Invasion triangle: an organizational framework for species invasion

Re-Envisioning the Plant Disease Triangle: Full Integration of the Host Microbiota and a Focal Pivot to Health Outcomes

The disease triangle is a structurally simple but conceptually rich model that is used in plant pathology and other fields of study to explain infectious disease as an outcome of the three-way relationship between a host, a pathogen, and their environment. It also serves as a guide for finding solutions to treat, predict, and prevent such diseases. With the omics-driven, evidence-based realization that the abundance and activity of a pathogen are impacted by proximity to and interaction with a diverse multitude of other microorganisms colonizing the same host, the disease triangle evolved into a tetrahedron shape, which features an added fourth dimension representing the host-associated microbiota. Another variant of the disease triangle emerged from the recently formulated pathobiome paradigm, which deviates from the classical "one pathogen" etiology of infectious disease in favor of a scenario in which disease represents a conditional outcome of complex interactions between and among a host, its microbiota (including microbes with pathogenic potential), and the environment. The result is a version of the original disease triangle where "pathogen" is substituted with "microbiota." Here, as part of a careful and concise review of the origin, history, and usage of the disease triangle, I propose a next step in its evolution, which is to replace the word "disease" in the center of the host-microbiota-environment triad with the word "health." This triangle highlights health as a desirable outcome (rather than disease as an unwanted state) and as an emergent property of host-microbiota-environment interactions. Applied to the discipline of plant pathology, the health triangle offers an expanded range of targets and approaches for the diagnosis, prediction, restoration, and maintenance of plant health outcomes. Its applications are not restricted to infectious diseases only, and its underlying framework is more inclusive of all microbial contributions to plant well-being, including those by mycorrhizal fungi and nitrogen-fixing bacteria, for which there never was a proper place in the plant disease triangle. The plant health triangle also may have an edge as an education and communication tool to convey and stress the importance of healthy plants and their associated microbiota to a broader public and stakeholdership.

Adaptive responses to living in stressful habitats: Do invasive and native plant populations use different strategies?

Plants inhabit stressful environments characterized by a variety of stressors, including mine sites, mountains, deserts, and high latitudes. Populations from stressful and reference (non-stressful) sites often have performance differences. However, while invasive and native species may respond differently to stressful environments, there is limited understanding of the patterns in reaction norms of populations from these sites. Here, we use phylogenetically controlled meta-analysis to assess the performance of populations under stress and non-stress conditions. We ask whether stress populations of natives and invasives differ in the magnitude of lowered performance under non-stress conditions and if they vary in the degree of performance advantage under stress. We also assessed whether these distinctions differ with stress intensity. Our findings revealed that natives not only have greater adaptive advantages but also more performance reductions than invasives. Populations from very stressful sites had more efficient adaptations, and performance costs increased with stress intensity in natives only. Overall, the results support the notion that adaptation is frequently costless. Reproductive output was most closely associated with adaptive costs and benefits. Our study characterized the adaptive strategies used by invasive and native plants under stressful conditions, thereby providing important insights into the limitations of adaptation to extreme sites.

Lytic and Molecular Evidence of the Widespread Coriander Leaf Spot Disease Caused by Alternaria dauci

Coriandrum sativum L. is a globally significant economic herb with medicinal and aromatic properties. While coriander leaf blight disease was previously confined to India and the USA, this study presents new evidence of its outbreak in Africa and the Middle East caused by Alternaria dauci. Infected leaves display irregular chlorotic to dark brown necrotic lesions along their edges, resulting in leaf discoloration, collapse, and eventual death. The disease also impacts inflorescences and seeds, significantly reducing seed quality. Koch's postulates confirmed the pathogenicity of the fungus through the re-isolation of A. dauci from artificially infected leaves, and its morphology aligns with typical A. dauci features. Notably, this study identified strong lytic activity (cellulase: 23.76 U, xylanase: 12.83 U, pectinase: 51.84 U, amylase: 9.12 U, and proteinase: 5.73 U), suggesting a correlation with pathogenicity. Molecular characterization using ITS (ON171224) and the specific Alt-a-1 gene (OR236142) supports the fungal morphology. This research provides the first comprehensive documentation of the pathological, lytic, and molecular evidence of A. dauci leaf blight disease on coriander. Future investigations should prioritize the development of resistant coriander varieties and sustainable disease management strategies, including the use of advanced molecular techniques for swift and accurate disease diagnosis to protect coriander from the devastating impact of A. dauci.

Meta-analysis identifies native priority as a mechanism that supports the restoration of invasion-resistant plant communities

The restoration of invasion-resistant plant communities is an important strategy to combat the negative impacts of alien invasions. Based on a systematic review and meta-analysis of seed-based ecological restoration experiments, here we demonstrate the potential of functional similarity, seeding density and priority effect in increasing invasion resistance. Our results indicate that native priority is the most promising mechanism to control invasion that can reduce the performance of invasive alien species by more than 50%. High-density seeding is effective in controlling invasive species, but threshold seeding rates may exist. Overall seeding functionally similar species do not have a significant effect. Generally, the impacts are more pronounced on perennial and grassy invaders and on the short-term. Our results suggest that biotic resistance can be best enhanced by the early introduction of native plant species during restoration. Seeding of a single species with high functional similarity to invasive alien species is unpromising, and instead, preference should be given to high-density multifunctional seed mixtures, possibly including native species favored by the priority effect. We highlight the need to integrate research across geographical regions, global invasive species and potential resistance mechanisms.

Biodiversity of Pathogenic and Toxigenic Seed-Borne Mycoflora of Wheat in Egypt and Their Correlations with Weather Variables

Surveillance investigations for pathogenic and toxigenic fungi are important to refine our understanding of their epidemiology and help in predicting their outbreaks. During 2019, 198 samples of wheat grains were collected from 25 wheat-growing governorates in Egypt to detect and identify seed-borne mycoflora in vitro. Forty-four fungal species belonging to 20 genera were identified. Molecular data for these fungi were analyzed to construct a phylogenetic tree. Occurrence and biodiversity indicators were calculated. Two prevalent pathogens (average incidence > 40%) were Alternaria alternata and Cladosporium spp. Ustilago tritici was present in only seven of the 25 governorates, and less abundant than Tilletia tritici, the causal agent of stinking smut. Sinai governorate recorded the greatest species diversity, while the greatest species richness was in Qena and Sohag governorates. Canonical correspondence analysis of data for 20 fungal genera with temperature, relative humidity, precipitation, wind speed or solar radiation revealed that relative humidity was the most influential weather variable. It showed that occurrence and distribution of the 20 genera corresponded well with three out of four Egyptian climatic regions: Mediterranean, semi-arid, and arid. Knowing pathogen occurrence and distribution in Egypt is the first step to developing future disease management strategies to limit yield losses and improve food security. Despite this study being conducted on the wheat-growing areas in Egypt, our findings are useful for other wheat-growing countries that share the same climatic conditions. The correlation between a given fungus and the climatic variables can be useful in other ecosystems.

Drivers of Solidago species invasion in Central Europe-Case study in the landscape of the Carpathian Mountains and their foreground

AIM

The invasion process is a complex, context-dependent phenomenon; nevertheless, it can be described using the PAB framework. This framework encompasses the joint effect of propagule pressure (P), abiotic characteristics of the environment (A), and biotic characteristics of both the invader and recipient vegetation (B). We analyzed the effectiveness of proxies of PAB factors to explain the spatial pattern of Solidago canadensis and S. gigantea invasion using invasive species distribution models.

LOCATION

Carpathian Mountains and their foreground, Central Europe.

METHODS

The data on species presence or absence were from an atlas of neophyte distribution based on a 2 × 2 km grid, covering approximately 31,200 km2 (7,752 grid cells). Proxies of PAB factors, along with data on historical distribution of invaders, were used as explanatory variables in Boosted Regression Trees models to explain the distribution of invasive Solidago. The areas with potentially lower sampling effort were excluded from analysis based on a target species approach.

RESULTS

Proxies of the PAB factors helped to explain the distribution of both S. canadensis and S. gigantea. Distributions of both species were limited climatically because a mountain climate is not conducive to their growth; however, the S. canadensis distribution pattern was correlated with proxies of human pressure, whereas S. gigantea distribution was connected with environmental characteristics. The varied responses of species with regard to distance from their historical distribution sites indicated differences in their invasion drivers.

MAIN CONCLUSIONS

Proxies of PAB are helpful in the choice of explanatory variables as well as the ecological interpretation of species distribution models. The results underline that human activity can cause variation in the invasion of ecologically similar species.

Traits explain invasion of alien plants into tropical rainforests

1. The establishment of new botanic gardens in tropical regions highlights a need for weed risk assessment tools suitable for tropical ecosystems. The relevance of plant traits for invasion into tropical rainforests has not been well studied.2. Working in and around four botanic gardens in Indonesia where 590 alien species have been planted, we estimated the effect of four plant traits, plus time since species introduction, on: (a) the naturalization probability and (b) abundance (density) of naturalized species in adjacent native tropical rainforests; and (c) the distance that naturalized alien plants have spread from the botanic gardens.3. We found that specific leaf area (SLA) strongly differentiated 23 naturalized from 78 non-naturalized alien species (randomly selected from 577 non-naturalized species) in our study. These trends may indicate that aliens with high SLA, which had a higher probability of naturalization, benefit from at least two factors when establishing in tropical forests: high growth rates and occupation of forest gaps. Naturalized aliens had high SLA and tended to be short. However, plant height was not significantly related to species' naturalization probability when considered alongside other traits.4. Alien species that were present in the gardens for over 30 years and those with small seeds also had higher probabilities of becoming naturalized, indicating that garden plants can invade the understorey of closed canopy tropical rainforests, especially when invading species are shade tolerant and have sufficient time to establish.5. On average, alien species that were not animal dispersed spread 78 m further into the forests and were more likely to naturalize than animal-dispersed species. We did not detect relationships between the measured traits and estimated density of naturalized aliens in the adjacent forests.6. Synthesis: Traits were able to differentiate alien species from botanic gardens that naturalized in native forest from those that did not; this is promising for developing trait-based risk assessment in the tropics. To limit the risk of invasion and spread into adjacent native forests, we suggest tropical botanic gardens avoid planting alien species with fast carbon capture strategies and those that are shade tolerant.

The failed invasion of Harmonia axyridis in the Azores, Portugal: Climatic restriction or wrong population origin?

We tested two questions: (i) whether the climatic conditions of the Azorean Islands in Portugal may have restricted the invasion of Harmonia axyridis across this archipelago and (ii) determine what population of this species could have a higher probability of invading the islands. We used MaxEnt to project the climate requirements of different H. axyridis populations from three regions of the world, and the potential global niche of the species in the Azorean islands. Then we assessed the suitability of the islands for each of the three H. axyridis populations and global potential niche through histograms analysis, Principal Component Analysis (PCA) of climate variables, and a variable-by-variable assessment of the suitability response curves compared with the climatic conditions of the Azores. Climatic conditions of the Azores are less suitable for the U.S. and native Asian populations of H. axyridis, and more suitable for European populations and the global potential niche. The PCA showed that the climatic conditions of the islands differed from the climatic requirements of H. axyridis. This difference is mainly explained by precipitation of the wettest month, isothermality, and the minimum temperature of the coldest month. We concluded that the climatic conditions of the Azores could have influenced the establishment and spread of H. axyridis on these islands from Europe. Our results showed that abiotic resistance represented by the climate of the potentially colonizable zones could hinder the establishment of invasive insects, but it could vary depending of the origin of the colonizing population.

Pattern and causes of the establishment of the invasive bacterial potato pathogen Dickeya solani and of the maintenance of the resident pathogen D. dianthicola

Invasive pathogens can be a threat when they affect human health, food production or ecosystem services, by displacing resident species, and we need to understand the cause of their establishment. We studied the patterns and causes of the establishment of the pathogen Dickeya solani that recently invaded potato agrosystems in Europe by assessing its invasion dynamics and its competitive ability against the closely related resident D. dianthicola species. Epidemiological records over one decade in France revealed the establishment of D. solani and the maintenance of the resident D. dianthicola in potato fields exhibiting blackleg symptoms. Using experimentations, we showed that D. dianthicola caused a higher symptom incidence on aerial parts of potato plants than D. solani, while D. solani was more aggressive on tubers (i.e. with more severe symptoms). In co-infection assays, D. dianthicola outcompeted D. solani in aerial parts, while the two species co-existed in tubers. A comparison of 76 D. solani genomes (56 of which have been sequenced here) revealed balanced frequencies of two previously uncharacterized alleles, VfmB_Pro and VfmB_Ser , at the vfmB virulence gene. Experimental inoculations showed that the VfmB_Ser population was more aggressive on tubers, while the VfmB_Pro population outcompeted the VfmB_Ser population in stem lesions, suggesting an important role of the vfmB virulence gene in the ecology of the pathogens. This study thus brings novel insights allowing a better understanding of the pattern and causes of the D.solani invasion into potato production agrosystems, and the reasons why the endemic D. dianthicola nevertheless persisted.

Harmonising the fields of invasion science and forest pathology

Invasive alien species are widely recognised as significant drivers of global environmental change, with far reaching ecological and socio-economic impacts. The trend of continuous increases in first records, with no apparent sign of saturation, is consistent across all taxonomic groups. However, taxonomic biases exist in the extent to which invasion processes have been studied. Invasive forest pathogens have caused, and they continue to result in dramatic damage to natural forests and woody ecosystems, yet their impacts are substantially underrepresented in the invasion science literature. Conversely, most studies of forest pathogens have been undertaken in the absence of a connection to the frameworks developed and used to study biological invasions. We believe this is, in part, a consequence of the mechanistic approach of the discipline of forest pathology; one that has been inherited from the broader discipline of plant pathology. Rather than investigating the origins of, and the processes driving the arrival of invasive microorganisms, the focus of pathologists is generally to investigate specific interactions between hosts and pathogens, with an emphasis on controlling the resulting disease problems. In contrast, central to the field of invasion science, which finds its roots in ecology, is the development and testing of general concepts and frameworks. The lack of knowledge of microbial biodiversity and ecology, speciation and geographic origin present challenges in understanding invasive forest pathogens under existing frameworks, and there is a need to address this shortfall. Advances in molecular technologies such as gene and genome sequencing and metagenomics studies have increased the “visibility” of microorganisms. We consider whether these technologies are being adequately applied to address the gaps between forest pathology and invasion science. We also interrogate the extent to which the two fields stand to gain by becoming more closely linked.

Historical distribution and current drivers of guppy occurrence in Brazil

Humans introduce non-native species by means such as the deliberate release of fish into fresh waters and through commercial trade. The guppy Poecilia reticulata Peters, 1859, is commonly kept in aquaria and controls disease vectors, and now it occurs in many areas outside its natural distribution. Its initial habitat in Brazil was identified, and a study was performed to determine whether the density of guppies can be explained by the density of human population, per-capita gross domestic product, level of human impact on the areas where guppies have been found and fish-sampling effort. A total of 1402 guppy records were found; the southeastern region had the oldest records; and the southeastern, northeastern and central-western regions had the maximum records. Low tolerance to the colder climate may be the reason for the lack of guppy records in the southernmost states. It was also observed that the occurrence of this fish is positively, yet weakly, related to the density of human population, indicating that improved regulations regarding its use in controlling disease vectors, the aquarium trade and education of aquarium hobbyists could help prevent the spread of this species and its potential impacts in Brazil.

Competition between the invasive Impatiens glandulifera and UK native species: the role of soil conditioning and pre-existing resident communities

Himalayan balsam (Impatiens glandulifera) is a highly invasive annual herb that has become extremely prevalent in riparian zones across the UK. The competitive ability of I. glandulifera, both in terms of resource exploitation and allelopathy (i.e., the release of biochemicals that may be toxic to neighbouring plants), is considered a key determinant of its success. Little is known, however, about the effects of the resident community on the establishment and growth of I. glandulifera. Here, we aim to increase our understanding of the competitive ability of this highly invasive plant by investigating the effects of soil conditioning on the performance of four co-occurring native species (Tanacetum vulgare, Urtica dioica, Chelidonium majus and Arabidopsis thaliana). In addition, we also aim to investigate the effect that the pre-existing species composition have on the performance of I. glandulifera seedlings by establishing artificial communities (monocultures and mixtures of four UK native species, including U. dioica). We found negative effects of soil conditioning by I. glandulifera in all four species, either by reducing above-ground biomass, chlorophyll content or both. Monocultures of U. dioica were the only artificial communities that reduced growth of I. glandulifera, and we did not find any support for the idea that a more diverse community may be more resistant to invasion. Our results confirm the high competitive ability of I. glandulifera and highlight how the identity of the natives in the resident community may be key to limit its success.

Spatial global assessment of the pest Bagrada hilaris (Burmeister) (Heteroptera: Pentatomidae): current and future scenarios

BACKGROUND

The insect Bagrada hilaris (Burmeister) an important pest worldwide, mainly due to the serious economic losses incurred and the large number of zones invaded. However, current and future spatial distributions of this pest, and the total area of cropland potentially affected have not been estimated. Here, we aim to: (1) estimate the potential geographic distribution of B. hilaris; (2) quantify the total area of cropland potentially affected worldwide, and in two recently colonized zones (California and Chile); and (3) estimate future changes in distribution under different climate change scenarios.

RESULTS

We found that B. hilaris shows high environmental suitability in Mediterranean and arid regions, potentially affecting 1 108 184.1 km² of cropland worldwide. The most affected continents were Asia and America, with 309 659.8 and 294 638.6 km² of cropland at risk. More than 50% of cropland areas are at risk in seven countries. In California and central Chile, 43.7% and 50% of susceptible crops are at a high level of risk, respectively. Climate change scenarios predict an increase in the potential distribution of B. hilaris worldwide; America being the most affected continent.

CONCLUSIONS

Our results provide a spatially explicit baseline from which to focus efforts on the prevention, management and control of this pest worldwide. © 2018 Society of Chemical Industry.

Community Assembly Theory as a Framework for Biological Invasions

Biological invasions present a global problem underlain by an ecological paradox that thwarts explanation: how do some exotic species, evolutionarily naïve to their new environments, outperform locally adapted natives? We propose that community assembly theory provides a framework for addressing this question. Local community assembly rules can be defined by evaluating how native species' traits interact with community filters to affect species abundance. Evaluation of exotic species against this benchmark indicates that exotics that follow assembly rules behave like natives, while those exhibiting novel interactions with community filters can greatly underperform or outperform natives. Additionally, advantages gained by exotics over natives following disturbance can be explained by accounting for extrinsic assembly processes that bias exotic traits toward ruderal strategies.

Potential global distribution of Diabrotica species and the risks for agricultural production

BACKGROUND

Despite efforts in the last few decades to prevent biological invasions, agricultural pests continue to spread as a result of globalization and international trade. This study was conducted to identify suitable areas for the occurrence of four Diabrotica species and to assess the potential impact of these species in a scenario of invasion followed by spread throughout the estimated suitable regions.

RESULTS

Our findings reveal that a large proportion of the suitable areas for Diabrotica species overlap with cultivated areas. Niche analyses also demonstrated that these species occupy a small proportion of the suitable habitats available to them, indicating that, if new areas are invaded, there is a risk of spread throughout adjacent regions.

CONCLUSION

Most of the suitable areas for Diabrotica species overlap with highly productive agricultural areas, suggesting that a potential spread of these species may cause economic loss. Our study provides a valuable contribution to the development of tools aiming to predict the potential spread of these species throughout the world. © 2018 Society of Chemical Industry.

Conceptualising the interactive effects of climate change and biological invasions on subarctic freshwater fish

Climate change and species invasions represent key threats to global biodiversity. Subarctic freshwaters are sentinels for understanding both stressors because the effects of climate change are disproportionately strong at high latitudes and invasion of temperate species is prevalent. Here, we summarize the environmental effects of climate change and illustrate the ecological responses of freshwater fishes to these effects, spanning individual, population, community and ecosystem levels. Climate change is modifying hydrological cycles across atmospheric, terrestrial and aquatic components of subarctic ecosystems, causing increases in ambient water temperature and nutrient availability. These changes affect the individual behavior, habitat use, growth and metabolism, alter population spawning and recruitment dynamics, leading to changes in species abundance and distribution, modify food web structure, trophic interactions and energy flow within communities and change the sources, quantity and quality of energy and nutrients in ecosystems. Increases in temperature and its variability in aquatic environments underpin many ecological responses; however, altered hydrological regimes, increasing nutrient inputs and shortened ice cover are also important drivers of climate change effects and likely contribute to context-dependent responses. Species invasions are a complex aspect of the ecology of climate change because the phenomena of invasion are both an effect and a driver of the ecological consequences of climate change. Using subarctic freshwaters as an example, we illustrate how climate change can alter three distinct aspects of species invasions: (1) the vulnerability of ecosystems to be invaded, (2) the potential for species to spread and invade new habitats, and (3) the subsequent ecological effects of invaders. We identify three fundamental knowledge gaps focused on the need to determine (1) how environmental and landscape characteristics influence the ecological impact of climate change, (2) the separate and combined effects of climate and non-native invading species and (3) the underlying ecological processes or mechanisms responsible for changes in patterns of biodiversity.

Worldwide Alien Invasion: A Methodological Approach to Forecast the Potential Spread of a Highly Invasive Pollinator

The ecological impacts of alien species invasion are a major threat to global biodiversity. The increasing number of invasion events by alien species and the high cost and difficulty of eradicating invasive species once established require the development of new methods and tools for predicting the most susceptible areas to invasion. Invasive pollinators pose serious threats to biodiversity and human activity due to their close relationship with many plants (including crop species) and high potential competitiveness for resources with native pollinators. Although at an early stage of expansion, the bumblebee species Bombus terrestris is becoming a representative case of pollinator invasion at a global scale, particularly given its high velocity of invasive spread and the increasing number of reports of its impacts on native bees and crops in many countries. We present here a methodological framework of habitat suitability modeling that integrates new approaches for detecting habitats that are susceptible to Bombus terrestris invasion at a global scale. Our approach did not include reported invaded locations in the modeling procedure; instead, those locations were used exclusively to evaluate the accuracy of the models in predicting suitability over regions already invaded. Moreover, a new and more intuitive approach was developed to select the models and evaluate different algorithms based on their performance and predictive convergence. Finally, we present a comprehensive global map of susceptibility to Bombus terrestris invasion that highlights priority areas for monitoring.

Occurrence and distribution of tomato seed-borne mycoflora in Saudi Arabia and its correlation with the climatic variables

One hundred samples of tomato seeds were collected in 2011 and 2012 from tomato-cultivated fields in Saudi Arabia and screened for their seed-borne mycoflora. A total of 30 genera and 57 species of fungi were recovered from the collected seed samples using agar plate and deep-freezing blotter methods. The two methods differed as regards the frequency of recovered seed-borne fungi. Seven fungi among those recovered from tomato seeds, which are known as plant pathogens, were tested for their pathogenicity and transmission on tomato seedlings. The recovery rate of these pathogens gradually decreased from root up to the upper stem, and did not reach to the stem apex. The distribution of tomato seed-borne fungi was also investigated throughout Saudi Arabia. In this concern, Al-Madena governorate recorded the highest incidence of fungal flora associated with tomato seeds. The impact of meteorological variables on the distribution of tomato seed-borne mycoflora was explored using the ordination technique (canonical correspondence analysis). Among all climatic factors, relative humidity was the most influential variable in this regard. Our findings may provide a valuable contribution to our understanding of future global disease change and may be used also to predict disease occurrence and fungal transfer to new uninfected areas.

Canopy and knowledge gaps when invasive alien insects remove foundation species

The armored scale Aulacaspis yasumatsui invaded the northern range of the cycad Cycas micronesica in 2003, and epidemic tree mortality ensued due to a lack of natural enemies of the insect. We quantified cycad demographic responses to the invasion, but the ecological responses to the selective removal of this foundation species have not been addressed. We use this case to highlight information gaps in our understanding of how alien invasive phytophagous insects force cascading adverse ecosystem changes. The mechanistic role of unique canopy gaps, oceanic island examples and threatened foundation species with distinctive traits are three issues that deserve research efforts in a quest to understand this facet of ecosystem change occurring across multiple settings globally.