Disentangling the diversity of definitions for the pollination ecosystem service and associated estimation methods

Observed declines in pollinator populations due to human pressures is of critical concern because pollination is an essential regulating ecosystem service (ES). Pollination has a major role in human food production and in maintaining flowering plant diversity. Estimating the pollination ES and its trends is thus essential for informing policy and management. However, the pollination ES remains poorly defined in practice as it is a polysemic term. In practice it is considered as: the ecosystem capacity to support pollination, the social demand for pollination or the actual pollination flow, which is the match between capacity and demand. Its quantification is complicated by the coexistence of numerous indicators. In this paper, we aimed to disentangle the diversity of definitions of the pollination service along with their associated indicators and field methods in the ecosystem service literature. From 131 reviewed papers, most studies were under temperate climate with a focus on hymenopteran pollinators, overlooking the importance of hoverflies and other dipteran pollinators. We identified four main pollination ES definitions that can be separated in two categories: (1) pollinator presence and pollen transfer, which are related to the capacity of the pollination ES; and (2) pollination success and production of fruits or seeds for human consumption, which are related to the flow of the pollination ES. Importantly, when quantified simultaneously pollination capacity and flow showed varying levels of congruence. Half of the reported relationships were congruent, but almost as many were neutral. Direct indicators of the two definitional categories characterised the respective actors of the pollination function. Studies of pollination capacity characterised pollinating agents (pollinator communities), while pollination flow studies described seeds and/or fruits produced from animal pollination. Both pollination capacity definitions share insect observations and captures as most frequent methods (70% of analysed studies), with a prevalence of indicators of pollinator diversity and abundance. Pollination flow estimation methods mainly describe the amount (number, weight) and quality of seeds/fruits. Pollinators depend on availability of feeding and nesting resources for their survival, and on their ability to reach them within landscapes. Thus, common indirect indicators of pollination are environmental variables related to semi-natural habitats and flower resources. We end this review with guidelines for relevant choices by ecosystem services scholars and practitioners of indicators and methods according to study questions and constraints (technical, financial, time and skills). Further investigations will be necessary to disentangle pollination capacity and flow relationships.