Wolf-Christian Saul

Mountain regions worldwide harbour key and unique biodiversity, yet they are increasingly threatened by multiple human pressures projected to intensify throughout the 21 ^st century. While invasive species are widely recognized as one of the main drivers of global biodiversity loss, until recently, their pressure in alpine and montane systems remained relatively low. But recent evidence suggests that this situation is changing. Climate warming, land-use change, pollution, and increasing rates of alien species introductions are enabling invasive plants to expand into previously less disturbed mountain habitats. Understanding the mechanisms behind this process is essential for predicting future invasions and preventing their impacts. In this study, we assess the magnitude and mechanisms of the negative environmental impacts caused by alien plant species in mountain ecosystems worldwide. Specifically, we obtained a list of 564 species recorded as naturalized across 18 mountain regions, from standardized road surveys along elevational gradients conducted by the Mountain Invasion Research Network (MIREN). We evaluated these species using the Environmental Impact Classification for Alien Taxa (EICAT), an objective and transparent method recognized by the IUCN. EICAT classifies alien species according to the severity of their impacts on native species populations and allows for the classification of the mechanisms driving these impacts. Documented impacts occurring in both mountain and non-mountain ecosystems were included in the assessment. Our preliminary results suggest that empirical evidence on the environmental impacts of most alien plant species remains limited or entirely lacking, particularly in mountain environments. For the species with available information, the most frequently reported impact mechanisms include competition with native species as well as indirect effects mediated through interactions with other organisms (such as disruption of pollinator networks) and allelopathic effects. Overall, this study provides a global overview of the potential impacts of alien plants in mountain ecosystems and establishes a robust scientific foundation for prioritization and the development of targeted management recommendations.

The removal of invasive alien plants is a common practice to passively restore native biodiversity and ecosystem functioning. However, invasive plants can generate legacy effects: persistent alterations in evolutionary, ecological, and/or socio-economic processes that hinder or prevent recovery after their removal. Management actions themselves can generate and exacerbate legacy effects. Despite increasing awareness, legacies remain under-reported and poorly understood, particularly those related to management actions. Here we propose an expanded definition of legacy effects, and a conceptual framework to identify and analyze the factors influencing their magnitude, persistence in time, and temporal dynamics. We propose classifying legacies as either evolutionary, ecological, or socio-economic according to (i) the mechanisms that generate them, (ii) the level of biological organization affected (species, population, community, or ecosystem), and (iii) the role of management actions in generating, exacerbating, and mitigating persistent impacts. (i) the mechanisms that generate them, (ii) the level of biological organization affected (species, population, community, or ecosystem), and (iii) the role of management actions in generating, exacerbating, and mitigating persistent impacts. Our framework provides a structured approach to predict post-removal dynamics and anticipate constraints on recovery to a desired reference state. We show that recognizing and addressing legacy effects is essential for improving invasive species management and for designing restoration strategies that are context-dependent and robust to long-term ecosystem change.

Invasive alien species are one of the major threats to global biodiversity, ecosystem integrity, nature’s contribution to people and human health. While scenarios about potential future developments have been available for other global change drivers for quite some time, we largely lack an understanding of how biological invasions might unfold in the future across spatial scales. Based on previous work on global invasion scenarios, we developed a workflow to downscale global scenarios to a regional and policy-relevant context. We applied this workflow at the European scale to create four European scenarios of biological invasions until 2050 that consider different environmental, socio-economic and socio-cultural trajectories, namely the European Alien Species Narratives (Eur-ASNs). We compared the Eur-ASNs with their previously published global counterparts (Global-ASNs), assessing changes in 26 scenario variables. This assessment showed a high consistency between global and European scenarios in the logic and assumptions of the scenario variables. However, several discrepancies in scenario variable trends were detected that could be attributed to scale differences. This suggests that the workflow is able to capture scale-dependent differences across scenarios. We also compared the Global- and Eur-ASNs with the widely used Global and European Shared Socioeconomic Pathways (SSPs), a set of scenarios developed in the context of climate change to capture different future socio-economic trends. Our comparison showed considerable divergences in the scenario space occupied by the different scenarios, with overall larger differences between the ASNs and SSPs than across scales (global vs. European) within the scenario initiatives. Given the differences between the ASNs and SSPs, it seems that the SSPs do not adequately capture the scenario space relevant to understanding the complex future of biological invasions. This underlines the importance of developing independent, but complementary, scenarios focused on biological invasions. The downscaling workflow we presented and implemented here provides a tool to develop such scenarios across different regions and contexts. This is a major step towards an improved understanding of all major drivers of global change including biological invasions.

Future dynamics of biological invasions are highly uncertain because they depend on multiple environmental, societal and socio-economic drivers. We adopted a qualitative scenario approach to explore the future of invasive alien species (IAS) in Europe and created an overall strategy for their management that considers different plausible future developments. The scenarios and strategy were developed during two online workshops with a multidisciplinary team of experts. First, we downscaled four global scenarios of biological invasions to the European level. Second, we developed a management strategy structured into 19 goals that cover a broad array of IAS-related topics (i.e. policy, research, public awareness and biosecurity), and provided solutions for achieving these goals considering the European scenarios. Third, we identified four interrelated recommendations around which any long-term strategy for managing IAS in Europe can be structured: (i) a European biosecurity regime, (ii) a dedicated communication strategy, (iii) data standardization and management tools, and (iv) a monitoring and assessment system. Finally, we identified the feasibility of the IAS management strategy, finding strong differences among the four scenarios. High levels of technological development, public environmental awareness, and effectiveness of IAS policies facilitated the implementation of the overall management strategy. Together, our results indicate that it is time for a new management of biological invasions in Europe based on a more integrative perspective across sectors and countries to be better prepared for future challenges.

Species introduced through human-related activities beyond their native range, termed alien species, have various impacts worldwide. The IUCN Environmental Impact Classification for Alien Taxa (EICAT) is a global standard to assess negative impacts of alien species on native biodiversity. Alien species can also positively affect biodiversity (for instance, through food and habitat provisioning or dispersal facilitation) but there is currently no standardized and evidence-based system to classify positive impacts. We fill this gap by proposing EICAT+, which uses 5 semiquantitative scenarios to categorize the magnitude of positive impacts, and describes underlying mechanisms. EICAT+ can be applied to all alien taxa at different spatial and organizational scales. The application of EICAT+ expands our understanding of the consequences of biological invasions and can inform conservation decisions.

In a comment on our recent article (Heger et al. 2019), Bridgewater and Hemming (2020) call for putting the umbrella concept “ecological novelty” into a policy context. They suggest linking ecological novelty to the “nature-based solution” paradigm and comment that “Global policy discussions around the ‘New Deal for nature’ seem to only embrace traditional conservation paradigms, leaving ecological novelty in a policy vacuum” (Bridgewater and Hemming 2020). We agree with Bridgewater and Hemming (2020) that political frameworks need new ways of expressing at which forms of “nature” conservation actions should aim, and we also share their opinion that the umbrella concept of “ecological novelty” can...

Background and aims: Since its emergence in the mid‐20th century, invasion biology has matured into a productive research field addressing questions of fundamental and applied importance. Not only has the number of empirical studies increased through time, but also has the number of competing, overlapping and, in some cases, contradictory hypotheses about biological invasions. To make these contradictions and redundancies explicit, and to gain insight into the field’s current theoretical structure, we developed and applied a Delphi approach to create a consensus network of 39 existing invasion hypotheses.Results: The resulting network was analysed with a link‐clustering algorithm that revealed five concept clusters (resource availability, biotic interaction, propagule, trait and Darwin’s clusters) representing complementary areas in the theory of invasion biology. The network also displays hypotheses that link two or more clusters, called connecting hypotheses, which are important in determining network structure. The network indicates hypotheses that are logically linked either positively (77 connections of support) or negatively (that is, they contradict each other; 6 connections).Significance: The network visually synthesizes how invasion biology’s predominant hypotheses are conceptually related to each other, and thus, reveals an emergent structure – a conceptual map – that can serve as a navigation tool for scholars, practitioners and students, both inside and outside of the field of invasion biology, and guide the development of a more coherent foundation of theory. Additionally, the outlined approach can be more widely applied to create a conceptual map for the larger fields of ecology and biogeography.

Our ability to predict invasions has been hindered by the seemingly idiosyncratic context-dependency of individual invasions. However, we argue that robust and useful generalisations in invasion science can be made by considering “invasion syndromes” which we define as “a combination of pathways, alien species traits, and characteristics of the recipient ecosystem which collectively result in predictable dynamics and impacts, and that can be managed effectively using specific policy and management actions”. We describe this approach and outline examples that highlight its utility, including: cacti with clonal fragmentation in arid ecosystems; small aquatic organisms introduced through ballast water in harbours; large ranid frogs with frequent secondary transfers; piscivorous freshwater fishes in connected aquatic ecosystems; plant invasions in high-elevation areas; tall-statured grasses; and tree-feeding insects in forests with suitable hosts. We propose a systematic method for identifying and delimiting invasion syndromes. We argue that invasion syndromes can account for the context-dependency of biological invasions while incorporating insights from comparative studies. Adopting this approach will help to structure thinking, identify transferrable risk assessment and management lessons, and highlight similarities among events that were previously considered disparate invasion phenomena.