A summary of the relative importance of the different threatening processes is shown in the figure below.
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.
Agricultural expansion and intensification
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. These threats include those related to intensive arable farming (loss of uncultivated habitats and widespread use of insecticides and herbicides), livestock farming (resulting in grazing and stocking regimes that are damaging to grasslands and fragile Mediterranean ecosystems) and the continued presence of commercial timber plantations.
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 and 97-99% of the historically managed grassland in Sweden. 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. The loss of semi-natural grasslands also negatively impacts on localised and specialised solitary species (e.g., Andrena hattorfiana and A. humilis in Sweden).
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. 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 non threatened 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, and commercial Bombus in the US. Exposure to sub-lethal doses of neonicotinoids have been linked with increased levels of the gut pathogen Nosema in honey bees and colony loss by impairing overwinter survival in honey bees. 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.
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. Traits such as body size, foraging range, food storage capacity etc. vary highly between bee species and thus, as does the potential sensitivity to the direct or indirect effects of pesticides. It seems clear that traits related to honey bees makes them more robust than many wild bees to resist the effects of pesticides. Nevertheless, our knowledge regarding effects is based primarily on honey bees. Gill and Raine (2014) have, however, shown that prolonged exposure of sublethal 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 food needs for pollinators and food delivery is disrupted. Herbicide application can have a significant local effect on bees, especially those species that are specialised pollen foragers.
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) and poor for many bees, especially some Bombus species and Fabaceae specialists.
Residential and commercial (including coastal infrastructure) development
Urban sprawl and infrastructure development have continued apace in the late 20th and early 21st century, with the expansion of cities, ports, tourist resorts and associated recreational areas. Global trade in goods grew by an average of 6.9% a year between 1997 and 2006 and sea ports in northern Europe have been enacting major extension projects to accommodate this (e.g., EUROMAX-Terminal in Rotterdam with an expansion in area of 1,000 ha). Mass tourism in coastal regions has seen an increase in both the size of the local population and number of hotels (e.g., Malaga; Costa del Sol, Spain saw an increase in the number of hotels from 150 to 300 between 1983 and 2000 and a rise in population of 71.6% from 1950-2000). It is estimated that by 2020 there will be some 350 million tourists visiting the Mediterranean coastal region alone. Along the Mediterranean coasts of Spain and France and all along the coast of mainland Italy, 75-80% of the coastal sand dunes have been destroyed by tourism, urbanisation and industry. Sand dune systems in Greece and Portugal are under growing urbanisation pressures as well. The increasing numbers of tourists in the Costa del Sol has caused an expansion in the number recreational facilities, from tennis courts, marinas and camping grounds to golf courses (many of which are coastal). These threatened systems support bees such as Osmia balearica and Osmia uncicornis.
Tourism associated with skiing is an extremely important economic factor in the Alpine regions of Europe, and the area affected by ski pistes or by infrastructure development related to tourism is still increasing. Work for this Red List suggests that highly restricted, threatened montane bees such as Bombus brodmannicus are believed to be at risk from future skiing-related development. In all, some 166 non-threatened and 19 threatened bee species are regarded as under threat from expanding urban sprawl and infrastructure development, and 21 non-threatened species and 4 threatened species are regarded as under threat from human disturbances associated with tourism.
Other ecosystem modifications, including mining and quarrying
In low lying coastal areas where flood risk from tidal surges is a potentially damaging threat to human life and livelihood, hardening and strengthening of sea walls, and the creation of new defences (such as in the Dutch Delta area) have impacted on coastal habitats, especially saltmarshes. Maintaining the existing coastline generally will cause considerable loss of saltmarsh with direct impact on specialised bee species such as Colletes halophilus (a European endemic with a restricted range).
Priority habitats with sandy soils in Atlantic Europe include areas that support lowland heathland (a temperate dwarf shrub community). These globally important habitats have been under threat for many years now from urban expansion, widespread plantation of commercial forestry and mineral extraction. Urban development can impact habitats through physical degradation and fragmentation, road building, pollution, increased fire risk and waste disposal though landfill. Ten non-threatened species are regarded as under threat from mining and quarrying, and 56 non-threatened species and 10 threatened species are regarded as under threat from a variety of other modifications to ecosystems.
Fire and fire susceptibility
181 species appear to be threatened by fires. Within the Mediterranean basin, between 1980 and 1990, an average of 700,000 ha of phryganic shrublands, heathland and grasslands, were burnt each year by a total of some 60,000 fires, with Greece, Spain, Portugal and France accounting for more than half of this total. Greater fire frequencies and fire extent as a result of climate change have been noted in the Mediterranean basin. In the exceptionally hot and dry summers in the early 21st century, large fires were both widespread and common in the Mediterranean basin. Although lightning is known to be a cause of some fires, it is now considered that about 95% of the fires in the Mediterranean area are of human origin. Whilst fire is an important element in maintaining Mediterranean shrubland, an increased frequency of fire in Mediterranean shrubby ecosystems allied with grazing of immediate post-fire communities can decrease bee diversity. 179 non-threatened species and two threatened species are regarded as under threat from an increased susceptibility to fire.
Climate change (including habitat shifting and alteration, droughts and temperature extremes)
Changes in climate are also considered to be an important driver of increased extinction risk and 136 bee species appear to be threatened by it. In the steppic regions of eastern Europe, an increase in summer rainfall has led to habitat conversion from dry xeric grasslands to meadow and scrub to the detriment of bee species (e.g., Bombus fragrans) that are restricted to these dry habitats. Studies by Maracchi et al. (2005) and Olesen and Bindi (2002) also show that climatic change in Europe will lead to more widespread and prolonged heat waves and summer droughts and an increase in temperature across the Boreal, Arctic and Alpine regions will severely impact the vegetation composition. This is already having an effect on the species associated with these habitats, as the bumblebee species of these biomes come under increased threat of extinction. The European Red List shows that 113 non-threatened species and 23 threatened species are regarded as threatened by climate change.
Major threats to bees in Europe