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Invasion ecology has been criticised for its lack of general principles. To explore this criticism, we conducted a meta-analysis that examined characteristics of invasiveness (i.e. the ability of species to establish in, spread to, or become abundant in novel communities) and invasibility (i.e. the susceptibility of habitats to the establishment or proliferation of invaders). There were few consistencies among invasiveness characteristics (3 of 13): established and abundant invaders generally occupy similar habitats as native species, while abundant species tend to be less affected by enemies; germination success and reproductive output were significantly positively associated with invasiveness when results from both stages (establishment/spread and abundance/impact) were combined. Two of six invasibility characteristics were also significant: communities experiencing more disturbance and with higher resource availability sustained greater establishment and proliferation of invaders. We also found that even though 'propagule pressure' was considered in only ~29% of studies, it was a significant predictor of both invasiveness and invasibility (55 of 64 total cases). Given that nonindigenous species are likely introduced non-randomly, we contend that 'propagule biases' may confound current paradigms in invasion ecology. Examples of patterns that could be confounded by propagule biases include characteristics of good invaders and susceptible habitats, release from enemies, evolution of 'invasiveness', and invasional meltdown. We conclude that propagule pressure should serve as the basis of a null model for studies of biological invasions when inferring process from patterns of invasion.

Aim Geographical variation in numbers of established non-native species provides clues to the underlying processes driving biological invasions. Specifically, this variation reflects landscape characteristics that drive non-native species arrival, establishment and spread. Here, we investigate spatial variation in damaging non-native forest insect and pathogen species to draw inferences about the dominant processes influencing their arrival, establishment and spread. Location The continental USA, including Alaska (Hawaii not included). Methods We assembled the current geographical ranges (county-level) of 79 species of damaging non-indigenous forest insect and pathogen species currently established in the continental USA. We explored statistical associations of numbers of species per county with habitat characteristics associated with propagule pressure and with variables reflecting habitat invasibility. We also analysed relationships between the geographical area occupied by each pest species and the time since introduction and habitat characteristics. Results The geographical pattern of non-native forest pest species richness is highly focused, with vastly more species in the north-eastern USA. Geographical variation in species richness is associated with habitat factors related to both propagule pressure and invasibility. Ranges of the non-native species are related to historical spread; range areas are strongly correlated with time since establishment. The average (all species) radial rate of range expansion is 5.2 km yr -1 , and surprisingly, this rate did not differ among foliage feeders, sap-feeders, wood borers and plant pathogens. Main conclusions Forest pest species are much more concentrated in the north-eastern region of the USA compared with other parts of the country. This pattern most likely reflects the combined effects of propagule pressure (pest arrival), habitat invasibility (pest establishment) and invasion spread. The similarity in historical spread among different types of organisms indicates the importance of anthropogenic movement in spread.

Invasive plants disrupt ecosystems by influencing species interactions, aiding co-invader integration, and worsening competition for native plants. Successful forest invasive species management relies on grasping these interaction patterns at the community level. This study aims to assess the community attributes of invasive plants in tropical forests of Northeast India, examining their dominance in various habitats, elucidating their influence at the community level and some species-specific effects on associated co-invaders and non-invasive groups. The present investigation recorded 101 invasive plant species mostly of neotropical origin, comprising of grasses, sedges, forbs, shrubs, trees, and vines. The findings revealed both positive and negative effects of invasive species richness and their percentage at community level and throughout different habitats. However, the presence of dominant invasive species caused a substantial decrease in species diversity of plant community in the study area, depicting a significant negative association with the non-invasive group and a notable positive association with the co-invasive group. These findings offer valuable insights by specifying the community-level impacts of exotic invaders on the natural plant population which will be very helpful in devising effective management plans to counter biotic invasion in the biodiversity rich tropical forested landscapes.

The geographical distributions of non-native forest insects and pathogens (pests) result from a multitude of interacting abiotic and biotic factors. Following arrival, the presence of suitable host trees and environmental conditions are required for pests to establish and spread, but the role of forest biodiversity in this process is not well-understood. We analyzed county-level data for 22 non-native forest pests in the conterminous United States, developing species-specific models to investigate the effects of spatial contagion, human activities, and host and non-host tree biomass or richness on the occurrence of pest species. Species-specific models indicated that (i) the spatial contagion of invasions was the most common driver of invasion incidence, (ii) facilitation effects from host biomass and richness were present in approximately half of the invasions and almost entirely observed in invasions by sap-feeding insects or pathogens, and (iii) there was substantial variation in the direction and magnitude of the effects of non-host tree biomass and richness on invasion. Our analyses highlighted the prominent role of spatially derived propagule pressure in driving intracontinental invasions whereas effects of forest biodiversity were variable and precluded broad generalizations about facilitation and dilution effects as drivers of forest pest invasions at large spatial scales.

Determining the invasibility of habitats by alien species is crucial for understanding their spread potential, the habitats the most at risk and to implement adequate management actions. This is urgent for introduced taxa that show high invasion potential across broad geographical scales. We here assess these processes in invasive Pelophylax water frogs which are widespread colonizers across Western Europe and for which the invasibility of habitats remains to be quantified. Specifically, we used hierarchical occupancy models in a Bayesian framework to identify local- and landscape-scale features that can enhance occupancy of the most common invasive water frog, the marsh frog (P.ridibundus), in southern France. Water frogs were highly detectable and showed high occupancy across the invaded landscape. The invaders expressed a very broad habitat tolerance for both local- and landscape-scale variables while their invasion was facilitated by the occurrence of deep, permanent ponds with abundant aquatic vegetation and high sun exposure. Cross-validation showed a good transferability of models across space. The high invasibility of a wide range of habitats by Pelophylax water frogs is alarming and unveils their invasiveness, contributing therefore to explain their success of invasion over broad geographic scales.

AIM: Stronger inferences about biological invasions may be obtained when accounting for multiple invasion measures and the spatial heterogeneity occurring across large geographic areas. We pursued this enquiry by utilizing a multimeasure, multiregional framework to investigate forest plant invasions at a subcontinental scale. LOCATION: United States of America (USA). METHODS: Using empirical data from a national survey of USA forests, we compiled and mapped invasion richness (number of invasive species) and invasion prevalence (percentage of plots invaded) for 2524 counties. We then modelled each of these invasion measures as functions of 22 factors reflective of propagule pressure and/or habitat invasibility for eastern and western forests separately using simultaneous autoregressive spatial error models. RESULTS: Eastern forests had higher mean invasion richness (6.1) and prevalence (48%) than western forests (3.2 and 10%, respectively). Spatial patterns of invasion richness and prevalence differed, especially in the West. Propagule pressure factors were always positively associated with both invasion measures. Factors associated with human legacy were nearly six times more strongly associated with western than eastern invasions. Unlike propagule pressure factors, habitat invasibility factors shared inconsistent associations with invasion measures. MAIN CONCLUSIONS: The weaker associations between human legacy and invasions in the heavily invaded East, compared to the less‐invaded West, suggest a declining effect of propagule pressure over time with increasing invasion intensity. The importance of propagule pressure in less‐invaded western forests suggests that spatial variability in propagule inputs, coupled with lags between establishment and commonness, drives the spatial differences between invasion richness and prevalence during early invasion stages. Meanwhile, declining spatial disagreement between invasion measures and the relative unimportance of propagule pressure, in heavily invaded eastern forests, suggest that species‐specific variation in response to habitat invasibility drives spatial differences between invasion measures during later invasion stages. These insights further illustrate the importance of spatial heterogeneity in invasive plant management and policy at macroscales.

Pinus are among the highly invasive species that have spread outside their plantation area after their introduction in the Southern Hemisphere. The case of Pinus kesiya invasion is observed in the high plateau of Madagascar, inside the sclerophyll Tapia woodland which is dominated by the endemic Uapaca bojeri tree species. The analysis of this invasion was carried out using 375 plots of 100 m2 each in Tapia woodland. Data on the vegetation structure, the plot characteristics and the propagule pressure were collected. We recorded a total of 740 pines distributed in 29.8% of the plots. The generalized linear model built on P. kesiya at the three different life stages allowed us to highlight a different explicative variable on the species’ presence and abundance separately. The factors explaining pine occurrence varied according to the pine life stage. In the seedling stage, the distance of the plot from the propagule source combined with the longitudinal position of the plot explained 18% of the pine presence. In the sapling and adult stages, the vegetation structure was the main important factor (22% and 11% of variation explained regarding presence and abundance). The frequency of U. bojeri and the degree of disturbance were the most important factors characterizing this vegetation structure. Based on these results, a strategy to control pine invasion in the Tapia woodland may focus on enrichment with U. bojeri and limitation of the plantation of P. kesiya in proximity.

Despite intensive research, we still have no general understanding of why plant invasions occur. Many different mechanisms of plant invasions have been proposed, but studies designed to investigate them often produce inconsistent results. It remains unclear whether this unsatisfying state of affairs reflects the complexity of the real world (in which every invasion is unique) or the failure to identify the key processes driving most plant invasions. Here we argue that greater generalization is possible, but only if we recognize that the ecological and evolutionary processes enabling a species to advance into a new area change during the course of an invasion. In our view, an invasion can often usefully be subdivided into a primary phase, in which the abundance of an often preadapted species increases rapidly (typically in resource-rich, disturbed habitats), and a secondary phase, in which further spread is contingent upon plastic responses or genetic adaptation to new ecological circumstances. We present various examples to show how this partitioning of the invasion phase sensu stricto produces new hypotheses about the processes underlying plant invasions. Some of these hypotheses can be conveniently tested by investigating plant invasions along strong environmental gradients such as those that occur in mountainous regions.

On a local scale, invasiveness of introduced species and invasibility of habitats together determine invasion success. A key issue in invasion ecology has been how to quantify the contribution of species invasiveness and habitat invasibility separately. Conventional approaches, such as comparing the differences in traits and/or impacts of species between native and/or invaded ranges, do not determine the extent to which the performance of invaders is due to either the effects of species traits or habitat characteristics. Here we explore the interaction between two of the most widespread earthworm invaders in the world (Asian Amynthas agrestis and European Lumbricus rubellus ) and study the effects of species invasiveness and habitat invasibility separately through an alternative approach of \"third habitat\" in Tennessee, USA. We propose that feeding behaviors of earthworms will be critical to invasion success because trophic ecology of invasive animals plays a key role in the invasion process. We found that (1) the biomass and isotopic abundances (δ 13 C and δ 15 N) of A. agrestis were not impacted by either direct effects of L. rubellus competition or indirect effects of L. rubellus -preconditioned habitat; (2) A. agrestis disrupted the relationship between L. rubellus and soil microorganisms and consequently hindered litter consumption by L. rubellus ; and (3) compared to L. rubellus , A. agrestis shifted its diet more readily to consume more litter, more soil gram-positive (G+) bacteria (which may be important for litter digestion), and more non-microbial soil fauna when soil microorganisms were depleted. In conclusion, A. agrestis showed strong invasiveness through its dietary flexibility through diet shifting and superior feeding behavior and its indirectly negative effect of habitat invasibility on L. rubellus via changes in the soil microorganism community. In such context, our results expand on the resource fluctuation hypothesis and support the superior competitor hypothesis. This work presents additional approaches in invasion ecology, provides some new dimensions for further research, and contributes to a greater understanding of the importance of interactions between multiple invading species.

Invasive alien plants have invaded various habitats, posing a threat to biodiversity. Several hypotheses have been proposed to explain the mechanisms of invasion, but few studies have considered the characteristics of the invaded communities and the effects of human interference in the invasion. In this study, we compared the invasibility of three different habitats: abandoned land, eucalyptus plantations, and natural secondary forests. We explored the effects of species diversity, phylogenetic diversity, and disturbance factors on the invasibility of different habitats. The results showed that the invasibility of abandoned land was the highest and the invasibility of the natural secondary forest was the lowest. Phylogenetic indicators affected the invasibility of abandoned land and eucalyptus plantations, and disturbance factors affected the invasibility of all three habitats, while the characteristics of the invaded communities had a weak impact. Our research provided supporting evidence for Darwin’s naturalization hypothesis and his disturbance hypothesis but found no relationship between biotic resistance and invasibility. This study indicated that the differences among habitats should be considered when we prove Darwin’s naturalization hypothesis in nature reserves.