Explore the vast range of titles available.

Urban environments are often seen as unique or degraded habitats that both present hardships for some sensitive species and provide opportunities to others. Non-indigenous species (NIS) are commonly referenced in the latter group, and are comprised of species that can tolerate the unique conditions or capitalize on the opportunities found in urban environments. Moreover, these urban beneficiaries may be those that normally cannot overcome competitive interactions in intact native communities, but find opportunity to flourish in urban habitats. We ask the question: do NIS benefit from urbanization? We answer this question using three strategies. First, we explore the problem conceptually, using community assembly theory. Second, we perform a broad literature review. Finally, we analyze studies with sufficient information using a meta-analysis. We show that the available evidence supports the proposition that NIS benefit from urbanization, with NIS obtaining higher abundances and greater diversity in more urbanized habitats. There were only 43 studies that measured NIS abundance and diversity while adequately quantifying the degree of urbanization surrounding plots, and effect sizes (measured by Hedge’s D) reveal that NIS obtain higher abundances in more urbanized habitats, and especially for invertebrates. Despite the intense interest in NIS dynamics and impacts, we note a general dearth of robust studies that adequately quantify ‘urbanization’, and we end with a general call for more detailed research.

Urban areas have unique assemblages of species which are governed by novel ecological processes. People living in these environments have specific needs and demands in terms of ecosystem services (ES). Urban ecosystems are transformed in many ways by human activities and their floras comprise a high proportion of alien plant species, many of which were intentionally introduced to provide, augment or restore ES. Urban environments also have novel disturbance regimes and provide colonization sites for the establishment, dispersal and proliferation of alien plant species; such conditions often generate biological invasions which may cause marked changes to ES. We review the roles that alien plants play in providing urban ES and ecosystem disservices (EDS) globally. We identify the main ES and EDS associated with alien plants, and highlight the key species involved. A literature search revealed 335 papers, representing studies in 58 cities or urban areas in 27 countries. These studies recorded 337 alien plant species, contributing to 39 different ES and 27 EDS–310 species were recorded as contributing to ES and 53 species to EDS. A small number of alien plant taxa were frequently recorded as providing multiple ES in many urban ecosystems; the 10 most recorded species accounted for 21% of the ES recorded. Some of these species also result in significant EDS; three species accounted for 30% of the EDS recorded. Cultural services (notably aesthetics) are the most reported ES provided by alien plants in urban areas of developed countries, while provisioning services (notably food production) are most reported in developing countries. The most commonly studied EDS provided by alien plants is the impact on human health (notably allergic reactions). Eighty percent of studies on alien plants and ES and EDS have been done in developed countries. To elucidate the full range of effects of alien plants, more work is needed in developing countries. Urban planners and managers need to be mindful of both the positive and negative impacts of alien plant species to maximise the provision of ES.

Globalisation has resulted in the movement of organisms outside their natural range, often with negative ecological and economic consequences. As cities are hubs of anthropogenic activities, with both highly transformed and disturbed environments, these areas are often the first point of entry for alien species. We compiled a global database of cities with more than one million inhabitants that data had on alien species occurrence. We then identified the most prominent pathways of introduction and vectors of spread of alien species in these cities. Most species were intentionally introduced to cities and were released or escaped from confinement. The majority of alien species then spread within cities through natural means (primarily unaided dispersal). Pathway prominence varied across the taxonomic groups of alien species: the most prominent pathway for plants and vertebrates was the escape pathway; for invertebrates the stowaway and contaminant pathways were most likely to facilitate introductions. For some organisms, pathway prominence varied with the geographical and climatic characteristics of the city. The characteristics of the cities also influenced the prominence of vectors of spread for alien species. Preventing the natural spread of alien species within cities, and into adjacent natural environments will be, at best, difficult. To prevent invasions, both the intentional and unintentional introduction of potentially harmful alien species to cities must be prevented. The pathways of introduction and vectors of spread identified here should be prioritised for management.

Urban trees have been increasingly appreciated for the many benefits they provide. As concentrated hubs of human-mediated movement, the urban landscape is, however, often the first point of contact for exotic pests including insects and plant pathogens. Consequently, urban trees can be important for accidentally introduced forest pests to become established and potentially invasive. Reductions in biodiversity and the potential for stressful conditions arising from anthropogenic disturbances can predispose these trees to pest attack, further increasing the likelihood of exotic forest pests becoming established and increasing in density. Once established in urban environments, dispersal of introduced pests can proceed to natural forest landscapes or planted forests. In addition to permanent long-term damage to natural ecosystems, the consequences of these invasions include costly attempts at eradication and post establishment management strategies. We discuss a range of ecological, economic and social impacts arising from these incursions and the importance of global biosecurity is highlighted as a crucially important barrier to pest invasions. Finally, we suggest that urban trees may be viewed as ‘sentinel plantings’. In particular, botanical gardens and arboreta frequently house large collections of exotic plantings, providing a unique opportunity to help predict and prevent the invasion of new pests, and where introduced pests with the capacity to cause serious impacts in forest environments could potentially be detected during the initial stages of establishment. Such early detection offers the only realistic prospect of eradication, thereby reducing damaging ecological impacts and long term management costs.

It has been suggested that existing frameworks for guiding management of invasive species in rural areas and protected areas are inadequate for dealing with invasions in urban settings. A framework for selecting appropriate goals for managing invasive species in urban areas was developed by Gaertner et al. (Landsc Urban Plan 151:1–9, 2016 ). This framework groups species into three management approaches (control priority, active engagement, and tolerance) depending on their real or perceived benefits and their potential to generate negative impacts. This study tests the practical application of the framework using the example of Cape Town. We assess the suitability of the framework to support decision-making for managing invasive species in urban ecosystems using a questionnaire-based survey of members of the public, and an e-mail-based survey and a workshop with invasion biology researchers and managers. Specifically, we (1) determine the differences in perceptions regarding the benefits and impacts of invasive species between the public, managers and researchers; (2) investigate how consistently managers and researchers group invasive species into the three management categories; and (3) identify, with the help of managers and researchers, issues linked to the framework and give suggestions to overcome the identified issues. We found no clear pattern in the perceptions of the public, managers and researchers regarding perceived benefits and negative impacts of invasive species. Instead, the answers were widely scattered among all groups for most of the species that were considered. However, using the framework leads to a higher consistency among managers in placing the species into management categories, compared to invasive species grouping without guidance of the framework. We conclude that decision-support frameworks can assist managers in placing invasive species into management categories. However, even more specific guidelines on the use of invasive species management frameworks in urban areas are needed.

Tree species have been planted widely beyond their native ranges to provide or enhance ecosystem services such as timber and fibre production, erosion control, and aesthetic or amenity benefits. At the same time, non-native tree species can have strongly negative impacts on ecosystem services when they naturalize and subsequently become invasive and disrupt or transform communities and ecosystems. The dichotomy between positive and negative effects on ecosystem services has led to significant conflicts over the removal of non-native invasive tree species worldwide. These conflicts are often viewed in only a local context but we suggest that a global synthesis sheds important light on the dimensions of the phenomenon. We collated examples of conflict surrounding the control or management of tree invasions where conflict has caused delay, increased cost, or cessation of projects aimed at invasive tree removal. We found that conflicts span a diverse range of taxa, systems and countries, and that most conflicts emerge around three areas: urban and near-urban trees; trees that provide direct economic benefits; and invasive trees that are used by native species for habitat or food. We suggest that such conflict should be seen as a normal occurrence in invasive tree removal. Assessing both positive and negative effects of invasive species on multiple ecosystem services may provide a useful framework for the resolution of conflicts.

Alien species are often first introduced to urban areas, so it is unsurprising that towns and cities are often hotspots for invasions. However, while large cities are usually the first sites of introduction, small towns are more numerous and have a greater chance of launching invasions into natural areas as they have proportionally larger interfaces with their surroundings. In this paper we develop a set of scenarios as hypotheses to explore the role of small towns in facilitating within-country dispersal of alien plants. In particular, we developed ten scenarios for how introductions to small towns, agricultural and natural areas can lead to landscape-scale invasions. We tested a part of these scenarios using a case study of a highly invaded region in South Africa (the Berg River catchment in the Western Cape). We specifically investigated the main plant invasion routes between 12 small towns and their surrounding agricultural and natural areas. This was accomplished by conducting urban-specific alien plant surveys in towns, then comparing these results to regional databases of naturalized and/or invasive plants. Many of the alien plants found in urban areas were listed as invasive or naturalized in the catchment (over 30% of the total alien species pool). Despite marked environmental gradients across the study area, we found no relationships between the alien plant species richness in towns and climatic variables or with levels of anthropogenic disturbances. All towns hosted large numbers of invasive plant species and nearly half of the alien species found in towns were naturalized or invasive in surrounding areas. The likelihood of alien plants being naturalized or invasive outside urban areas increased in proportion to their local abundance in towns and if they were tall and woody. Ornamental horticulture was the main reason for introduction of these alien species (69%). Small towns can and do harbour significant populations of plant taxa that are able to spread to surrounding natural areas to launch invasions. Comparing lists of species from urban alien plant surveys with those from naturalisation records for the region is a useful protocol for identifying species which may be moving along the introduction–naturalization–invasion continuum.

Management of invasive species often raises substantial conflicts of interest. Since such conflicts can hamper proposed management actions, managers, decision makers and researchers increasingly recognize the need to consider the social dimensions of invasive species management. In this exploratory study, we aimed (1) to explore whether species’ taxonomic position (i.e. animals vs. plants) and type of invaded landscape (i.e. urban vs. non-urban) might influence public perception about the management of invasive species, and (2) to assess the potential of public awareness to increase public support for invasive species management. We reviewed the scientific literature on the conflicts of interest around the management of alien species and administered two-phased questionnaires (before and after providing information on the target species and its management) to members of the public in South Africa and the UK (n = 240). Our review suggests that lack of public support for the management of invasive animals in both urban and non-urban areas derives mainly from moralistic value disagreements, while the management of invasive plants in non-urban areas mostly causes conflicts based on utilitarian value disagreements. Despite these general trends, conflicts are context dependent and can originate from a wide variety of different views. Notably, informing the public about the invasive status and negative impacts of the species targeted for management appeared to increase public support for the management actions. Therefore, our results align with the view that increased public awareness might increase the public support for the management of invasive species, independent of taxonomic position and type of landscape.

Ornamental horticulture is the most important pathway for alien plant introductions worldwide, and consequently, invasive spread of introduced plants often begins in urban areas. Although most introduced ornamental garden-plant species are locally not naturalized yet, many of them have shown invasion potential elsewhere in the world, and might naturalize when climate changes. We inventoried the planted flora of 50 public and 61 private gardens in Radolfzell, a small city in southern Germany, to investigate whether local naturalization success of garden plants is associated with their current planting frequency, climatic suitability (as assessed with climatic niche modelling) and known naturalization status somewhere in the world. We identified 954 introduced garden-plant species, of which 48 are already naturalized in Radolfzell and 120 in other parts of Germany. All currently naturalized garden plants in Radolfzell have a climatic suitability probability of ≥ 0.75 and are naturalized in ≥ 13 out of 843 regions globally. These values are significantly higher than those of garden plants that have not become locally naturalized yet. Current planting frequencies, however, were not related to current naturalization success. Using the identified local naturalization thresholds of climatic suitability and global naturalization frequency, and climate projections for the years 2050 and 2070, we identified 45 garden-plant species that are currently not naturalized in Radolfzell but are likely to become so in the future. Although our approach cannot replace a full risk assessment, it is well-suited and applicable as one element of a screening or horizon scanning-type approach.

Globalization has increased the frequency of inadvertent introductions of plant pathogens. Many catastrophic invasions of both natural and agricultural systems have been initiated through anthropogenic dissemination pathways. Phytophthora species are a group of invasive plant pathogens causing many of the most important plant disease epidemics. A review of Phytophthora species descriptions published following the publication of the first DNA-based Phytophthora phylogeny was conducted to highlight patterns of recent introductions and to provide insights for early pathogen detection initiatives. Seventy-two publications from 2001 to 2016 describing 98 Phytophthora species were evaluated. Of the 91 species with data on geographic location isolation, 22% were described from type specimens isolated from urban environments, 33% from agricultural environments and 45% from natural environments. Within the urban environment, ornamental plant trading nurseries were the most important sources. Specifically, for Phytophthora ramorum, a species causing multiple epidemics globally, the largest proportion of first report publications were from urban environments, including nurseries. We therefore suggest that detection programs for invasive plant pathogens within the urban environment would be valuable. In this regard, specialized monitoring and citizen science projects that target urban areas where live plant-trading industries are concentrated would be particularly effective to both promote early detection and to facilitate a rapid response to new species invasions.