Non‐native species have multiple abundance–impact curves

The abundance–impact curve is helpful for understanding and managing the impacts of non‐native species. Abundance–impact curves can have a wide range of shapes (e.g., linear, threshold, sigmoid), each with its own implications for scientific understanding and management. Sometimes, the abundance–impact curve has been viewed as a property of the species, with a single curve for a species. I argue that the abundance–impact curve is determined jointly by a non‐native species and the ecosystem it invades, so that a species may have multiple abundance–impact curves. Models of the impacts of the invasive mussel Dreissena show how a single species can have multiple, noninterchangeable abundance–impact curves. To the extent that ecosystem characteristics determine the abundance–impact curve, abundance–impact curves based on horizontal designs (space‐for‐time substitution) may be misleading and should be used with great caution, it at all. It is important for scientists and managers to correctly specify the abundance–impact curve when considering the impacts of non‐native species. Diverting attention from the invading species to the invaded ecosystem, and especially to the interaction between species and ecosystem, could improve our understanding of how non‐native species affect ecosystems and reduce uncertainty around the effects of management of populations of non‐native species.     

Scaling the consequences of interactions between invaders from the individual to the population level

The impact of human‐induced stressors, such as invasive species, is often measured at the organismal level, but is much less commonly scaled up to the population level. Interactions with invasive species represent an increasingly common source of stressor in many habitats. However, due to the increasing abundance of invasive species around the globe, invasive species now commonly cause stresses not only for native species in invaded areas, but also for other invasive species. I examine the European green crab Carcinus maenas, an invasive species along the northeast coast of North America, which is known to be negatively impacted in this invaded region by interactions with the invasive Asian shore crab Hemigrapsus sanguineus. Asian shore crabs are known to negatively impact green crabs via two mechanisms: by directly preying on green crab juveniles and by indirectly reducing green crab fecundity via interference (and potentially exploitative) competition that alters green crab diets. I used life‐table analyses to scale these two mechanistic stressors up to the population level in order to examine their relative impacts on green crab populations. I demonstrate that lost fecundity has larger impacts on per capita population growth rates, but that both predation and lost fecundity are capable of reducing population growth sufficiently to produce the declines in green crab populations that have been observed in areas where these two species overlap. By scaling up the impacts of one invader on a second invader, I have demonstrated that multiple documented interactions between these species are capable of having population‐level impacts and that both may be contributing to the decline of European green crabs in their invaded range on the east coast of North America.     

Phenotypic responses of invasive species to removals affect ecosystem functioning and restoration

Reducing the abundances of invasive species by removals aims to minimize their ecological impacts and enable ecosystem recovery. Removal methods are usually selective, modifying phenotypic traits in the managed populations. However, there is little empirical evidence of how removal‐driven changes in multiple phenotypic traits of surviving individuals of invasive species can affect ecosystem functioning and recovery. Overcoming this knowledge gap is highly relevant because individuals are the elemental units of ecological processes and so integrating individual‐level responses into the management of biological invasions could improve their efficiency. Here we provide novel demonstration that removals by trapping, angling and biocontrol from lakes of the globally invasive crayfish Procambarus clarkii induced substantial changes in multiple phenotypic traits. A mesocosm experiment then revealed that these changes in phenotypic traits constrain recovery of basic ecosystem functions (decomposition of organic matter, benthic primary production) by acting in the opposite direction than the effects of reduced invader abundance. However, only minor ecological impacts of invader abundance and phenotypic traits variation remained a year after its complete eradication. Our study provides quantitative evidence to an original idea that removal‐driven trait changes can dampen recovery of invaded ecosystems even when the abundance of invasive species is substantially reduced. We suggest that the phenotypic responses of invaders to the removal programme have strong effects on ecosystem recovery and should be considered within the management of biological invasions, particularly when complete eradication is not achievable.     

Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds

Biological plant invasions pose a serious threat to native biodiversity and have received much attention, especially in terrestrial habitats. In freshwater ecosystems impacts of invasive plant species are less studied. We hypothesized an impact on organisms from the water column and from the sediment. We then assessed the impact of three aquatic invasive species on the plants and macroinvertebrates: Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum. Our research on 32 ponds in Belgium indicated that the reduction in the native plant species richness was a common pattern to invasion. However, the magnitude of impacts were species specific. A strong negative relationship to invasive species cover was found, with submerged vegetation the most vulnerable to the invasion. Invertebrate richness, diversity and abundance were measured in sediments of invaded and uninvaded ponds along a gradient of H. ranunculoides, L. grandiflora, and M. aquaticum species cover. We found a strong negative relationship between invasive species cover and invertebrate abundance, probably due to unsuitable conditions of the detritus for invertebrate colonization. Taxonomic compositions of aquatic invertebrate assemblages in invaded ponds differed from uninvaded ponds. Sensitive benthos, such as mayflies were completely absent in invaded ponds. The introduction of H. ranunculoides, L. grandiflora, and M. aquaticum in Belgian ponds has caused significant ecological alterations in the aquatic vegetation and the detritus community of ponds.     

Direct and indirect impacts of predation by fish on the zooplankton community: an experimental analysis using tanks

The impact of Pseudorasbora parva, a common zooplanktivorous fish species in Japan, on a zooplankton community was analyzed in experimental tanks, half of which were stocked with the fish. Different zooplankton species showed different responses to the introduction of the fish. In the presence of the fish, the populations of the large cladoceran Ceriodaphnia and the predatory copepod Mesocyclops were reduced, but the population of the herbivorous copepod Eodiaptomus and the small cladocerans Bosmina fatalis and Bosminopsis deitersi increased relative to the controls. The increase of Mesocyclops seen in the control tanks might have suppressed the populations of the small cladocerans, which are vulnerable to invertebrate predation. The results suggest that the population densities of the large prey items preferred by the fish, Ceriodaphnia and Mesocyclops, were controlled directly by fish predation, but the population densities of the smaller and less preferred zooplankton were controlled indirectly through the food-web cascade.     

Impact assessment revisited: improving the theoretical basis for management of invasive alien species

The theoretical underpinnings of the assessment of invasive alien species impacts need to be improved. At present most approaches are unreliable to quantify impact at regional scales and do not allow for comparison of different invasive species. There are four basic problems that need to be addressed: (1) Some impacted ecosystem traits are spatially not additive; (2) invader effects may increase non-linearly with abundance or there may be effect thresholds impairing estimates of linear impact models; (3) the abundance and impact of alien species will often co-vary with environmental variation; and (4) the total invaded range is an inappropriate measure for quantifying regional impact because the habitat area available for invasion can vary markedly among invasive species. Mathematical models and empirical data using an invasive alien plant species (Heracleum mantegazzianum) indicate that ignoring these issues leads to impact estimates almost an order of magnitude from the real values. Thus, we propose a habitat-sensitive formula for regional impact assessment that is unaffected by non-linearity. Furthermore, we make some statistical suggestions on how to assess invader effects properly and we discuss the quantification of the invaded range. These improvements are crucial for impact assessment with the overall aim of prioritizing management of invasive species.     

Variation in the impact of non‐native seaweeds along gradients of habitat degradation: a meta‐analysis and an experimental test

Biological invasions are acknowledged among the main drivers of global changes in biodiversity. Despite compelling evidence of species interactions being strongly regulated by environmental conditions, there is a dearth of studies investigating how the effects of non‐native species vary among areas exposed to different anthropogenic pressures. Focusing on marine macroalgae, we performed a meta‐analysis to test whether and how the direction and magnitude of their effects on resident communities and species varies in relation to cumulative anthropogenic impact levels. The relationship between human impact levels and non‐native species impact intensity emerged only for a reduced subset of the response variables examined. Yet, there was a trend for the effects of non‐native species on community biomass and abundance and on species abundance to become less negative at heavily impacted sites. By contrast, the magnitude of negative effects of seaweed on community evenness tended to increase with human impact levels. The hypothesis of decreasing severity of invader’ impacts along a gradient of habitat degradation was also tested experimentally at a regional scale by comparing the effects of the removal of non‐native alga, Caulerpa cylindracea, on resident assemblages among rocky reefs exposed to different anthropogenic pressures. Assemblages at urban and pristine site did not differ when invaded, but did so when C. cylindracea was removed. Our results suggest that, despite the generally weak relationship between human impacts levels and non‐native species impacts, more negative impacts can be expected in less stressful environments (i.e. less degraded or pristine sites), where competitive interactions are presumably the driving force structuring resident communities. Implementing strategies for controlling the establishment of non‐native seaweeds should be, thus, considered a priority for preserving biodiversity in relatively pristine areas. On the other hand, control of invaders at degraded sites could be warranted to lessen their role as propagule sources. Synthesis Local anthropogenic stressors that severely alter biotic and abiotic conditions may underpin context‐dependency in the impacts of biological invasions. We used a meta‐analysis and an experimental test to examine the relationship between cumulative human impacts and ecological impact of non‐native seaweeds on resident assemblages. Our results suggest that more negative impacts of non‐native seaweeds on the abundance and biomass of resident assemblages can be expected in less degraded or pristine sites. Possibly, stronger impacts prevail at pristine sites, where assemblages are mainly structured by biotic interactions. Hence, management efforts should be mostly directed to prevent the establishment and spread of non‐native seaweeds in pristine areas. On the other hand, weak, but positive effects of seaweeds at the most degraded sites add to the ongoing debate on the role of non‐native species in rehabilitation plans.     

Can chaoborid larvae control their zooplanktonic prey populations in a neotropical lake?

Although it is well known that predatory Chaoborus Lichtenstein 1800 larvae have a strong negative impact on zooplankton populations in many Holarctic lakes, this top‐down control is still not entirely clear in the Neotropics. In Lake Carioca, a Brazilian monomictic lake, chaoborids became one of the main zooplankton predators after the depletion of planktivorous fish by non‐native piscivorous fish. The aim of this research was to test the hypothesis that Chaoborus spp. larvae control the density of their prey populations. Enclosures simulated conditions with higher (Pr+) and lower (Pr?) predatory Chaoborus spp. larva densities over 30 days in two periods (stratification and mixing). Our results indicated that the dominant copepod species was affected only in the stratification period when its population was smaller. In this circumstance, predated individuals were probably not replaced in the same proportion by reproductive output of the smaller population. Rotifers were not sensitive to changes in chaoborids densities in both periods. The lack of a strong top‐down effect recorded by our mesocosm experiments emphasises the importance of continuing to investigate the ecological roles played by this invertebrate predator in aquatic food webs and if its ability to regulate the zooplankton in most Holarctic lakes can (or cannot) be widely generalised to Neotropical communities. Such a result also suggests that alternative research approaches, besides in‐situ experiments, may be necessary to better clarify this topic. Specifically for cladocerans, parallel paleolimnological analyses in Lake Carioca have supported the idea of a possible negative impact of Chaoborus larvae on bosminids after the introduction of non‐native piscivorous fishes. Abstract in Portuguese is available with online material.     

Predation of seabirds by invasive rats: multiple indirect consequences for invertebrate communities

Invasive species are a global problem but most studies have focused on their direct rather than indirect ecological effects. We studied litter and soil‐inhabiting invertebrate communities on 18 islands off northern New Zealand, to better understand the indirect ecological consequences of rat (Rattus) invasion. Nine islands host high densities of burrowing procellariid seabirds that transport large amounts of nutrients from the ocean to the land. The other nine have been invaded over the past 50–150 years by rat species that have severely reduced the density of seabirds by preying on eggs and chicks. Invaded islands had lower densities of seabird burrows but deeper forest litter than did the uninvaded islands, indicative of rats reducing disturbance effects of seabirds. However, despite deeper litter on the invaded islands, eight of the 19 orders of invertebrates that we measured were significantly less abundant on invaded islands. Furthermore, three soil‐inhabiting micro‐invertebrate groups that we measured were significantly less abundant on invaded islands. These differences probably result from rats thwarting transfer of resources by seabirds from the ocean to the land. We also investigated community‐level properties of each of three test groups of invertebrates (minute land snails, spiders and soil nematodes) to illustrate this process. Spiders were equally abundant on both groups of islands, but showed lower species richness on the invaded islands. The other two groups showed no difference in species richness with island invasion status, but were more abundant on uninvaded islands. Reduced abundance of soil nematodes on invaded islands provides strong evidence of indirect consequences of seabird reduction by rats, because nematodes are unavailable to rats as prey. We predict that if rats are eradicated from islands, components of below‐ground invertebrate dependent on seabird‐mediated soil conditions may take considerable time to recover because they require subsequent seabird recolonisation.     

Indirect food web effects of <Emphasis Type="Italic">Bythotrephes</Emphasis> invasion: responses by the rotifer <Emphasis Type="Italic">Conochilus</Emphasis> in Harp Lake,Canada

As a recent invader of North American lakes, Bythotrephes longimanus has induced large changes in crustacean zooplankton communities through direct predation effects. Here we demonstrate that Bythotrephes can also have indirect food web effects, specifically on rotifer fauna. In historical time series data, the densities of the colonial rotifer Conochilus unicornis significantly increased after Bythotrephes invasion in Harp Lake, Ontario. No such changes were observed in a non-invaded reference lake, the nearby Red Chalk Lake. Evidence for two mechanisms explaining the Conochilus increase was examined based on changes to the crustacean zooplankton community over time. Rapid and severe declines in several herbivorous species of cladoceran zooplankton after Bythotrephes detection indicated a decrease in exploitative competition pressure on Conochilus. Secondly, a later and significant decline to virtual extinction of native invertebrate predators (Mesocyclops and Leptodora) could account for the observed Conochilus increase which also began 1–2 years after invasion by Bythotrephes. Ultimately, it appears that both reduced competition followed by a loss of native invertebrate predators were necessary to lead to the large Conochilus densities observed following invader establishment. From this analysis of long-term community data, it appears that Bythotrephes has important indirect, as well as direct, food web effects in newly invaded North American lakes with implications for trophic relationships.     

Invasive Predator, <Emphasis Type="Italic">Bythotrephes</Emphasis>, has Varied Effects on Ecosystem Function in Freshwater Lakes

Bythotrephes longimanus is an invertebrate predator that has invaded the North American Great Lakes and a number of inland lakes, where it preys on crustacean zooplankton. We examined the effect of Bythotrephes on two measures of ecosystem function during a four-month observational study of freshwater lakes on the boreal shield. Bythotrephes-invaded lakes had significantly lower epilimnetic zooplankton abundance and production compared to reference lakes. On average, Bythotrephes consumed 34% of zooplankton production when it was present in lakes. There was some evidence of changes in the timing of zooplankton production, as well as shifts to cooler, less productive habitats, which may lessen the overall effect of the invader on the transfer of energy to higher trophic levels. We experimentally demonstrated a weak trophic cascade where invader predation reduced zooplankton biomass, and subsequently increased phytoplankton growth. However, the response was small in magnitude and not biologically relevant at the whole lake-scale. The most conspicuous effect of Bythotrephes that we measured was a diversion of energy away from native predators at higher trophic levels. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Impacts of the invasive Argentine ant on native ants and other invertebrates in coastal scrub in south‐eastern Australia

Invasive ants threaten native biodiversity and ecosystem function worldwide. Although their principal direct impact is usually the displacement of native ants, they may also affect other invertebrates. The Argentine ant, Linepithema humile (Dolichoderinae), one of the most widespread invasive ant species, has invaded native habitat where it abuts peri‐urban development in coastal Victoria in south‐eastern Australia. Here we infer impacts of the Argentine ant on native ants and other litter and ground‐dwelling invertebrates by comparing their abundance and taxonomic composition in coastal scrub forest either invaded or uninvaded by the Argentine ant. Species composition of native ants at bait stations and extracted from litter differed significantly between Argentine ant‐invaded and uninvaded sites and this was consistent across years. Argentine ants had a strong effect on epigeic ants, which were either displaced or reduced in abundance. The native ant Rhytidoponera victoriae (Ponerinae), numerically dominant at uninvaded sites, was completely absent from sites invaded by the Argentine ant. However, small hypogeic ants, including Solenopsis sp. (Myrmicinae) and Heteroponera imbellis (Heteroponerinae), were little affected. Linepithema humile had no detectable effect upon the abundance and richness of other litter invertebrates. However, invertebrate group composition differed significantly between invaded and uninvaded sites, owing to the varied response of several influential groups (e.g. Collembola and Acarina). Floristics, habitat structure and measured environmental factors did not differ significantly between sites either invaded or uninvaded by Argentine ants, supporting the contention that differences in native ant abundance and species composition are related to invasion. Changes in the native ant community wrought by Argentine ant invasion have important implications for invertebrate communities in southern Australia and may affect key processes, including seed dispersal.     

Effect of mosquitofish (Gambusia affinis) and sestonic food abundance on the invertebrate community within a constructed treatment wetland

1. The effect of mosquitofish (Gambusia affinis) predation on the invertebrate community in a hypereutrophic constructed treatment wetland in southern California was investigated at two nutrient levels that influenced sestonic food abundance. 2. Gambusia affinis and insect predators in the wetland had a significant impact on larval mosquito density in the wetland irrespective of nutrient level. At the end of the 5‐month study, cladoceran abundance in predator exclusion enclosures was 2–3 orders of magnitude greater than in the treatments that allowed access by planktivores. Chironomids were the most abundant insect group collected in emergence traps, and midge production from the high nutrient location of the wetland was greater than from the low nutrient location, but was not affected significantly by G. affinis. The presence of abundant alternative prey in this highly enriched wetland may have weakened the predation impact of G. affinis on mosquitoes. 3. The abundances of six invertebrate groups in dipper samples and of four insect groups in emergence trap collections were analysed using a multivariate distance‐based linear model. Fish treatment and location interactions with sampling date explained significant amounts of the variation in the abundance of invertebrate groups. 4. Multivariate multiple regression analysis showed that chlorophyll‐a concentration explained a large portion of the variability in non‐predatory insect and zooplankton abundance at the high nutrient location, whereas bacterial density explained a large portion of the variability in the abundances of these taxa at the low nutrient location. Predatory insects were not directly coupled to the bottom‐up influence of bacterial abundance and chlorophyll‐a.     

Interactions between invading benthivorous fish and native whitefish in subarctic lakes

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Responses of invertebrates to herbicide in Salix cinerea invaded wetlands: Restoration implications

The use of herbicides to control weeds, particularly large invasions, has now become an essential management tool in many ecological restoration projects. The herbicide glyphosate is routinely used to control the invasive weed, Grey Willow (Salix cinerea), within New Zealand wetlands. However, little is known about the effects of glyphosate on invertebrates. We determine the short‐term effects of glyphosate on the abundance and composition of the nontarget canopy invertebrate community in wetlands invaded by Grey Willow in New Zealand. Initially, the application of glyphosate and a surfactant showed no detectable effect on the canopy invertebrates examined in this study. However, 27 days after herbicide application, significant Grey Willow canopy loss caused dramatic decreases in the abundance of invertebrates in the glyphosate‐treated plots compared with the unsprayed plots. Invertebrates appeared to be sensitive to changes in vegetation structure, such as canopy loss. These results agree with previous studies that have shown that the negative impacts of glyphosate on invertebrate communities are related to indirect effects via habitat modification as the herbicide‐treated vegetation dies. From a terrestrial invertebrate perspective, this study suggests that the use of glyphosate herbicide is suitable for the control of invasive weeds within wetland restoration projects as it appears to have negligible impact on the canopy invertebrate assemblage.     

Experimental plant invasion reduces arthropod abundance and richness across multiple trophic levels

Plant invasions are known to have negative impacts on native plant communities, yet their influence on higher trophic levels has not been well documented. Past studies investigating the effects of invasive plants on herbivores and carnivores have been largely observational in nature and thus lack the ability to tease apart whether differences are a cause or consequence of the invasion. In addition, understanding how plant traits and plant species compositions change in invaded habitats may increase our ability to predict when and where invasive plants will have effects that cascade to animals. To assess effects on arthropods, we experimentally introduced a non‐native plant (Microstegium vimineum, Japanese stiltgrass) in a community re‐assembly experiment. We also investigated possible mechanisms through which the invader could affect associated arthropods, including changes in native plant species richness, above‐ground plant biomass, light availability and vegetation height. In experimentally invaded plots, arthropod abundance was reduced by 39%, and species richness declined by 19%. Carnivores experienced greater reductions in abundance than herbivores (61% vs 31% reduction). Arthropod composition significantly diverged between experimentally invaded and control plots, and particular species belonging to the abundant families Aphididae (aphids), Formicidae (ants) and Phalacridae (shining flower beetles) contributed the most to compositional differences. Among the mechanisms we investigated, only the reduction in native plant species richness caused by invasion was strongly correlated with total arthropod abundance and richness. In sum, our results demonstrate negative impacts of M. vimineum invasion on higher trophic levels and suggest that these effects occur, in part, indirectly through invader‐mediated reductions in the richness of the native plant community. The particularly strong response of carnivores suggests that plant invasion could reduce top–down control of herbivorous species for native plants.     

Pine invasion decreases density and changes native tree communities in woodland Cerrado

ABSTRACT

Background: Invasive plants can negatively impact native communities, but the majority of the effects of these invasions have been demonstrated only for temperate ecosystems. Tropical ecosystems, including the Cerrado, a biodiversity hotspot, are known to be invaded by numerous non-native species, but studies of their impacts are largely lacking.

Aims: Our research aimed at quantifying how Pinus spp. presence and density affected Cerrado plant communities.

Methods: We sampled areas invaded and non-invaded by Pinus spp. to determine if pine invasion affected native tree richness, diversity, evenness, and density. We also evaluated if community composition differed between invaded and non-invaded sites.

Results: We found invaded plots had lower native tree densities than non-invaded plots and that Pinus spp. invasions changed native tree communities by reducing native species abundances.

Conclusion: Invasive pines had negative impacts on the native Cerrado tree community by reducing native plant density and changing species abundances. Reduced density and abundance at early invasion stages can result in reduction in biodiversity in the long term.     

Invasional 'meltdown' on an oceanic island

Islands can serve as model systems for understanding how biological invasions affect community structure and ecosystem function. Here we show invasion by the alien crazy ant Anoplolepis gracilipes causes a rapid, catastrophic shift in the rain forest ecosystem of a tropical oceanic island, affecting at least three trophic levels. In invaded areas, crazy ants extirpate the red land crab, the dominant endemic consumer on the forest floor. In doing so, crazy ants indirectly release seedling recruitment, enhance species richness of seedlings, and slow litter breakdown. In the forest canopy, new associations between this invasive ant and honeydew‐secreting scale insects accelerate and diversify impacts. Sustained high densities of foraging ants on canopy trees result in high population densities of host‐generalist scale insects and growth of sooty moulds, leading to canopy dieback and even deaths of canopy trees. The indirect fallout from the displacement of a native ‘keystone’ species by an ant invader, itself abetted by introduced/cryptogenic mutualists, produces synergism in impacts to precipitate invasional ‘meltdown’ in this system.     

Co-invasion of similar invaders results in analogous ecological impact niches and no synergies

Exotic species can cause changes to their invaded ecosystems, which can be large and long lasting. Despite most landscapes being invaded by multiple exotic plant species, >90 % of impact studies only characterize the impacts of single species. Therefore, our knowledge of invasive plant impacts does not reflect the co-invaded nature of most landscapes, potentially ignoring complex interactions among exotic species. Our objective was to characterize potential invader synergies (positive interactions) on biotic and abiotic ecological parameters among the important forest invaders Japanese stiltgrass (Microstegium vimineum) and wavyleaf basketgrass (Oplismenus undulatifolius), which co-invade eastern US deciduous forests. To characterize synergies, we used a factorial selective removal study, as well as an observational study to further explore invader cover-impact relationships. Although both invaders can reduce native plant richness by 70 % individually or in combination, there were no impact synergies. Total cover of any combination of the two invaders had a negative quadratic effect on total, exotic, and native plant richness; i.e., all community metrics were greatest at intermediate levels of total invader cover and lowest at maximum invader cover. Native richness was more greatly affected than exotic richness by the co-invasion. Soil metrics had no clear trend in either study. Japanese stiltgrass and wavyleaf basketgrass appear to have overlapping impact niches—the number, magnitude, and direction of biotic and abiotic changes to the invaded ecosystem—that only vary in impact magnitude, not breadth. As a result of their overlapping impact niches and non-synergies in this co-invaded system, the addition of the recent invader wavyleaf basketgrass has not resulted in additional changes to the invaded forests. Future impact studies should focus on multiple species and identifying synergies, especially as they relate to invader cover, which informs ecological interactions and management prioritization.     

Introduction of the non-indigenous amphipod Gammarus pulex alters population dynamics and diet of juvenile trout Salmo trutta

Summary 1. The amphipod Gammarus pulex , introduced to Irish rivers with the aim of enhancing trout feeding, is displacing the native Gammarus duebeni celticus . These two species are generally associated with different environmental conditions and macroinvertebrate communities, confounding assessment of effects of the invader as compared with the native on fish populations. Here, we uncouple effects of the two Gammarus species from environmental gradients.
2. A weir dissects a lowland stretch of the River Lissan, slowing the upstream invasion by G. pulex and resulting in contiguous G. pulex , mixed species and G. d. celticus reaches. Total invertebrate abundance and biomass in the benthos were significantly higher in the G. pulex reach, driven by high invader abundance, with low abundance of other taxa. Gammarus pulex was particularly prominent in night-time drift.
3. Correspondingly, densities and biomass of 0+ trout were significantly higher in the G. pulex reach, while instantaneous loss rates were lower. Fish growth rates were similar among the three reaches.
4. In the G. pulex reach, this invader dominated the diet of 0+ trout, leading to ingestion of significantly higher invertebrate biomass than fish in the other reaches. Fish generally preyed on Gammarus in proportion to its abundance, but exhibited some positive selection for G. pulex in the invaded reach.
5. The negative effects of the invader on native invertebrates are contrasted with positive effects on juvenile trout. This indicates changes in energy flux after invasion, with differential resource use or assimilation by G. pulex probable underlying mechanisms. As the frequency of amphipod invasions increases globally, investigations of their role as strong interactors at multiple levels of ecological organisation is required if the consequences of deliberate and unintentional introductions are to be predicted, and ultimately, prevented.