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Predicting how increasing rates of global trade will result in new establishments of potentially damaging invasive species is a question of critical importance to the development of national and international policies aimed at minimizing future invasions. Centuries of historical movement and establishment of invading species may have depleted the supply of species available for future invasions, and it has been suggested that the problem of invasions will diminish as a result of this. However, the extent to which source pool depletion affects future invasions remains unclear., Here we describe a mechanistic model that captures the simultaneous effects of depletion of source species pools along with increases in pathway rates (e.g. imports) to predict future numbers of new invasions. We assume that the distribution of species abundance within invasion pathways is positively skewed, which is modelled using a log-normal distribution. Given their high propagule pressure, the most abundant species are likely to invade first, while the many rare species are likely to invade only under high pathway volumes. We apply this model to the case study of bark beetle, Scolytinae, invasions in the USA., Source species pools in Europe and Asia (225 and 655 species of Scolytinae, respectively) are much larger than numbers that have historically established (16 and 32). Parameterization of the model indicates a highly skewed species abundance distribution in the pathway and this is confirmed by species frequencies in port inspection records, thus explaining why only a small fraction of species has historically invaded., Forecasts from the model indicate that with increasing rates of imports, more species from these regions are likely to invade in the future despite the depletion of the most abundant species from source species pools. Previous statistical models tend to underestimate future establishments in the presence of increasing import rates due to their failure to account for key underlying mechanisms., Policy implications. The mechanistic model developed here is widely applicable for predicting future invasions of all taxa and provides insights into how increases in rates of imports counteract the species pool depletion effect, resulting in the continued establishment of new species. [ABSTRACT FROM AUTHOR]

Exotic tree species are widely used in forest plantations for their often high productivity and performance compared to native trees. However, these advantages may be compromised by herbivore damage., A list of European insect species that have expanded their host range to one of 28 exotic tree species introduced to Europe was compiled from a systematic literature review. The number of successful expansions was analysed using three predictors: (i) phylogenetic relatedness between exotic and European tree species; (ii) area covered by exotic tree species in Europe; and (iii) time since their introduction into Europe., In total, 590 host expansions of native insects to exotic trees were found, mainly of polyphagous species (43%); 25% of the cases reported some type of damage. Bark and wood borers, and defoliators were the dominant guilds., The number of recruited native insect species and cases where major damage occurred was positively correlated with the geographical extent of exotic trees in Europe and the presence of congeneric native trees., Synthesis and applications. The use of exotic tree species creates opportunities for native insect herbivores to expand their host range and increase their damage if they are widely planted next to native congeners. Risk assessment studies are recommended when introducing new tree species for forestry plantations. Risk assessments should include trials on susceptibility to any potential damaging organisms in the introduced range. [ABSTRACT FROM AUTHOR]

Although many non‐native species arrive in novel environments, only a fraction successfully establish. A crucial factor affecting the colonization process of invading species is propagule pressure. The positive association between propagule pressure and colonization probability is driven both by stochastic dynamics and the 'Allee effect'. Although the role of Allee effects in invading populations is theoretically compelling, they are difficult to quantify in the field because the earliest phases of biological invasions are typically not observed.We conducted parallel studies using two species of bark beetles, Hylurgus ligniperda in New Zealand and Ips pini in North America, to (i) assess the role of propagule pressure on colonization success, (ii) empirically test for Allee effects and (iii) estimate Allee thresholds. We evaluated each of these factors by experimentally testing the effect of propagule pressure (numbers released) on simulated tree colonization success in two different settings: (1) field conditions where beetles could disperse freely and (2) enclosed cages where dispersal loss was prevented.Colonization success was positively associated with numbers released (i.e. propagule pressure) for both species, though colonization occurred at lower numbers for H. ligniperda than for I. pini. A demographic Allee effect was only detected in H. ligniperda and the Allee threshold was smaller when beetles were released 1 m from host billets than when they were released 10 m away. Greater colonization success at lower release densities may explain the invasion success of H. ligniperda worldwide. Higher release densities required for successful colonization may explain why I. pini is a poor establisher.Synthesis and applications. Our results linking invasion failure to small founding population densities generally support the theoretical literature on the role of propagule pressure and Allee effects in biological invasions. Agencies such as the International Plant Protection Committee (IPPC) setting phytosanitary measures such as the International Standards for Phytosanitary Measures No. 15 (ISPM 15) should consider using the colonization thresholds estimated here to limit numbers of colonizing individuals below invasion thresholds. Additionally, agencies conducting trapping efforts at ports‐of‐entry should be aware of these thresholds and widely report when trapping numbers exceed these thresholds. [ABSTRACT FROM AUTHOR]

Plant stress from causes such as drought is thought to increase the susceptibility of plants to herbivore attack and damage. Although this forms the basis of the widely cited plant stress hypothesis, there are few cases where this has been tested thoroughly., We compared the frequency of attack and damage of an invasive pine bark beetle Hylates ater, on seedlings of Pinus radiata, using a 2 × 2 factorial design with artificially induced stress and insecticide application to distinguish effects of beetle attack and direct effects of stress., Contrary to expectations based on the plant stress hypothesis, twice as many unstressed seedlings (22%) were attacked than stressed seedlings (11%). However, stressed seedlings were twice as likely to experience sustained bark beetle feeding resulting in girdling, than unstressed seedlings (31% and 15%, respectively)., In response to beetle attack, unstressed seedlings showed much stronger resin production than stressed seedlings, which is likely to explain the greater survival of unstressed seedlings after attack., Experimental drought stress caused a 50% reduction in shoot growth., Synthesis and applications. Maximizing seedling vigour and minimizing (pre-)planting stress are key for the resilience of planted forests. Managers should emphasize a stress-free transition of seedlings from nursery to field as well as correct planting techniques, in order to safeguard this vulnerable stage. This is particularly crucial since drought stress and insect invasions are likely to increase as a result of global change. [ABSTRACT FROM AUTHOR]

Invasions by non‐native pests and diseases represent serious threats to biodiversity, agriculture and human health. Under current border arrival rates associated with international trade not all such invasions can be prevented, so early detection and eradication (forced extinction) are important strategies for preventing establishment and long‐term impacts. Removal of host plants has historically been a common tool used alone and in concert with other tools for eradication of plant pests but there is little scientific theory specific to the management of invasive species to guide the application of this eradication strategy.We drew upon extensive conservation biology literature documenting the effect of habitat destruction or fragmentation driving extinction. We applied a previously developed spatially explicit model of gypsy moth, Lymantria dispar, spatial dynamics to explore how fragmentation affects population persistence. This model accounts for a component Allee effect driven by mate‐finding failure that interacts with dispersal.We observed a nonlinear dependency of population persistence on the fraction of habitat cover and the level of habitat fragmentation. Simulation of active habitat fragmentation via the removal of habitat in swaths of varying widths or the application of pesticide in varying swaths showed that removal of hosts or pesticide application in narrow swaths (i.e. 40 m wide) caused the greatest probabilities of extinction. Generally, habitat removal was more effective than one‐off pesticide treatments at causing extinction.Synthesis and applications. Spatially explicit modelling of Allee dynamics in invading gypsy moth populations showed that host removal can be an effective method to eradicate invasive plant‐feeding insects especially when habitat fragmentation is applied at a desirable level. Furthermore, this can be used as an alternative to, or in conjunction with, pesticide treatments, to provide more options for carrying out eradications and to increase the probability of eradication success. [ABSTRACT FROM AUTHOR]

Pest regulation is an important ecosystem service provided by biodiversity, as plants growing in species-rich communities often experience associational resistance to herbivores. However, little is known about the respective influence of the quantity and identity of associated species on herbivory in focal plants., Using a meta-analysis to compare insect herbivory in pure and mixed forests, we specifically tested the effects of the relative abundance of focal tree species and of phylogenetic distance between focal and associated tree species on the magnitude of associational resistance., Overall, insect herbivory was significantly lower in mixed forests, but the outcome varied greatly depending on the phylogenetic relatedness among tree species and the degree of herbivore feeding specialization., Specialist herbivore damage or abundance was positively related to relative abundance of their host trees, regardless of the phylogenetic distance between host and associated tree species., By contrast, tree diversity triggered associational resistance to generalist herbivores only when tree mixtures included tree species phylogenetically distant to the focal species., Synthesis and applications. Our study demonstrates that the establishment of mixed forests per se is not sufficient to convey associational resistance to herbivores if the identity of tree species associated in mixtures is not taken into account. As a general rule, mixing phylogenetically more distinct tree species, such as mixtures of conifers and broadleaved trees, results in more effective reduction in herbivore damage. [ABSTRACT FROM AUTHOR]