The Native Bees of Lolland (Denmark) Revisited after 100 Years: The Demise of the Specialists.

There is a global concern over insect declines, including both species loss and population declines. In particular, declines of species, such as bees that anchor trophic interactions and shoulder many of the essential ecosystem services, have been the focus of broad public concern. However, our understanding of what characterizes those species that are lost because of declines over long periods is limited by a scarcity of comparative studies. We here compare the species composition from a collection of bees sampled over two decades (2000-2019) from the island of Lolland in Denmark, with a collection also sampled over two decades (1900-1919), but a century ago by Jørgensen and his contemporaries. We further test if (1) the probability that bee species that were sampled a century ago are also found today depends on their degree of floral specialization; (2) and use information from pollen samples from bees from the historical records to assess if certain floral resources have been lost. In total, 203 species were recorded in the two periods, but less than half, 92 species, occurred in both sampling periods. A total of 174 species of bees were recorded from 1900-1919, and 121 species were recorded from 2000-2019, including 29 species not reported in 1900-1919. Notably, we report a reduction in the species composition among forage specialist bees from 26.4% to 15.7% of the bee fauna, a consistent and highly significant decline both when correcting for parasitic and social species. Pollen swabs from bees collected in the first period, 1900-1919, did not identify any plants that are not available today but documented a series of plants that were important to bees back then. These plants are still common today, such as Taraxacum and Salix. Our findings highlight the importance of local and historical faunistic studies, such as that of Jørgensen, who was a resident schoolteacher on the island of Lolland in southern Denmark, for documenting how changes over time affect the species composition in bee communities.

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

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.

Distribution, phenology and host plants of Danish bees (Hymenoptera, Apoidea).

Based on the examination of 47.629 specimens of bees from Denmark (54.6ºN to 57.8ºN) we summarize for the first time the distribution, phenology and host-plants for all 286 known species occurring in the country. The mapped occurrence records distinguish whether species are found only in 1974 or earlier, 1975 or later, or in both periods. Danish bees are recorded active from February 29 through October 21 and have been collected in 79% of the 10x10 km Universal Transverse Mercator (UTM) squares covering the country; seven UTM squares have over 100 reported species. Some UTM squares are poorly sampled, including less than five species recorded from 31% of the sampled squares. In addition bees are recorded from 282 different plants representing 50 families with the majority of observations from Asteraceae and Fabaceae (Leguminosae). The data, compiled from all major bee collections in the country, will provide policy makers and scientists a tool for establishing strategies for pollinator conservation and further research.