Contrasting morphometric responses to increasing urbanisation in congeneric sparrow species

Increased urbanisation influences the morphometric traits of various species, often resulting in urban individuals being smaller than their non-urban counterparts. Urbanisation can affect fundamental eco-evolutionary patterns and impact species' ability to adapt to and occupy rapidly changing environments through morphological changes. We investigated the morphometric responses of two passerine species, the non-native house sparrow (Passer domesticus) and its native congener, the Cape sparrow (Passer melanurus), along gradients of spatial and temporal urbanisation in South Africa over a 52-year period. The house sparrow was significantly heavier, larger and in better condition with increasing urban infrastructure and lower urban vegetation cover, while the Cape sparrow showed opposing trends along these gradients. Temporally, the house sparrow's body mass increased consistently over the 52-year study period, suggesting changes in morphology were concomitant with increasing urbanisation over time. This study demonstrates distinct differences in the morphological responses of the non-native house sparrow and the native Cape sparrow to increasing urban development. These morphological responses may also underpin community-level changes caused by urbanisation, enhancing the capabilities of non-native species to thrive over their native counterparts in these environments.

Are Rattus rattus fleas invasive? Evaluation of flea communities in invasive and native rodents in Chile

Co-invasion, characterized by the simultaneous introduction of hosts and parasites with the latter establishing themselves in native hosts, is a phenomenon of ecological concern. Rattus rattus, a notorious invasive species, has driven the extinction and displacement of numerous avian and mammalian species and serves as a key vector for diseases affecting both humans and wildlife. Among the parasites hosted by R. rattus are fleas, which exhibit obligate parasitic behaviour, a generalist nature and high prevalence, increasing the likelihood of flea invasion. Simultaneously, invasive species can serve as hosts for native parasites, leading to potential amplification or dilution of parasite populations in the environment. In Chile, R. rattus has been present since the 17th century because of the arrival of the Spanish colonizers through the ports and has spread throughout urban, rural and wild Chilean territories. This study aims to evaluate whether co-invasion of native fleas of invasive rats occurs on native rodents in Chile and to determine whether black rats have acquired flea native to Chile during their invasion. For this, we captured 1132 rodents from 26 localities (20° S-53° S). Rattus rattus was found coexisting with 11 native rodent species and two species of introduced rodents. Among the native rodents, Abrothrix olivacea and Oligoryzomys longicaudatus exhibited more extensive sympatry with R. rattus. We identified 14 flea species associated with R. rattus, of which only three were native to rats: Xenopsylla cheopis, Leptopsylla segnis and Nosopsyllus fasciatus. These three species presented a higher parasite load in black rats compared to native fleas. Leptopsylla segnis and N. fasciatus were also found associated with native rodent species that cohabit with R. rattus. The remaining species associated with R. rattus were fleas of native rodents, although they were less abundant compared to those associated with native rodents, except for Neotyphloceras pardinasi and Sphinctopsylla ares. Although there has been evidence of flea transmission from rats to native species, the prevalence and abundance were relatively low. Therefore, it cannot be definitively concluded that these fleas have established themselves in native rodent populations, and hence, they cannot be classified as invasive fleas. This study underscores R. rattus' adaptability to diverse environmental and geographical conditions in Chile, including its capacity to acquire fleas from native rodents. This aspect has critical implications for public health, potentially facilitating the spread of pathogens across various habitats where these rats are found.

Microsatellites and mitochondrial evidence of multiple introductions of the invasive raccoon Procyon lotor in France

Raccoons (Procyon lotor) are worldwide invaders, due to deliberate or accidental releases, and their impacts exceed hundred of billions of Euros in Europe only. In France, raccoons have currently established three separate, expanding populations. Identifying the current spatial genetic structure, dispersal events and phylogeography of these populations is needed to infer the invasion history and identify management units. We used wild and captive individuals sampled in France and Belgium to characterize the genetic diversity and current population genetic structure of French raccoon populations and identify potential genetic connectivity with the Belgium population using both mitochondrial DNA and microsatellite loci. Results confirm that French populations are the result of at least three independent introductions. While the three populations display low genetic diversity and sign of recent bottleneck, they are still expanding, suggesting that in addition to their ecological plasticity, the remaining genetic diversity is sufficient to successfully adapt to their new environment and allow a quick colonization. Particular attention must be given to the North-Eastern population, which shows genetic admixture with the Belgium population, as admixed individuals may exhibit hybrid vigor facilitating their expansion. The comparison of captive and wild individuals did not allow to identify a potential captive origin of the wild populations. The current regulation in France allowing captivity in zoos without enforcement to tighten the biosecurity of detention facilities might dampen any management measure as few introduced founders might be enough to create new populations.

Climate change and the potential distribution of the glassy-winged sharpshooter (Homalodisca vitripennis), an insect vector of Xylella fastidiosa

Biological invasions represent a major threat for biodiversity and agriculture. Despite efforts to restrict the spread of alien species, preventing their introduction remains the best strategy for an efficient control. In that context preparedness of phytosanitary authorities is very important and estimating the geographical range of alien species becomes a key information. The present study investigates the potential geographical range of the glassy-winged sharpshooter (Homalodisca vitripennis), a very efficient insect vector of Xylella fastidiosa, one of the most dangerous plant-pathogenic bacteria worldwide. We use species distribution modeling (SDM) to analyse the climate factors driving the insect distribution and we evaluate its potential distribution in its native range (USA) and in Europe according to current climate and different scenarios of climate change: 6 General Circulation Models (GCM), 4 shared socioeconomic pathways of gas emission and 4 time periods (2030, 2050, 2070, 2090). The first result is that the climate conditions of the European continent are suitable to the glassy-winged sharpshooter, in particular around the Mediterranean basin where X. fastidiosa is present. Projections according to future climate conditions indicate displacement of climatically suitable areas towards the north in both North America and Europe. Globally, suitable areas will decrease in North America and increase in Europe in the coming decades. SDM outputs vary according to the GCM considered and this variability indicated areas of uncertainty in the species potential range. Both potential distribution and its uncertainty associated to future climate projections are important information for improved preparedness of phytosanitary authorities.

Invasive Drosophila suzukii outnumbers native controphics and causes substantial damage to fruits of forest plants

Impacts of biological invasions are diverse and can have far-reaching consequences for ecosystems. The spotted wing drosophila, Drosophila suzukii, is a major invasive pest of fruits, which negatively affects fruit and wine production. However, little is known about the ecological impact of this fly species on more natural ecosystems it has invaded, such as forests. In this study, we investigated the use of potential host plants by D. suzukii at 64 sites in different forest communities in Switzerland from mid-June to mid-October 2020. We examined more than 12,000 fruits for egg deposits of D. suzukii to assess its direct impact on the plants. We recorded symptoms of fruit decay after egg deposition to determine if D. suzukii attacks trigger fruit decay. In addition, we monitored the drosophilid fauna with cup traps baited with apple cider vinegar, as we expected that D. suzukii would outnumber and potentially outcompete native controphics, especially other drosophilids. Egg deposits of D. suzukii were found on the fruits of 31 of the 39 potential host plant species studied, with 18 species showing an attack rate > 50%. Overall, fruits of Cotoneaster divaricatus (96%), Atropa bella-donna (91%), Rubus fruticosus corylifolius aggr. (91%), Frangula alnus (85%) and Sambucus nigra (83%) were attacked particularly frequently, resulting also in high predicted attack probabilities that varied among forest communities. Later and longer fruiting, black fruit colour, larger fruit size and higher pulp pH all positively affected attack rates. More than 50% of the plant species showed severe symptoms of decay after egg deposition, with higher pulp sugar content leading to more severe symptoms. The high fruit attack rate observed was reflected in a high abundance and dominance of D. suzukii in trap catches, independent of forest community and elevation. Drosophila suzukii was by far the most abundant species, accounting for 86% (81,395 individuals) of all drosophilids. The abundance of D. suzukii was negatively associated with the abundance of the native drosophilids. Our results indicate that the invasive D. suzukii competes strongly with other frugivorous species and that its presence might have far-reaching ecosystem-level consequences. The rapid decay of fruits attacked by D. suzukii leads to a loss of resources and may disrupt seed-dispersal mutualisms through the reduced consumption of fruits by dispersers such as birds.

The Future of Invasion Science Needs Physiology

Incorporating physiology into models of population dynamics will improve our understanding of how and why invasions succeed and cause ecological impacts, whereas others fail or remain innocuous. Targeting both organismal physiologists and invasion scientists, we detail how physiological processes affect every invasion stage, for both plants and animals, and how physiological data can be better used for studying the spatial dynamics and ecological effects of invasive species. We suggest six steps to quantify the physiological functions related to demography of nonnative species: justifying physiological traits of interest, determining ecologically appropriate time frames, identifying relevant abiotic variables, designing experimental treatments that capture covariation between abiotic variables, measuring physiological responses to these abiotic variables, and fitting statistical models to the data. We also provide brief guidance on approaches to modeling invasions. Finally, we emphasize the benefits of integrating research between communities of physiologists and invasion scientists.

Evolutionary change in metabolic rate of Daphnia pulicaria following invasion by the predator Bythotrephes longimanus

Metabolic rate is a trait that may evolve in response to the direct and indirect effects of predator-induced mortality. Predators may indirectly alter selection by lowering prey densities and increasing resource availability or by intensifying resource limitation through changes in prey behavior (e.g., use of less productive areas). In the current study, we quantify the evolution of metabolic rate in the zooplankton Daphnia pulicaria following an invasive event by the predator Bythotrephes longimanus in Lake Mendota, Wisconsin, US. This invasion has been shown to dramatically impact D. pulicaria, causing a ~60% decline in their biomass. Using a resurrection ecology approach, we compared the metabolic rate of D. pulicaria clones originating prior to the Bythotrephes invasion with that of clones having evolved in the presence of Bythotrephes. We observed a 7.4% reduction in metabolic rate among post-invasive clones compared to pre-invasive clones and discuss the potential roles of direct and indirect selection in driving this change.

Predator suppression by a toxic invader does not cascade to prey due to predation by alternate predators

Invasive species often have catastrophic direct effects on native species through increased competition and predation. Less well understood are indirect, cascading effects across trophic levels. To reveal trophic disruptions caused by invasive species, it is necessary to document interactions between species at different trophic levels and across guilds. Here, we take this approach to quantify the impact of the invasion of cane toads at a riparian site in the Kimberley, northwest Australia. These toads are toxic to many native vertebrate predators and following toad arrival we observed the expected severe population decline in monitor lizards. Contrary to expectations however, this did not facilitate species in the next trophic level down: the abundance of their reptilian prey, as well as nest success of a songbird whose nests were predominantly depredated by monitor lizards, remained unchanged. Instead, detailed observations suggest a change in the suite of nest predators, with monitor lizards being replaced by other—mainly avian—predators, possibly reflecting their release from competitors that are more efficient nest predators. Hence, our findings highlight complex indirect effects of an invasive species across trophic levels and indicate that trophic cascades can go undetected when failing to include direct observations on predator–prey interactions.

Sequential invasions by fruit flies (Diptera: Tephritidae) in Pacific and Indian Ocean islands: A systematic review

The aim of our review was to examine the cases of Tephritidae invasions across island systems in order to determine whether they follow a hierarchical mode of invasion. We reviewed the literature on factors and mechanisms driving invasion sequences in Pacific and Southwest Indian Ocean islands and gathered every record of invasion by a polyphagous tephritid in island groups. From invasion date or period, we defined an invasion link when a new fruit fly established on an island where another polyphagous tephritid is already resident (that was indigenous or a previous invader). Across surveyed islands, we documented 67 invasion links, involving 24 tephritid species. All invasion links were directional, i.e., they involved a series of invasions by invaders that were closely related to a resident species but were increasingly more competitive. These sequential establishments of species are driven by interspecific competition between resident and exotic species but are also influenced by history, routes, and flows of commercial exchanges and the bridgehead effect. This information should be used to improve biosecurity measures. Interactions between trade flow, invasive routes, and the presence of invasive and resident species should be integrated into large‐scale studies.

Global Warming Could Magnify Insect-Driven Apparent Competition Between Native and Introduced Host Plants in Sub-Antarctic Islands

Pristine sub-Antarctic islands terrestrial ecosystems, including many endemic species, are highly threatened by human-induced cosmopolitan plant invasion. We propose that native plant suppression could be further facilitated by the subsequent invasion by generalist pest species that could exacerbate their competitive exclusion through the process of apparent competition. By comparing the biological parameters of an invasive aphid species, Myzus ascalonicus, on one native (Acaena magellanica) and one invasive (Senecio vulgaris) plant species, we showed that survival and fecundity were higher and development time lower on the native plant species than on the invasive one. Moreover, comparing the effect of a temperature increase on the population dynamics of M. ascalonicus on the two plants, we showed that the relative profitability of the native species is further amplified by warming. Hence, while pest population doubling time is 28% higher on the invasive plant under current temperature, it would become 40% higher with an increase in temperature of 3°C. Consequently, our findings demonstrate that global warming could exacerbate competitive exclusion of native plants by invasive plants in sub-Antarctic islands by its indirect effect on the apparent competition mediated by generalist phytophagous pests.

ALIEN SPECIES OF FISH HELMINTHS OF BELARUS

The article provides data on the species composition of alien helminth species, which infest introduced fish species in pond fish farms, and also the literature data on natural reservoirs of Belarus. Seven alien helminth species have been revealed in introduced fish in reservoirs of Belarus ( Khawia sinensis Hsü, 1935, Schyzocotyle acheilognathi (Yamaguti, 1934) , Caryophyllaeus fimbriceps Annenkova-Chlopina, 1919, Philometroides cyprini (Ishii, 1931), Dactylogyrus dulkeiti Bychowsky, 1936, Bothriocephalus claviceps (Goeze, 1782) , Anguillicola crassus Kuwahara 1974). All species of helminths excluding the nematode A.crassus and the cestode B.claviceps - parasites of the eels Anguilla anguilla , were introduced to Belarus with the seeding of Cyprinus carpio haematopterus (Linnaeus, 1758) from the Far East. The invasive species of helminths ( K. sinensis, S. acheilognathi, P. cyprini, A. crassus ), which affect fishery by causing the death of farmed fish, are singled out.

Antigonon leptopus invasion is associated with plant community disassembly in a Caribbean island ecosystem

Invasions by non-native plant species are widely recognized as a major driver of biodiversity loss. Globally, (sub-)tropical islands form important components of biodiversity hotspots, while being particularly susceptible to invasions by plants in general and vines in particular. We studied the impact of the invasive vine A. leptopus on the diversity and structure of recipient plant communities on the northern Caribbean island St. Eustatius. We used a paired-plot design to study differences in species richness, evenness and community structure under A. leptopus-invaded and uninvaded conditions. Community structure was studied through species co-occurrence patterns. We found that in plots invaded by A. leptopus, species richness was 40–50% lower, and these plots also exhibited lower evenness. The magnitude of these negative impacts increased with increasing cover of A. leptopus. Invaded plots also showed higher degrees of homogeneity in species composition. Species co-occurrence patterns indicated that plant communities in uninvaded plots were characterized by segregation, whereas recipient plant communities in invaded plots exhibited random co-occurrence patterns. These observations suggest that invasion of A. leptopus is not only associated with reduced species richness and evenness of recipient communities in invaded sites, but also with a community disassembly process that may reduce diversity between sites. Given that A. leptopus is a successful invader of (sub-)tropical islands around the globe, these impacts on plant community structure highlight that this invasive species could be a particular conservation concern for these systems.

The Role of Monk Parakeets as Nest-Site Facilitators in Their Native and Invaded Areas

Simple Summary

Invasive species can be harmful to native species, although this fact could be more complex when some natives eventually benefit from invaders. Faced with this paradox, we show how the invasive monk parakeet, the only parrot species that builds its nests with sticks, can host other species as tenants, increasing nest-site availability for native but also exotic species. This same pattern is observed in the native range of the species, and when parakeets occupy urban or rural habitats, although the richness of tenants was higher in invaded areas and rural habitats. Tenants participated in the cooperative defense against predators, benefiting parakeets with their presence. As tenants can be both native and invasive species, management plans should consider the complex network of interactions developed with the invader.

Abstract

While most of the knowledge on invasive species focuses on their impacts, little is known about their potential positive effects on other species. Invasive ecosystem engineers can disrupt recipient environments; however, they may also facilitate access to novel resources for native species. The monk parakeet (Myiopsitta monachus) is a worldwide invader and the only parrot that builds its own communal nests, which can be used by other species. However, the ecological effects of these interspecific interactions are barely known. We compared the role of the monk parakeet as a nest-site facilitator in different rural and urban areas, both invaded and native, across three continents and eight breeding seasons. A total of 2690 nests from 42 tenant species, mostly cavity-nesting birds, were recorded in 26% of 2595 monk parakeet nests. Rural and invaded areas showed the highest abundance and richness of tenant species. Multispecies communal nests triggered interspecific aggression between the monk parakeet host and its tenants, but also a cooperative defense against predators. Despite the positive effects for native species, monk parakeets also facilitate nesting opportunities to other non-native species and may also transmit diseases to tenants, highlighting the complexity of biotic interactions in biological invasions.

Exotic plants accumulate and share herbivores yet dominate communities via rapid growth

Herbivores may facilitate or impede exotic plant invasion, depending on their direct and indirect interactions with exotic plants relative to co-occurring natives. However, previous studies investigating direct effects have mostly used pairwise native-exotic comparisons with few enemies, reached conflicting conclusions, and largely overlooked indirect interactions such as apparent competition. Here, we ask whether native and exotic plants differ in their interactions with invertebrate herbivores. We manipulate and measure plant-herbivore and plant-soil biota interactions in 160 experimental mesocosm communities to test several invasion hypotheses. We find that compared with natives, exotic plants support higher herbivore diversity and biomass, and experience larger proportional biomass reductions from herbivory, regardless of whether specialist soil biota are present. Yet, exotics consistently dominate community biomass, likely due to their fast growth rates rather than strong potential to exert apparent competition on neighbors. We conclude that polyphagous invertebrate herbivores are unlikely to play significant direct or indirect roles in mediating plant invasions, especially for fast-growing exotic plants.

Differing impacts of two major plant invaders on urban plant-dwelling spiders (Araneae) during flowering season

Plant invasions can have major impacts on ecosystems and influence global species diversity. In Central Europe, Himalayan balsam (Impatiens glandulifera) and American goldenrods (Solidago canadensis and S. gigantea) are important invaders often establishing dense and homogeneous stands, especially in urban and other disturbed habitats. We investigated their impacts on plant-dwelling spiders (abundance, family structure, guild structure) and potential spider prey items during flowering season within an urbanized landscape using a paired design comparing invaded and native reference vegetation plots. In general, flowering American goldenrods and Himalayan balsam had no significant impacts on the spider family composition. Invasion of American goldenrods further had no effect on total spider abundance and potential prey item abundance. In contrast, goldenrods showed a significantly increased crab spider (Thomisidae) abundance while being less inhabited by web builders. Himalayan balsam negatively influenced free hunters and running crab spider (Philodromidae) abundance, while we found no effects on other groups and total spider abundance. For Himalayan balsam, potential prey item abundance was higher than in native vegetation stands. Notwithstanding that our results only represent a snapshot of the system, they suggest that large-scale removal of urban goldenrod stands during flowering season might negatively influence local spider abundance, especially of crab spiders. Management efforts should therefore be accompanied by compensation measures to avoid disruptive effects on local plant-dwelling spider communities.

Evidence of indirect biotic resistance: native ants decrease invasive plant fitness by enhancing aphid infestation

The biotic resistance hypothesis asserts that native species may hinder the invasion of exotic species, which can occur either directly or indirectly by influencing interactions between exotic and local species. Aphid-tending ants may play a key role in the indirect biotic resistance to plant invasion. Ants may protect aphids, thus increasing their negative effect on exotic plants, but may also deter chewing herbivores, thus benefiting exotic plants. We studied native aphid-tending ants (Dorymyrmex tener, Camponotus distinguendus, and Dorymyrmex richteri) on exotic nodding thistles (Carduus thoermeri), which are attacked by thistle aphids (Brachycaudus cardui) and thistle-head weevils (Rhinocyllus conicus). We evaluated the impact of ants, aphids, and weevils on thistle seed set. We compared ant species aggressiveness towards aphid predators and weevils and performed ant-exclusion experiments to determine the effects of ants on aphid predators and weevils. We analysed whether ant species affected thistle seed set through their effects on aphids and/or weevils. The ant D. tener showed the most aggressive behaviour towards aphid predators and weevils. Further, D. tener successfully removed aphid predators from thistles but did not affect weevils. Excluding D. tener from thistles increased seed set. Analyses supported a negative indirect pathway between the aggressive D. tener and thistle seed set through aphid populations, while the other ant species showed no indirect effects on thistle reproduction. Therefore, aggressive aphid-tending ants may enhance biotic resistance by increasing aphid infestation on exotic invasive plants. This study highlights the importance of indirect biotic resistance in modulating the success of invasive species.

Invasive grass (Microstegium vimineum) indirectly benefits spider community by subsidizing available prey

Invasive plant species cause a suite of direct, negative ecological impacts, but subsequent, indirect effects are more complex and difficult to detect. Where identified, indirect effects to other taxa can be wide-ranging and include ecological benefits in certain habitats or locations.Here, we simultaneously examine the direct and indirect effects of a common, invasive grass species (Microstegium vimineum) on the invertebrate communities of understory deciduous forests in the eastern United States. To do this, we use two complementary analytic approaches to compare invaded and reference plots: (a) community composition analysis of understory arthropod taxa and (b) analysis of isotopic carbon and nitrogen ratios of a representative predatory spider species.Invaded plots contained a significantly greater abundance of nearly all taxa, including predators, herbivores, and detritivores. Spider communities contained over seven times more individuals and exhibited greater species diversity and richness in invaded plots.Surprisingly, however, the abundant invertebrate community is not nutritionally supported by the invasive plant, despite 100% ground cover of M. vimineum. Instead, spider isotopic carbon ratios showed that the invertebrate prey community found within invaded plots was deriving energy from the plant tissue of C3 plants and not the prevalent, aboveground M. vimineum. Synthesis and applications. We demonstrate that invasive M. vimineum can create non-nutritional ecological benefits for some invertebrate taxa, with potential impacts to the nutritional dynamics of invertebrate-vertebrate food webs. These positive impacts, however, may be restricted to habitats that experience high levels of ungulate herbivory or reduced vegetative structural complexity. Our results highlight the importance of fully understanding taxon- and habitat-specific effects of invading plant species when prioritizing invasive species removal or management efforts.

Reproductive characteristics of American bullfrogs (Lithobates catesbeianus) in their invasive range of the Pacific Northwest, USA

Invasive species pose a major threat to global biodiversity. The effects of invasive species can be strongly influenced and potentially mediated by their reproductive characteristics, such as fecundity, egg production, and duration and number of reproductive events. Selection for smaller body size at first reproduction can also play a role in their establishment, facilitating colonization and spread. The American bullfrog, native to the eastern U.S. (Lithobates catesbeianus), is a species that has invaded more than 40 countries across 4 continents. This species has become especially prevalent in the western United States since its introduction in the early 1900s. This study characterized reproductive characteristics of bullfrogs with emphasis on the minimum size at which males and females reach sexual maturity in the Willamette Valley, Oregon, USA invasion range. We collected and dissected 121 individuals in 2013 and 2017, quantifying characteristics of sexual maturity including snout-vent length, total length, sex, tympanum diameter, presence of distended oviducts or eggs for females, and testes length and sperm activity in males. Our results showed that the minimum reproductive size of both males and females was smaller relative to bullfrogs in their native range as well as in populations across their invasive range. Reduction in size at reproductive maturity is likely impacting the invasive success of American bullfrogs and this study gives us insight on management actions to control the invasion. Applying this insight, managers can adjust their definition of reproductively active adults, increasing the target population of culling and other control methods.

A trophic cascade initiated by an invasive vertebrate alters the structure of native reptile communities

Invasive vertebrates are frequently reported to have catastrophic effects on the populations of species which they directly impact. It follows then, that if invaders exert strong suppressive effects on some species then other species will indirectly benefit due to ecological release from interactions with directly impacted species. However, evidence that invasive vertebrates trigger such trophic cascades and alter community structure in terrestrial ecosystems remains rare. Here, we ask how the cane toad, a vertebrate invader that is toxic to many of Australia's vertebrate predators, influences lizard assemblages in a semi-arid rangeland. In our study area, the density of cane toads is influenced by the availability of water accessible to toads. We compared an index of the abundance of sand goannas, a large predatory lizard that is susceptible to poisoning by cane toads and the abundances of four lizard families preyed upon by goannas (skinks, pygopods, agamid lizards and geckos) in areas where cane toads were common or rare. Consistent with the idea that suppression of sand goannas by cane toads initiates a trophic cascade, goanna activity was lower and small lizards were more abundant where toads were common. The hypothesis that suppression of sand goannas by cane toads triggers a trophic cascade was further supported by our findings that small terrestrial lizards that are frequently preyed upon by goannas were more affected by toad abundance than arboreal geckos, which are rarely consumed by goannas. Furthermore, the abundance of at least one genus of terrestrial skinks benefitted from allogenic ecosystem engineering by goannas where toads were rare. Overall, our study provides evidence that the invasion of ecosystems by non-native species can have important effects on the structure and integrity of native communities extending beyond their often most obvious and frequently documented direct ecological effects.

Diversity within mutualist guilds promotes coexistence and reduces the risk of invasion from an alien mutualist

Biodiversity is an important component of healthy ecosystems, and thus understanding the mechanisms behind species coexistence is critical in ecology and conservation biology. In particular, few studies have focused on the dynamics resulting from the co-occurrence of mutualistic and competitive interactions within a group of species. Here we build a mathematical model to study the dynamics of a guild of competitors who are also engaged in mutualistic interactions with a common partner. We show that coexistence as well as competitive exclusion can occur depending on the competition strength and on strength of the mutualistic interactions, and we formulate concrete criteria for predicting invasion success of an alien mutualist based on propagule pressure, alien traits (such as its resource exchange ability) and composition of the recipient community. We find that intra guild diversity promotes the coexistence of species that would otherwise competitively exclude each other, and makes a guild less vulnerable to invasion. Our results can serve as a useful framework to predict the consequences of species manipulation in mutualistic communities.

Combined effects of land-use intensification and plant invasion on native communities

Land-use intensification (LUI) and biological invasions are two of the most important global change pressures driving biodiversity loss. However, their combined impacts on biological communities have been seldom explored, which may result in misleading ecological assessments or mitigation actions. Based on an extensive field survey of 445 paired invaded and control plots of coastal vegetation in SW Spain, we explored the joint effects of LUI (agricultural and urban intensification) and invasion on the taxonomic and functional richness, mean plant height and leaf area of native plants. Our survey covered five invasive species with contrasting functional similarity and competitive ability in relation to the native community. We modeled the response of native communities for the overall and invader-specific datasets, and determined if invader-native functional differences could influence the combined impacts of LUI and invasion. Overall, we found that urban intensification reduced taxonomic richness more strongly at invaded plots (synergistic interactive effects). In contrast, functional richness loss caused by urban intensification was less pronounced at invaded plots (antagonistic interactive effects). Overall models showed also that urban intensification led to reduced mean leaf area, while agriculture was linked to higher mean plant height. When exploring invader-specific models, we observed that the combined effects of agricultural and urban intensification with invasion were heterogeneous. At invaded plots, invader-native functional differences accounted for part of this variability. Our findings demonstrate the importance of considering the interactive effects of global change pressures for a better assessment and management of ecosystems.

Walleye growth declines following zebra mussel and Bythotrephes invasion

Invasive species represent a threat to aquatic ecosystems globally; however, impacts can be heterogenous across systems. Documented impacts of invasive zebra mussels (Dreissena polymorpha) and spiny water fleas (Bythotrephes cederströmii; hereafter Bythotrephes) on native fishes are variable and context dependent across locations and time periods. Here, we use a hierarchical Bayesian analysis of a 35-year dataset on two fish species from 9 lakes to demonstrate that early life growth of ecologically important fishes are influenced by these aquatic invasive species. Walleye (Sander vitreus) in their first year of life grew more slowly in the presence of either invader after correcting for temperature (measured by degree days), and were on average 12 or 14% smaller at the end of their first summer following invasion by Bythotrephes or zebra mussels, respectively. Yellow perch (Perca flavescens) growth was less affected by invasion. Yellow perch on average grew more slowly in their first year of life following invasion by zebra mussels, although this effect was not statistically distinguishable from zero. Early life growth of both walleye and yellow perch was less tightly coupled to degree days in invaded systems, as demonstrated by increased variance surrounding the degree day-length relationship. Smaller first-year size is related to walleye survival and recruitment to later life stages and has important implications for lake food webs and fisheries management. Future research quantifying effects of zebra mussels and Bythotrephes on other population-level processes and across a wider gradient of lake types is needed to understand the mechanisms driving observed changes in walleye growth.

Stuck amongst introduced species: Trophic ecology reveals complex relationships between the critically endangered Niau kingfisher and introduced predators, competitors and prey

The introduction of mammals on oceanic islands currently threatens or has caused the extinction of many endemic species. Cats and rats represent the major threat for 40 % of currently endangered island bird species. Direct (predation) and/or indirect (exploitative competition for food resource) trophic interactions are key mechanisms by which invaders cause the decrease or extinction of native populations. Here, we investigated both direct and indirect trophic interactions amongst four predator species (i.e. animals that hunt, kill and feed on other animals), including three introduced mammals (Felis silvestris catus, Rattus rattus and Rattus exulans) and one critically endangered native bird, the Niau kingfisher (Todiramphus gertrudae). All four species’ diets and prey availability were assessed from sampling at the six main kingfisher habitats on Niau Island during the breeding season. Diet analyses were conducted on 578 cat scats, 295 rat digestive tracts (218 R. exulans and 77 R. rattus) and 186 kingfisher pellets. Despite simultaneous use of morphological and PCR-based methods, no bird remains in cat and rat diet samples could be assigned to the Niau kingfisher, weakening the hypothesis of current intense predation pressure. However, we determined that Niau kingfishers mainly feed on introduced and/or cryptogenic prey and highlighted the potential for exploitative competition between this bird and both introduced rat species (for Dictyoptera, Coleoptera and Scincidae). We recommend removing the cats and both rat species, at least within kingfisher breeding and foraging areas (e.g. mechanical or chemical control, cat sterilisation, biosecurity reinforcement), to simultaneously decrease predation risk, increase key prey availability and boost kingfisher population dynamics.

Mutual Facilitation Among Invading Nuttall's Waterweed and Quagga Mussels

Nuttall's waterweed (Elodea nuttallii) is the most abundant invasive aquatic plant species in several European countries. Elodea populations often follow a boom-bust cycle, but the causes and consequences of this dynamics are yet unknown. We hypothesize that both boom and bust periods can be affected by dreissenid mussel invasions. While mutual facilitations between these invaders could explain their rapid parallel expansion, subsequent competition for space might occur. To test this hypothesis, we use data on temporal changes in the water quality and the abundance of E. nuttallii and the quagga mussel Dreissena rostriformis bugensis in a temperate shallow lake. Lake Müggelsee (Germany) was turbid and devoid of submerged macrophytes for 20 years (1970-1989), but re-colonization with macrophytes started in 1990 upon reductions in nutrient loading. We mapped macrophyte abundance from 1999 and mussel abundance from 2011 onwards. E. nuttallii was first detected in 2011, spread rapidly, and was the most abundant macrophyte species by 2017. Native macrophyte species were not replaced, but spread more slowly, resulting in an overall increase in macrophyte coverage to 25% of the lake surface. The increased abundance of E. nuttallii was paralleled by increasing water clarity and decreasing total phosphorus concentrations in the water. These changes were attributed to a rapid invasion by quagga mussels in 2012. In 2017, they covered about one-third of the lake area, with mean abundances of 3,600 mussels m^-2, filtering up to twice the lake's volume every day. The increasing light availability in deeper littoral areas supported the rapid spread of waterweed, while in turn waterweed provided surface for mussel colonization. Quantities of dreissenid mussels and E. nuttallii measured at 24 locations were significantly correlated in 2016, and yearly means of E. nuttallii quantities increased with increasing mean dreissenid mussel quantities between 2011 and 2018. In 2018, both E. nuttallii and dreissenid abundances declined. These data imply that invasive waterweed and quagga mussels initially facilitated their establishment, supporting the invasional meltdown hypothesis, while subsequently competition for space may have occurred. Such temporal changes in invasive species interaction might contribute to the boom-bust dynamics that have been observed in Elodea populations.

Using Network Theory to Understand and Predict Biological Invasions

Understanding and predicting biological invasions is challenging because of the complexity of many interacting players. A holistic approach is needed with the potential to simultaneously consider all relevant effects and effectors. Using networks to describe the relevant anthropogenic and ecological factors, from community-level to global scales, promises advances in understanding aspects of invasion from propagule pressure, through establishment, spread, and ecological impact of invaders. These insights could lead to development of new tools for prevention and management of invasions that are based on species' network characteristics and use of networks to predict the ecological effects of invaders. Here, we review the findings from network ecology that show the most promise for invasion biology and identify pressing needs for future research.

Morphological and molecular analysis of cryptic native and invasive freshwater snails in Chile

Species delimitation in minute freshwater snails is often difficult to perform using solely shell morphology. The problem intensifies when invasive species spread within the distribution range of morphologically similar native species. In Chile, the Truncatelloidean snails are represented by the native genera Heleobia and Potamolithus plus the invasive mudsnail Potamopyrgus antipodarum, which can easily be confused. Using an integrative approach, we performed molecular phylogenetic analysis and studied reproductive and morphological features to identify superficially similar forms inhabiting the central area of the country. Truncatelloidean snails were identified in 40 of 51 localities sampled, 10 containing Potamopyrgus antipodarum, 23 Heleobia and 7 Potamolithus. Based on these results and previously published data, the known distribution of the mudsnail in Chile encompasses 6 hydrological basins, including 18 freshwater ecosystems. The finding of the mudsnails in several type localities of native species/subspecies of “Heleobia” that were not find in situ suggests species replacement or significant extinction of native fauna, a hypothesis supported by the restudy of type material that shows that endemic forms belong to the genus Potamolithus. This study shows the usefulness of integrative taxonomy not only resolving complex taxa with cryptic morphology but also measuring the extent of an ongoing invasion.

Hope and caution: rewilding to mitigate the impacts of biological invasions

Rewilding is a novel approach to ecological restoration. Trophic rewilding in particular aims to reinstate ecological functions, especially trophic interactions, through the introduction of animals. We consider the potential for trophic rewilding to address biological invasions. In this broad review, we note some of the important conceptual and ethical foundations of rewilding, including a focus on ecosystem function rather than composition, reliance on animal agency, and an appeal to an ethic of coexistence. Second, we use theory from invasion biology to highlight pathways by which rewilding might prevent or mitigate the impacts of an invasion, including increasing biotic resistance. Third, we use a series of case studies to illustrate how reintroductions can mitigate the impacts of invasions. These include reintroductions and positive management of carnivores and herbivores including European pine martens (Martes martes), Eurasian otters (Lutra lutra), dingoes (Canis dingo), Tasmanian devils (Sarcophilus harrisii) and tule elk (Cervus canadensis nannodes). Fourth, we consider the risk that rewilding may enable a biological invasion or aggravate its impacts. Lastly, we highlight lessons that rewilding science might take from invasion biology.This article is part of the theme issue 'Trophic rewilding: consequences for ecosystems under global change'.

Foraging connections: Patterns of prey use linked to invasive predator diel movement

Invasive predators can profoundly impact native communities, especially in insular ecosystems where functionally equivalent predators were evolutionarily absent. Beyond direct consumption, predators can affect communities indirectly by creating or altering food web linkages among existing species. Where invasive predators consume prey from multiple distinct resource channels, novel links may couple the dynamics of disjunct modules and create indirect interactions between them. Our study focuses on invasive populations of Eleutherodactylus coqui (Anura: Leptodactylidae) on Hawaii Island. Coqui actively forage in the understory and lower canopy at night but return to the forest floor and belowground retreats by day. Recent dietary studies using gut contents and naturally occurring stable isotopes indicate higher than expected consumption of litter arthropods, which in these Hawaiian forests are primarily non-native species. We used laboratory studies to observe diurnal and nocturnal foraging behavior, and experimental field additions of C₄ vegetation as a litter tracer to distinguish epigaeic sources from food web pools in the C₃ canopy. Lab trials revealed that prey consumption during diurnal foraging was half that consumed during nocturnal foraging. Analysis of δ¹³C isotopes showed incorporation of C₄ carbon into litter arthropods within one month, and Bayesian mixing models estimated that 15–25% of the carbon in coqui tissue was derived from litter sources. These results support recent findings that E. coqui are not quiescent diurnally but instead actively forage. Such activity by a mobile invasive predator may introduce a novel linkage that integrates detrital and foliar resource pools, potentially distributing influences of invasive litter arthropods through the broader system to amplify impacts on native species.

Secondary invasion re-redefined: The distinction between invader-facilitated and invader-contingent invasions as subclasses of secondary invasion

Linked Article: https://doi.org/10.1002/ece3.3964