Evaluating competition for forage plants between honey bees and wild bees in Denmark

Abstract

A recurrent concern in nature conservation is the potential competition for forage plants between wild bees and managed honey bees. Specifically, that the highly sophisticated system of recruitment and large perennial colonies of honey bees quickly exhaust forage resources leading to the local extirpation of wild bees. However, different species of bees show different preferences for forage plants. We here summarize known forage plants for honey bees and wild bee species at national scale in Denmark. Our focus is on floral resources shared by honey bees and wild bees, with an emphasis on both threatened wild bee species and foraging specialist species. Across all 292 known bee species from Denmark, a total of 410 plant genera were recorded as forage plants. These included 294 plant genera visited by honey bees and 292 plant genera visited by different species of wild bees. Honey bees and wild bees share 176 plant genera in Denmark. Comparing the pairwise niche overlap for individual bee species, no significant relationship was found between their overlap and forage specialization or conservation status. Network analysis of the bee-plant interactions placed honey bees aside from most other bee species, specifically the module containing the honey bee had fewer links to any other modules, while the remaining modules were more highly inter-connected. Despite the lack of predictive relationship from the pairwise niche overlap, data for individual species could be summarized. Consequently, we have identified a set of operational parameters that, based on a high foraging overlap (>70%) and unfavorable conservation status (Vulnerable+Endangered+Critically Endangered), can guide both conservation actions and land management decisions in proximity to known or suspected populations of these species.

Fig 1. Number of different pollen types present in multiple pollen samples from across Denmark as a function of week number 14 to 40 (C.S.I. data merged for 2014 and 2015).

A polynomial trend line (R² = 0.1499) for the data is added.

Fig 2. Histogram with the number of wild bee species (y-axis) that has foraging overlap in percent with honey bees in intervals of 10 (x-axis).

The honey bee is excluded and intervals are 90 < i ≤ 100, etc. except 0 ≤ i ≤ 10. ‘Threatened species’ includes those that are in the IUCN categories Critically Endangered (CR), Endangered (EN), and Vulnerable (VU), while ‘Non-threatened’ includes those that are Least Concern (LC) and Near Threatened (NT). Additional categories used are ‘Data Deficient’ (DD) and ‘Regionally Extinct’ (RE). Honey bees in Denmark are ‘NA’ (Not Applicable), i.e., considered not eligible for a national assessment because of extensive management in the country.

Fig 3. Box-plot of different functional groups of wild bees and their foraging overlap with honey bees.

The honey bee and wild bees with no recorded forage plants are excluded.

Fig 4

Number of runs in which (a) plant genera or (b) bee species is placed in the same module as honey bees.

Fig 5. Graph of the five modules.

a) Size of circles (modules) are proportional to number of both plant and bee species in each module, thickness of lines connecting modules are proportional to number of interactions between the modules. b-f) the five different modules identified from a, squares are bee species, circles are plant genera. Modules b to f are presented in the same position as they appear in a, with module b in top. Honey bee is the central square (blue) in the module b.