Practice makes the expert: The importance of training volunteers in the generation of phenological data from photographs of biodiversity observation platforms

Each database had a set of approximately 30 to 50 observations. Each observation consisted of a URL link that redirected the rater to the observation on the website (Fig 2) as well as fixed fields that could be scored by the volunteers, and that corresponded to each phenophase. Each observation had date and geographical information as well as information about observer. In addition to the phenophases to be scored, a field defined as "Whole plant" was included to identify whether the observation was made on a whole individual or on a section of an individ- ual. Volunteers were also asked to record the time taken to score each database. Phenophases could be rated with one of three ratings, "Yes” (Y), "No” (N), and "Don’t know" (Dk), but each observation could have more than one phenophase at the same time. "Yes" and "No" ratings correspond to whether the phenophase is distinguishable or not in the observed photographic record while "Don’t know" was used in cases where the observed photograph(s) was unclear, did not correspond to the species of interest, or was clearly a cultivated individual (e.g., in a pot). Each volunteer was assigned at least three different databases (30 to 50 records per data- base), to ensure that each observation was independently reviewed and scored by three trained and three untrained volunteers. In addition, the expert group rated a total of three L. nepetifo- lia databases (84 records) and four N. glauca databases (214 records). The groups of trained

Fig 2. Two views of observations reviewed by volunteers on the Naturalista website. A record of each species used in this study, Leonotis nepetifolia (top) and Nicotiana glauca (bottom), is shown. The N. glauca observation is composed of two photographs, marked on the image with a blue box. The photograph of L. nepetifolia shows the phenophases of new growth, immature flower, mature flower, and dried fruit. The photograph of N. glauca shows the immature flower, mature flower, and dried fruit phenophases. https://doi.org/10.1371/journal.pone.0282750.g002 PLOS ONE The training of volunteers as a promoter of phenological studies PLOS ONE | https://doi.org/10.1371/journal.pone.0282750 March 7, 2023 6 / 17