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No way back: changes in biotic interactions promoted by invasive alien plants
Abstract
Plant invasions are one of the main direct drivers of species loss in nature, seriously altering the habitat structure and leading to substantial effects on ecosystem functioning. Globalization, habitat fragmentation, and climate change facilitate the migration and colonization of many exotic species that induce changes on native communities through disruption of biotic interactions. These changes are currently taking place at rates unprecedented in the period newly defined as the Anthropocene. Many questions related to plant invasions have been answered in the last decades. However, directions of most recent studies tend to be very global or only focus in a particular assemblage of species, with few considering the many functional and trophic roles of entire groups. It is widely known that environmental disturbances clearly facilitate the spread of invasive plants. Nevertheless, the changes in the plant-herbivore interactions and the alteration of the composition of invertebrate communities have received little attention, as well as which environmental factors may favour the invasion success of unaltered ecosystems. To carry out this doctoral dissertation, a greenhouse experiment and several fieldwork studies were developed on vulnerable native ecosystems in the northwestern Iberian Peninsula. This region presents some of the most problematic plant species worldwide: three invasive woody Acacia species (Acacia dealbata Link, Acacia longifolia (Andr.) Willd. and Acacia melanoxylon R. Br.) and the invasive Carpobrotus edulis (L.) N.E. Br. The main objective of this doctoral dissertation is to elucidate whether the impact generated by these plant invasions on the native plant communities alter the biotic interactions, especially affecting the plant-herbivore interactions, and changing the composition of invertebrate communities due to the presence of these invasive plants. The results showed that A. dealbata can emerge and growth in unaltered plant communities, favouring their expansion under natural conditions and adjusting the growth to the ecosystem characteristics. Besides, the microhabitat conditions of areas invaded by A. dealbata play an important role for the establishment of native species, even being the main factor affecting seedling establishment rather than changes in soil properties or allelopathy. Moreover, the results obtained indicate that A. dealbata and C. edulis can accumulate new natural enemies in the introduced range, but the damages found did not seem enough to exert an effective natural control. Carpobrotus edulis may also alter the associated herbivore community structure by changing the plant-herbivore interactions and influencing the occurrence of herbivore assemblages. Finally, it is noted that the invasive plants studied alter the biodiversity and ecosystem functioning by reducing the invertebrate assemblages and functional diversity. Overall, the results presented here provide evidence that invasive plants alter the biotic interactions, especially affecting the plant-herbivore interactions and changing the composition of invertebrate communities. It is important to explore the direct and indirect effects produced by plant invasions on the taxonomic and functional diversity to detect alterations of the associated organisms at the ecosystem scale. Predicting such changes poses a serious challenge to conserve the ecosystems in a time of rapid environmental disturbance. This information is vital to define long-term management strategies for reducing the spread of these invasive plants.
A spittlebug of the genus Aphrophora Germar, 1821 (Hemiptera: Cercopoidea: Aphro-phoridae) occurs in profusion on many coastal California populations of invasive exotic ice-plant, Carpobrotus edulis (L.) N. E. Brown (Aizoaceae), a spreading, mat-forming, perennial succulent. Described here as Aphrophora moscoviciae sp. nov., it occurs along the Paci c Coast and San Francisco Bay, from Marin County south to at least Ventura County, and achieves remarkably high population levels in some localities, with a maximum density of 2700 nymphs/ m 2 , an apparent world record for spittlebugs. While it appears to be endemic to California, native hosts are unknown. It shares habitats with three demonstrated spittlebug vectors of the plant pathogen Xylella fastidiosa Wells et al. (Xanthomonadaceae), including one, Philaenus spumarius (Linnaeus, 1758), that is common on iceplant. This suggests that it could play a role in broader transmission networks among cultivated and wild plants.
Global transport of organisms by humans provides novel resources to wild species, which often respond maladaptively. Native herbivorous insects have been killed feeding on toxic exotic plants, which acted as 'ecological traps'1-4. We document a novel 'eco-evolutionary trap' stemming from the opposite effect; that is, high fitness on an exotic resource despite lack of adaptation to it. Plantago lanceolata was introduced to western North America by cattle-ranching. Feeding on this exotic plant released a large, isolated population of the native butterfly Euphydryas editha from a longstanding trade-off between maternal fecundity and offspring mortality. Because of this release-and despite a reduced insect developmental rate when feeding on this exotic-Plantago immediately supported higher larval survival than did the insects' traditional host, Collinsia parviflora 5 . Previous work from the 1980s documented an evolving preference for Plantago by ovipositing adults 6 . We predicted that if this trend continued the insects could endanger themselves, because the availability of Plantago to butterflies is controlled by humans, who change land management practices faster than butterflies evolve 6 . Here we report the fulfilment of this prediction. The butterflies abandoned Collinsia and evolved total dependence on Plantago. The trap was set. In 2005, humans withdrew their cattle, springing the trap. Grasses grew around the Plantago, cooling the thermophilic insects, which then went extinct. This local extinction could have been prevented if the population had retained partial use of Collinsia, which occupied drier microhabitats unaffected by cattle removal. The flush of grasses abated quickly, rendering the meadow once again suitable for Euphydryas feeding on either host, but no butterflies were observed from 2008 to 2012. In 2013-2014, the site was naturally recolonized by Euphydryas feeding exclusively on Collinsia, returning the system to its starting point and setting the stage for a repeat of the anthropogenic evolutionary cycle.
Aims
The tree legume Acacia dealbata Link is an aggressive Australian invader that severely affects abiotic and biotic compartments of ecosystems worldwide. This invasive species outcompetes native plant communities through direct competition, changes in microhabitat and soil properties under the canopy and the release of allelopathic compounds. However, these effects are usually studied separately and under controlled conditions. The objective of this study was to evaluate the combined effect of these modifications exerted by A. dealbata on the establishment of native and invasive species in the field.
Methods
A full factorial experiment was performed in order to test the combined effect of microhabitat, soil type and allelopathy on the establishment of the invasive A. dealbata and three native species in the field (NW Spain). We sowed seeds of native or invasive species in pots with different soil type (soil collected under A. dealbata or Pinus pinaster), allelopathy status (soil treated or untreated with activated carbon) and microhabitat (under the canopy of A. dealbata or P. pinaster). The number and total biomass of established plants were evaluated after eight months. Soil abiotic properties were determined in all soils.
Important Findings
The establishment of native and invasive species was significantly affected by the microhabitat, with a higher number of seedlings under the native forest (P. pinaster). The establishment of A. dealbata seedlings was also significantly promoted by its own soil. A negligible effect of allelopathy was detected on the establishment and growth of both invasive and native species. We conclude that (1) the main factor affecting seedling establishment was microhabitat rather than changes in soil properties or allelopathy, (2) soil modifications by A. dealbata promote the establishment of its own seedlings, contributing to the progress of invasion, and (3) allelochemicals released by A. dealbata do not seem to play a key role for the establishment of native and invasive plants under field conditions in the European non-native range.
Arbuscular Mycorrhizal Fungi (AMF) are obligate soil biotrophs that establish intimate relationships with 80% of terrestrial
plant families. AMF obtain carbon from host plants and contribute to the acquisition of mineral nutrients, mainly phosphorus.
The presence of invasive plants has been identified as a soil disturbance factor; often conditioning the structure and function
of soil microorganisms. Despite the investigation of many aspects related to the invasion of Acacia dealbata, the effect produced on the structure of AMF communities has never been assessed. We hypothesize that A. dealbata modifies the structure of AMF community, influencing the establishment and growth of plants that are dependent on these mutualisms.
To validate our hypothesis we carried out DGGE analysis and also grew plants of Plantago lanceolata in pots using roots of native shrublands or from A. dealbata, as inoculum of AMF. Cluster analyses from DGGE indicated an alteration in the structure of AMF communities in invaded soils.
After 15 weeks, we found that plants grown in pots containing native roots presented higher stem and root growth and also
produced higher biomass in comparison with plants grown with A. dealbata inoculum. Furthermore, plants that presented the highest biomass and growth exhibited the maximum mycorrhizal colonization
and phosphorus content. Moreover, fluorescence measurements indicated that plants grown with A. dealbata inoculum even presented higher photosynthetic damage. Our results indicate that the presence of the invader A. dealbata modify the composition of the arbuscular fungal community, conditioning the establishment of native plants.
Arthropods were collected from 20 roadside patches of the invasive creeping daisy (Sphagneticola trilobata) in Suva, fiji, using a sweep net. In total, 841 individuals were collected, assigned to 104 recognizable Taxonomic units (rTus) in nine major taxonomic groups. Even though the plant has been present in Suva for only 40 years, the use of rarefaction methods suggests it may be valuable to arthropods, including native and endemic taxa, and play host to around 120 species in the Suva area alone. Honey bees were observed on the flowers, suggesting the plant may be beneficial to honey producers as a source of supplementary pollen and nectar. Although the fauna appeared to be dominated by exotic species, some native fijian insect species were recorded, indicating there may be some minor conservation potential for this plant as an alternative host for invertebrates and enhancing local biodiversity, especially in an urban setting.
Plant species that growth close to or under the canopy of Acacia dealbata Link (Fabaceae, subfamily: Mimosoideae) within its non-native range, survive with difficulty or not at all, especially if they are native. This phenomenon has been attributed to allelopathy; one of the strategies used by A. dealbata to trigger an invasion process. Native species Quillaja saponaria Molina (tree) and Helenium aromaticum (Hook.) H.L. Bailey (herb), share A. dealbata’s range in South-central Chile. This study was performed on the Mediterranean Biobío Region of Chile. We evaluated the effect of leaves, flowers, pods and seeds of A. dealbata on the germination and early growth of these native species. Biological assays were carried out under laboratory conditions, based on aqueous extracts and the direct effect of plant material. Leaf litter prevented the germination of both species and seeds of the invasive species impeded the germination of Q. saponaria. Other plant parts from A. dealbata also induced reductions of hypocotyl and radicle lengths in the native species, reaching over 50 % in some treatment values. All plant parts caused radicle necrosis, preventing the formation of root hairs and, consequently, jeopardizing the survival possibility of the recipient species. The results show that A. dealbata can interfere with the establishment of pioneer herbaceous species in ecological succession and can also affect trees if they are reached by the invasion front.
Studies on the impacts of invasive plants are common but most are short term and fail to consider the
temporal context of invasion. The present work investigated particular invasions on two different time scales
and asked: (1) Do the impacts of an invasive tree on plant communities change after decades of invasion? (2)
Are patterns of impacts dynamic over a period of five years? (3) Can multiple parameters reveal impacts that
are unnoticed when single-value parameters are measured alone? Contrasting plant communities (long
invaded, recently invaded and non-invaded) of a Portuguese coastal dune were compared during a 5-year
period to assess the impacts of the invasive Acacia longifolia. Plant diversity, richness, cover, plant traits,
indicator species analysis, similarity between areas and species turnover were analysed to reveal spatial and
temporal patterns of change. Native species richness declined in both invaded areas. As invasion time
increased, species shared with natural habitats decreased along with native plant cover, diversity and
species turnover. Many species typical of dunes were replaced by generalists and exotics species, and richness
of species with some level of nitrophily increased, particularly in recently invaded areas. Life form spectrum
was radically transformed in invaded areas with several small-sized life forms being replaced by one
single microphanerophyte. Germination of A. longifolia was greatest in long invaded areas. Findings show that
modification of plant communities intensifies with invasion time and that invasion promotes a diversity
of structural and functional changes which are dynamic over a few years (medium-term scale) but which tend to
stabilize after several decades (long-term scale). An analysis of parameters in combination revealed changes
that were not apparent when the same parameters were considered separately.
All around the globe, humans have greatly altered the abiotic and biotic environment with ever-increasing speed. One defining feature of the Anthropocene epoch is the erosion of biogeographical barriers by human-mediated dispersal of species into new regions, where they can naturalize and cause ecological, economic and social damage. So far, no comprehensive analysis of the global accumulation and exchange of alien plant species between continents has been performed, primarily because of a lack of data. Here we bridge this knowledge gap by using a unique global database on the occurrences of naturalized alien plant species in 481 mainland and 362 island regions. In total, 13,168 plant species, corresponding to 3.9% of the extant global vascular flora, or approximately the size of the native European flora, have become naturalized somewhere on the globe as a result of human activity. North America has accumulated the largest number of naturalized species, whereas the Pacific Islands show the fastest increase in species numbers with respect to their land area. Continents in the Northern Hemisphere have been the major donors of naturalized alien species to all other continents. Our results quantify for the first time the extent of plant naturalizations worldwide, and illustrate the urgent need for globally integrated efforts to control, manage and understand the spread of alien species.
La invasión por acacias australianas altera profundamente tanto las características abióticas del suelo como la estructura de las comunidades microbianas edáficas, modificando los procesos y servicios de los ecosistemas invadidos. En general, las acacias invasoras conllevan un gran aumento del contenido de hojarasca, carbono y nitrógeno, así como modificaciones en los ciclos biogeoquímicos de estos elementos y una disminución de la disponibilidad hídrica en los ecosistemas invadidos. Producen también modificaciones en la diversidad funcional y genética de las comunidades de bacterias y hongos del suelo. Durante la invasión por algunas especies de acacias australianas se ha comprobado además que se produce una co-invasión de bacterias fijadoras de nitrógeno exóticas asociadas a las acacias. Estas bacterias exóticas pueden asociarse a leguminosas nativas que existen en el ecosistema invadido lo que conlleva una disrupción de la simbiosis entre leguminosas y rizobios nativos. Todos estos cambios en las propiedades abióticas y bióticas del suelo pueden tener consecuencias negativas para las especies vegetales nativas, reduciendo su crecimiento o impidiendo su establecimiento. Además, los cambios introducidos por las acacias refuerzan la invasión ya que favorecen la germinación, crecimiento y capacidad competidora de estas especies. Los estudios realizados sugieren que estos procesos de retroalimentación positivos entre las acacias y los suelos que invaden aumentan con el tiempo de invasión y dificultan la recuperación natural de los ecosistemas.
Despite the dynamic nature of ecological interactions, most studies on species networks offer static representations of their structure, constraining our understanding of the ecological mechanisms involved in their spatio-temporal stability. This is the first study to evaluate plant-herbivore interaction networks on a small spatio-temporal scale. Specifically, we simultaneously assessed the effect of host plant availability, habitat complexity and seasonality on the structure of plant-herbivore networks in a coastal tropical ecosystem. Our results revealed that changes in the host plant community resulting from seasonality and habitat structure are reflected not only in the herbivore community, but also in the emergent properties (network parameters) of the plant-herbivore interaction network such as connectance, selectiveness and modularity. Habitat conditions and periods that are most stressful favored the presence of less selective and susceptible herbivore species, resulting in increased connectance within networks. In contrast, the high degree of selectivennes (i.e. interaction specialization) and modularity of the networks under less stressful conditions was promoted by the diversification in resource use by herbivores. By analyzing networks at a small spatio-temporal scale we identified the ecological factors structuring this network such as habitat complexity and seasonality. Our research offers new evidence on the role of abiotic and biotic factors in the variation of the properties of species interaction networks.
Aim of study: Acacia dealbata is a naturalized tree of invasive behaviour that has expanded from small plots associated with vineyards into forest ecosystems. Our main objective is to find evidence to support the notion that disturbances, particularly forest fires, are important driving factors in the current expansion of A. dealbata.
Area of study: We mapped it current distribution using three study areas and assesses the temporal changes registered in forest cover in these areas of the valley of the river Miño.
Material and methods: The analyses were based on visual interpretation of aerial photographs taken in 1985 and 2003 of three 1 × 1 km study areas and field works.
Main result: A 62.4%, 48.6% and 22.2% of the surface area was covered by A. dealbata in 2003 in pure or mixed stands. Furthermore, areas composed exclusively of A. dealbata make up 33.8%, 15.2% and 5.7% of the stands. The transition matrix analyses between the two dates support our hypothesis that the areas currently covered by A. dealbata make up a greater proportion of the forest area previously classified as unwooded or open forest than those without A. dealbata cover. Both of these surface types are the result of an important impact of fire in the region. Within each area, A. dealbata is mainly located on steeper terrain, which is more affected by fires.
Research highlights: A. dealbata is becoming the dominant tree species over large areas and the invasion of this species gives rise to monospecific stands, which may have important implications for future fire regimes.
Invasive species currently pose a major environmental challenge. Understanding their development and the factors associated with their expansion is the first step towards developing effective control measures. This work proposes the use of detailed spatio-temporal information from forest monitoring systems to assess the demographic rates, spatio-temporal patterns and spread determinants of invasive plants in forest ecosystems. For this purpose, we selected two of the most widespread non-native plants in Europe: Acacia dealbata and Acacia melanoxylon. Focusing on the forested area of northwest Spain and based on the comparison of two cycles of the Spanish National Forest Inventory, this study analyzes the dynamics of Acacia species between 1998 and 2008 in regards to changes in their spatial distribution, dominance, abundance, diametrical (dbh) structure and regeneration. In addition, the forested area was classified into forest types to identify the forests which are more susceptible to invasion. Finally, through general linear models, this study analyzes the relative importance of abiotic and biotic factors determining the spread of Acacia species over the studied period. The results confirm a rapid expansion in the presence of Acacia species in the forests of NW Spain, with annual spread rates around 0.1%. These two species are increasing their dominance across most forest types in the study area, where they are becoming the dominant species in the regeneration of some of them. Environmental factors and connectivity between Acacia populations are identified as the main factors associated with their spread into new areas. Additionally, the combination of disturbances and biotic factors associated with stand structure (total basal area, richness and tree cover) appear to determine the vulnerability or resistance of some forest to their spread. The early stage of invasion detected highlights the potential of Acacia species to continue spreading. This aspect, in conjunction with the high degree of disturbances (mainly fires) in this region, could be critical in determining the configuration of future forest landscapes in NW Spain. This study demonstrates the value of considering broad-scale periodic forest surveys to monitor biological invasions in forests ecosystems. The spatially-explicit data obtained from these surveys can contribute not only to furthering our knowledge with regard to invasion biology but also to developing more efficient conservation and management strategies.
Field guide about invasive plant species in Portugal. Includes invasive and potentially invasive plants. (in Portuguese)
The climatic conditions predicted for the twenty-first century may aggravate the extent and impacts of plant invasions, by favouring those invaders more adapted to altered conditions or by hampering the native flora. We aim to predict the fate of native and invasive tree species in the oak forests of Northwest Spain, where the exotic invaders Acacia dealbata and Eucalyptus globulus co-occur with the natives Quercus robur and Quercus pyrenaica and the naturalized Pinus pinaster. We selected adult, dominant trees of each species, collected increment cores, measured the ring width and estimated the basal area increment (BAI, cm2 year−1). Climate/growth models were built by using linear mixed-effect models, where the previous-year BAI and seasonal temperature and precipitation were the fixed factors and the individual the random factor. These models were run to project the fate of studied species in the A2 and B2 CO2 emission scenarios until 2100. The models explained over 50 % of BAI variance in all species but E. globulus, where growth probably occurs whenever a minimum environmental requirement is met. Warm autumns favoured BAI of both natives, probably due to an extension of leaf lifespan, but hampered A. dealbata and P. pinaster BAI, maybe because of water imbalance and/or the depletion of carbon reserves. The projections yielded a positive BAI trend for both Quercus along the twenty-first century, but negative for the invader A. dealbata and clearly declining for the naturalized P. pinaster. Our results disagree with previous literature pointing at climate change as a driver of invasive species’ success and call for further studies regarding the effect of climate change on co-occurring natives and invaders.
Background: The invasive success of Acacia dealbata is partially attributed to the release of allelopathic compounds and changes in soil characteristics. However, both mechanisms have been studied separately, which does not reflect field conditions. Aims: To test whether interactions of allelopathy, soil origin and soil microbiota affect plant germination and growth of A. dealbata. Methods: Soil sampling and collection of natural canopy leachates were conducted in Coimbra, Portugal. Germination and seedling growth were tested in sterilised and control soils from native forests and invasive acacia stands, watered either with acacia or native canopy leachates. Sapling growth was further studied in soils from native forests and invasive acacia stands. Results: No interaction between Acacia leachates and soil microbiota was detected. Leachates of A. dealbata had a negative effect on the growth of its own seedlings in native soils. Soil microbiota reduced seedling growth in native forests. Saplings of A. dealbata grew bigger in the invaded soil than in soil from native forests. Conclusions: Soil microorganisms have a positive effect on the initial establishment and growth of A. dealbata. The effect of Acacia leachates depends on the origin of the soil. Soil previously modified by A. dealbata seems to favour its own growth.
Tree invasions can lead to changes in the plant community composition and reductions in the number of native species. Acacia dealbata Link (silver wattle) was introduced to Chile as an ornamental. This species originated in Australia and Tasmania has been documented as invader in several ecosystems in the world. In Chile, this species is strongly associated to roads, rivers and human disturbances, performing as an invader in natural and semi-natural ecosystems. However, A. dealbata impacts at community levéis have been scarcely studied. In order to determínate the relationship between A. dealbata invasión and the species richness and composition of plants in natural communities of south-central of Chile, we recorded the plant composition in three study conditions: wader Acacia canopy, inside an adjacent native forest and in the edges between both conditions. We selected five study sites near to Concepción, Chile (37° S) and in each condition we surveyed three transects of 20x2 m2 with a total of 15 transects at the three conditions. Results showed that the total species richness is significantly lower under Acacia canopy compared with the stand edge and native forest. Native species richness is higher within native forest and alien species richness is significantly higher in the stand edge. Plant composition also changes in the presence of A dealbata. This study demonstrates the positive relationship between the presence of A dealbata stands and the reduction of plant diversity in the south-central área of Chile.
Plant–soil feedbacks can exacerbate competition between invasive and native species, although the net effect of the interaction between soil biota and competition is likely to be species-specific. Very few studies have addressed the combined effect of soil and competition on plant performance and invasion by exotic woody species. This study explores plant growth and competition between Acacia dealbata and Pinus pinaster in three different soils—native, disturbed and invaded—in Portugal. The invasion of native P. pinaster forests by A. dealbata can be explained by the stronger competition ability of the exotic tree species. Competition is stronger in the native soil, allowing the establishment of A. dealbata in this soil and the displacement of P. pinaster. During invasion, A. dealbata changes soil conditions and establishes positive plant–soil feedbacks that promote its own germination and growth and increase P. pinaster mortality. Soil disturbance by the introduction of a different exotic species, Eucalyptus globulus, did not promote invasion by A. dealbata. We found a significant effect of soil legacy on both growth and competitive ability of the invasive A. dealbata. The ability of A. dealbata to outcompete the native P. pinaster in its own soil and the positive plant–soil feedbacks established after invasion are important mechanisms for A. dealbata invasion.
Predation by exotic species has caused the extinction of many native animal species on islands, whereas competition from exotic plants has caused few native plant extinctions. Exotic plant addi- tion to islands is highly nonrandom, with an almost perfect 1 to 1 match between the number of naturalized and native plant species on oceanic islands. Here, we evaluate several alternative implica- tions of these findings. Does the consistency of increase in plant richness across islands imply that a saturation point in species richness has been reached? If not, should we expect total plant richness to continue to increase as new species are added? Finally, is the rarity of native plant extinctions to date a misleading measure of the impact of past invasions, one that hides an extinction debt that will be paid in the future? By analyzing historical records, we show that the number of naturalized plant species has increased linearly over time on many individual islands. Further, the mean ratio of naturalized to native plant species across islands has changed steadily for nearly two centuries. These patterns suggest that many more species will become naturalized on islands in the future. We also discuss how dynamics of invasion bear upon alternative saturation scenarios and the implications these scenarios have for the future retention or extinction of native plant species. Finally, we identify invasion-motivated research gaps (propagule pressure, time-lags to extinction, abundance shifts, and loss of area) that can aid in forecasting extinction and in developing a more comprehensive theory of species extinctions. between extinction and colonization. At the saturation point, the addition of new colonizing species results in the local extinction of a like number of previously established species. We refer to this as ''extinction-based saturation,'' which is consistent with Island Biogeography Theory (IBT) (6, 7). Second, the total number of species in a place could be maintained by suppression of colonization. A saturation point is reached if the invasion of new species is inhibited by species already present. We refer to this as ''colonization-based saturation,'' which is consistent with the resistance to invasion in niche-based models of species diversity, such as Stochastic Niche Theory (SNT), where the total number of species in a place is determined by how finely resources can be partitioned among species (9). These alternative pathways for species saturation have very different implications for the impact of invading species on native plant extinctions. Extinction-based saturation (like IBT) explicitly predicts species turnover, i.e., change in species com- position, with the addition of species to islands (6, 7). Therefore, once a saturation point has been reached, the continued addition of exotic species should result in the extinction of native species, and the rate of extinction increases as the rate of colonization increases. If this holds true for islands, then we may eventually have dramatic increases in the number of native plant extinc- tions—particularly if exotics continue to be added in large numbers. In contrast, colonization-based saturation (like SNT) explicitly predicts that the addition of exotic species will become progressively less likely to occur as species richness increases (10). Colonization-based saturation (like SNT) also predicts that existing species should benefit from a priority effect that makes them unlikely to be displaced by incoming species—as long as invading species are more or less equivalent to native species in their efficiency in using resources (10). If this holds true on islands, then we should expect the rate of naturalization of exotic species to diminish dramatically and few currently established species (both native and exotic) to be at risk of extinction in the future. These are two very distinct views of the future, and distinguishing among these and other alternatives is important. Doing so will help us to advance our basic understanding of ecological and evolutionary theory while simultaneously advanc- ing our understanding of a pressing applied issue, namely the future of plant diversity on islands worldwide. Here, we evaluate the role of species invasions on the extinc- tion of native species. We begin by examining the International Union for Conservation of Nature database for patterns of extinction in plants and terrestrial vertebrates that have occurred This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, ''In the Light of Evolution II: Biodiversity and Extinction,'' held December 6-8,
The allelopathic potentials of throughfall (rain passing through the canopy) and macerate (apical branches mixed with distilled water) leachates of Acacia dealbata grove during the flowering were studied. The test plants were corn (Zea mays) and 4 plant spp. (Dicranum sp. Hedera hibernica, Leucobryum sp. and Dactylis glomerata) present both inside and outside the grove, while Leucobryum sp. and Dactylis glomerata were present only outside the grove. A. dealbata leachates showed allelopathic interference to test species. Although the germination of corn was inhibited by throughfall and macerate leachates, but the latter was more inhibitory. Besides, leachates also inhibited the photosynthesis of corn. The macerate leachates also stimulated the respiration of species located outside the A. dealbata grove. This suggested that A. dealbata can interact with native species through allelopathic interference and may be responsible for biodiversity loss in introduced ranges.
Acacia dealbata Link (Mimosaceae) is a woody leguminous tree introduced to Europe from Australia that has become a serious environmental problem in north-western Spain, where it forms dense monospecific patches threatening native-plant biodiversity. We describe the effects of invasion by A. dealbata on understorey plant composition in shrubland, oak-and pine-forest communities in north-western Spain. Plant species richness and composition, plant density and total plant cover were measured and compared among patches with different invasion status (invaded, transition and non-invaded patches) in each studied community. A clear effect of the community type and invasion on the understorey vegetation was observed. In general, composition of shrubland was different from that of oak and pine forests. We found significant effects of invasion status on species richness, plant density and total plant cover; values were significantly lower in invaded than non-invaded patches. Invasion by A. dealbata also was associated with changes in species composition. In total, eight native species, including the endemic Daboecia cantabrica (thuds) K. Koch, were confined to non-invaded patches and were replaced by other natives in invaded and transition patches. Our results suggest that although A. dealbata represented a serious threat to all of the study communities, the severity of the impact depended on the community type.
Invasion by exotic species constitutes a major threat to natural ecosystems. One of the most invasive species of current concern in Europe is Acacia dealbata Link (Mimosaceae), the expansion of which is reducing populations of native species, and hence biodiversity. To investigate the potential involvement of allelopathic mechanisms in this process, we evaluated the germination and seedling growth performance of Lactuca sativa L. (Compositae), Zea mays L. (Gramineae), Dactylis glomerata L. (Gramineae), Arabidopsis thaliana (L.) Heynh. (Cruciferae) and A. dealbata, in the presence of mixtures of throughfall and litter leachate and of aqueous soil extracts collected under A. dealbata at different times of the year. Each test species responded differently to the solutions being assayed suggesting that A. dealbata allelochemicals may affect a variety of physiological growth parameters. In most cases the observed effects were stimulatory rather than inhibitory, which may have long-term deleterious effects on native plant populations since premature growth during periods of poor resources can be detrimental. Interestingly, increased radical growth of A. dealbata was detected with either extract suggesting a positive autoalleloapthy effect.
■ Abstract Contributions from the field of population biology hold promise for understanding and managing invasiveness; invasive species also offer excellent oppor- tunities to study basic processes in population biology. Life history studies and demo- graphic models may be valuable for examining the introduction of invasive species and identifying life history stages where management will be most effective. Evolution- ary processes may be key features in determining whether invasive species establish and spread. Studies of genetic diversity and evolutionary changes should be useful for
Study of the impacts of biological invasions, a pervasive component of global change, has generated remarkable understanding of the mechanisms and consequences of the spread of introduced populations. The growing field of invasion science, poised at a crossroads where ecology, social sciences, resource management, and public perception meet, is increasingly exposed to critical scrutiny from several perspectives. Although the rate of biological invasions, elucidation of their consequences, and knowledge about mitigation are growing rapidly, the very need for invasion science is disputed. Here, we highlight recent progress in understanding invasion impacts and management, and discuss the challenges that the discipline faces in its science and interactions with society.
Aim The biophysical impacts of invasive Australian acacias and their effects on ecosystem services are explored and used to develop a framework for improved restoration practices.
Location South Africa, Portugal and Chile.
Methods A conceptual model of ecosystem responses to the increasing severity (density and duration) of invasions was developed from the literature and our knowledge of how these impacts affect options for restoration. Case studies are used to identify similarities and differences between three regions severely affected by invasions of Australian acacias: Acacia dealbata in Chile, Acacia longifolia in Portugal and Acacia saligna in South Africa.
Results Australian acacias have a wide range of impacts on ecosystems that increase with time and disturbance, transform ecosystems and alter and reduce ecosystem service delivery. A shared trait is the accumulation of massive seed banks, which enables them to become dominant after disturbances. Ecosystem trajectories and recovery potential suggest that there are important thresholds in ecosystem state and resilience. When these are crossed, options for restoration are radically altered; in many cases, autogenic (self-driven and self-sustaining) recovery to a pre-invasion condition is inhibited, necessitating active intervention to restore composition and function.
Main conclusions The conceptual model demonstrates the degree, nature and reversibility of ecosystem degradation and identifies key actions needed to restore ecosystems to desired states. Control and restoration operations, particularly active restoration, require substantial short- to medium-term investments, which can reduce losses of biodiversity and ecosystem services, and the costs to society in the long term. Increasing restoration effectiveness will require further research into linkages between impacts and restoration. This research should involve scientists, practitioners and managers engaged in invasive plant control and restoration programmes, together with society as both the investors in, and beneficiaries of, more effective restoration.
Aim The degree to which eco‐physiological traits critical to seedling establishment are related to differences in geographic range size among species is not well understood. Here, we first tested the idea that seedling eco‐physiological attributes associated with establishment differ between narrowly distributed and geographically widespread plant species. Secondly, we tested the notion that species occupying wide geographic ranges have greater phenotypic plasticity in response to the environment than contrasted species with more restricted distributions.
Location Eastern Australia.
Methods We compared five pairs of geographically restricted and widespread Acacia species grown under glasshouse conditions for differences in seedling relative growth rate and associated allocational, morphological and physiological traits. We then examined whether widespread species displayed greater phenotypic plasticity in these traits than narrowly distributed species in response to changes in soil water availability.
Results Neither relative growth rate nor any measure of biomass accumulation or allocation differed significantly between seedlings of narrowly distributed and widespread species. In addition, the plasticity of biomass allocation was not greater in widespread species. However, the leaflets of widespread species had higher photosynthetic capacity and greater plasticity of water use efficiency than the leaflets of narrowly distributed species.
Main conclusions We demonstrated fundamental differences in the physiology and plasticity of leaflets of widespread and narrowly distributed species. The greater plasticity of these seedling leaflet traits may allow widespread Acacia species to utilize a wider range of environmental conditions in relation to soil moisture than restricted Acacia species. However, we did not find corresponding differences in mean or plasticity of seedling growth and allocational traits. In general, we suggest that relationships between rarity and species traits are both context and taxon specific.
The tree Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain, where its expansion
is assumed to reduce populations of native species and threaten local plant biodiversity. In order to investigate the potential
involvement of allelopathic mechanisms in this process, net photosynthetic and respiration rates of four test native understory
species (Hedera hibernica (G. Kirchn.) Bean, Dicranum sp., Dactylis glomerata L. and Leucobryum sp.) were evaluated using a Clark-type electrode in the presence of canopy leachate collected under A. dealbata stands at four times of the year for 2years and macerate from their apical branches at the same sites and times. The first
two test species were present both inside and outside of A. dealbata stands, while the last two were only located outside the stands. We found that there were significant differences in respiration
and net photosynthetic rates between the control and A. dealbata extracts in all test species. The respiration rate was increased by both canopy leachate and macerate extracts on certain
collection dates, but net photosynthetic rate was stimulated by macerate and inhibited by canopy leachate on other dates.
The main phenological stages of A. dealbata in which respiration and net photosynthetic rates were more affected were blossoming, inflorescence formation and in periods
after severe drought, in this decreasing order. Our results also showed that Dicranum sp., Leucobryum sp. and D. glomerata were more affected by aqueous extracts than other species during a 2-year period. We suggest that the observed inhibitory
or stimulatory effects on the physiological parameters studied could have an adverse effect on the understory species, and
that allelopathic interference seems to participate in this process.
KeywordsAllelopathic interference–Aqueous extracts–Biodiversity–Invasion–Native species–Respiration and net photosynthetic rates
Trees act as ecosystem engineers and invasions by exotic tree species profoundly impact recipient communities. Recently, research
on invasive trees has dramatically increased, enabling the assessment of general trends in tree invasion. Analysing 90 studies
dealing with 45 invasive tree species, we conducted a quantitative review and a meta-analysis to estimate the relevance of
eight leading hypotheses for explaining tree invasions. We also tested whether species functional traits (growth rate, density/cover,
germination, biomass and survival) equally promote tree invasiveness. Overall, our results suggest that several hypotheses,
linked to invasibility or invasiveness, are pertinent to explain tree invasions. Furthermore, more than one hypothesis has
been supported for a given species, which indicates that multiple factors lead to the success of invasive tree species. In
addition, growth rate appears to be the most efficient predictor of invasiveness for invasive trees and could thus be used
as a means to identify potential alien tree invasions. We conclude that further investigations are needed to test the consistency
of some hypotheses across a broader pool of invasive tree species, whilst experimental studies with the same tree species
across a larger range of sites would help to reveal the full suite of factors that affect tree invasions.
KeywordsTree invasion–Systematic review–Meta-analysis–Invasiveness–Invasibility–Functional traits
One of the most cited hypotheses explaining the inordinate success of a small proportion of introduced plants that become
pests is the ‘natural enemies hypothesis’. This states that invasive introduced plants spread rapidly because they are liberated
from their co-evolved natural enemies. This hypothesis had not been properly tested until recently. Previous reviews on this
topic have been narrative and vote counting in nature. In this review, we carried out quantitative synthesis and meta-analysis
using existing literature on plants and their herbivores to test the different components of the enemy release hypothesis.
We found supporting evidence in that (1) insect herbivore fauna richness is significantly greater in the native than introduced
ranges, and the reduction is skewed disproportionally towards specialists and insects feeding on reproductive parts; and (2)
herbivore damage levels are greater on native plants than on introduced invasive congeners. However, herbivore damage levels
are only marginally greater for plants in native than in introduced ranges, probably due to the small numbers of this type
of study. Studies quantifying herbivore impacts on plant population dynamics are too scarce to make conclusions for either
comparison of plants in native vs introduced ranges or of co-occurring native and introduced congeners. For future research,
we advocate that more than two-way comparisons between plants in native and introduced ranges, or native and introduced congeners
are needed. In addition, the use of herbivore exclusions to quantify the impacts of herbivory on complete sets of population
vital rates of native vs introduced species are highly desirable. Furthermore, three-way comparisons among congeners of native
plants, introduced invasive, and introduced non-invasive plants can also shed light on the importance of enemy release. Finally,
simultaneously testing the enemy release hypothesis and other competing hypotheses will provide significant insights into
the mechanisms governing the undesirable success of invasive species.
The degree of specialization in the legume-rhizobium mutualism and the variation in the response to different potential symbionts are crucial factors for understanding the process of invasion by exotic legumes and the consequences for the native resident plants and bacteria. The enhanced novel mutualism hypothesis predicts that exotic invasive legumes would take advantage of native rhizobia present in the invaded soils. However, recent studies have shown that exotic legumes might become invasive by using exotic introduced microsymbionts, and that they could be a source of exotic bacteria for native legumes. To unravel the role of novel and old symbioses in the progress of invasion, nodulation and symbiotic effectiveness were analyzed for exotic invasive plants and native co-occurring legumes in a Mediterranean coastal dune ecosystem. Although most of the studied species nodulated with bacteria from distant origins these novel mutualisms were less effective in terms of nodulation, nitrogenase activity and plant growth than the interactions of plants and bacteria from the same origin. The relative effect of exotic bradyrhizobia was strongly positive for exotic invasive legumes and detrimental for native shrubs. We conclude that (1) the studied invasive legumes do not rely on novel mutualisms but rather need the co-introduction of compatible symbionts, and (2) since exotic rhizobia colonize native legumes in invaded areas, the lack of effectiveness of these novel symbiosis demonstrated here suggests that invasion can disrupt native belowground mutualisms and reduce native legumes fitness.
Ecology Letters (2011) 14: 702–708
Biological invasions cause ecological and economic impacts across the globe. However, it is unclear whether there are strong patterns in terms of their major effects, how the vulnerability of different ecosystems varies and which ecosystem services are at greatest risk. We present a global meta-analysis of 199 articles reporting 1041 field studies that in total describe the impacts of 135 alien plant taxa on resident species, communities and ecosystems. Across studies, alien plants had a significant effect in 11 of 24 different types of impact assessed. The magnitude and direction of the impact varied both within and between different types of impact. On average, abundance and diversity of the resident species decreased in invaded sites, whereas primary production and several ecosystem processes were enhanced. While alien N-fixing species had greater impacts on N-cycling variables, they did not consistently affect other impact types. The magnitude of the impacts was not significantly different between island and mainland ecosystems. Overall, alien species impacts are heterogeneous and not unidirectional even within particular impact types. Our analysis also reveals that by the time changes in nutrient cycling are detected, major impacts on plant species and communities are likely to have already occurred.
Tree invasions have escalated in importance in the last few decades (more species, greater area invaded, more types of impacts, increasing complexity of management challenges), and are increasingly studied from many perspectives. This research spans many disciplines, including ecology, population biology, genetics, remote sensing, ecological modelling, risk analysis, resource economics and, increasingly, the humanities. There has been substantial progress in understanding patterns and processes , but many unanswered questions remain. Only a few invasive trees have been well studied, many of them in only a small part of their invasive range. Invasive trees often have substantial impacts, especially when they invade formerly treeless vegetation. Trees have several features that make them useful for understanding key aspects of biological invasions (the determinants of invasiveness and invasibility), but also the full spectrum of human perceptions and values that frames biological invasions as an environmental problem. This editorial provides background and summarizes the main outputs from a workshop held in Argentina in September 2012 that set out to summarize current knowledge on key topics and to determine the most important challenges facing researchers and managers. The sixteen papers in the special issue of Biological Invasions span disciplines, geographic regions and taxa and provide novel insights on pathways and historical perspectives, detection and monitoring, determinants of invasive-ness, function and impact, and the many challenges that face managers.
Aim Australian acacias (1012 recognized species native to Australia, which were previously grouped in Acacia subgenus Phyllodineae) have been moved extensively around the world by humans over the past 250 years. This has created the opportunity to explore how evolutionary, ecological, historical and sociological factors interact to affect the distribution, usage, invasiveness and perceptions of a globally important group of plants. This editorial provides the background for the 20 papers in this special issue of Diversity and Distributions that focusses on the global cross-disciplinary experiment of introduced Australian acacias. Location Australia and global. Methods The papers of the special issue are discussed in the context of a unified framework for biological invasions. Distributions of species were mapped across Australia, their representation in bioclimatic zones examined and the potential global distribution of the group modelled. By collating a variety of different lists, we determined which Australian acacias have reached different stages in the introduction-naturalization-invasion continuum in different parts of the world. Paradigms and key research questions relating to barriers to invasion, stages of invasion and management perceptions are sketched. Results According to our global database of Australian acacia records, 386 species have been moved outside Australia by human agency, 71 species are naturalized or weedy, and 23 are unequivocally invasive. Climatic models suggest that about a third of the world's land surface is climatically suitable for Australian acacias. Many species are commercially important crops or are useful for other purposes and have been extensively planted, and many different human perceptions of Australian acacias exist in different parts of the world. The papers in the special issue cover all the barriers, stages and processes that define biological invasions and touch on many aspects: history and the human dimension; aspects of the species pool; species traits; biotic interactions; climate and niche; and management. Main conclusions Australian acacias are an excellent model group for examining interactions between evolutionary, ecological and socioeconomic drivers of species introductions. New insights have emerged on the biological, ecological and evolutionary correlates of naturalization and invasion, but human usage factors permeate all explanatory models. Understanding and managing introduced Australian acacias requires a fundamental and integrative appreciation of both intrinsic (e.g. species traits) and extrinsic (e.g. human usage and perceptions) aspects.
Plant invasions can drastically change the structure of native communities, but it is not fully understood whether alien species occupy phylogenetic and functional space within the range occupied by natives, or provide a novel set of evolutionary origins and traits to the invaded communities.
Here, we evaluated this open question with data on a large number of plant communities from different terrestrial habitats. We used ~27,000 vegetation plots from 26 terrestrial habitats in the Czech Republic and compared phylogenetic and functional diversity (PD and FD, respectively) and community trait means in invaded and non‐invaded plots. We tested for differences (1) between invaded vs. non‐invaded plots, (2) among natives in invaded vs. non‐invaded plots, and (3) in invaded plots only, with and without aliens. To minimize habitat filtering effects on PD and FD, we ran these tests within the habitat‐specific species pools of the 26 vegetation types.
In general, PD, FD and trait mean values changed with invasion, with changes being rather consistent across the habitats considered. Invaded plots were less phylogenetically, but more functionally diverse than non‐invaded plots. The greater FD in invaded plots, compared to non‐invaded ones, was due to greater dissimilarity between natives. In fact, native species in invaded plots showed higher PD and FD than native species in non‐invaded plots, while alien species reduced PD and FD in invaded plots. Changes in the trait means with invasion were due to differences in native species in invaded and non‐invaded plots, rather than to an effect of alien species. Within most habitats, the trait means and variance of all aliens were similar to those of all natives, while in some habitats, the variability in traits was greater between aliens that belonged to phylogenetically closer clades.
Synthesis. Our results suggest that alien species more often occupy a phylogenetic and functional space within the range formed by the native species in a community. They do so either by filling empty gaps or by excluding natives from the existing phylogenetic and functional space, rather than occupying or creating a phylogenetic and trait space outside of it.
This chapter examines how the novel ecosystem concept is applied with reference to sites affected by alien plant invasions in different parts of the world. It summarizes what is known about the mechanisms and processes whereby plant invasions are known to generate impacts in invaded ecosystems. The chapter also explains how such drivers and impacts link with key facets of current discussions and debates around novel ecosystems. Finally, it discusses the usefulness of the novel ecosystems concept for understanding and managing plant invasions in the face of rapid global change. The concept of ecosystem resilience and associated thresholds has been suggested as a helpful framework for identifying the degree of ecosystem degradation.
Non-native plants have invaded nearly all ecosystems and represent a major component of global ecological change. Plant invasions frequently change the composition and structure of vegetation communities, which can alter animal communities and ecosystem processes. We reviewed 87 articles published in the peer-reviewed literature to evaluate responses of arthropod communities and functional groups to non-native invasive plants. Total abundance of arthropods decreased in 62% of studies and increased in 15%. Taxonomic richness decreased in 48% of studies and increased in 13%. Herbivorous arthropods decreased in response to plant invasions in 48% of studies and increased in 17%, likely due to direct effects of decreased plant diversity. Predaceous arthropods decreased in response to invasive plants in 44% of studies, which may reflect indirect effects due to reductions in prey. Twenty-two percent of studies documented increases in predators, which may reflect changes in vegetation structure that improved mobility, survival, or web-building for these species. Detritivores increased in 67% of studies, likely in response to increased litter and decaying vegetation; no studies documented decreased abundance in this functional group. Although many researchers have examined effects of plant invasions on arthropods, sizeable information gaps remain, specifically regarding how invasive plants influence habitat and dietary requirements. Beyond this, the ability to predict changes in arthropod populations and communities associated with plant invasions could be improved by adopting a more functional and mechanistic approach. Understanding responses of arthropods to invasive plants will critically inform conservation of virtually all biodiversity and ecological processes because so many organisms depend on arthropods as prey or for their functional roles, including pollination, seed dispersal, and decomposition. Given their short generation times and ability to respond rapidly to ecological change, arthropods may be ideal targets for restoration and conservation activities. Efectos de las Plantas Invasoras sobre los Artrópodos.
Invasive tree Acacia dealbata was introduced in the Iberian Peninsula in the 19th century. Nowadays, it is spreading in wide areas in which native vegetation has been altered or removed by human-activities, as in the case of Quercus robur forests in the northwest of Spain. In this article we assess the impact of A. dealbata invasion on soil properties (pH, organic matter, total N, NH4+-N and NO3--N), light characteristics (direct and diffuse photon flux density and leaf area index) and soil seed bank and established vegetation. We selected three sites where native Q. robur forests and A. dealbata invaded patches grow nearby. Before 2008 autumn rains, we took soil samples under five trees per species (A. dealbata and Q. robur) and site to determine soil seed bank floristic composition and soil properties. In spring 2009 we monitored the floristic composition of the vegetation growing under the same trees. Our results show that A. dealbata presence increases total N, NH4+-N and NO3--N and decreases pH in soils but does not affect light characteristics. Acacia dealbata modifies soil seed bank composition by decreasing species richness, seed density and the percentage of bryophyte and fern spores, and by increasing the percentage of Asteraceae and exotic species. Understorey of invaded A. dealbata forests shows a decrease in species richness and plant cover. Finally, A. dealbata reduces the similarity between the soil seed bank and the established vegetation. These results suggest that A. dealbata invasion produces a deep impact on Q. robur forests that hampers their regeneration, even after A. dealbata removal.
Acacia dealbata Link, an Australian tree legume, is one of the most invasive species in south-eastern Europe. The invasive success of A. dealbata is partially attributed to its ability to release allelopathic compounds that affect native plant species, but the allelopathic effect on soil microbes has been little explored. Here, we used natural leachates to assess the bioactivity of these allelochemicals on soil microorganisms in native Mediterranean pine and mixed forests. Soil samples were treated either with acacia canopy leachate or the corresponding canopy leachate. Soil microbial communities were analyzed using Biolog Ecoplates™ and PCR–DGGE. Allelochemicals naturally released by A. dealbata clearly modified soil bacterial functional diversity in the pine forest where acacia leachate significantly increased the consumption of carbohydrates and amino acids and reduced the utilization of carboxylic acids. Acacia leachates also lead to a significant reduction in bacterial richness and diversity in the pine forest soil. However, the soil microorganisms of mixed oak forest were insensitive to allelochemical activity. Our results show that the allelopathic effects of A. dealbata on soil microbes depend on ecosystem type and that soil bacteria are more sensitive than soil fungi to the allelochemicals released by A. dealbata. We conclude that the higher sensitivity of pine forest soil microbiota to allelochemicals introduced by A. dealbata can contribute to the process of invasion.
Aim Woody plants were not widely considered to be important invasive alien species until fairly recently. Thousands of species of trees and shrubs have, however, been moved around the world. Many species have spread from planting sites, and some are now among the most widespread and damaging of invasive organisms. This article presents a global list of invasive alien trees and shrubs. It discusses taxonomic biases, geographical patterns, modes of dispersal, reasons for introductions and key issues regarding invasions of non‐native woody plants around the world.
Location Global.
Methods An exhaustive survey was made of regional and national databases and the literature. Correspondence with botanists and ecologists and our own observations in many parts of the world expanded the list. Presence of invasive species was determined for each of 15 broad geographical regions. The main reasons for introduction and dissemination were determined for each species.
Results The list comprises 622 species (357 trees, 265 shrubs in 29 plant orders, 78 families, 286 genera). Regions with the largest number of woody invasive alien species are: Australia (183); southern Africa (170); North America (163); Pacific Islands (147); and New Zealand (107). Species introduced for horticulture dominated the list (62% of species: 196 trees and 187 shrubs). The next most important reasons for introduction and dissemination were forestry (13%), food (10%) and agroforestry (7%). Three hundred and twenty‐three species (52%) are currently known to be invasive in only one region, and another 126 (20%) occur in only two regions. Only 38 species (6%) are very widespread (invasive in six or more regions). Over 40% of invasive tree species and over 60% of invasive shrub species are bird dispersed.
Main conclusions Only between 0.5% and 0.7% of the world’s tree and shrub species are currently invasive outside their natural range, but woody plant invasions are rapidly increasing in importance around the world. The objectively compiled list of invasive species presented here provides a snapshot of the current dimensions of the phenomenon and will be useful for screening new introductions for invasive potential.
Acacia dealbata Link is an Australian woody legume that has become a serious environmental problem in Northwest Spain where it forms dense monospecific patches modifying the structure of different native ecosystems and threatening native aboveground biodiversity. In spite of the dramatic changes observed in the vegetation of invaded sites little is known about the consequences of invasion for soil microorganisms. To investigate the effect of A. dealbata invasion on the structure of soil fungi and bacteria communities, samples were taken from invaded and non-invaded areas from three different ecosystems in Northwest Spain: pine forest, shrubland and grassland. In each ecosystem type, soil samples were taken in areas of native vegetation, areas invaded by A. dealbata and in the transition zone between native and invaded vegetation. Soil microorganisms were analyzed in the different samples by PCR-DGGE using general primers for eubacteria and fungi. Soil analyses were also performed to evaluate the effect of A. dealbata invasion on soil fertility.The invasion by A. dealbata consistently increased soil N, C, organic matter and exchangeable P content in the three studied ecosystems. A clear effect of the invasion on the overall structure of microorganism communities was only observed in the shrubland where soil fungal communities in the invaded and transition areas clustered together and apart from the native soil. Significant differences in soil microorganisms richness and diversity between invaded and not invaded soils were only found in the grassland. Grassland invasion by A. dealbata lead to a significant increase of bacterial richness and to a significant reduction in fungal richness and diversity. Our results show that although the changes on soil chemistry due to A. dealbata invasion are consistent among the studied ecosystems, the effect on soil microorganisms depends on the ecosystem type affected by the invasion.
The exotic trees Ailanthus altissima,
Robinia pseudoacacia, Acer negundo and Elaeagnus angustifolia coexist with the native trees Fraxinus angustifolia and Ulmus minor in river banks of central Spain. Similarly, the exotic trees Acacia dealbata and Eucalyptus globulus co-occur with the natives Quercus pyrenaica and Pinus pinaster in Northwest Spain. We aimed to identify the environmental conditions that favour or hamper the establishment success of these species. In spring 2008, seeds of the studied species were sown under an experimental gradient of light (100, 65, 35, 7% of full sunlight) combined with three levels of soil moisture (mean soil water potential = −0.97, −1.52 and −1.77 MPa.). During the first growing season we monitored seed emergence and seedling survival. We found that the effect of light on the establishment success was stronger than the effect of soil moisture. Both exotic and native species of central Spain showed a good performance under high light, A. negundo being the most shade tolerant. Water shortage diminished E. angustifolia and A. altissima success. Among NW Spain species, A. dealbata and P. pinaster were found to be potential competitors for colonizing high-irradiance scenarios, while Q. pyrenaica and E. globulus were more successful under moderate shade. High soil moisture favoured E. globulus but not A. dealbata establishment. These results contribute to understand some of the factors controlling for spatial segregation between coexisting native and exotic tree species, and can help to take decisions orientated to the control and management of these exotic species.
Ecosystems worldwide are losing some species and gaining others, resulting in an interchange of species that is having profound
impacts on how these ecosystems function. However, research on the effects of species gains and losses has developed largely
independently of one another. Recent conceptual advances regarding effects of species gain have arisen from studies that have
unraveled the mechanistic basis of how invading species with novel traits alter biotic interactions and ecosystem processes.
In contrast, studies on traits associated with species loss are fewer, and much remains unknown about how traits that predispose
species to extinction affect ecological processes. Species gains and losses are both consequences and drivers of global change;
thus, explicit integration of research on how both processes simultaneously affect ecosystem functioning is key to determining
the response of the Earth system to current and future human activities.