The negative ecological impacts of a globally introduced species decrease with time since introduction

Contemporary perspectives on the ecological impacts of invasive freshwater fishes

Introductions of non-native freshwater fish continue to increase globally, although only a small proportion of these introductions will result in an invasion. These invasive populations can cause ecological impacts in the receiving ecosystem through processes including increased competition and predation pressure, genetic introgression and the transmission of non-native pathogens. Definitions of ecological impact emphasize that shifts in the strength of these processes are insufficient for characterizing impact alone and, instead, must be associated with a quantifiable decline of biological and/or genetic diversity and lead to a measurable loss of diversity or change in ecosystem functioning. Assessments of ecological impact should thus consider the multiple processes and effects that potentially occur from invasive fish populations where, for example, impacts of invasive common carp Cyprinus carpio populations are through a combination of bottom-up and top-down processes that, in entirety, cause shifts in lake stable states and decreased species richness and/or abundances in the biotic communities. Such far-reaching ecological impacts also align to contemporary definitions of ecosystem collapse, given they involve substantial and persistent declines in biodiversity and ecosystem functions that cannot be recovered unaided. Thus, while not all introduced freshwater fishes will become invasive, those species that do develop invasive populations can cause substantial ecological impacts, where some of the impacts on biodiversity and ecosystem functioning might be sufficiently harmful to be considered as contributing to ecosystem collapse.

Species Home-Making in Ecosystems: Toward Place-Based Ecological Metrics of Belonging

Globalization has undeniably impacted the Earth’s ecosystems, but it has also influenced how we think about natural systems. Three fourths of the world’s forests are now altered by human activity, which challenges our concepts of native ecosystems. The dichotomies of pristine vs. disturbed as well as our view of native and non-native species, have blurred; allowing us to acknowledge new paradigms about how humans and nature interact. We now understand that the use of militaristic language to define the perceived role of a plant species is holding us back from the fact that novel systems (new combinations of all species) can often provide valuable ecosystem services (i.e., water, carbon, nutrients, cultural, and recreation) for creatures (including humans). In reality, ecosystems exist in a gradient from native to intensely managed – and “non-nativeness” is not always a sign of a species having negative effects. In fact, there are many contemporary examples of non-native species providing critical habitat for endangered species or preventing erosion in human-disturbed watersheds. For example, of the 8,000–10,000 non-native species introduced to Hawai‘i, less than 10% of these are self-sustaining and 90 of those pose a danger to native biota and are considered invasive. In this paper, we explore the native/non-native binary, the impacts of globalization and the political language of invasion through the lens of conservation biology and sociology with a tropical island perspective. This lens gives us the opportunity to offer a place-based approach toward the use of empirical observation of novel species interactions that may help in evaluating management strategies that support biodiversity and ecosystem services. Finally, we offer a first attempt at conceptualizing a site-specific approach to develop “metrics of belonging” within an ecosystem.

Genotype-by-environment interactions drive the maintenance of genetic variation in a Salmo trutta L. hybrid zone

Allopatric gene pools can evolve in different directions through adaptive and nonadaptive processes and are therefore a source of intraspecific diversity. The connection of these previously isolated gene pools through human intervention can lead to intraspecific diversity loss, through extirpation of native populations or hybridization. However, the mechanisms leading to these situations are not always explicitly documented and are thus rarely used to manage intraspecific diversity. In particular, genotype-by-environment (GxE) interactions can drive postzygotic reproductive isolation mechanisms that may result in a mosaic of diversity patterns, depending on the local environment. We test this hypothesis using a salmonid species (Salmo trutta) in the Mediterranean (MED) area, where intensive stocking from non-native Atlantic (ATL) origins has led to various outcomes of hybridization with the native MED lineage, going from MED resilience to total extirpation via full hybridization. We investigate patterns of offspring survival at egg stage in natural environments, based on parental genotypes in interaction with river temperature, to detect potential GxE interactions. Our results show a strong influence of maternal GxE interaction on embryonic survival, mediated by maternal effect through egg size, and a weak influence of paternal GxE interaction. In particular, when egg size is large and temperature is cold, the survival rate of offspring originating from MED females is three times higher than that of ATL females' offspring. Because river temperatures show contrast at small scale, this cold adaptation for MED females' offspring constitutes a potent postzygotic mechanism to explain small-scale spatial heterogeneity in diversity observed in MED areas where ATL fish have been stocked. It also indicates that management efforts could be specifically targeted at the environments that actively favor native intraspecific diversity through eco-evolutionary processes such as postzygotic selection.

Human influence on brown trout juvenile body size during metapopulation expansion

Change in body size can be driven by social (density) and non-social (environmental and spatial variation) factors. In expanding metapopulations, spatial sorting by means of dispersal on the expansion front can further drive the evolution of body size. However, human intervention can dramatically affect these founder effects. Using long-term monitoring of the colonization of the remote Kerguelen islands by brown trout, a facultative anadromous salmonid, we analyse body size variation in 32 naturally founded and 10 human-introduced populations over 57 years. In naturally founded populations, we find that spatial sorting promotes slow positive changes in body size on the expansion front, then that body size decreases as populations get older and local density increases. This pattern is, however, completely different in human-introduced populations, where body size remains constant or even increases as populations get older. The present findings confirm that changes in body size can be affected by metapopulation expansion, but that human influence, even in very remote environments, can fully alter this process.

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.

Interpopulation Variability in Dietary Traits of Invasive Bleak Alburnus alburnus (Actinopterygii, Cyprinidae) Across the Iberian Peninsula

The bleak Alburnus alburnus is native to most of Europe. This cyprinid fish is a successful invader in the Iberian Peninsula. No studies exist on its foraging strategies on a large scale for this ecoregion. The aim of the present study was to compare dietary traits of invasive bleak among the main Iberian rivers and a ‘reference’ native bleak population from France. Bleak were sampled during May–June 2019 from the Iberian Rivers Ebro, Tagus, Guadiana, Segura and Guadalquivir and the River Saône (France). Diptera larvae and zooplankton were common food categories in the River Saône. Insect nymphs were more important in the River Ebro. The intake of plant material was higher in the River Tagus. Flying insects were more consumed in the River Guadiana. Nektonic insects were important in the River Guadalquivir. Detritus was a frequent food category for all populations, in terms of occurrence and mass. Dietary parameters followed a unimodal response in relation to the latitudinal gradient, with the maximum values for the Tagus and Guadiana populations. Overall, results suggest that this wide interpopulation variability will contribute to the species’ successful establishment throughout Mediterranean Europe, which poses a serious risk to its highly valuable native fish fauna.

Global determinants of prey naiveté to exotic predators

Prey naiveté-the failure of prey to recognize novel predators as threats-is thought to exacerbate the impact that exotic predators exert on prey populations. Prey naiveté varies under the influence of eco-evolutionary mediating factors, such as biogeographic isolation and prey adaptation, although an overall quantification of their influence is lacking. We conducted a global meta-analysis to test the effects of several hypothesized mediating factors on the expression of prey naiveté. Prey were overall naive towards exotic predators in marine and freshwater systems but not in terrestrial systems. Prey naiveté was most pronounced towards exotic predators that did not have native congeneric relatives in the recipient community. Time since introduction was relevant, as prey naiveté declined with the number of generations since introduction; on average, around 200 generations may be required to erode naiveté sufficiently for prey to display antipredator behaviour towards exotic predators. Given that exotic predators are a major cause of extinction, the global predictors and trends of prey naiveté presented here can inform efforts to meet conservation targets.

Cumulative meta-analysis identifies declining but negative impacts of invasive species on richness after 20 yr

A principal impact of invasive species is that they reduce local species richness. However, it is unknown whether the magnitude of the richness decrease has been consistent over the past two decades of published research. We used cumulative meta-analysis to synthesize evidence from 240 articles evaluating whether this cumulative evidence base generally supports, or refutes, the association between invasive species presence and richness declines. First, we determined whether evidence accumulation lowered the mean effect size of invasive species on local native richness through time; termed the "decline effect." Then, as mean effect sizes changed over time, we identified when accumulated evidence reached sufficiency, indicating that the mean effect direction (positive or negative) was unlikely to be reversed by unpublished research. We also assessed whether the mean effect size reached a threshold of stability over publication years. To date, no research has tested mechanisms of the decline effect, and here we determine whether publication bias, sample size, time since invasion, or invader trophic position are driving a decline effect in the published evidence base. We found a clear decline in the cumulative mean effect of invasive species on local native species richness as published evidence accumulated between 1999 and 2016. Despite this decline, an average negative association was stable and sufficiently robust to unpublished studies by 2007, showing a 21% mean richness decrease by 2016. Contrary to our expectation, the decline effect manifested consistently regardless of invasive species trophic position, time since invasion, or journal rank. Within taxonomic subgroups, trees, insects, and herbaceous plants exhibit a decline effect, yet still show sufficient and stable negative impacts on richness. However, many other taxonomic subgroups (e.g., crustaceans, fish, mammals) lack evidence for average negative impacts on richness, or have not met sufficiency or stability thresholds.

Contrasting ecological impacts of geographically close invasive populations

Intraspecific trait variability is now well recognized as a key component of biodiversity explaining how individuals within a species can differentially interact with their environment. To date, however, this concept has rarely been incorporated in the study of biological invasions, despite its provision of new insights into invasive species management. Here, we used an experimental approach to investigate how invasive red swamp crayfish (Procambarus clarkii) derived from geographically close ecosystems can differentially impact prey community structure and relevant ecosystem processes. We also compared the magnitude of the effects induced by invasive species introduction with those induced by intraspecific variability. Our results showed that effects of intraspecific variability can be strong for direct interactions such as resource (e.g., leaf litter, snails) consumption and of similar magnitude to the effects induced by the introduction of the invasive species when considering indirect interactions (e.g., primary production, ecosystem metabolism). Overall, these results highlighted that invasive populations of the same species are not ecologically equivalent, with each population acting differently on their recipient ecosystem.

Accounting for intraspecific diversity when examining relationships between non-native species and functional diversity

Quantifying changes in functional diversity, the facet of biodiversity accounting for the biological features of organisms, has been advocated as one of the most integrative ways to unravel how communities are affected by human-induced perturbations. The present study assessed how functional diversity patterns varied among communities that differed in the degree to which non-native species dominated the community in temperate lake fish communities and whether accounting for intraspecific functional variability could provide a better understanding of the variation of functional diversity across communities. Four functional diversity indices were computed for 18 temperate lake fish communities along a gradient of non-native fish dominance using morphological functional traits assessed for each life-stage within each species. First, we showed that intraspecific variability in functional traits was high and comparable to interspecific variability. Second, we found that non-native fish were functionally distinct from native fish. Finally, we demonstrated that there was a significant relationship between functional diversity and the degree to which non-native fish currently dominated the community and that this association could be better detected when accounting for intraspecific functional variability. These findings highlighted the importance of incorporating intraspecific variability to better quantify the variation of functional diversity patterns in communities facing human-induced perturbations.