Modern agriculture is becoming increasingly pollinator-dependent. However, the global stock of domesticated honey bees is growing at a slower rate than its demand while wild bees are declining worldwide. This uneven scenario of high pollinator demand and low pollinator availability can translate into increasing pollination limitation, reducing the yield of pollinator-dependent crops. However, overall assessments of crop pollination limitation and the factors determining its magnitude are missing.

We assembled the first global database of pollination limitation in pollinator-dependent crops, encompassing 107 metadata comparing the quantity/quality of fruits/seeds produced by pollen supplemented vs naturally pollinated flowers. This database, based on 52 published studies, cover 30 crops in 52 crop systems. We conducted a meta-analysis comparing crop yield in pollen-supplemented vs. open-pollinated flowers. We assessed the overall magnitude of pollination limitation and whether this magnitude was influenced by (a) the presence/absence of managed honey bees, (b) crop compatibility system (i.e., self-compatible/self-incompatible), and (c) the interaction between these two factors. 

Following the protocols and good practices stated by Nakagawa et al.  2017, we conducted a systematic literature search of published studies in SCOPUS, the largest abstract and citation multidisciplinary database of peer-reviewed literature, covering publications from 1900 to July 2021. We searched for studies reporting at least one estimate of crop yield (e.g., seed- and/or fruit-set, individual seed or fruit weight, fruit or seed production per plant or area unit) after natural (open) pollination and pollen supplementation (i.e., open flowers manually supplemented with outcross pollen) treatments in pollinator-dependent crops. A pollinator-dependent crop was defined as any crop that exhibits at least some yield reduction when pollinators are excluded.  For a study to be included in the meta-analysis, it had to report the mean value, some measure of dispersion that could be ultimately transformed to standard deviation, and sample size for any of the above-mentioned measures of crop yield (“yield” hereafter) under both natural and supplemental pollination treatments in a pollinator-dependent crop. We only selected studies in which pollen supplementation was performed by adding outcross pollen manually or mechanically, at least once during flower lifespan, to the stigmas of otherwise open-pollinated flowers. For those studies reporting this information in figures, we extracted the data using WebPlotDigitalizer, an open web-based tool that extracts statistics from plots (https://automeris.io/WebPlotDigitizer/). For each selected study, we also gathered information regarding (i) the presence or absence of managed honey bees in the study crop, and (ii) crop compatibility system (i.e., SC or SI) (see Table S1 in Supporting Information). Studies that did not provide information regarding the presence/absence of managed honey bees were only considered for the evaluation of the overall pollination limitation (i.e., first question). Information regarding crop compatibility system (SC or SI) was gathered from each selected study when reported, and from Klein et al. 2007 or other literature sources when not reported in the selected study. Moreover, the crop Actinia deliciosa (kiwi) was included in the SI group, as it is dioecious and thus completely dependent on pollen transfer between plants for fruit production.