Rural Households' Demand Status for Mitigation of Prosopis juliflora (Sw.) DC Invasion and Its Determinant Factors in Ethiopia: Empirical Evidence from Afar National Regional State.

Ethiopia is among the world's poorest nations, and its economy is growing extremely slowly; thus, the government's budget to manage environmental amenities is not always sufficient. Thus, for the provision of environmental management services such as the eradication of Prosopis juliflora, the participation of local households and other stakeholders is crucial. This study is therefore initiated with the objective of assessing rural households' demands for mitigating Prosopis juliflora invasion in the Afar Region of Ethiopia. A multistage sampling technique was employed to obtain the 313 sample rural households that were used in the analysis, and those sample households were selected randomly and independently from the Amibara and Awash Fentale districts of Afar National Regional State, Ethiopia. In doing this, a seemingly unrelated bivariate probit model was used to determine factors affecting rural households' demands for mitigating Prosopis juliflora invasion. Consequently, as per the inferential statistical results, there was a significant mean/percentage difference between willing and nonwilling households for the hypothesized variables, except for some variables such as farm experience; years lived in the area, distance from the market, and dependency ratio. Furthermore, the seemingly unrelated bivariate probit model result indicates that sex, family size, tenure security, livestock holding, frequency of extension contact, and years lived in the area were important factors influencing the willingness to participate in Prosopis juliflora management practices positively, whereas age, off-farm/nonincome, and bid value affected willingness to pay negatively and significantly. Hence, to improve the participation level of households, policymakers should target these variables.

Predicting impact of a biocontrol agent: integrating distribution modeling with climate-dependent vital rates.

Species distribution models can predict the suitable climatic range of a potential biological control agent (BCA), but they provide little information on the BCA's potential impact. To predict high population buildup, a prerequisite of biocontrol impact, studies are needed that assess the effect of environmental factors on vital rates of a BCA across the environmental gradient of the BCA's suitable habitats, especially for the region where the BCA is considered for field release. We extended a published species distribution model with climate-dependent vital rates of Ophraella communa, a recently and accidentally introduced potential BCA of common ragweed, Ambrosia artemisiifolia in Europe. In field and laboratory experiments, we collected data on climate-dependent parameters assumed to be the most relevant for the population buildup of O. communa, i.e., temperature driving the number of generations per year and relative humidity (RH) determining egg hatching success. We found that O. communa concluded one generation in 334 cumulative degree days, and that egg hatching success strongly decreased from > 80% to < 20% when RH drops from 55% to 45% during the day. We used these values to spatially explicitly project population densities across the European range suitable for both A. artemisiifolia and the beetle and found that the present distribution of the beetle in Europe is within the range with the highest projected population growth. The highest population density of O. communa was predicted for northern Italy and parts of western Russia and western Georgia. Field observations of high impact on A. artemisiifolia with records of 80% aerial pollen reduction in the Milano area since the establishment of O. communa are in line with these predictions. The relative importance of temperature and RH on the population density of O. communa varies considerably across its suitable range in Europe. We propose that the combined statistical and mechanistic approach outlined in this paper helps to more accurately predict the potential impact of a weed BCA than a species distribution model alone. Identifying the factors limiting the population buildup of a BCA across the suitable range allows implementation of more targeted release and management strategies to optimize biocontrol efficacy.

Agro-ecological options for fall armyworm (Spodoptera frugiperda JE Smith) management: Providing low-cost, smallholder friendly solutions to an invasive pest.

Fall armyworm (FAW), a voracious agricultural pest native to North and South America, was first detected on the African continent in 2016 and has subsequently spread throughout the continent and across Asia. It has been predicted that FAW could cause up to $US13 billion per annum in crop losses throughout sub-Saharan Africa, thereby threatening the livelihoods of millions of poor farmers. In their haste to respond to FAW governments may promote indiscriminate use of chemical pesticides which, aside from human health and environmental risks, could undermine smallholder pest management strategies that depend to a large degree on natural enemies. Agro-ecological approaches offer culturally appropriate low-cost pest control strategies that can be readily integrated into existing efforts to improve smallholder incomes and resilience through sustainable intensification. Such approaches should therefore be promoted as a core component of integrated pest management (IPM) programmes for FAW in combination with crop breeding for pest resistance, classical biological control and selective use of safe pesticides. Nonetheless, the suitability of agro-ecological measures for reducing FAW densities and impact need to be carefully assessed across varied environmental and socio-economic conditions before they can be proposed for wide-scale implementation. To support this process, we review evidence for the efficacy of potential agro-ecological measures for controlling FAW and other pests, consider the associated risks, and draw attention to critical knowledge gaps. The evidence indicates that several measures can be adopted immediately. These include (i) sustainable soil fertility management, especially measures that maintain or restore soil organic carbon; (ii) intercropping with appropriately selected companion plants; and (iii) diversifying the farm environment through management of (semi)natural habitats at multiple spatial scales. Nevertheless, we recommend embedding trials into upscaling programmes so that the costs and benefits of these interventions may be determined across the diverse biophysical and socio-economic contexts that are found in the invaded range.

Origin matters: diversity affects the performance of alien invasive species but not of native species.

At local scales, it has often been found that invasibility decreases with increasing resident plant diversity. However, whether resident community diversity similarly resists invasion by alien versus native species is seldom studied. We examined this issue by invading constructed native plant assemblages that varied in species and functional richness with invasive alien or native Asteraceae species. Assemblages were also invaded with spotted knapweed, Centaurea stoebe, a native European aster that has been previously used in diversity-invasibility experiments in North America. We also conducted a field survey to explore the generality of the patterns generated from our experimental study. Both experimental and observational work revealed that increasing diversity reduced the performance of alien but not native invaders. Centaurea stoebe invading its native community performed poorly regardless of resident diversity, whereas in a parallel, previously published study conducted in North America, C. stoebe easily invaded low-diversity but not high-diversity assemblages. Our results suggest that diversity is an attribute of resident communities that makes them more or less susceptible to invasion by novel invasive alien but not native plant species.

Biogeographic effects on early establishment of an invasive alien plant.

PREMISE OF THE STUDY: Biotic resistance is often studied in the context of how interactions between native biota and invading species influence the success of those invaders. Seldom, however, is the strength of "resistance" compared biogeographically, where the ability of a species to impede invader establishment is contrasted between an invader's native and introduced recipient community. METHODS: We conducted an experiment to examine how community diversity influences seedling recruitment of a plant invader where it is native and contrasted with results previously published from introduced ranges. In Switzerland, we created recipient communities that varied in species and functional richness and invaded them, or not, with seeds of Centaurea stoebe, a native European plant that has been previously used in an identical experiment in North America, where it is a prominent invader. KEY RESULTS: The biogeographic comparison revealed that the recipient community largely prevented C. stoebe seedling establishment at home (Switzerland), but not away (Montana, USA), and that diversity of the resident vegetation did not contribute to the effects observed in the introduced range. CONCLUSIONS: Our results provide evidence that differences in the biogeographic conditions and/or overall level of competition of resident community between the native and introduced range considerably suppresses seedling recruitment of the invasive plant, rather than resident diversity itself. In the case of C. stoebe, the surprisingly low establishment success in the experiments conducted in the native compared with the introduced range is likely to be influenced by the higher level of competition with resident community, by abiotic environmental conditions or interactions between these two factors in the native range. Release from factors suppressing seedling recruitment at home may contribute to the successful invasion of C. stoebe in North America.

When a plant invader meets its old enemy abroad: what can be learnt from accidental introductions of biological control agents.

Accidental introductions of biological weed control (BWC) agents (i) offer opportunities to assess host use of agents with a potentially broader fundamental host-range than those approved for field release directly in target areas; (ii) urge national authorities to rapidly respond as they may threaten native species or crops, and by this (iii) help advancing post-release studies, a neglected aspect of BWC. Through detailed insights gained from studying the recent accidental introduction of the ragweed leaf beetle Ophraella communa into Europe, we derive suggestions for overcoming barriers to adoption of BWC by re-evaluating the predictive power of pre-release studies and, thus, the presently strict criteria for deciding upon their release that might exclude safe and efficient agents. By using the allergenic weed Ambrosia artemisiifolia and the accidentally introduced BWC agent O. communa as study system, we also hope to raise the awareness of authorities to consider biological control more prominently as a key approach for pest management in the 'One Health' concept, which aims to sustainably balance and optimize the health of people, animals, plants and ecosystems. © 2023 Society of Chemical Industry.

Furostanol Saponins and Ecdysteroids from Plants of the Genus Helleborus as Phagostimulants and Predator Deterrents for Larvae of Two Monophadnus Sawfly Species.

Sawfly species of the genus Monophadnus are specialised on Ranunculaceae plants from which the larvae can sequester furostanol saponins into the haemolymph, mainly (25R)-26-[(α-L-rhamnopyranosyl)oxy]-22α-methoxyfurost-5-en-3ß-yl-O-ß-D-glucopyranosyl-(1→3)-O-[6-acetyl-ß-D-glucopyranosyl-(1→3)]-O-ß-D-glucopyranoside (compound 1). In this work, TLC, GC-MS, and HPLC-DAD-ESI/MS analyses together with feeding, repeated simulated attacks, and ant deterrence bioassays were conducted to extend the chemoecological knowledge about two sawfly species specialised on H. foetidus L. (Monophadnus species A) and H. viridis L. (Monophadnus species B). Larvae of Monophadnus species B were mostly feeding on the squares treated with the n-butanol fraction from H. foetidus, compound 1 being its primary non-nutritional stimulant. In contrast, all H. viridis fractions stimulated feeding, with n-hexane marginally more active. ß-sitosterol within n-hexane was determined as the nutritional stimulant. Quantitative analyses demonstrated that leaves of H. viridis but not H. foetidus contain the ecdysteroids 20-hydroxyecdysone and polypodine B. Moreover, the haemolymph of Monophadnus species B larvae reared on H. viridis contained the glycosides of polypodine B and 20-hydroxyecdysone at a concentration of 2.5 to 6.8 µmol/g fresh weight of haemolymph. This concentration is several thousand times higher than the concentration range of the aglycones in their host plant (3.63 × 10-4 to 2.23 × 10-4 µmol total ecdysteroids/g fresh weight of leaves), suggesting bioaccumulation. The larvae of both species fed on H. foetidus do not show any traces of ecdysteroids in their haemolymph, indicating a facultative role of these compounds in their defence as well as their inability to endogenously synthesise these compounds. The haemolymph containing ecdysteroids was a significant feeding deterrent against Myrmica rubra L. ant workers (one of their natural predators) at 0.8 mg/mL. The larvae kept effective deterrent levels of glycosylated ecdysteroids (≅175 mM) between simulated attacks on days 1 and 2, but the levels clearly decreased on day 3 (≅75 mM). Most larvae (89%) survived a first attack but only 23% a consecutive second one. As a conclusion, we report for the first time that two Monophadnus species feeding on H. viridis sequester phytoecdysteroids into the larval haemolymph in the form of glycosides. In addition, compound 1 possesses defensive and phagostimulant activities, and we present evidence for a combined effect of furostanol saponins and ecdysteroids as repellents against ants.

Non-destructive environmental safety assessment of threatened and endangered plants in weed biological control.

Assessing the risk of nontarget attack (NTA) for federally listed threatened and endangered (T&E) plant species confamilial to invasive plants targeted for classical biological control, is one of the most important objectives of pre-release environmental safety assessments in the United States. However, evaluating potential NTA on T&E species is often complicated by restrictive agency requirements for obtaining propagules, or the ability to propagate plants and rear agents to the appropriate phenostages synchronously for testing, or both. Here, we assessed whether plant cues associated with a host recognition can be used for testing the attractiveness of four T&E and one rare single population plant species non-destructively for a candidate biocontrol agent. We used the seed-feeding weevil, Mogulones borraginis, a candidate for the biological control of the invasive plant, Cynoglossum officinale (Boraginaceae) as the study system. We collected olfactory and visual cues in the form of flowering sprigs from T&E plant species confamilial to the invasive plant in a non-destructive manner and used them to measure behavioral responses and searching time of weevils. Female weevils preferred C. officinale to all tested plant species in dual-choice bioassays using either olfactory or visual cues in a modified y-tube device. Furthermore, female weevils were repelled by the combined olfactory and visual cues from all tested T&E plant species in a dual-choice test against controls (e.g., purified air in an empty arm), indicating that it would be extremely unlikely for the weevil to attack any of these species upon release in the United States. Principal component analysis based on 61 volatile organic compounds effectively separated the five confamilial plant species and C. officinale, corroborating the results of behavioral bioassays. We conclude that studies on pre-alighting host selection behavior and the underlying physiological mechanisms of how organisms select host plants they exploit can aid in environmental safety testing of weed biological control agents.

Biological control for One Health.

Biological control has been effectively exploited by mankind since 300 CE. By promoting the natural regulation of pests, weeds, and diseases, it produces societal benefits at the food-environment-health nexus. Here we scrutinize biological control endeavours and their social-ecological outcomes through a holistic 'One-Health' lens, recognizing that the health of humans, animals, plants, and the wider environment are linked and interdependent. Evidence shows that biological control generates desirable outcomes within all One Health dimensions, mitigating global change issues such as chemical pollution, biocide resistance, biodiversity loss, and habitat destruction. Yet, its cross-disciplinary achievements remain underappreciated. To remedy this, we advocate a systems-level, integrated approach to biological control research, policy, and practice. Framing biological control in a One Health context helps to unite medical and veterinary personnel, ecologists, conservationists and agricultural professionals in a joint quest for solutions to some of the most pressing issues in planetary health.

Dissecting impact of plant invaders: do invaders behave differently in the new range?

Knowledge from basic plant ecology suggests that impact of one plant species on another is driven by either competition for the same limiting resources, or by unique plant traits. These processes might be context specific, explaining a differential impact of exotic plant invaders in the native vs. introduced range. With the help of a conceptual framework, we aimed at identifying the relationship between invader biomass and impact in the invasive Centaurea stoebe by conducting pairwise competition experiments with 15 European (old) and 15 North American (new) neighboring species. Old neighbors grew larger and could use available soil moisture more efficiently for growth than new neighbors. Interestingly, biomass of C. stoebe explained a substantial amount of the variation in biomass of the coevolved neighbors, but not of the new "naive" neighbors. Thus, impact in the home range appears to be driven by competition for the same limiting resources, but by other factors in the introduced range, possibly by exploitation of resources that are not used by the new neighbors or by interference competition. This distinction has important consequences for the management of invasive species, as in our study ecosystem recovery is less likely after simple biomass reduction.

Plant interspecific differences in arbuscular mycorrhizal colonization as a result of soil carbon addition.

Soil nutrient availability and colonization by arbuscular mycorrhizal fungi are important and potentially interacting factors shaping vegetation composition and succession. We investigated the effect of carbon (C) addition, aimed at reducing soil nutrient availability, on arbuscular mycorrhizal colonization. Seedlings of 27 plant species with different sets of life-history traits (functional group affiliation, life history strategy and nitrophilic status) were grown in pots filled with soil from a nutrient-rich set-aside field and amended with different amounts of C. Mycorrhizal colonization was progressively reduced along the gradient of increasing C addition in 17 out of 27 species, but not in the remaining species. Grasses had lower colonization levels than forbs and legumes and the decline in AM fungal colonization was more pronounced in legumes than in other forbs and grasses. Mycorrhizal colonization did not differ between annual and perennial species, but decreased more rapidly along the gradient of increasing C addition in plants with high Ellenberg N values than in plants with low Ellenberg N values. Soil C addition not only limits plant growth through a reduction in available nutrients, but also reduces mycorrhizal colonization of plant roots. The effect of C addition on mycorrhizal colonization varies among plant functional groups, with legumes experiencing an overproportional reduction in AM fungal colonization along the gradient of increasing C addition. We therefore propose that for a better understanding of vegetation succession on set-aside fields one may consider the interrelationship between plant growth, soil nutrient availability and mycorrhizal colonization of plant roots.

Synergistic effects of grass competition and insect herbivory on the weed Rumex obtusifolius in an inundative biocontrol approach.

Outcomes of weed biological control projects are highly variable, but a mechanistic understanding of how top-down and bottom-up factors influence the success of weed biological control is often lacking. We grew Rumex obtusifolius, the most prominent native weed in European grasslands, in the presence and absence of competition from the grass Lolium perenne and subjected it to herbivory through targeted inoculation with root-boring Pyropteron spp. To explore whether the interactive effects of competition and inundative biological control were size-dependent, R. obtusifolius was planted covering a large range of plant sizes found in managed grasslands. Overall, competition from the grass sward reduced aboveground biomass and final root mass of R. obtusifolius about 62- and 7.5-fold, respectively, and increased root decay of R. obtusifolius from 14 to 58%. Herbivory alone increased only root decay. However, grass competition significantly enhanced infestation by Pyropteron spp. and, as a consequence, enhanced the impact of herbivory on aboveground biomass and final root mass. The synergistic effect was so strong that R. obtusifolius plants grown from initially smaller roots did no longer develop. Inoculating R. obtusifolius with Pyropteron species in grasslands should be further pursued as a promising inundative biological control strategy in the weed's native range.

Water abstraction of invasive Prosopis juliflora and native Senegalia senegal trees: A comparative study in the Great Rift Valley Area, Ethiopia.

Besides direct water abstraction, natural water scarcity in semi-arid and arid regions may be further exacerbated by human-assisted changes in vegetation composition, including the invasion by non-native plant species. Water abstraction by the invasive tree Prosopis juliflora and by the native Senegalia senegal was compared in the dry Great Rift Valley, Ethiopia. Transpiration rates were quantified using the heat ratio method on six trees each of P. julifora and S. senegal, growing adjacent to each other in the same environment. Water use for P. juliflora trees ranges from 1 to 26 L/day (an average of 4.74 ± 1.97), and that of S. senegal trees from 1 to 38 L/day (an average of 5.48 ± 5.29 during two study years). For both species, soil heat, latent heat, and soil moisture status influenced the rates of sap flow of trees; in addition, water use by P. juliflora trees was related to vapor pressure deficit; the higher the vapor pressure deficit, the higher the water abstraction by P. juliflora. Stand densities of pure P. juliflora and S. senegal were 1200-1600 trees and 400-600 trees per ha, respectively. At the stand scale, P. juliflora consumed approximately 6636 L/day/ha (transpiration: 242 mm per year) and S. senegal stands consumed 2723 L/day/ha (transpiration: 87 mm per year). That is, P. juliflora stands consumed three times more water than S. senegal stands, because of two reasons: (1) P. juliflora stands are denser than S. senegal stands, and denser stands consume more water than less dense stands, and (2) P. juliflora is evergreen and uses water all year-round, while S. senegal sheds its leaves during the peak dry seasons. Our findings suggest that, compared to S. senegal, P. juliflora invasion results in severe impacts on groundwater resources of the drylands of Ethiopia, with direct and indirect consequences to ecosystem services and rural livelihoods.

Can the soil seed bank of Rumex obtusifolius in productive grasslands be explained by management and soil properties?

Rumex obtusifolius is a problematic weed in temperate grasslands worldwide as it decreases yield and nutritional value of forage. Because the species can recruit from the seed bank, we determined the effect of management and soil properties on the soil seed bank of R. obtusifolius in intensively managed, permanent grasslands in Switzerland (CH), Slovenia (SI), and United Kingdom (UK). Following a paired case-control design, soil cores were taken from the topsoil of grassland with a high density of R. obtusifolius plants (cases) and from nearby parcels with very low R. obtusifolius density (controls). Data on grassland management, soil nutrients, pH, soil texture, and density of R. obtusifolius plants were also collected. Seeds in the soil were germinated under optimal conditions in a glasshouse. The number of germinated seeds of R. obtusifolius in case parcels was 866 ±152 m-2 (CH, mean ±SE), 628 ±183 m-2 (SI), and 752 ±183 m-2 (UK), with no significant difference among countries. Densities in individual case parcels ranged from 0 up to approximately 3000 seeds m-2 (each country). Control parcels had significantly fewer seeds, with a mean of 51 ±18, 75 ±52, and 98 ±52 seeds m-2 in CH, SI, and UK, respectively, and a range between 0 and up to 1000 seeds m-2. Across countries, variables explaining variation in the soil seed bank of R. obtusifolius in case parcels were soil pH (negative relation), silt content (negative), land-use intensity (negative), and aboveground R. obtusifolius plant density (positive). Because a large soil seed bank can sustain grassland infestation with R. obtusifolius, management strategies to control the species should target the reduction in the density of mature plants, prevention of the species' seed production and dispersal, as well as the regulation of the soil pH to a range optimal for forage production.

Experimental prosopis management practices and grassland restoration in three Eastern African countries.

Woody species have been introduced in many parts of the world to provide economic benefits, but some of those species are now among the worst invaders, causing widespread economic and environmental damage. Management of woody species to restore original ecosystem services, such as biodiverse grassland that can provide fodder and sequester carbon, are needed to limit the impacts of alien species. However, the best management methods, i.e., the most economically efficient and effective way to remove trees and the most effective way to restore or rehabilitate the cleared land, are not developed for many species. In Eastern Africa, prosopis (Prosopis julifora) has invaded large areas of savanna and grassland, thereby affecting, among other things, fodder and water for livestock, access to dry season grazing lands and ultimately pastoral livelihoods. We tested three prosopis treatments (manual uprooting and cut stump and basal bark herbicide application) in combination with three incremental restoration interventions (divots, divots + mulching, divots + mulching + grass seed sowing). The three-year study was replicated in Ethiopia (Afar National Regional State), Kenya (Baringo county) and Tanzania (Moshi district). Prosopis survival and vegetation development, both diversity and biomass, were recorded. The prosopis treatments were all highly effective (between 85 and 100% tree mortality in almost all cases), but the two treatments that involved the complete removal of the aboveground biomass (manual and cut stump) yielded a more productive and more diverse vegetation than the treatment that killed the trees standing (basal bark). Compared to the effect of prosopis removal, the effect of restoration interventions on vegetation composition was small, indicating that most species re-established from the soil seed bank. The results show that it is possible to restore land previously invaded by prosopis. Despite the different rates of vegetation establishment and variation in species composition, the restoration interventions resulted in vegetation that in some cases contained a substantial fraction of perennial grasses. The method chosen to control prosopis depends on the availability of resources, including herbicides, and the need to remove rootstocks if the intention is to plant crops. Supplementary Information: The online version contains supplementary material available at 10.1186/s43170-023-00163-5.

Can plant resistance to specialist herbivores be explained by plant chemistry or resource use strategy?

At both a macro- and micro-evolutionary level, selection of and performance on host plants by specialist herbivores are thought to be governed partially by host plant chemistry. Thus far, there is little evidence to suggest that specialists can detect small structural differences in secondary metabolites of their hosts, or that such differences affect host choice or performance of specialists. We tested whether phytochemical differences between closely related plant species are correlated with specialist host choice. We conducted no-choice feeding trials using 17 plant species of three genera of tribe Senecioneae (Jacobaea, Packera, and Senecio; Asteraceae) and a more distantly related species (Cynoglossum officinale; Boraginaceae) containing pyrrolizidine alkaloids (PAs), and four PA-sequestering specialist herbivores of the genus Longitarsus (Chrysomelidae). We also assessed whether variation in feeding by specialist herbivores is attributable to different resource use strategies of the tested plant species. Plant resource use strategy was quantified by measuring leaf dry matter content, which is related to both plant nutritive value and to plant investment in quantitative defences. We found no evidence that intra-generic differences in PA profiles affect feeding by specialist herbivores. Instead, our results indicate that decisions to begin feeding are related to plant resource use strategy, while decisions to continue feeding are not based on any plant characteristics measured in this study. These findings imply that PA composition does not significantly affect host choice by these specialist herbivores. Leaf dry matter content is somewhat phylogenetically conserved, indicating that plants may have difficulty altering resource use strategy in response to selection pressure by herbivores and other environmental factors on an evolutionary time scale.

Impact of invasive Lantana camara on maize and cassava growth in East Usambara, Tanzania.

The impacts of invasive alien plant species on native plants are generally well documented, but little is known about the mechanisms underlying their impacts on crop growth. A better understanding of immediate as well as legacy effects and of direct and indirect impacts of invasive alien plant species is essential for an improved management of invaded cropland. We investigated how Lantana camara impacts the growth of two subsistence crops (maize and cassava) through competition for resources, allelopathy and the indirect plant-plant interactions. We carried out two pot experiments using soils from invaded abandoned, invaded cultivated and non-invaded cultivated crop fields. In the first experiment maize and cassava were grown alone or together with L. camara and half of the pots were treated with activated carbon to suppress allelochemicals. The effect of the soil microbial community on L. camara-crop interactions was assessed in a second experiment using autoclaved soil with 5% of soil from the three soil types. We found that L. camara reduced the growth of maize by 29%, but cassava was not affected. We did not find evidence of allelopathic effects of L. camara. Inoculation of autoclaved soil with microorganisms from all soil types increased biomass of cassava and reduced the growth of maize. Because L. camara only caused impacts when growing simultaneously with maize, the results suggest that removal of L. camara will immediately mitigate its negative impacts on maize.

Ecological impacts of invasive alien plants: a meta-analysis of their effects on species, communities and ecosystems.

Biological invasions cause ecological and economic impacts across the globe. However, it is unclear whether there are strong patterns in terms of their major effects, how the vulnerability of different ecosystems varies and which ecosystem services are at greatest risk. We present a global meta-analysis of 199 articles reporting 1041 field studies that in total describe the impacts of 135 alien plant taxa on resident species, communities and ecosystems. Across studies, alien plants had a significant effect in 11 of 24 different types of impact assessed. The magnitude and direction of the impact varied both within and between different types of impact. On average, abundance and diversity of the resident species decreased in invaded sites, whereas primary production and several ecosystem processes were enhanced. While alien N-fixing species had greater impacts on N-cycling variables, they did not consistently affect other impact types. The magnitude of the impacts was not significantly different between island and mainland ecosystems. Overall, alien species impacts are heterogeneous and not unidirectional even within particular impact types. Our analysis also reveals that by the time changes in nutrient cycling are detected, major impacts on plant species and communities are likely to have already occurred.

Plant invasions, generalist herbivores, and novel defense weapons.

One commonly accepted mechanism for biological invasions is that species, after introduction to a new region, leave behind their natural enemies and therefore increase in distribution and abundance. However, which enemies are escaped remains unclear. Escape from specialist invertebrate herbivores has been examined in detail, but despite the profound effects of generalist herbivores in natural communities their potential to control invasive species is poorly understood. We carried out parallel laboratory feeding bioassays with generalist invertebrate herbivores from the native (Europe) and from the introduced (North America) range using native and nonnative tetraploid populations of the invasive spotted knapweed, Centaurea stoebe. We found that the growth of North American generalist herbivores was far lower when feeding on C. stoebe than the growth of European generalists. In contrast, North American and European generalists grew equally well on European and North American tetraploid C. stoebe plants, lending no support for an evolutionary change in resistance of North American tetraploid C. stoebe populations against generalist herbivores. These results suggest that biogeographical differences in the response of generalist herbivores to novel plant species have the potential to affect plant invasions.

Invasive knotweed affects native plants through allelopathy.

PREMISE OF STUDY: There is increasing evidence that many plant invaders interfere with native plants through allelopathy. This allelopathic interference may be a key mechanism of plant invasiveness. One of the most aggressive current plant invaders is the clonal knotweed hybrid Fallopia × bohemica, which often forms monocultures in its introduced range. Preliminary results from laboratory studies suggest that allelopathy could play a role in this invasion. METHODS: We grew experimental communities of European plants together with F. × bohemica. We used activated carbon to test for allelopathic effects, and we combined this with single or repeated removal of Fallopia shoots to examine how mechanical control can reduce the species' impact. KEY RESULTS: Addition of activated carbon to the soil significantly reduced the suppressive effect of undamaged F. × bohemica on native forbs. The magnitude of this effect was similar to that of regular cutting of Fallopia shoots. Regular cutting of Fallopia shoots efficiently inhibited the growth of rhizomes, together with their apparent allelopathic effects. CONCLUSIONS: The ecological impact of F. × bohemica on native forbs is not just a result of competition for shared resources, but it also appears to have a large allelopathic component. Still, regular mechnical control successfully eliminated allelopathic effects. Therefore, allelopathy will create an additional challenge to knotweed management and ecological restoration only if the allelochemicals are found to persist in the soil. More research is needed to examine the mechanisms underlying Fallopia allelopathy, and the long-term effects of soil residues.