Seasonal variation in the general availability of floral resources for pollinators in northwest Europe: A review of the data
Ciaran Harris, Nicholas J. Balfour, Francis L.W. Ratnieks
Abstract
Key resources for animal populations can vary seasonally. For pollinators, the availability of floral resources, nectar and pollen, can fluctuate temporally due to flowering phenology and competition. Because conservation strategies for bees and other pollinators often involve enhancing floral resources, targeting periods when these are in short supply would increase their effectiveness. However, there is currently no consensus regarding which months or seasons are most challenging for foraging pollinators in any region of the globe. Here, we carried out a systematic literature review of studies that reported measures of foraging favourability in northwest Europe. Our main aims were to determine: 1) whether foraging conditions vary seasonally; 2) whether certain months or seasons were consistently identified as challenging for pollinator foraging. A total of 18 studies were identified that met the inclusion criteria and were primarily studies that used honeybees as indicators of seasonal changes in foraging favourability (15 studies). There was a general decline in foraging favourability throughout the year, with spring and early summer more likely to be categorised as favourable than late summer or autumn. August was most frequently identified as challenging for pollinator foraging, with 71 % of the studies reporting it as unfavourable. April was most frequently identified as the most favourable month for foraging, with 87 % of the studies reporting it as favourable. These results suggest that efforts to improve floral resource supply for pollinators may be more effective if targeted at late summer months, particularly August. • Availability of floral resources, pollen and nectar, can vary seasonally in northwest Europe. • Foraging conditions are favourable in spring and unfavourable in late summer. • Conservation efforts should prioritise providing additional flowers in late summer.