19 March 2015 (IUCN) – With the majority (56.7%) of European bee species being listed as Data Deficient, any overview of the threats to the continental apifauna will necessarily be incomplete. However, for conservation and management of bee diversity to be undertaken effectively, it is critical to have a clear understanding of taxonomy and ecology of the species present. National governments, through the Convention on Biological Diversity, recognise the existence of a taxonomic impediment and, through the Darwin Declaration, intend to address the situation (Environment Australia 1998). This shortfall in taxonomic expertise is very apparent in our understanding of bees. A major threat to effective deployment of conservation actions for the bees of Europe is an inability to understand and identify the species present and to monitor the state of populations effectively.
According to the European Red List, 212 species had no threats identified, while for 1,067 species threats remain unknown. Identified threats for the remaining species (663) are presented below, and a summary of the relative importance of the different threatening processes is shown in Figure 11.
Many of the environmental threats to bee diversity are associated with modern agriculture and, in particular, shifting agricultural practice and the increasing intensification of farming (Figure 11). These threats include those related to intensive arable farming (loss of uncultivated habitats and widespread use of insecticides and herbicides (Sydenham, et al., 2014, Gill and Raine 2014)), livestock farming (resulting in grazing and stocking regimes that are damaging to grasslands and fragile Mediterranean ecosystems) (Vulliamy, et al., 2006) and the continued presence of commercial timber plantations (Navarro-Cerrillo, et al., 2013).
According to the European Red List, 366 species are affected by changes in agricultural practice, which can lead to large scale habitat loss and habitat degradation, especially in temperate regions. Shifts from grassland hay cropping regimes to the more intensive silage production (i.e. late season to early season cropping) or increased grazing, has resulted in large scale losses of herb-rich grasslands e.g., 97% loss of enclosed semi‐natural grasslands in England and Wales (Bullock, et al., 2011) and 97-99% of the historically managed grassland in Sweden (Dahlström, et al., 2008). Loss of season-long flowering impacts particularly strongly on long-lived social insects, especially bumblebees (Bombus spp.), and in more intensively farmed regions of Europe, bumblebees are especially susceptible (Carvell, et al., 2006, Rundlöf, et al., 2008). The loss of semi-natural grasslands also negatively impacts on localised and specialised solitary species (e.g., Andrena hattorfiana and A. humilis in Sweden) (Franzén and Nilsson 2004).
In other parts of Europe, traditional land use has been abandoned, allowing for development of scrub and ultimately woodland. This is especially true in places that are generally unsuitable for more intensive farming, and in places such as the Baltic States it is abandonment, rather than habitat fragmentation, that is the key driver of species composition in semi-natural grasslands (Dauber, et al., 2006). 331 non-threatened species and 35 threatened species are regarded as under threat from agricultural expansion, intensification and shifts in agricultural practice, and 307 non-threatened species and 16 threatened species are regarded as under threat from livestock farming (often in conjunction with an increased susceptibility to fire in the Mediterranean region).
Pollution, pesticides, and herbicides Among the many threats linked to modern agriculture is the widespread use of agri-chemicals. The results of the European Red List show that 252 species of nonthreatened bees, and 7 threatened bee species are regarded as threatened by agricultural and forestry effluents; either by direct contact, or via a sub-lethal effect on the bees themselves (mainly due to insecticide application) or by damaging the floral resources (mainly due to herbicide application) on which bees depend.
The pesticide story is complex, but studies have shown that exposure to neonicotinoid pesticides can lead directly to the loss of honey bees (e.g., Tapparo, et al., 2012, Pisa, et al., 2015), and commercial Bombus in the US (e.g., Gradish, et al., 2010). Exposure to sub-lethal doses of neonicotinoids have been linked with increased levels of the gut pathogen Nosema in honey bees (Pettis, et al., 2012) and colony loss by impairing overwinter survival in honey bees (Lu, et al., 2014). Elston, et al., (2013) report that sub-lethal effects of thiamethoxam, a neonicotinoid pesticide, in conjunction with propiconazole, a DMI fungicide, affect colony initiation in bumblebee (Bombus terrestris) colonies (see also Godfray, et al., 2014).
A number of laboratory studies (e.g., Goulson 2013, Sandrock, et al., 2014) describe the sub-lethal effects of neonicotinoid pesticides on some species of bees, and growing evidence from field studies indicates that levels of systemic pesticides (neonicotinoids and fipronil) that have been documented in the environment are sufficient to cause adverse impacts on a wide range of non-target organisms, including bees (Pisa, et al., 2015). Traits such as body size, foraging range, food storage, etc. vary highly between bee species and as a result, so does the potential sensitivity to the direct or indirect effects of pesticides (Williams, et al., 2010). It seems clear that honey bee traits make them more robust than other wild bee species to resist the effects of pesticides (Desneux, et al., 2007). Nevertheless, our knowledge about the effects of pesticides is based primarily on honey bees. Gill and Raine (2014) have, however, shown that prolonged exposure of sub-lethal doses of Imidachloprid (a neonicotinoid) affects natural foraging behaviour of commercially reared Bombus terrestris in the field.
Herbicide application can also impact negatively on bee diversity, as it can reduce the availability of flowers on which bees depend and delay the flowering so the timing between the period when food is most needed by pollinators and food availability is disrupted (Boutin, et al., 2014). Herbicide application can have a significant local effect on bees, especially those species that are specialised pollen foragers (Nabhan and Buchmann 1995).
Increasing application of nitrogen-based fertilisers is typical of the widespread intensification of agriculture over much of the continent. Fertiliser use, in addition to encouraging the growth of the target crops, also promotes rank grassland, low in flowering plants (especially Fabaceae) (Wilson, et al., 1999) and poor for many bees, especially some Bombus species and Fabaceae specialists.