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Worldwide, large-scale biodiversity monitoring schemes are developing and involve many non-specialist volunteers. If the opening of schemes to non-specialists allows for the gathering of huge amounts of data, their quality represents a controversial issue. In the framework of the French Garden Butterfly Observatory (FGBO), we studied non-specialist volunteer identification errors based on identifications provided during a one-shot experiment. With 3492 butterfly pictures sent by 554 non-specialist volunteers, we directly measured identification errors and misidentification rates for each butterfly species or species group targeted by the FGBO. The results showed that when non-specialist volunteers identified butterflies at the species level, identification errors (i.e., the misidentification rate) reached 20.9%. It was only 5.0% when FGBO species groups were used. This study provides novel insights into the trade-off between data quantity and quality provided by non-specialist volunteers and shows that if protocols, research questions and identification levels are adapted, participatory monitoring schemes relying on non-specialists represent a powerful and reliable tool to study common species at a large scale and on a long-term basis.

Along with worldwide urbanization, upheavals in habitat and temperature are major threats for biodiversity. However, due to their interdependence, their relative roles as drivers of animal community composition remain entangled. Here, we investigated how taxonomic and functional compositions of arthropod communities were related to uncorrelated habitat and temperature gradients, and compared landscape (i.e., urbanization, Urban Heat Island (UHI)) to local variables (i.e., vegetation height and cover, near-ground temperature). We sampled 20,499 spiders (137 species) on 36 grasslands in Rennes (northwestern France). Unlike rural areas, urban sites were characterized by short vegetation and intense UHI, hosted species-poor communities, and were composed of small thermophilic species. UHI intensification and local loss of habitat complexity (short and dense vegetation) were associated with declining large and heat-sensitive species. These results highlight the prevalent role of urban warming, rather than land cover change, as an urban filter. Further, we show that landscape-scale UHI, not local temperature, filters species according to their functional attributes. UHI can therefore be considered as a thermal barrier, filtering species according to their physiological capacity to cope with urban thermal conditions. Finally, to counterbalance biotic homogenization, we argue for the importance of implementing complex habitat structures at the local scale within urban green infrastructure.

Citizen science (CS) is now very popular in ecology. The number of scientific publications referencing CS has increased steadily over the past 15 years, with more than 1150 publications today. However, the multiplicity of research involved suggests that this number is highly underestimated. Based on this paradox, a literature review on CS shows that while its formalization in 2009 facilitated its referencing, about 70% of the publications are not referenced using CS as keyword. To understand this under-representation, an analysis of 149 publications related to the famous Christmas Bird Count program shows that this underestimation is not mainly related to the diversity of keywords used to describe CS, but rather to the fact that CS is mainly considered as a method (four publications out of five). The results also show that taking into account the whole text of a publication would represent a substantial improvement for the analysis of scientific databases, whatever the field of research.