Context This survey relates to COST (European Cooperation in Science and Technology) Action CA17122 - Alien CSI - Increasing understanding of alien species through citizen science (see https://alien-csi.eu/). The main aim of this survey was to collect information on Citizen Science projects/initiatives involving alien species in European Member States and some neighbouring countries. The survey was performed using a google forms. Definitions We defined Citizen Science projects as project which actively involved citizens in scientific enquiry generating new knowledge or understanding on alien species. Citizens may act as contributors, collaborators, or as project leader and have a meaningful role in the project. 'Alien Species' are defined as any live specimen of a species, subspecies or lower taxon of animals, plants, fungi or micro-organisms introduced outside its natural range; it includes any part, gametes, seeds, eggs or propagules of such species, as well as any hybrids, varieties or breeds that might survive and subsequently reproduce. Alien Species thus includes both species that are invasive and species that are alien but not invasive. An 'Invasive Alien Species' is defined as an alien species whose introduction or spread has been found to threaten or adversely impact upon biodiversity and/or related ecosystem services. Survey methodology The survey was made available on Google Forms and disseminated online, collecting responses from June 27, 2019 to April 6, 2020. It was shared with all COST Action CA17122 participants and in each country one person coordinated contacts with existing citizen science projects involving alien and/or invasive species and requested that they complete the survey. Thus, all projects were active in EU member states and neighbouring countries, though some may also be active outside of Europe. To increase reach, the survey was also disseminated through the European Citizen Science Association (ECSA) newsletter and mailing list and respondents were asked to share it with colleagues and local networks via snowball sampling. Questions and attribute values Survey questions and attribute values were developed using JRC metadata standards for CS projects (Bio Innovation Service 2018) and the project metadata model of PPSR Core, a set of global, transdisciplinary data and metadata standards for Public Participation in Scientific Research (https://core.citizenscience.org/). The survey included 62 questions in nine sections: Contact information of the respondent; General characterization of the project, including a brief summary, geographical scope, time scale, hosting entities, funding, etc.; Information on project scope, including target audience, taxonomic and environmental scope, project aims, type of data collected, etc.; Policy-related information, namely if the project has policy relevance and inclusion of species listed in the EU IAS Regulation; Information on engagement, such as type of involvement of citizens in the design of the project, engagement methods and social media used, skills needed to participate and frequency of contributions; Information on feedback and support provided to participants by the project, e.g., if projects provide materials for species identification, guidelines, training activities, information on how data from the project are used, feedback mechanisms and support; Data quality and data management, namely validation mechanism for records, registration type, methods of recording, whether data are open and accessible to citizen scientists, data form used to store data, data standards and data licence used, whether a public data management plan was drafted for the project, and the vocabulary used with respect to biological invasions (origin, occurrence status, degree of establishment and pathway of introduction); Performance indicators of projects, namely, usage of apps, number of participants and number of records, whether learning is assessed, number and type of publications using data from the project; Notes and remarks. Files raw_data.xlsx: includes the non-processed survey responses, supplemented with a project_ID. All GDPR sensitive data such as email addresses were omitted. Each row represents one project. projects_excluded.csv: includes all projects that were omitted from the analysis and the specific criteria for this exclusion. processed_data.csv: includes the cleaned, processed survey responses, used for analysis. The R code used for the analysis is available on this github repository. survey.pdf: a pdf extract from the original Google Forms, including all questions and their specifications.

This work was supported by Action CA17122 Increasing understanding of alien species through citizen science (Alien-CSI), supported by COST (European Cooperation in Science and Technology) through workshops and a short term scientific mission (n° ECOST-STSM-CA17122-42402).