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

information that contributes to scientific progress [25,26]. This activity allows gathering and analyzing data with a broad temporal and geographic scope, which would otherwise be impos- sible or cost restrictive. For this reason, data and sampling citizen science protocols have been to study bird diversity patterns [27], abundance and distribution of tree species [28], assess- ment of anthropogenic impact on natural communities [29], environmental monitoring [30], and even for the classification of stars [31]. Phenological studies have used citizen data through the National Phenology Network (NPN) portal (https://www.usanpn.org/), a platform that brings together phenological data recorded by both citizen scientists and experts, some of which has led to peer-reviewed publications ranging from phenological descriptions of a spe- cies [32] to proposals for the control of invasive species [33], and models of phenological response to climate change scenarios [34] (see https://www.usanpn.org/publications for a list of articles generated using NPN data). Although phenological data are one of the indicators used in Global Climate Change (GCC) assessments, most countries do not have a unified platform to gather phenological information. The NPN-USA is one of the few platforms that collects, stores, and shares national-level phenological information for more than 1,300 species, including data on pres- ence/absence and intensity of different phenophases [35]. Since 2007, this platform has estab- lished a general monitoring approach to phenology, with monitoring protocols for plants,

insects, fish, amphibians and reptiles, and birds and mammals. The plant protocol distin- guishes 24 phenophases, which may or may not be present in any of the 11 groups in which these organisms are classified [35]. For the recording of observations, the system provides stan- dardized formats in which the presence/absence of each phenophase and the number of struc- tures of interest in each phenophase (for example, from 3 to 10 flowers) or the proportion of the individual that presents each phenophase of interest (for example, 5–24% of the plant with leaves changing color) is recorded, but it does not require the registration of visual evidence such as photographs. To increase the quality of the data obtained, the program has informative and illustrated material on botanical principles and definition of phenophases [35]. In Europe, the Pan-European Phenology Project (PEP725) brings together and standardizes phenological data that have been collected by European phenology networks over the years and makes them freely available in a single unified database. In this project, each collaborating phenology net- work collects data (through volunteers such as PhenoWatch and the Swedish National Phenol- ogy Network) under established guidelines, which are then curated by a PEP725 committee. This project contains data from more than six decades, 265 species and 46 phenophases that follow the BBCH scale [36]. In India, the SeasonWatch citizen science project has promoted the phenological monitoring of eight vegetative and reproductive phenophases of more than

130 common Indian species since 2010. This project allows users to record, on a weekly basis, the presence and abundance of leaves, flowers, and fruits in different phenological stages, with the help of illustrated resources (https://www.seasonwatch.in). Apart from the citizen science networks designed to gather phenological data, scientists have access to other sources to extract phenological information. Biodiversity observation plat- forms such as iNaturalist and the associated partners sites in different countries, such as Natur- alista in Mexico, include photographic documentation with a geolocation, which helps relate geographic and temporal data and presence of species. This data has contributed to the genera- tion of animal and plant species distributions [37–40], but the derived information as an input for phenological information has rarely been assessed [but see 25,42]. Although this platform has the option of recording phenological annotations for observations, that is, indicating the phenophase of the registered organism, these annotations are rarely made. For example, in Naturalista, which is the collaborative platform between the National Commission for the Knowledge and Use of Biodiversity (CONABIO) in Mexico and iNaturalist, few records for 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 3 / 17