Expansion of both the human population and its land use have led to habitat loss for pollinators. Critically, due to the loss of food resources and places to nest undisturbed, pollinators have been unable to sustain population levels. One of the primary drives of land use change is agriculture, which since the middle of the 20th century has been exacerbating the problem of habitat loss with its use of pesticides. Whether its fungicide, herbicide or insecticide directly, synthetic chemicals can be lethal to pollinating insects, persisting in the environment to have knock on and cumulative effects.
To compound the problem, the transport of commercial honey bee hives and proliferation of invasive species means that diseases and pathogens are being spread to from domestic to wild bees, and visa versa. Parasites like the terrifying Varroa destructor are making honey bees more vulnerable to other diseases, worsening the situation.
Though mostly documented in bees, factors like habitat loss, the accelerated spread of disease and pesticide poisoning affect many other pollinators and non-pollinators across the planet.