Climate matching drives spread rate but not establishment success in recent unintentional bird introductions

Alien range size, habitat breadth, origin location, and domestication of alien species matter to their impact risks

Invasive alien species are a major driver of biodiversity loss. Currently, the process of biological invasions is experiencing a constant acceleration, foreshadowing a future increase in the threat posed by invasive alien species to global biodiversity. Therefore, it is necessary to assess the impact risks of invasive alien species and related factors. Here, we constructed a dataset of negative environmental impact events to evaluate the impact risks of alien species. We collected information on 1071 established alien terrestrial vertebrates and then gathered negative environmental impacts for 108 of those species. Generalized linear mixed-effects model and phylogenetic generalized least-squares regression model were used to examine the characteristic (including life-history traits, characteristics related to distribution, and introduction event characteristics) correlates of species' impact risks at the global scale for 108 established alien terrestrial vertebrates (mammals, birds, reptiles and amphibians). Our results showed that a total of 3158 negative environmental impacts were reported for 108 harmful species across 71 countries worldwide. Factors associated with impact risks varied slightly among taxa, but alien range size, habitat breadth, origin location, and domestication were significantly correlated with impact risks. Our study aims to identify the characteristics of alien species with high-impact risks to facilitate urgent assessment of alien species and to protect the local ecological environment and biodiversity.

Climate match is key to predict range expansion of the world's worst invasive terrestrial vertebrates

Understanding the determinants of the range expansion of invasive alien species is crucial for developing effective prevention and control strategies. Nevertheless, we still lack a global picture of the potential factors influencing the invaded range expansion across taxonomic groups, especially for the world's worst invaders with high ecological and economic impacts. Here, by extensively collecting data on 363 distributional ranges of 19 of world's worst invasive terrestrial vertebrates across 135 invaded administrative jurisdictions, we observed remarkable variations in the range expansion across species and taxonomic groups. After controlling for taxonomic and geographic pseudoreplicates, model averaging analyses based on generalized additive mixed-effect models showed that species in invaded regions having climates more similar to those of their native ranges tended to undergo a larger range expansion. In addition, as proxies of propagule pressure and human-assisted transportation, the number of introduction events and the road network density were also important predictors facilitating the range expansion. Further variance partitioning analyses validated the predominant role of climate match in explaining the range expansion. Our study demonstrated that regions with similar climates to their native ranges could still be prioritized to prevent the spread of invasive species under the sustained global change.

Patterns and drivers of climatic niche dynamics during biological invasions of island-endemic amphibians, reptiles, and birds

Shifts between native and alien climatic niches pose a major challenge for predicting biological invasions. This is particularly true for insular species because geophysical barriers could constrain the realization of their fundamental niches, which may lead to underestimates of their invasion potential. To investigate this idea, we estimated the frequency of shifts between native and alien climatic niches and the magnitude of climatic mismatches using 80,148 alien occurrences of 46 endemic insular amphibian, reptile, and bird species. Then, we assessed the influence of nine potential predictors on climatic mismatches across taxa, based on species' characteristics, native range physical characteristics, and alien range properties. We found that climatic mismatch is common during invasions of endemic insular birds and reptiles: 78.3% and 55.1% of their respective alien records occurred outside of the environmental space of species' native climatic niche. In comparison, climatic mismatch was evident for only 16.2% of the amphibian invasions analyzed. Several predictors significantly explained climatic mismatch, and these varied among taxonomic groups. For amphibians, only native range size was associated with climatic mismatch. For reptiles, the magnitude of climatic mismatch was higher for species with narrow native altitudinal ranges, occurring in topographically complex or less remote islands, as well as for species with larger distances between their native and alien ranges. For birds, climatic mismatch was significantly larger for invasions on continents with higher phylogenetic diversity of the recipient community, and when the invader was more evolutionarily distinct. Our findings highlight that apparently common niche shifts of insular species may jeopardize our ability to forecast their potential invasions using correlative methods based on climatic variables. Also, we show which factors provide additional insights on the actual invasion potential of insular endemic amphibians, reptiles, and birds.

Mechanistic models project bird invasions with accuracy

Invasive species pose a major threat to biodiversity and inflict massive economic costs. Effective management of bio-invasions depends on reliable predictions of areas at risk of invasion, as they allow early invader detection and rapid responses. Yet, considerable uncertainty remains as to how to predict best potential invasive distribution ranges. Using a set of mainly (sub)tropical birds introduced to Europe, we show that the true extent of the geographical area at risk of invasion can accurately be determined by using ecophysiological mechanistic models that quantify species' fundamental thermal niches. Potential invasive ranges are primarily constrained by functional traits related to body allometry and body temperature, metabolic rates, and feather insulation. Given their capacity to identify tolerable climates outside of contemporary realized species niches, mechanistic predictions are well suited for informing effective policy and management aimed at preventing the escalating impacts of invasive species.

Abiotic and Biotic Factors from the Past as Predictors of Alien Bird Richness and Temporal Beta-Diversity

The challenge of predicting the distribution of alien species has long been a focus of invasion ecology. Herein, we assessed biotic and abiotic factors from the 1980s as potential predictors of alien bird species patterns 20 years later in the state of New York. To assess the ability of each factor to predict future alien species patterns, we analysed the influence of biotic (native taxonomic, functional and phylogenetic diversity, and human population density) and abiotic (climate and land use) factors from the 1980s on the observed alien species richness patterns in the 2000s and the temporal change in the composition of the alien communities between the 1980s and the 2000s using both single-predictor and multivariate models. Alien species richness from the 1980s was a reliable predictor of the alien species richness and temporal beta-diversity patterns in the 2000s. Among abiotic factors, maximum temperature and agricultural land-uses constituted sufficient predictors of future alien species richness and better predictors than the native biotic factors. The performance of single-predictor models was generally weaker in predicting temporal alien beta-diversity; however, past alien species richness and maximum temperature again outperformed the other factors. Predictions and management decisions should focus on warm and agricultural areas, as well as areas with an already high number of established alien species.

Constructing an Ensemble Model and Niche Comparison for the Management Planning of Eucalyptus Longhorned Borer Phoracantha semipunctata under Climate Change

Phoracantha semipunctata is a destructive invasive alien forest pest worldwide. It primarily damages the eucalyptus via adults, affecting almost all parts of the eucalyptus. Its larvae develop in almost all major tissues of the plant. Phoracantha semipunctata spreads both via the migration of adults and global trade in intercontinental translocation. Currently, this pest has spread to six continents worldwide, except Antarctica, resulting in substantial economic losses. Based on global occurrence data and environmental variables, the potential global geographical distribution of P. semipunctata was predicted using an ensemble model. The centroid shift, overlap, unfilling, and expansion scheme were selected to assess niche dynamics during the global invasion process. Our results indicated that the AUC and TSS values of the ensemble model were 0.993 and 0.917, respectively, indicating the high prediction accuracy of the model. The distribution pattern of P. semipunctata is primarily attributed to the temperature seasonality (bio4), mean temperature of the warmest quarter (bio10), and human influence index variables. The potential geographical distribution of P. semipunctata is primarily in western and southwestern Asia, western Europe, western and southern North America, southern South America, southern Africa, and eastern and southern Oceania. The potential geographical distribution of P. semipunctata showed a downward trend in the 2030s and the 2050s. The distribution centroid showed a general tendency to shift southward from the near-current to future climate. Phoracantha semipunctata has largely conserved its niche during the global invasion process. More attention should be paid to the early warning, prevention, and control of P. semipunctata in the countries and regions where it has not yet become invasive.

Potential for invasion of traded birds under climate and land-cover change

Humans have moved species away from their native ranges since the Neolithic, but globalization accelerated the rate at which species are being moved. We fitted more than half million distribution models for 610 traded bird species on the CITES list to examine the separate and joint effects of global climate and land-cover change on their potential end-of-century distributions. We found that climate-induced suitability for modelled invasive species increases with latitude, because traded birds are mainly of tropical origin and much of the temperate region is 'tropicalizing.' Conversely, the tropics are becoming more arid, thus limiting the potential from cross-continental invasion by tropical species. This trend is compounded by forest loss around the tropics since most traded birds are forest dwellers. In contrast, net gains in forest area across the temperate region could compound climate change effects and increase the potential for colonization of low-latitude birds. Climate change has always led to regional redistributions of species, but the combination of human transportation, climate, and land-cover changes will likely accelerate the redistribution of species globally, increasing chances of alien species successfully invading non-native lands. Such process of biodiversity homogenization can lead to emergence of non-analogue communities with unknown environmental and socioeconomic consequences.

Annual Censuses and Citizen Science Data Show Rapid Population Increases and Range Expansion of Invasive Rose-Ringed and Monk Parakeets in Seville, Spain

Population changes of invasive species can go unnoticed long before population explosions, so long-term monitoring programs are needed to assess changes in population size. Although invasive populations of rose-ringed (Psittacula krameri) and monk parakeets (Myiopsitta monachus) are present worldwide, their current status and dynamics are mostly poorly known. Here, we provide a long-term population monitoring of both parakeet species established in a Mediterranean urban area. Between 2013 and 2021, we conducted systematic population censuses in the city of Seville and collected their occurrence and spatial distribution data from citizen science platforms. Our censuses showed a rapid population growth of both species: rose-ringed parakeets increased from 1200 to 6300 individuals, while monk parakeets increased from 70 to 1487 individuals. These population trends were weakly reflected by the number of parakeet observations and the number of cells with parakeet observations but not by the number of individuals recorded in citizen science platforms. Moreover, for the monk parakeet, the number of cells with observations was related to the spatial spread of its nests across the study area. Although resource-intensive, long-term monitoring programs are essential to assess population changes and develop effective management actions for invasive species. Thus, contrasting this information with data taken through citizen science platforms can validate the utility of the latter for assessing population status of invasive species.

Drivers of alien species composition in bird markets across the world

The global pet trade is a major pathway for the introduction of invasive alien species. The composition of species selected for transport is driven by market demands, which may be influenced by a combination of both historical and cultural factors. We compared Eastern (Taiwan) and Western (Australia and the Iberian Peninsula) bird markets to explore factors associated with the species composition and geographic origin of the birds for sale. We used a bespoke randomization test to compare species composition, geographic origins, and species overlap at different taxonomic levels among bird markets across countries. Alien species identified in the study accounted for more than 10% of the world's bird species. Parrots and songbirds were the most common alien bird taxa traded across all markets. In both Iberian and Australian markets, there was a strong bias toward parrots, waxbills, gamebirds, and finches. In Taiwan, species traded more than expected were parrots, waxbills, starlings, and leafbirds. Neotropical species were the most traded group in the three markets. Afrotropical species were also traded more than expected in Iberian and Australian markets, while the Taiwanese traded more alien species from neighboring Asian regions. The bird trade focuses on the same few bird groups worldwide. The composition and origin of species preferred in the Western markets may be influenced by colonial histories, cultural similarity, and strict regulations on wildlife importation, while species preferences in Eastern markets are strongly influenced by regional culture and proximity. Propagule pressure is a dominant factor influencing the success of biological invasions; it is important to recognize differences in the composition of bird markets among regions because they can translate into different invasion risks, among other factors.

Nesting innovations allow population growth in an invasive population of rose-ringed parakeets

Certain traits of recipient environments, such as the availability of limiting resources, strongly determine the establishment success and spread of non-native species. These limitations may be overcome through behavioral plasticity, allowing them to exploit alternative resources. Here, we show how a secondary cavity nester bird, the rose-ringed parakeet Psittacula krameri, innovates its nesting behavior as a response to the shortage of tree cavities for nesting in its invasive range in Tenerife (Canary Islands). We observed that some breeding pairs excavated their own nest cavities in palms, thus becoming primary cavity nester, whereas others occupied nests built with wood sticks by another invasive species, the monk parakeet Myiopsitta monachus. The use of these novel nesting strategies increased the number of breeding pairs by up to 52% over 6 years, contributing to a 128.8% increase of the whole population. Innovative nests were located at greater heights above ground and were more aggregated around conspecifics but did not result in greater breeding success than natural cavities. Occupation of monk parakeet colonies by rose-ringed parakeets also benefited the former species through a protective-nesting association against nest predators. Our results show how an invasive species innovate nesting behaviors and increase nest-site availability in the recipient environment, thus facilitating its population growth and invasion process. Potential behavioral innovations in other invasive rose-ringed parakeet populations may be overlooked, and should be considered for effective management plans.

Local climate adaptation and gene flow in the native range of two co-occurring fruit moths with contrasting invasiveness

Invasive species pose increasing threats to global biodiversity and ecosystems. While previous studies have characterized successful invaders based on ecological traits, characteristics related to evolutionary processes have rarely been investigated. Here we compared gene flow and local adaptation using demographic analyses and outlier tests in two co-occurring moth pests across their common native range of China, one of which (the peach fruit moth, Carposina sasakii) has maintained its native distribution, while the other (the oriental fruit moth, Grapholita molesta) has expanded its range globally during the past century. We found that both species showed a pattern of genetic differentiation and an evolutionary history consistent with a common southwestern origin and northward expansion in their native range. However, for the noninvasive species, genetic differentiation was closely aligned with the environment, and there was a relatively low level of gene flow, whereas in the invasive species, genetic differentiation was associated with geography. Genome scans indicated stronger patterns of climate-associated loci in the noninvasive species. While strong local adaptation and reduced gene flow across its native range may have decreased the invasiveness of C. sasakii, this requires further validation with additional comparisons of invasive and noninvasive species across their native range.

Distance to native climatic niche margins explains establishment success of alien mammals

One key hypothesis explaining the fate of exotic species introductions posits that the establishment of a self-sustaining population in the invaded range can only succeed within conditions matching the native climatic niche. Yet, this hypothesis remains untested for individual release events. Using a dataset of 979 introductions of 173 mammal species worldwide, we show that climate-matching to the realized native climatic niche, measured by a new Niche Margin Index (NMI), is a stronger predictor of establishment success than most previously tested life-history attributes and historical factors. Contrary to traditional climatic suitability metrics derived from species distribution models, NMI is based on niche margins and provides a measure of how distant a site is inside or, importantly, outside the niche. Besides many applications in research in ecology and evolution, NMI as a measure of native climatic niche-matching in risk assessments could improve efforts to prevent invasions and avoid costly eradications.

Selection on individuals of introduced species starts before the actual introduction

Biological invasion is a global problem with large negative impacts on ecosystems and human societies. When a species is introduced, individuals will first have to pass through the invasion stages of uptake and transport, before actual introduction in a non-native range. Selection is predicted to act during these earliest stages of biological invasion, potentially influencing the invasiveness and/or impact of introduced populations. Despite this potential impact of pre-introduction selection, empirical tests are virtually lacking. To test the hypothesis of pre-introduction selection, we followed the fate of individuals during capture, initial acclimation, and captivity in two bird species with several invasive populations originating from the international trade in wild-caught pets (the weavers Ploceus melanocephalus and Euplectes afer). We confirm that pre-introduction selection acts on a wide range of physiological, morphological, behavioral, and demographic traits (incl. sex, age, size of body/brain/bill, bill shape, body mass, corticosterone levels, and escape behavior); these are all traits which likely affect invasion success. Our study thus comprehensively demonstrates the existence of hitherto ignored selection acting before the actual introduction into non-native ranges. This could ultimately change the composition and functioning of introduced populations, and therefore warrants greater attention. More knowledge on pre-introduction selection also might provide novel targets for the management of invasive species, if pre-introduction filters can be adjusted to change the quality and/or quantity of individuals passing through such that invasion probability and/or impacts are reduced.

Predicting the Future Distribution of Ara rubrogenys, an Endemic Endangered Bird Species of the Andes, Taking into Account Trophic Interactions

Species distribution models (SDMs) are commonly used with climate only to predict animal distribution changes. This approach however neglects the evolution of other components of the niche, like food resource availability. SDMs are also commonly used with plants. This also suffers limitations, notably an inability to capture the fertilizing effect of the rising CO2 concentration strengthening resilience to water stress. Alternatively, process-based dynamic vegetation models (DVMs) respond to CO2 concentration. To test the impact of the plant modelling method to model plant resources of animals, we studied the distribution of a Bolivian macaw, assuming that, under future climate, DVMs produce more conservative results than SDMs. We modelled the bird with an SDM driven by climate. For the plant, we used SDMs or a DVM. Under future climates, the macaw SDM showed increased probabilities of presence over the area of distribution and connected range extensions. For plants, SDMs did not forecast overall response. By contrast, the DVM produced increases of productivity, occupancy and diversity, also towards higher altitudes. The results offered positive perspectives for the macaw, more optimistic with the DVM than with the SDMs, than initially assumed. Nevertheless, major common threats remain, challenging the short-term survival of the macaw.

Environmental resistance predicts the spread of alien species

The unabating rise in the number of species introduced outside of their native range makes predicting the spread of alien species an urgent challenge. Most predictions use models of the ecological niche of a species to identify suitable areas for invasion, but these predictions may have limited accuracy. Here, using the global alien avifauna, we demonstrate an alternative approach for predicting alien spread based on the environmental resistance of the landscape. This approach does not require any information on the ecological niche of the invading species and, instead, uses gradients of biotic similarity among native communities in the invaded region to predict the most likely routes of spread. We show that environmental resistance predicts patterns of spread better than a null model of random dispersal or a model based on climate matching to the native range of each species. Applying this approach to simulate future spread reveals major regional differences in projected invasion risk, shaped by proximity to existing invasion hotspots as well as gradients in environmental resistance. Our results show how environmental resistance may provide a general and complementary approach for predicting invasion risk that can be rapidly deployed even when information on the niche or the identity of potential invaders is unknown.

Distribution of alien tetrapods in the Iberian Peninsula

We present a dataset that assembles occurrence records of alien tetrapods (amphibians, reptiles, birds and mammals) in the Iberian Peninsula, a coherent biogeographically unit where introductions of alien species have occurred for millennia. These data have important potential applications for ecological research and management, including the assessment of invasion risks, formulation of preventive and management plans, and research at the biological community level on alien species. This dataset summarizes inventories and data sources on the taxonomy and distribution of alien tetrapods in the Iberia Peninsula, comprising known locations from published literature, expert knowledge and citizen science platforms. An expert-based assessment process allowed the identification of unreliable records (misclassification or natural dispersion from native range), and the classification of species according to their status of reproduction in the wild. Distributional data was harmonized into a common area unit, the 10 × 10 km Universal Transverse Mercator (UTM) system (n = 6,152 cells). The year of observation and/or year of publication were also assigned to the records. In total, we assembled 35,940 unique distribution records (UTM × species × Year) for 253 species (6 amphibians, 16 reptiles, 218 birds and 13 mammals), spanning between 1912 and 2020. The species with highest number of distribution records were the Mediterranean painted frog Discoglossus pictus (n = 59 UTM), the pond slider Trachemys scripta (n = 471), the common waxbill Estrilda astrild (n = 1,275) and the house mouse Mus musculus (n = 4,043), for amphibians, reptiles, birds and mammals, respectively. Most alien species recorded are native to Africa (33%), followed by South America (21%), Asia (19%), North America (12%) and Oceania (10%). Thirty-six species are classified by IUCN as threatened in their native range, namely 2 Critically Endangered (CR), 6 Endangered (EN), 8 Vulnerable (VU), and 20 species Near Threatened (NT). Species maps are provided in DataSet1, as well R code and GIS layers to update them as new records are obtained.

Behavioral responses to numerical differences when two invasive ants meet: the case of Lasius neglectus and Linepithema humile

Two of the world’s most invasive ants, Linepithema humile and Lasius neglectus, are destined to overlap in range as they continue to spread throughout Europe. Although L. humile arrived first, and is therefore more numerically abundant, L. neglectus is the more behaviorally dominant of the two. We performed lab trials to determine whether L. humile could use numerical abundance to overcome the behavioral dominance of L. neglectus and whether the ants’ behavioral patterns shifted when the species co-occurred. We found that L. neglectus was more aggressive when less abundant, whereas the opposite was true of L. humile. When L. neglectus was outnumbered, it employed aggressive behaviors, such as biting or chemical attacks, more frequently than L. humile; it also utilized a behavioral sequence that included mandible opening and biting. Our results for these species support the hypothesis that species modulate their behavior towards competitors, which facilitates the understanding of how multiple invasive ant species can co-occur in a given area. Moreover, our study shows that the co-occurrence of invasive species could result from the use of two strategies: (1) the Bourgeois strategy, in which aggressiveness changes based on numerical dominance and (2) the dear-enemy strategy, in which aggressiveness is reduced when competitors co-occur. Since these strategies may lead to territory partitioning, we suggest that the behavioral flexibility displayed by both species when they overlap may allow local co-occurrence and increase their likelihood of co-occurrence during their range expansion in Europe, which could have a negative cumulative impact on invaded areas.

Opportunistic or Non-Random Wildlife Crime? Attractiveness Rather Than Abundance in the Wild Leads to Selective Parrot Poaching

Illegal wildlife trade, which mostly focuses on high-demand species, constitutes a major threat to biodiversity. However, whether poaching is an opportunistic crime within high-demand taxa such as parrots (i.e., harvesting proportional to species availability in the wild), or is selectively focused on particular, more desirable species, is still under debate. Answering this question has important conservation implications because selective poaching can lead to the extinction of some species through overharvesting. However, the challenges of estimating species abundances in the wild have hampered studies on this subject. We conducted a large-scale survey in Colombia to simultaneously estimate the relative abundance of wild parrots through roadside surveys (recording 10,811 individuals from 25 species across 2221 km surveyed) and as household, illegally trapped pets in 282 sampled villages (1179 individuals from 21 species). We used for the first time a selectivity index to test selection on poaching. Results demonstrated that poaching is not opportunistic, but positively selects species based on their attractiveness, defined as a function of species size, coloration, and ability to talk, which is also reflected in their local prices. Our methodological approach, which shows how selection increases the conservation impacts of poaching for parrots, can be applied to other taxa also impacted by harvesting for trade or other purposes.

High Prevalence of Novel Beak and Feather Disease Virus in Sympatric Invasive Parakeets Introduced to Spain From Asia and South America

The psittacine beak and feather disease (PBFD) is a globally widespread infectious bird disease that mainly affects species within the Order Psittaciformes (parrots and allies). The disease is caused by an avian circovirus (the beak and feather disease virus, BFDV), which is highly infectious and can lead to severe consequences in wild and captive populations during an outbreak. Both legal and illegal trading have spread the BFDV around the world, although little is known about its prevalence in invasive parrot populations. Here, we analyze the BFDV prevalence in sympatric invasive populations of rose-ringed (Psittacula krameri) and monk parakeets (Myiopsitta monachus) in Southern Spain. We PCR-screened 110 blood samples (55 individuals from each species) for BFDV and characterized the genotypes of five positives from each species. About 33% of rose-ringed parakeets and 37% of monk parakeets sampled were positive for BFDV, while neither species showed disease symptoms. The circovirus identified is a novel BFDV genotype common to both species, similar to the BFDV genotypes detected in several parrot species kept in captivity in Saudi Arabia, South Africa and China. Our data evidences the importance of an accurate evaluation of avian diseases in wild populations, since invasive parrots may be bringing BFDV without showing any visually detectable clinical sign. Further research on the BFDV prevalence and transmission (individual–individual, captive–wild and wild–captive) in different bird orders and countries is crucial to understand the dynamics of the viral infection and minimize its impact in captive and wild populations.

A protective nesting association with native species counteracts biotic resistance for the spread of an invasive parakeet from urban into rural habitats

Background

Non-native species are often introduced in cities, where they take advantage of microclimatic conditions, resources provided by humans, and competitor/predator release to establish and proliferate. However, native communities in the surrounding rural or natural areas usually halt their spread through biotic resistance, mainly via top-down regulative processes (predation pressure). Here, we show an unusual commensal interaction between exotic and native bird species that favours the spread of the former from urban to rural habitats.

Results

We show how Monk parakeets Myiopsitta monachus, an invasive species often introduced in cities worldwide, associated for breeding with a much larger, native species (the white stork Ciconia ciconia) to reduce predation risk in central Spain, thus allowing their colonization of rural areas. Parakeets selected stork nests close to conspecifics and where breeding raptors were less abundant. Parakeets always flushed when raptors approached their nests when breeding alone, but stayed at their nests when breeding in association with storks. Moreover, when storks abandoned a nest, parakeets abandoned it in the following year, suggesting that storks actually confer protection against predators.

Conclusions

Our results show how a protective-nesting association between invasive and native species can counteract biotic resistance to allow the spread of an invasive species across non-urban habitats, where they may become crop pests. Monk parakeet populations are now growing exponentially in several cities in several Mediterranean countries, where they coexist with white storks. Therefore, management plans should consider this risk of spread into rural areas and favour native predators as potential biological controllers.

New aliens in Australia: 18 years of vertebrate interceptions

Abstract ContextAustralia has a high diversity of endemic vertebrate fauna. Yet, transnational human activities continue to increase the rate of transportation, introduction and establishment of new alien vertebrates in Australia, to the detriment of environmental and socioeconomic services. Eradication of invasive vertebrates is often costly and without guarantee of success; therefore, methods for detecting, intercepting and preventing the transport of alien species earlier in the invasion pathway provide substantial benefit. AimTo anticipate emergent threats to Australian biosecurity posed by the transport and introduction of new alien vertebrates over time. MethodsWe collated vertebrate interception data from various mainland Australian State, Territory and Commonwealth government reporting agencies, including data from a previously published study, at both pre-border and post-border stages from 1999 to 2016. Using generalised linear and generalised additive modelling, we predicted trends in interception frequency using predictors such as vertebrate taxa, detection category and alien status. Key resultsInterception frequency increased over time for all vertebrate classes, for pre-border stowaways and for post-border captive and at-large interceptions, with no saturation in the accumulation of new species over time. Five species were responsible for almost half of all incidents, of which red-eared sliders (Trachemys scripta elegans), boa constrictors (Boa constrictor) and corn snakes (Pantherophis guttatus) are prominent in Australia’s illegal alien pet trade. Rose-ringed parakeets (Psittacula krameri) are prominent in the legal alien cage-bird trade, which remains poorly regulated. Asian common toads (Duttaphrynus melanostictus) were frequently detected as stowaways, and most stowaway incidents originated from Southeast Asia, particularly Indonesia, via shipping. Data deficiency for pre-border incidents increased rapidly in 2015 and 2016. ConclusionsAustralia is subject to a persistent and increasing risk of alien vertebrate introductions and incursions over time, owing partly to emergent trends in the alien pet trade as well as increased global trade and tourism. ImplicationsThe future of Australia’s biosecurity remains dependent on stringent border security to prevent the arrival of novel species, but our findings also highlight the importance of ongoing management and control of high-risk species already present, often illegally, within Australia.

Cities may save some threatened species but not their ecological functions

Background

Urbanization is one of the main causes of biodiversity loss worldwide. Wildlife responses to urbanization, however, are greatly variable and, paradoxically, some threatened species may achieve much larger populations in urban than in natural habitats. Urban conservation hotspots may therefore help some species avoid regional or even global extinctions, but not conserve their often overlooked ecological functions in the wild. We aim to draw attention to this issue using two species of globally threatened parrots occurring in the Dominican Republic: the Hispaniolan amazon (Amazona ventralis) and the Hispaniolan parakeet (Psittacara chloropterus).

Methods

We conducted a large-scale roadside survey in June 2017 across the country to estimate the relative abundance of parrots in natural habitats, rural habitats, and cities. We combined this with informal interviews with local people to collect information on past and current human impacts on parrot populations. We also looked for foraging parrots to assess their potential role as seed dispersers, an ecological function that has been overlooked until very recently.

Results

Relative abundances of both parrot species were negligible in rural areas and very low in natural habitats. They were generally between one and two orders of magnitude lower than that of congeneric species inhabiting other Neotropical ecosystems. Relative abundances were six times higher in cities than in natural habitats in the case of the Hispaniolan parakeet and three times higher in the case of the Hispaniolan amazon. People indicated hunting for a source food and to mitigate crop damage as causes of parrot population declines, and a vigorous illegal trade for parrots (131 individuals recorded, 75% of them poached very recently), mostly obtained from protected areas where the last small wild populations remain. We observed parrots foraging on 19 plant species from 11 families, dispersing the fruits of 14 species by carrying them in their beaks and consuming them in distant perching trees. They discarded undamaged mature seeds, with the potential to germinate, in 99.5% of cases (n = 306), and minimum dispersal distances ranged from 8 to 155 m (median = 37 m).

Discussion

The loss of ecological functions provided by some species when they disappear from natural habitats and only persist in cities may have long-term, unexpected effects on ecosystems. Our example demonstrates how two cities may soon be the last refuges for two endemic parrots if overharvesting continues, in which case their overlooked role as seed dispersers would be completely lost in nature. The functional extinction of these species could strongly affect vegetation communities in an island environment where seed-dispersal species are naturally scarce. While conservation plans must include urban populations of threatened species, greater efforts are needed to restore their populations in natural habitats to conserve ecological functions.

Nest-site competition and killing by invasive parakeets cause the decline of a threatened bat population

The identification of effects of invasive species is challenging owing to their multifaceted impacts on native biota. Negative impacts are most often reflected in individual fitness rather than in population dynamics of native species and are less expected in low-biodiversity habitats, such as urban environments. We report the long-term effects of invasive rose-ringed parakeets on the largest known population of a threatened bat species, the greater noctule, located in an urban park. Both species share preferences for the same tree cavities for breeding. While the number of parakeet nests increased by a factor of 20 in 14 years, the number of trees occupied by noctules declined by 81%. Parakeets occupied most cavities previously used by noctules, and spatial analyses showed that noctules tried to avoid cavities close to parakeets. Parakeets were highly aggressive towards noctules, trying to occupy their cavities, often resulting in noctule death. This led to a dramatic population decline, but also an unusual aggregation of the occupied trees, probably disrupting the complex social behaviour of this bat species. These results indicate a strong impact through site displacement and killing of competitors, and highlight the need for long-term research to identify unexpected impacts that would otherwise be overlooked.

Selection on a behaviour-related gene during the first stages of the biological invasion pathway

Human-induced biological invasions are common worldwide and often have negative impacts on wildlife and human societies. Several studies have shown evidence for selection on invaders after introduction to the new range. However, selective processes already acting prior to introduction have been largely neglected. Here, we tested whether such early selection acts on known behaviour-related gene variants in the yellow-crowned bishop (Euplectes afer), a pet-traded African songbird. We tested for nonrandom allele frequency changes after trapping, acclimation and survival in captivity. We also compared the native source population with two independent invasive populations. Allele frequencies of two SNPs in the dopamine receptor D4 (DRD4) gene-known to be linked to behavioural activity in response to novelty in this species-significantly changed over all early invasion stages. They also differed between the African native population and the two invading European populations. The two-locus genotype associated with reduced activity declined consistently, but strongest at the trapping stage. Overall genetic diversity did not substantially decrease, and there is little evidence for new alleles in the introduced populations, indicating that selection at the DRD4 gene predominantly worked on the standing genetic variation already present in the native population. Our study demonstrates selection on a behaviour-related gene during the first stages of a biological invasion. Thus, pre-establishment stages of a biological invasion do not only determine the number of propagules that are introduced (their quantity), but also their phenotypic and genetic characteristics (their quality).