For the last 50 years, honey bee hives have increased by 45%, but in some parts, mostly in Europe and North America, their numbers have been declining. During the same 50-year period, non-animal pollinated crop production has doubled, whilst animal pollinated crop production has increased four times. The global agriculture is becoming more and more dependent on pollination (Kluser et al., 2011).
With the increase of their importance for pollination services, their decline is more alarming. Land-use change negatively affects honey bee populations. Fragmentation of the natural habitats and degradation are main sources of decreases of pollinators.
Fragmentation of the habitats results in reduction of food sources for resident animals (Kluser et al., 2011). However, when there are small but diverse patches close to the hive, bees are profiting from the diversity of vegetation collecting distinct food sources at short distances. With larger patches close by, foragers have to choose between diversity of resources (using more energy and having longer maximum foraging range), or having low quality resources (Beekman & Ratnieks, 2000).
Moreover, unhealthy ecosystems ease the development of parasites, affecting the populations of wild pollinators as well as managed populations (Kluser et al., 2011).
Another key factor in honey bee population decline is decrease of flower-rich biotopes due to ploughing (Kryger, Enkegaard, Stranberg, & Axelsen, 2011). During summer bees have to fly to farther distances to find resources (Couvillon, Schürch, & Ratnieks, 2014). In the study by Beekman et al. (2004) the mean values of foraging distances were different for small and large colonies. Small hives forage at smaller distances, while large hives foraging further.
In July mean value for small hive was 670m, while the value in August was 1430m; for larger hives these values were 620m and 2850m respectively (Beekman, Sumpter, Seraphides, & Ratnieks, 2004). Hagler et al. (2011) found that the foraging range of honey bees ranged from 45 m to 5983 m.
Air pollution is another issue honey bee population has to face to find available flower resources. Due to the effects of pollution on the insect attracting chemical production in flowers, the scents now can only travel 200m from the plants when in the 1800s it could reach 800m distance (McFrederick, Kathilankal, & Fuentes, 2008).
In parts of Europe crops have become main food source for the honey bees. In United Kingdom main flower source for honey bees have become oil seed rape Brassica napus. Scientists have mentioned that likelihood of “Colony Collapse Disorder” (CCD) increases with low-protein pollen such as blueberries and sunflowers . In agricultural landscapes bees have difficulty obtaining quality resources such as sufficient pollen with essential amino acids, also mineral salt and proteins. Decrease in wild flower resources and the diverse diet is crucial for successful larva development and their survival during the winter (Kluser et al., 2011).
Systemic insecticides used in agriculture, may cause toxic exposure of non-target pollinators, including honey bees. Laboratory studies have shown lethal effects of the highly toxic chemicals such as Imidacloprid, Clothianidin, Thiamethoxam and associated ingredients. There have been cases of population mortality by the unsustainable use of above-mentioned chemicals as well as Varroa treatment. In addition to that, there have been some examples of losing of sense of direction, impairing memory and brain metabolism (Kluser et al., 2011). Herbicide spraying also have indirect negative effects on honey bee populations as important plants and habitats are destroyed (Kluser et al., 2011).
Due to climate change, flower growing periods have been shortening, and flowering periods shifting. Spatial and temporal changes in the floral resource availability may hinder the livelihood of pollinators (Steltzer & Post, 2009). Bee community composition depends on floral composition and resources offered by them (Potts et al., 2003). Climate change sometimes may possibly decrease precipitation, in some regions, altering seasonal rainfall. Conceivably, causing asynchronization between pollinators and plant life-cycles.