populations. The model simulations suggest that managers could dedicate 50% of their management time to monitoring without risk of accelerating invasions or reducing the impact of their weed management program.

Bidirectional history of hybridization in California wild radish, Raphanus sativus (Brassicaceae), as revealed by chloroplast DNA

The evolutionary processes that take place in invasive plant populations are not well documented or understood. Interspecific hybridization between cultivated radish (Raphanus sativus) and R. raphanistrum is known to be responsible for the origin of the invasive California wild radish, but little is known about the nature of the hybridization events that produced the hybrid-derived lineage. We analyzed the trnL-rpl32 intergenic region of chloroplast DNA (cpDNA) obtained from 37 cultivated radish individuals from four different cultivars, 53 R. raphanistrum individuals from six European populations and 104 California wild radish individuals from 11 populations covering its entire range throughout the state. We found that cultivated radish and R. raphanistrum shared no cpDNA haplotypes but that they both shared haplotypes with California wild radish, evidence for bidirectional hybridization between the progenitor species in the creation of the California lineage. We also found evidence that multiple cultivars and multiple European source populations contributed to the diversity of cpDNA haplotypes within California. Studies like this will continue to be important for our understanding of the origin of invasive populations and the mechanisms by which they succeed.

General guidelines for invasive plant management based on comparative demography of invasive and native plant populations

Summary 1. General guidelines for invasive plant management are currently lacking. Population declines may be achieved by focusing control on demographic processes (survival, growth, fecundity) with the greatest impact on population growth rate. However, we often have little demographic information on populations in the early stages of an invasion when control can be most effective. Here we determine whether synthesis of existing demographic data on invasive and native plant populations can address this knowledge problem. 2. We compared population dynamics between invasive and native species using published matrix population models for 21 invasive and 179 native plant species. We examined whether the population growth rate responsiveness to survival, growth and fecundity perturbations varied between invasive and native species, and determined which demographic processes of invaders to target for reductions in population growth rate. 3. Invaders had higher population growth rates ( λ ) than natives, resulting in differences in demographic processes. Perturbations of growth and fecundity transitions (elasticities) were more important for population growth of invaders, whereas perturbations of survival had greater importance for population growth of natives. 4. For both invasive and native species, elasticities of λ to survival increased with life span and decreased with λ ; while elasticities to growth and fecundity decreased with life span and increased with λ . 5. For long-lived invaders, simulated reductions in either survival, growth or fecundity transitions were generally insufficient to produce population declines, whereas multiple reductions in either survival + growth or survival + fecundity were more effective. For short-lived invaders, simulated reductions in growth or fecundity and all pairwise multiple reductions produced population declines. 6. Synthesis and applications . Life history and population growth rate of invasive species are important in the selection of control targets. For rapidly growing populations of short-lived invaders, growth and fecundity transitions should be prioritized as control targets over survival transitions. For long-lived invaders, simultaneous reductions in more than one demographic process, preferably survival and growth, are usually required to ensure population decline. These general guidelines can be applied to rapidly growing new plant invasions and at the invasion front where detailed demographic data on invasive species are lacking.

Ecological Genetics of Plant Invasion: What Do We Know?

AbstractThe rate at which plant invasions occur is accelerating globally, and a growing amount of recent research uses genetic analysis of invasive plant populations to better understand the histories, processes, and effects of plant invasions. The goal of this review is to provide natural resource managers with an introduction to this research. We discuss examples selected from published studies that examine intraspecific genetic diversity and the role of hybridization in plant invasion. We also consider the conflicting evidence that has emerged from recent research for the evolution of increased competitiveness as an explanation for invasion, and the significance of multiple genetic characteristics and patterns of genetic diversity reported in the literature across different species invasions. High and low levels of genetic diversity have been found in different invading plant populations, suggesting that either selection leading to local adaptation, or pre-adapted characteristics such as phenotypic plasticity, can lead to aggressive range expansion by colonizing nonnative species. As molecular techniques for detecting hybrids advance, it is also becoming clear that hybridization is a significant component of some plant invasions, with consequences that include increased genetic diversity within an invasive species, generation of successful novel genotypes, and genetic swamping of native plant gene pools. Genetic analysis of invasive plant populations has many applications, including predicting population response to biological or chemical control measures based on diversity levels, identifying source populations, tracking introduction routes, and elucidating mechanisms of local spread and adaptation. This information can be invaluable in developing more effectively targeted strategies for managing existing plant invasions and preventing new ones. Future genetic research, including the use of high throughput molecular marker systems and genomic approaches such as microarray analysis, has the potential to contribute to better understanding and more effective management of plant invasions.