Invasive Plant Populations Research Articles

Scanning the horizon for invasive plant threats using a data-driven approach

Early detection and eradication of invasive plants are more cost-effective than managing well-established invasive plant populations and their impacts. However, there is high uncertainty around which taxa are likely to become invasive in a given area. Horizon scanning that combines a data-driven approach with rapid risk assessment and consensus building among experts can help identify invasion threats. We performed a horizon scan of potential invasive plant threats to Florida, USA—a state with a high influx of introduced species, conditions that are generally favorable for plant establishment, and a history of negative impacts from invasive plants. We began with an initial list of 2128 non-native plant taxa that are known invaders or crop pests. We built on previous invasive species horizon scans by developing data-based criteria to prioritize 100 taxa for rapid risk assessment. The semi-automated prioritization process included selecting taxa “on the horizon” (i.e., not yet in the target location and not on a noxious weed list) with climate matching, naturalization history, “weediness” record, and global commonness. We derived overall invasion risk scores with rapid risk assessment by evaluating the likelihood of each of the taxa arriving, establishing, and having an impact in Florida. Then, following a consensus-building discussion, we identified six plant taxa as high risk, with overall risk scores ranging from 75 to 100 out of a possible 125. The six taxa are globally distributed, easily transported to new areas, found in regions with climates similar to Florida’s, and can impact native plant communities, human health, or agriculture. Finally, we evaluated our initial and final lists for potential biases. Assessors tended to assign higher risk scores to taxa that had more available information. In addition, we identified biases towards four plant families and certain geographical regions of origin. Our horizon scan approach identified taxa conforming to metrics of high invasion risk and used a methodology refined for plants that can be applied to other locations.

Closer to Cayuga’s Waters

In recent years, an invasive plant species, Hydrilla verticillate, has been identified to pose significant threats to the local ecology of Cayuga Lake. This paper aims to provide a practical, quantitative, and real time evaluation system that helps monitor and predict the population dynamics of the plant. The system includes two main parts: sampling and simulation. In Section 3, we discuss in detail our stochastic Monte Carlo simulation based on the enhanced Invasive Plant Population Dynamic (IPPD) model. Human activities, such as boating, are identified to greatly accelerate the spread of Hydrilla. In Section 4, we employ statistical methods to promote confidence in the sample results, which in turn guarantees more accurate simulation results. We provide an adaptive algorithm to calculate the minimum sample size in real time. Other than the practical utility of our study, there is also the theoretical importance in that various approaches discussed in this paper, such as the IPPD model and the adaptive algorithm, can serve as the basis for future work on this subject.

Phenotypic plasticity of light use favors a plant invader in nitrogen-enriched ecosystems.

Eutrophication is believed to promote plant invasion, resulting in high growth performances of invasive plants and, therefore, the great potential for growth-induced intraspecific competition for light. Current hypotheses predict how eutrophication promotes plant invasion but fail to explain how great invasiveness is maintained under eutrophic conditions. In diverse native communities, co-occurring plants of varying sizes can avoid light competition by exploiting light complementarily; however, whether this mechanism applies to intraspecific competition in invasive plant populations remains unknown. Using a 2-year field nitrogen (N)-enrichment experiment on one of the global invasive plants, Spartina alterniflora, we found that the plasticity of light use reduced intraspecific competition and increased biomass production in S. alterniflora. This plasticity effect was enhanced when S. alterniflora had no nutrient limitations. In the N-enrichment treatments, the height difference among S. alterniflora ramets increased as light intensity decreased under the canopy. Compared with ambient N, under N enrichment, shorter individuals increased their light-use efficiency and specific leaf area in response to the reduced light intensity under the canopy. However, such ecophysiological plasticity was not found for taller individuals. Our findings revealed that the light-use plasticity of short individuals can be envisaged as a novel mechanism by which an invasive plant alleviates intraspecific competition and increases its invasiveness, challenging the prevailing perspective that the invasiveness of exotic plants is constrained by intraspecific competition.

The Role of Mass-Rearing in Weed Biological Control Projects in South Africa

It has been documented that the continual release of high numbers of biological control (biocontrol) agents for weeds increases the likelihood of agent establishment and has been shown to reduce the time between the first release and subsequent control of the target weed. Here we review the mass-rearing activities for weed biocontrol agents in South Africa between 2011 and 2020. Some 4.7 million individual insects from 40 species of biocontrol agent have been released on 31 weed species at over 2000 sites throughout South Africa during the last decade. These insects were produced at mass-rearing facilities at eight research institutions, five schools and 10 Non-Governmental Organizations. These mass-rearing activities have created employment for 41 fulltime, fixed contract staff, of which 11 are people living with physical disabilities. To improve the uptake of mass-rearing through community engagement, appropriate protocols are required to ensure that agents are produced in high numbers to suppress invasive alien plant populations in South Africa.

Geospatial Approaches to Monitoring the Spread of Invasive Species of Solidago spp.

Global climate change influences plant invasion which spreads all over the Europe. Invasive plants are predominantly manifest negative impacts, which require increased attention not only from ecologists. The research examines the possibilities offered by geospatial technologies in mapping the spatial spread of invasive plants of the genus Solidago. Invasive plant population was investigated at two localities, Malý Šariš and Chminianska Nová Ves in Slovakia, as well as the mapping of the area by multispectral imaging to determine the spectral reflectance curve of the monitored plant species. Using spatial analyses in the geographic information system, we evaluated changes in the plant density in the two localities. Based on the obtained results, we found that the number of individuals (ramets) in the Malý Šariš is significantly increasing, while in the examined area of Chminianska Nová Ves, there is a decrease in the number of Solidago spp. in the last monitored year. At the same time, we can state that in the areas with the highest increase in the number of ramets, the highest plant density per hectare was also recorded. We can also say that due to the spectral proximity of the surrounding vegetation, the spectral resolution in four spectral bands is insufficient for the classification of multispectral records in the case of Solidago spp. and cannot replace the advantages of high spectral resolution hyperspectral imaging, which significantly refines the feature space for Solidago spp. and the surrounding vegetation.

Decline of the invasive plant Asparagus asparagoides within the first seven years after release of biological control agents in Australia

There is ample anecdotal evidence and expert opinion on the adverse impact of imported biological control agents on populations of invasive plants, but still a paucity of quantitative data. Asparagus asparagoides (bridal creeper), a perennial climber with an extensive below-ground network of rhizomes and tubers, was considered one of the most invasive plants of natural ecosystems in southern Australia in the 1990s. A long-term experiment was conducted at 15 sites invaded by A. asparagoides across Australia to determine whether its growth and reproduction declined following the release of two biological control agents: a leafhopper (undescribed Erythroneurini formerly referred to as Zygina sp.) and rust fungus (Puccinia myrsiphylli). Data on A. asparagoides were collected annually at each site for up to 3 years before the release of one or both agents in 2000 or 2001, and up to 7 years after release to capture spatial and temporal variability. Our results showed a steady decrease in A. asparagoides seedling and shoot density, and total above-ground biomass in quadrats across all sites in the years following the release of the leafhopper and/or rust fungus. The number of fruits produced in quadrats greatly varied between sites, although for most sites, fruit numbers before the release of agents were higher than after release. Changes in A. asparagoides climbing onto standardized trellises, however, were not consistent across sites during the same period, with major reductions in plant measurements recorded at some sites but not others. Differences in climatic conditions between sites during A. asparagoides growing season did not explain these variable results. Incidence of leafhopper damage on A. asparagoides cladodes in quadrats and trellises was generally lower than that of rust infection.

Effective combination of management methods suppresses invasive Jerusalem artichoke

The main intention of our research is to determine an effective method to suppress invasive plant populations of Helianthus tuberosus spreading uncontrollably in the territory of the Poodří Protected Landscape Area (PLA) in the Czech Republic. This article brings together the results of selected management methods from 2017-2019 (mechanical, chemical and their combination) and their effect on the growth and reproductive characteristics of Helianthus tuberosus as well as the biodiversity of the local habitats. The combination of mowing and application herbicide based on clopyralid, fluroxypyr and MCPA proves to be the most effective. The partial results have been published before [1,2]. We present a complete research overview of the Helianthus tuberosus disposal.

The Effect of Cutting and Waterlogging on Plant-Related CO2 and N2O Fluxes Associated with the Invasive N-Fixing Species Gunnera tinctoria

The overall impact that plant invasions have on greenhouse gas emissions (GHG) by plant-mediated effects and how these interact with environmental and management factors is largely unknown. To address this, we report on the effects of leaf removal and waterlogging, either singularly or in combination, on the fluxes of CO2 and N2O associated with the invasive species Gunnera tinctoria. Both the removal of leaves with and without flooding resulted in higher CO2 emissions due to reductions in photosynthesis. Whilst waterlogging alone was also associated with a reduction in photosynthesis, this was slower than the effect of leaf removal. Significant N2O emissions were associated with intact plants, which increased immediately after leaf removal, or seven days after waterlogging with or without leaf removal. We found positive correlations between CO2 and N2O emissions and petiole and rhizome areas, indicating a size-dependent effect. Our results demonstrate that intact plants of G. tinctoria are a source of N2O emissions, which is enhanced, albeit transiently, by the removal of leaves. Consequently, management interventions on invasive plant populations that involve the removal of above-ground material, or waterlogging, would not only reduce CO2 uptake, but would further compromise the ecosystem GHG balance through enhanced N2O emissions.

Comparing Interpretation of High-Resolution Aerial Imagery by Humans and Artificial Intelligence to Detect an Invasive Tree Species

Timely, accurate maps of invasive plant species are critical for making appropriate management decisions to eliminate emerging target populations or contain infestations. High-resolution aerial imagery is routinely used to map, monitor, and detect invasive plant populations. While conventional image interpretation involving human analysts is straightforward, it can require high demands for time and resources to produce useful intelligence. We compared the performance of human analysts with a custom Retinanet-based deep convolutional neural network (DNN) for detecting individual miconia (Miconia calvescens DC) plants, using high-resolution unmanned aerial system (UAS) imagery collected over lowland tropical forests in Hawai’i. Human analysts (n = 38) examined imagery at three linear scrolling speeds (100, 200 and 300 px/s), achieving miconia detection recalls of 74 ± 3%, 60 ± 3%, and 50 ± 3%, respectively. The DNN achieved 83 ± 3% recall and completed the image analysis in 1% of the time of the fastest scrolling speed tested. Human analysts could discriminate large miconia leaf clusters better than isolated individual leaves, while the DNN detection efficacy was independent of leaf cluster size. Optically, the contrast in the red and green color channels and all three (i.e., red, green, and blue) signal to clutter ratios (SCR) were significant factors for human detection, while only the red channel contrast, and the red and green SCRs were significant factors for the DNN. A linear cost analysis estimated the operational use of a DNN to be more cost effective than human photo interpretation when the cumulative search area exceeds a minimum area. For invasive species like miconia, which can stochastically spread propagules across thousands of ha, the DNN provides a more efficient option for detecting incipient, immature miconia across large expanses of forested canopy. Increasing operational capacity for large-scale surveillance with a DNN-based image analysis workflow can provide more rapid comprehension of invasive plant abundance and distribution in forested watersheds and may become strategically vital to containing these invasions.

Plant invasion alters latitudinal pattern of plant-defense syndromes.

The relationship between herbivory and latitude may differ between native and introduced populations of invasive plants, which can generate latitudinal heterogeneity in the strength of enemy release. However, still little is known about how latitudinal heterogeneity in herbivore pressure influences latitudinal variation in defense phenotypes of invasive plants. We tested how latitudinal patterns in multi-variate defense syndromes differed between native (Argentinian) and introduced (Chinese) populations of the invasive herb Alternanthera philoxeroides. In addition, to better understand the drivers underlying latitudinal patterns, we also tested whether associations of defense syndromes with climate and herbivory differed between native and introduced ranges. We found that native plant populations clustered into three main defense syndromes associated with latitude. In contrast, we only found two defense syndromes in the introduced range. One matched the high-latitude syndrome from the native range, but was distributed at both the northern and southern range limits in the introduced range. The other was unique to the introduced range and occurred at mid-latitudes. Climatic conditions were associated with variation in syndromes in the native range, and climatic conditions and herbivory were associated with variation in syndromes in the introduced range. Together, our results demonstrate that plants may under the new environmental conditions in the introduced range show latitudinal patterns of defense syndromes that are different from those in their native range. This emphasizes that geographical dependence of population differentiation should be explicitly considered in studies on the evolution of defense in invasive plants.

Streetlights positively affect the presence of an invasive grass species.

Anthropogenic disturbances associated with urban ecosystems can create favorable conditions for populations of some invasive plant species. Light pollution is one of these disturbances, but how it affects the growth and establishment of invasive plant populations is unknown. Cheatgrass (Bromus tectorum) is a problematic invasive species where it has displaced native grassland communities in the United States, but to our knowledge, there have been no studies of the ecological factors that affect cheatgrass presence in urban ecosystems. We conducted field surveys in urban alleys in Denver, Colorado, to compare the presence of cheatgrass at sites with and without artificial light at night (hereafter artificial light) from streetlights. These streetlights are mounted on utility poles, which cause ground disturbance when installed in alleys; we were able to test the independent effect of poles on cheatgrass establishment because not all poles have streetlights on them. We found that cheatgrass was positively associated with the presence of streetlights and to a lesser extent poles. In addition to cheatgrass, we also found that other plants were positively associated with the presence of both poles and streetlights. Our results suggest that artificial light may benefit the occurrence of cheatgrass and other plant species in urban settings. While invasive populations of cheatgrass in wild habitats attract the most attention from managers, we suggest more consideration for this grass in urban environments where its growth and establishment benefit from anthropogenic changes.

Effects of soil nitrogen levels on growth and defense of the native and introduced genotypes of alligator weed

AbstractPlants growing in nutrient-rich environment are predicted to be less defended than conspecifics under nutrient limitation. However, less is known about the effects of nutrient levels on tolerance and induced resistance, and whether the effects differ between native and introduced populations of invasive plants. We performed a greenhouse experiment with introduced (the USA) and native (Argentina) genotypes of Alternanthera philoxeroides in order to study the effects of soil nitrogen levels on plant growth, constitutive and herbivore (Agasicles hygrophila)-induced chemical defense, and herbivory tolerance. We measured total biomass, elongation rate (as proxy of growth rate), carbon and nitrogen, and the concentration of triterpenoid saponins (defensive chemicals) in leaves and roots. Constitutive resistance (+33% higher leaf triterpenoid saponins in control treatment at low nitrogen level) and tolerance [less decreased total biomass after herbivory treatment (−24% and −15% for high and low nitrogen levels)] were favored at lower nitrogen level, while induced resistance was favored at higher nitrogen level (+24% increased leaf triterpenoid saponins after herbivory treatment at high nitrogen level). Constitutive resistance and tolerance exhibited trade-offs with growth rate, while induced resistance positively correlated with growth rate. Additionally, the introduced genotypes had −6% lower content of leaf carbon in the presence of herbivores than the native genotypes at low nitrogen level, but such difference was absent at high nitrogen level. Our results indicate that soil nitrogen levels influence the preference of different defensive strategies of plant, and interweave with herbivory to determine the performance of introduced genotypes.

Botany is the root and the future of invasion biology.

Botany is the root and the future of invasion biology.

Increasing flavonoid concentrations in root exudates enhance associations between arbuscular mycorrhizal fungi and an invasive plant

Many invasive plants have enhanced mutualistic arbuscular mycorrhizal (AM) fungal associations, however, mechanisms underlying differences in AM fungal associations between introduced and native populations of invasive plants have not been explored. Here we test the hypothesis that variation in root exudate chemicals in invasive populations affects AM fungal colonization and then impacts plant performance. We examined flavonoids (quercetin and quercitrin) in root exudates of native and introduced populations of the invasive plant Triadica sebifera and tested their effects on AM fungi and plant performance. We found that plants from introduced populations had higher concentrations of quercetin in root exudates, greater AM fungal colonization and higher biomass. Applying root exudates more strongly increased AM fungal colonization of target plants and AM fungal spore germination when exudate donors were from introduced populations. The role of root exudate chemicals was further confirmed by decreased AM fungal colonization when activated charcoal was added into soil. Moreover, addition of quercetin into soil increased AM fungal colonization, indicating quercetin might be a key chemical signal stimulating AM fungal associations. Together these results suggest genetic differences in root exudate flavonoids play an important role in enhancing AM fungal associations and invasive plants’ performance, thus considering root exudate chemicals is critical to unveiling mechanisms governing shifting plant-soil microbe interactions during plant invasions.

Arbuscular mycorrhizal colonization rate of an exotic plant, Galinsoga quadriradiata, in mountain ranges changes with altitude.

Arbuscular mycorrhizal fungi (AMF) are suggested to be important for invasions by many exotic plants. However, it is not yet known how associations between AMF and invasive plant populations change in mountains ranges and how changed associations affect further expansion of different populations in new habitats. We conducted a field survey to detect AMF colonization rate of the invasive Galinsoga quadriradiata along an elevational gradient ranging from 223 to 1947masl in the Qinling and Bashan Mountains, China. Additionally, a greenhouse experiment was conducted to compare plant growth performance among five elevational populations. In the field, total plant mass and seed production, as well as root AMF colonization rate, significantly decreased with elevation. When populations were grown in a novel soil environment in the greenhouse, the high-altitude populations achieved higher seed and total mass at lower AMF colonization rate than the low-altitude populations. Moreover, high AMF association was related to high intraspecific competition within low-altitude populations and limited seed production. Our results revealed that the associations between AMF and G. quadriradiata decrease with altitude in mountain ranges, and this may indicate that differentiation of association between AMF and elevational populations occurs during range expansion of G. quadriradiata. The results of the greenhouse experiment suggest that the high-altitude populations are more aggressive than the low-altitude populations in a non-stressful environment.

High genetic diversity in the clonal aquatic weed Alternanthera philoxeroides in the United States

AbstractThe distribution of genetic diversity in invasive plant populations can have important management implications. Alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.] was introduced into the United States around 1900 and has since spread throughout much of the southern United States and California. A successful biological control program was initiated in the late 1960s that reduced A. philoxeroides in the southern United States, although control has varied geographically. The degree to which variation among genotypes may be responsible for variation in control efficacy has not been well studied due to a lack of genetic data. We sampled 373 plants from 90 sites across the United States and genotyped all samples at three chloroplast regions to help inform future management efforts. Consistent with clonal spread, there was high differentiation between sites, yet we found six haplotypes and high haplotype diversity (mean h = 0.48) across states, suggesting this plant has been introduced multiple times. Two of the haplotypes correspond to previously described biotypes that differ in their susceptibility to herbicides and herbivory. The geographic distribution of the three common haplotypes varied by latitude and longitude, while the other haplotypes were widespread or localized to one or a few sites. All the haplotypes we screened are hexaploid (6n = 102), which may enhance biological control. Future studies can use these genetic data to determine whether genotypes differ in their invasiveness or respond differently to control measures. Some states, for instance, have mainly a single haplotype that may respond more uniformly to a single control strategy, whereas other states may require a variety of control strategies. These data will also provide the basis for identifying the source regions in South America, which may lead to the discovery of new biological control agents more closely matched to particular genotypes.

Transcription-mediated tissue-specific lignification of vascular bundle causes trade-offs between growth and defence capacity during invasion of Solidago canadensis.

Allocation of more resources to growth but less to defense causing growth vigor of invasive alien plant populations contributes to successful invasion. However, few studies has addressed to relationship between vascular development variation and this mechanism. In this study, a common garden experimentwas established to compare the growth and vascular bundle development between native and introduced populations of Solidago canadensis, which is a wide-distributed invasive species in China. Our results suggested that the rapid growth of introduced populations could be explained by the well-developed and highly lignified xylem; while native populations present more developed and highly lignified phloem, which contributed more resistance to the infection of Sclerotiun rofsii compared with introduced populations. This difference was resulted from tissue-specific tradeoff distribution of lignification related gene expression between xylem and phloem, which is regulated by upstream MYB transcription factors. Our study gives a novel insight of mechanism that explain invasion success: lignin-related gene transcription-mediated tissue-specific lignification of vascular bundle contributes tradeoffs in resource allocation between growth and defence capacity during successful invasion of S. canadensis.

Disease in Invasive Plant Populations.

Non-native invasive plants can establish in natural areas, where they can be ecologically damaging and costly to manage. Like cultivated plants, invasive plants can experience a relatively disease-free period upon introduction and accumulate pathogens over time. Diseases of invasive plant populations are infrequently studied compared to diseases of agriculture, forestry, and even native plant populations. We evaluated similarities and differences in the processes that are likely to affect pathogen accumulation and disease in invasive plants compared to cultivated plants, which are the dominant focus of the field of plant pathology. Invasive plants experience more genetic, biotic, and abiotic variation across space and over time than cultivated plants, which is expected to stabilize the ecological and evolutionary dynamics of interactions with pathogens and possibly weaken the efficacy of infectious disease in their control. Although disease is expected to be context dependent, the widespread distribution of invasive plants makes them important pathogen reservoirs. Research on invasive plant diseases can both protect crops and help manage invasive plant populations.

Increased free amino acid contents of the invasive weed Ageratina adenophora and the effects on its specialist herbivore Procecidochares utilis

AbstractIn order to verify the escape‐from‐enemy hypothesis from the changes of nutrient substance and fitness of natural enemies on alien plants, contents of free amino acids in native and invasive plant populations of Ageratina adenophora and life history parameters of specialist herbivore Procecidochares utilis reared on these plants were investigated. Our results showed that the contents of glycine, valine, γ‐aminobutyric acid, proline, serine, alanine, and arginine in the invasive plants were higher than those in the native plants of A. adenophora. There was a shorter developmental duration and higher fecundity of P. utilis when fed on the invasive plants. The results indicated a possible fitness tradeoff of natural enemies between invasive and native plants arose from nutrient substance changes.

Environmental Diagnosis of Degraded Pasture Areas and Soil Chemical Quality

The extensive grazing system is predominant in Brazilian livestock. The lack of adequate management in these areas hinders the performance of the activity and can bring problems related to the environmental degradation of pastures. Therefore, our objective was to perform environmental diagnosis in pasture areas on the Belo Horizonte farm, in the municipality of Dueré-Tocantins in northern Brazil. The environmental diagnosis was made in three pastures composed of Urochloa brizantha cv. Marandu (pasture 1, pasture 2 and pasture 3) in the rural property of Belo Horizonte, municipality of Dueré -Tocantins. Degradation was evaluated by the following methods: (a) structured interview of questionnaire to evaluate pasture formation, degradation, recovery and maintenance; (b) soil sampling of 0-20 cm layer and (c) visual assessment of the level of environmental degradation. Regarding the first method, the owner reported that the greatest difficulty in adopting conservation pasture practices was the high cost of implementation, which caused only a few practices to be performed. The soil analysis demonstrated that nutritional parameters were lower than the reference values adopted. In the third method, the five parameters: vigor and pasture quality; invasive plant population; soil cover; erosion were evaluated. In the use of these five parameters for evaluation, the individual results were established for each parameter, which ranges from 1 to 4. Only pastures 1 and 3 are in a state of degradation at different levels, ‘strong’ and ‘light/mild’, respectively. Reduced soil fertility was found to be one of the causal agents of pasture degradation. Pasture degradation is related to the reduction of soil chemical attributes, such as phosphorus (P), nitrogen (N) and high aluminum (Al) saturation.