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

platforms can provide phenological information with high geographic coverage and an increasing temporal coverage on general phenological patterns of species with wide distri- butions but has limited applicability in the identification of exact start and end dates. and peaks of the different phenophases. Introduction Phenology as a science and its application Phenological responses have a genetic component such that there is an ecological fit between plant development and environmental conditions. These external cues cause phenophases of coexisting individuals to respond more or less synchronously to environmental conditions [1– 3]. The close environment-development response has allowed for the use of phenological data to describe the synchronization of ecological interactions [4], determine the structure of plant communities [5,6], assess the dynamics of nutrient uptake and CO2 levels and other gases at the ecosystem level [7,8], and optimize agricultural and horticultural strategies [9–11], among oth- ers. In addition, the sensitivity of some phenophases to environmental temperature has made phenology an effective tool for monitoring the impacts of global climate change (GCC) on plants and animals [12]. For example, changes in growing season patterns [13], as well as shifts in flowering and fruit production [14] in recent decades have been detected and correlated with environmental changes attributed to GCC. These changes have in turn had repercussions on ecological components [4], agricultural practices [11,15,16], and ecosystem goods and services

[17–19]. Nonetheless, using phenological at a regional or global scale involves large amounts of quantitative and standardized data on phenological attributes coming from a wide spatial and temporal window, as well as time series of an established association between phenological response and environmental variables, usually accessible by national meteorological services. Many phenological studies are based on measurements from a sample of individuals within a population or community [20]. This type of research generates detailed and precise data on the timing and intensity of phenophases and their association with environmental require- ments in a population or a small geographic area, but intrinsic limitations in terms of number of species, selected methodology, and the geographic and temporal dimensions significantly restrict or make impossible potential extrapolation of phenological response patterns to envi- ronmental variations throughout an entire distribution range [7,20,21]. In order to identify patterns of phenological change at regional or global scales, or assess phenological changes in species with a wide spatial distribution and environmental tolerance, phenological monitoring can be carried out using lower resolution but high coverage data as input. That is, using large amounts of data from a wide geographical range, over several years, and not restricted by complex experimental designs nor controlled or monitored micro-envi- ronmental variables. For example, components from the phenology of communities or ecosys-

tems can be measured using remote sensing data (e.g. aerial photographs or satellite images [22], and the phenology at regional or higher scales of particular species that cannot be studied using remote sensing methods, could be studies using herbarium records, or through collabo- rations with networks of non-expert volunteers who make nature observations or monitor the phenological stages of plants and animals at national or regional levels [20,23,24]. Citizen science as an input for phenological studies Although there is no single definition of citizen science, this term is used to refer to projects that include the participation of non-expert volunteers in the generation and/or analysis of 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 2 / 17 National Strategy on IAS” provided funds to J.G. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.