Invasion Ecology Research Articles

An alien fisher in Patagonian South America

While doing fieldwork in the Cape Horn Biosphere Reserve at the southernmost tip of South America (Patagonia), we saw an American mink (Neovison vison) diving into a kelp forest and later emerging with an enormous golden kingklip (Genypterus blacodes), which it ate on the rocky shore. Mink were introduced to Chile and Argentina by fur farmers in the 1940s. Since then, this invasive, semiaquatic mammal has spread through Patagonia, feeding on mammals, birds, and fishes. Of the fishes, mink prey on small exotic salmonids, galaxiids, and notothenioids. Astonishingly, this mink had caught a demersal fish that was larger than itself. The kingklip, a marine predator that is important for small-scale fisheries, inhabits depths from the intertidal zone to 1000 m, but is most abundant between 50 m and 500 m. So, this was likely an unusual prey for a mink, raising new questions about mink's feeding habits. Are we detecting all native species that are affected by invasive mink? Despite using scat or stable isotope analyses to identify the prey items of invasive species, direct observations can also contribute essential data. The kingklip has very small, smooth-edged scales, which can make it undetectable in scat. How did the mink catch a marine demersal fish? The boulder-covered seafloor is a perfect habitat for a kingklip, but mink must dive to catch this prey. Most diving mink studies are done in freshwater environments where dives typically do not exceed 3 m depth. Our observation also provides questions about invasive mink ecology: has their diving behavior been learned in marine environments, and are mink expanding beyond their normal range in this novel environment? What are the likely consequences for the marine food web in Patagonia?

Robinia pseudoacacia-dominated vegetation types of Southern Europe: Species composition, history, distribution and management

Knowledge of the species composition of invaded vegetation helps to evaluate an ecological impact of aliens and design an optimal management strategy. We link a new vegetation analysis of a large dataset to the invasion history, ecology and management of Robinia pseudoacacia stands across Southern Europe and provide a map illustrating Robinia distribution. Finally, we compare detected relationships with Central Europe.We show that regional differences in Robinia invasion, distribution, habitats and management are driven both by local natural conditions (climate and soil properties, low competitive ability with native trees) and socioeconomic factors (traditional land-use). Based on the classification of 467 phytosociological relevés we distinguished five broad vegetation types reflecting an oceanity−continentality gradient. The stands were heterogeneous and included 824 taxa, with only 5.8% occurring in more than 10% of samples, representing mainly hemerobic generalists of mesophilous, nutrient-rich and semi-shady habitats. The most common were dry ruderal stands invading human-made habitats. Among native communities, disturbed mesic and alluvial forests were often invaded throughout the area, while dry forests and scrub dominated in Balkan countries. Continuous, long-term and large-scale cultivation represent a crucial factor driving Robinia invasions in natural habitats. Its invasion should be mitigated by suitable management taking into account adjacent habitats and changing cultivation practices to select for native species. Robinia invasion has a comparable pattern in Central and Southern Europe, but there is a substantial difference in management and utilization causing heterogeneity of many South-European stands.

Global assessment of ecological risks associated with farmed fish escapes

Aquaculture is the world's fastest growing food-producing sector and currently the main source of fish supply. However, environmental sustainability is one of the main challenges faced by the industry, in particular the inevitable occurrence of fish escapes, which are considered a major threat to marine ecosystems. Here we evaluated the risks associated with the impacts of introducing non-native species, the genetic introgression of farmed fish into wild stocks and the spread of pathogens and parasites through escapes of farmed fish at a global scale. Our analysis indicated that a nearly a third of marine ecoregions of the world are to some extent at risk from the impacts of fish escapes. We estimated that 26.5% of global production comprises non-native species, equating 1.74 million tonnes per year, with the Magellanic province in Southern Chile being a hotspot for ecological invasion impacts, owing to a large production of non-native salmonids. Genetic risk hotspots were also identified in East China and Yellow Sea, which support the world's largest and more diverse production of native fish. The combination of high pathogen diversity and production levels recorded for East China and the Mediterranean Sea resulted in the highest pathogenic risk predicted for these provinces. When considering the combined risk of these three stressors the warm temperate Northwest Pacific ranked highest in terms of overall risks. We highlight the need of preventive and mitigation measures to reduce fish escapes, particularly in sensitive ecoregions, considering risk assessment for farming non-native species and the critical role of policy makers in implementing these measures to allow the sustainable development of aquaculture.

Interactions between Invasive Monk Parakeets (Myiopsitta monachus) and Other Bird Species during Nesting Seasons in Santiago, Chile.

Simple SummaryThe monk parakeet (Myiopsitta monachus) is an invasive species, unique in the parrot family for its ability to build large nest structures. This species became globally distributed, promoted in part due to the pet trade market, and now is considered a pest because of the economic losses they produce. During the reproductive seasons of 2017 and 2018, we registered interactions between invasive monk parakeets and resident bird species in Santiago, Chile. We observed positive and negative interactions, and herein, we describe parakeets’ nest occupancy by nine bird species, two invasive and seven native. For this reason, the monk parakeet should be considered an ecosystem engineer, a species that is creating available breeding space of potential use for other species. Our results contribute to an assessment of implications of this ecological invasion of local urban wildlife, and raise concern on other impacts, such as disease transmission, as a consequence of these interactions.The monk parakeet (Myiopsitta monachus) is considered to be one of the most invasive bird species because its unique ability among parrots to build their own communal nests. Currently, they are considered an invasive species in 19 countries and a pest—even in their native distribution—because of economic losses derived from their impacts. During the reproductive seasons of 2017 and 2018, we registered interactions between invasive monk parakeets and resident bird species in Santiago, Chile. We observed agonistic and affiliative interactions, and further, we described monk parakeets’ nest occupancy by nine bird species, two invasive and seven native. For this reason, we consider that the monk parakeet is an allogenic ecosystem engineer with the potential to shape distribution and richness of sympatric species in urban environments. Our results contribute to an assessment of the implications of the monk parakeet’s ecological invasion to other synanthropic species, and raise concern of other potential impacts, such as pathogen transmission derived from these interactions.

Open Access journals need to become first choice, in invasion ecology and beyond

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CircumMed Pine Forest Database: an electronic archive for Mediterranean and Submediterranean pine forest vegetation data

Large thematic databases of vegetation plots are increasingly needed for vegetation studies and biodiversity research. In this paper, we present the CircumMed Pine Forest Database (GIVD ID: EU-00-026), which in November 2018, comprised records from 6317 pine-dominated vegetation plots (releves) and associated vegetation types from 20 countries of the Mediterranean Basin, Near East and Crimea. These vegetation plots were collected through a detailed literature search for plots not previously included in the European Vegetation Archive (EVA), in order to fill geographic gaps in data coverage. The database comprises Mediterranean (including Oromediterranean) pine-forest vegetation plots from both published and unpublished sources. All vegetation plots are georeferenced, although coordinates vary in accuracy. The database is accessible through EVA or from its Custodian. We anticipate the CircumMed Pine Forest Database will be a valuable resource for various types of broad-scale studies in the fields of vegetation classification, plant invasion ecology, macroecology and biological conservation.

Ecología de invasiones marinas en Chile continental: ¿Qué sabemos y que nos falta por saber?

La costa chilena presenta menos reportes de especies introducidas respecto a otras costas a nivel mundial, lo que podría corresponder a un fenómeno natural generado por condiciones oceanográficas, o bien a un artefacto producido por falta de información disponible. Mediante el análisis de 71 trabajos indexados en Web of Science, relacionados con ecología de invasiones marinas en la costa chilena, publicados entre 1998-2014, determinamos el estado actual de esta disciplina en Chile, e identificamos las tendencias de las investigaciones realizadas. La mayoría de las publicaciones se relacionan con especies en etapa de naturalización, y pocas con especies invasoras. El número de publicaciones por año incrementa linealmente, sugiriendo un creciente interés de la comunidad científica respecto al tema. La cantidad de información disponible (i.e., número de publicaciones) no es homogénea entre las regiones administrativas, y la mayoría de los trabajos son realizados sectores puntuales dentro de cada región (usualmente cercanos a la capital regional). Regiones con alto tráfico marítimo (e.g., Valparaíso, Magallanes) han sido muy poco estudiadas. A excepción de la región de Coquimbo y Los Lagos, los trabajos por región apuntan a unos pocos taxa. Muy pocos trabajos abarcan grandes escalas espaciales, aunque varios de ellos realizan experimentos a escalas de tiempo mayores (e.g., mensual, anual). Nuestros resultados permiten sugerir que el número de especies reportadas como introducidas en Chile podría estar subestimado. Observaciones en terreno, actualización periódica de catastros y utilización de herramientas moleculares, son propuestos como medidas para una mayor comprensión del tema a nivel nacional.

Insights from invasion ecology: Can consideration of eco-evolutionary experience promote benefits from root mutualisms in plant production?

Mutualistic plant–microbial functioning relies on co-adapted symbiotic partners as well as conducive environmental conditions. Choosing particular plant genotypes for domestication and subsequent cultivar selection can narrow the gene pools of crop plants to a degree that they are no longer able to benefit from microbial mutualists. Elevated mineral nutrient levels in cultivated soils also reduce the dependence of crops on nutritional support by mutualists such as mycorrhizal fungi and rhizobia. Thus, current ways of crop production are predestined to compromise the propagation and function of microbial symbionts, limiting their long-term benefits for plant yield stability. The influence of mutualists on non-native plant establishment and spread, i.e. biological invasions, provides an unexplored analogue to contemporary crop production that accounts for mutualistic services from symbionts like rhizobia and mycorrhizae. The historical exposure of organisms to biotic interactions over evolutionary timescales, or so-called eco-evolutionary experience (EEE), has been used to explain the success of such invasions. In this paper, we stress that consideration of the EEE concept can shed light on how to overcome the loss of microbial mutualist functions following crop domestication and breeding. We propose specific experimental approaches to utilize the wild ancestors of crops to determine whether crop domestication compromised the benefits derived from root microbial symbioses or not. This can predict the potential for success of mutualistic symbiosis manipulation in modern crops and the maintenance of effective microbial mutualisms over the long term.

Dendroecology of Pinus elliottii Engelm. reveals waves of invasion in a neotropical savanna

Despite advances in understanding the ecology and consequences of pine invasions, information on the patterns of structure, dynamics, and growth needed to manage these invasive populations are still poorly known. Here we used tree ring analysis to elucidate growth dynamics, age distribution and fire effects, by sampling three populations of slash pine invading Cerrado vegetation in southeastern Brazil. We found that the invasion occurred in waves, every 5–7 years. These pulses are likely related to the time needed by founder trees to reach maturity, and to climatic events. We found distinct growth patterns among the three study areas. Individual trees in the open sites, under low competition for light, showed an increasing trend of diameter and of the basal area growth rates with age. In the dense stands, however, the diameter increment rates gradually decreased with age, as the competition for light was intensified, and the basal area growth rate increased at lower rates. Growth rates were markedly reduced after fire, likely due to fire impact on the canopies, reducing photosynthesis. Fire scars in the wood allowed us to confirm that the formation of the tree rings occurs annually. When competition for light intensifies as the canopy of the stand closes, the invasive populations experience reduced individual growth of adult trees and constraints to the recruitment of young individuals. We did not find, however, evidence of self-thinning due to competition that could reduce the population of adult trees, open gaps and allow colonization by shade-tolerant native species. Therefore, a monodominant pine stand—the “pine desert” feature of the invaded sites—tends to be persistent over time.

Invasion Biology, Ecology, and Management of Western Flower Thrips.

Western flower thrips, Frankliniella occidentalis, first arose as an important invasive pest of many crops during the 1970s-1980s. The tremendous growth in international agricultural trade that developed then fostered the invasiveness of western flower thrips. We examine current knowledge regarding the biology of western flower thrips, with an emphasis on characteristics that contribute to its invasiveness and pest status. Efforts to control this pest and the tospoviruses that it vectors with intensive insecticide applications have been unsuccessful and have created significant problems because of the development of resistance to numerous insecticides and associated outbreaks of secondary pests. We synthesize information on effective integrated management approaches for western flower thrips that have developed through research on its biology, behavior, and ecology. We further highlight emerging topics regarding the species status of western flower thrips, as well as its genetics, biology, and ecology that facilitate its use as a model study organism and will guide development of appropriate management practices.

Environmental gradients influence differences in leaf functional traits between native and non-native plants.

Determining the characteristics of non-native plants that can successfully establish and spread is central to pressing questions in invasion ecology. Evidence suggests that some non-native species establish and spread in new environments because they possess characteristics (functional traits) that allow them to either successfully compete with native residents or fill previously unfilled niches. However, the relative importance of out-competing native species vs. filling empty niche space as potential mechanisms of invasion may depend on environmental characteristics. Here, we measured plant functional traits, proxies indicative of competitive and establishment strategies, to determine if these traits vary among native and invasive species and if their prevalence is dependent on environmental conditions. Using a natural environmental gradient in Hawai'i Volcanoes National Park, we evaluated how functional traits differ between native and non-native plant communities and if these differences change along an environmental gradient from hot, dry to cool, wet conditions. Functional trait differences suggested that both competition and open niche space may be important for invasion. Non-native communities tended to have traits associated with faster growth strategies such as higher specific leaf area and lower leaf thickness. However, native and non-native community traits became more dissimilar along the gradient, suggesting that non-native species may be occupying previously unfilled niche space at the hot, dry end of the gradient. We also found that most of the variation in functional trait values amongst plots was due to species turnover rather than intraspecific variation. These results highlight the role of environmental context when considering invasion mechanisms.

A test of the evolution of increased competitive ability in two invaded regions

Non-native plant species invasions can have significant ecological and economic impacts. Finding patterns that predict and explain the success of non-native species has thus been an important focus in invasion ecology. The evolution of increased competitive ability (EICA) hypothesis has been a frequently used framework to understand invasion success. Evolution of increased competitive ability predicts that (1) non-native populations will escape from coevolved specialist herbivores that were present within the native range and this release from specialist herbivores should result in relaxed selection pressure on specialist-related defense traits, (2) there will be a trade-off between allocation of resources for resistance against specialist herbivores and allocation to traits related to competitive ability, and (3) this shift will allow more allocation to competitive ability traits. We tested the predictions of EICA in the model plant Mimulus guttatus, a native of western North America (WNA). We compared how well the predictions of EICA fit patterns in two non-native regions, the United Kingdom (UK), an older more successful invasion, and eastern North America (ENA), a younger less successful invasion. We completed extensive herbivore surveys and grew plants derived from multiple populations in each region in a common greenhouse environment to test adherence to the predictions of EICA. We found evidence of specialist herbivore escape in the UK, but not the ENA plants. Compared to native plants the UK plants had lower levels of resistance traits, were taller, and produced larger and more flowers, while the ENA plants had mostly equivalent traits to the WNA plants. Plants from the UK conformed to the predictions of EICA more closely than those from ENA. The UK invasion is an older, more successful invasion, suggesting that support for EICA predictions may be highest in more successful invasions.

What's ploidy got to do with it? Understanding the evolutionary ecology of macroalgal invasions necessitates incorporating life cycle complexity.

Biological invasions represent grave threats to terrestrial, aquatic, and marine ecosystems, but our understanding of the role of evolution during invasions remains rudimentary. In marine environments, macroalgae account for a large percentage of invaders, but their complicated life cycles render it difficult to move methodologies and predictions wholesale from species with a single, free‐living ploidy stage, such as plants or animals. In haplodiplontic macroalgae, meiosis and fertilization are spatiotemporally separated by long‐lived, multicellular haploid and diploid stages, and gametes are produced by mitosis, not meiosis. As a consequence, there are unique eco‐evolutionary constraints that are not typically considered in invasions. First, selfing can occur in both monoicious (i.e., hermaphroditic) and dioicious (i.e., separate sexes) haplodiplontic macroalgae. In the former, fertilization between gametes produced by the same haploid thallus results in instantaneous, genome‐wide homozygosity. In the latter, cross‐fertilization between separate male and female haploids that share the same diploid parent is analogous to selfing in plants or animals. Separate sexes, therefore, cannot be used as a proxy for outcrossing. Second, selfing likely facilitates invasions (i.e., Baker's law) and the long‐lived haploid stage may enable purging of deleterious mutations, further contributing to invasion success. Third, asexual reproduction will result in the dominance of one ploidy and/or sex and the loss of the other(s). Whether or not sexual reproduction can be recovered depends on which stage is maintained. Finally, fourth, haplodiplontic life cycles are predicted to be maintained through niche differentiation in the haploid and diploid stages. Empirical tests are rare, but fundamental to our understanding of macroalgal invasion dynamics. By highlighting these four phenomena, we can build a framework with which to empirically and theoretically address important gaps in the literature on marine evolutionary ecology, of which biological invasions can serve as unnatural laboratories.

Induced water stress affects seed germination response and root anatomy in Robinia pseudoacacia (Fabaceae)

The different germination behaviours of the seeds under induced water limitations may be related to the different adaptive capacities acquired at the diverse collection sites, as a response to the different environmental parameters. The island of Pianosa resulted the most performant in term of germination responses and the co-occurrence of xeromorphic anatomical evidences at root level confirmed this trend. Regeneration from seeds is an important co-determinant in the invasion ecology of black locust. In the attempt of providing new information on its invasion potential in Mediterranean Europe under the future scenario of global warming, we investigated the effects of induced water-deficit regimes on: (1) seed germination performance and (2) root growth and anatomy. Ripe seeds were collected from four populations established in Tuscany (Central Italy): mechanically scarified seeds were incubated in a range (− 0.2/− 0.8 MPa) of constant water stress conditions at 21 °C. The final germination rate drastically declined with increasing induced water-deficit conditions, with the highest value at control and at − 0.2 MPa (ca 50–97%), and the lowest at − 0.6 MPa (ca 10–33%). The mean germination time decreased with increasing water stress. At root level, xeromorphism relies on the combination of different anatomical traits which co-optimize water uptake/loss: thinner roots, higher number of xylem vessels, vessels with small-sized lumen and thinner cell walls. Seeds collected in sites characterized by different environmental parameters display a noteworthy difference in the germination dynamics: as far as the beginning and ongoing of germination, as well as the germination response in time is concerned, the seeds from the island of Pianosa showed the highest performance; the major arid conditions in Pianosa could have caused a “stress imprint” able to facilitate a fast and protective response to future drought events. As a whole, our results confirmed the great phenotypic plasticity of black locust as a response to variable water availability and provided evidence for the potential high germination capacity in drier environments, as seems to be the future Mediterranean Europe.

Predicting invasiveness of exotic woody species using a traits-based framework.

Identifying potentially invasive species and preventing their introduction and establishment are of critical importance in invasion ecology and land management. Although an extensive body of research has been dedicated to identifying traits that confer invasiveness, our current knowledge is still often inconclusive due to limitations in geographic extent and/or scope of traits analyzed. Here, using a comprehensive set of 45 traits, we performed a case study of invasive traits displayed by exotic woody plants in the United States (U.S.) by comparing 63 invasive and 794 non-invasive exotic woody plant species naturalized across the country. We found that invasive woody species often bear the following two key traits: vegetative reproduction and long-distance seed dispersal (via water, birds or mammals). Boosted classification tree models based on these traits accurately predicted species invasiveness (86% accuracy on average). Presented findings provide a generalized understanding of the relative importance of functional traits in identifying potentially invasive woody species in the U.S. The knowledge generated in this study can be used to improve current classification systems of non-native woody plants used by various U.S. governmental agencies and land managers.

Scale and complexity implications of making New Zealand predator-free by 2050

ABSTRACT The goal to make New Zealand predator-free by 2050 has drawn strong praise and criticism, but these critiques have focused largely on economic or technological feasibility of long-term large-scale eradication. We suggest that achieving this goal is not a simple ‘scaling-up’ of current eradication efforts, but requires enduring co-ordination and integration of research, management and societal elements if a predator-free goal is to become a reality. Here we ask what are the key impediments to eradicating invasive species on a national scale? We highlight four interlinked issues that must be addressed to accomplish a predator-free New Zealand: (1) improved ecological understanding of interactive effects; (2) refinement and development of operational methods; (3) overcoming social and bioethical challenges; and (4) improving governance and partnerships with Māori. Understanding the linkages among these issues can also provide new insights into the biology and ecology of invasions, development of improved eradication methods, and social support or involvement in large-scale conservation management. Addressing these challenges will ultimately improve policy and management of biological invaders and set new international precedents.

Predicting the invasion of a southern African savannah by the black wattle (Acacia mearnsii)

Understanding the drivers of biological invasions in landscapes is a major goal in invasion ecology. The control of biological invasions has increasingly become critical in the past few decades because invasive species are thought to be a major threat to endemism. In this study, by examining the key variables that influence Acacia mearnsii, we sought to understand its potential invasion in eastern Zimbabwe. We used the maximum entropy (MaxEnt) method against a set of environmental variables to predict the potential invasion front of A. mearnsii. Our study showed that the predictor variables, i.e., aspect, elevation, distance from streams, soil type and distance from the nearest A. mearnsii plantation adequately explained (training AUC = 0.96 and test AUC = 0.93) variability in the spatial distribution of invading A. mearnsii. The front of invasion by A. mearnsii seemed also to occur next to existing A. mearnsii plantations. Results from our study could be useful in identifying priority areas that could be targeted for controlling the spread of A. mearnsii in Zimbabwe and other areas under threat from A. mearnsii invasion. We recommend that the plantation owners pay for the control of A. mearnsii invasion about their plantations.

Environmental factors affecting the invasion success and morphological responses of a globally introduced crayfish in floodplain waterbodies

Floodplain ecosystems that are characterized by high habitat heterogeneity and hydrological connectivity are considered hotspots for freshwater biodiversity. However, these biodiversity-rich areas have been seriously threatened by biological invasions. The signal crayfish (Pacifastacus leniusculus) is listed as 100 of the world’s worst invasive species, and is a major threat to freshwater biodiversity and ecosystem functioning. Here, we examined environmental factors relating to the invasion success of signal crayfish and their morphological responses in floodplain waterbodies. Classification and regression tree analyses showed that most of the influential factors differed between tributary and lake populations. In floodplain tributaries, the occurrence of crayfish was positively related with water temperature and abundance of leaf cover, while crayfish abundance was highest where large-wood was abundant. In floodplain lakes, crayfish were absent at oxygen-poor sites, and abundant at sites with high connectivity to a main channel. These results indicate that conservation practitioners should consider different environmental factors in accordance with strategies for invasive species management (i.e., offensive or defensive management). Furthermore, we demonstrated morphological differences between tributary and lake populations, with tributary crayfish having wider chelae. These morphological differences might have resulted from the physical differences between the two types of waterbodies, facilitating the rapid invasion of signal crayfish to floodplain waterbodies. Our study showed that invasion-risk assessments should consider both environmental factors and morphological responses to new environments to understand invasion ecology and to form effective conservation plans and to prioritize management actions.

Complex pattern formation driven by the interaction of stable fronts in a competition-diffusion system.

The ecological invasion problem in which a weaker exotic species invades an ecosystem inhabited by two strongly competing native species is modelled by a three-species competition-diffusion system. It is known that for a certain range of parameter values competitor-mediated coexistence occurs and complex spatio-temporal patterns are observed in two spatial dimensions. In this paper we uncover the mechanism which generates such patterns. Under some assumptions on the parameters the three-species competition-diffusion system admits two planarly stable travelling waves. Their interaction in one spatial dimension may result in either reflection or merging into a single homoclinic wave, depending on the strength of the invading species. This transition can be understood by studying the bifurcation structure of the homoclinic wave. In particular, a time-periodic homoclinic wave (breathing wave) is born from a Hopf bifurcation and its unstable branch acts as a separator between the reflection and merging regimes. The same transition occurs in two spatial dimensions: the stable regular spiral associated to the homoclinic wave destabilizes, giving rise first to an oscillating breathing spiral and then breaking up producing a dynamic pattern characterized by many spiral cores. We find that these complex patterns are generated by the interaction of two planarly stable travelling waves, in contrast with many other well known cases of pattern formation where planar instability plays a central role.

Differential tolerance of native and invasive tree seedlings from arid African deserts to drought and shade

Efforts to understand why some species become successful invaders and why some habitats are more at risk from invasive species is an important research focus in invasion ecology. With current global climate change, evaluation of the effects of shade and drought on cohabiting native and invasive species from extreme ecosystems is especially important. Acacia tortilis subsp. raddiana is a tree taxon native to arid African deserts. Prosopis glandulosa, native to the southwestern United States and Mexico, is invading African arid and semiarid regions that are habitat for A. t. subsp. raddiana. The aim of this study was to evaluate and compare the tolerance and responses of the seedlings of these two tree species to shade, water stress and their interactions. We measured and recorded growth rates and morphological, biochemical and physiological plant traits under two radiation and two water treatments in greenhouse conditions. Radiation intensity was a stronger driver of the performance of both species than water availability. Beyond the independent effects of shade and drought, the interactions of these factors yielded synergistic effects on seedlings of both tree species, affecting key plant traits. The seedlings of A. t. subsp. raddiana were able to implement important shifts in key functional traits in response to altering abiotic stress conditions, behaving as a stress-tolerant species that is well-adapted to the habitat it occupies in hot arid African deserts. In contrast, the fast-growing seedlings of P. glandulosa were stress-avoiding. The alien P. glandulosa seedlings were highly sensitive to water and shade stress. Moreover, they were particularly sensitive to drought in shade conditions. However, although alien P. glandulosa seedlings were exposed to high stress levels, they were able to avoid permanent damage to their photosynthetic apparatus by mechanisms such as increasing energy dissipation by heat emission and by adjusting the relative allocation of resources to above- and below-ground structures. Our results are useful for conservation planning and restoration of invaded hyperarid ecosystems.