Most of our bumblebee species are in decline and have been for a number of decades. The main reason for this is the loss and fragmentation of their habitat through urban expansion and a move towards more intensive agriculture using machinery and pesticides.
Historically, much of our countryside was not cultivated as it was not feasible or profitable to get a crop. However, improvements in technology meant that it became possible to cultivate a lot of previously unusable land. It has been estimated that 97% of the UK’s flower-rich traditional grasslands have been lost since start of the 20th century. Since all bumblebees rely entirely on flowers for food, this is clearly a problem.
Some widespread species are still doing fairly well as they are able to collect nectar and pollen from a wide range of plants, including those found in gardens. However, sadly this is not the case for the majority of species. They are more selective in their foraging habits, relying on wildflowers and specialist habitats. They also often have specific nesting requirement that are not easily replicated in a garden setting.