That is the question. There are several species of bees that are currently on the Endangered List and more which are precariously close to being added. Though there are a number of reasons why the bee population has declined, like pesticides and loss of habitats, the number one reason is Climate Change.
Bees are very important to the entire world. They pollinate billions of plants every year, including agricultural crops. Without them many plants we rely on for food would die off. And because bees pollinate wild plants as well, they provide food for wild animals.
Many tree species couldn't grow if bees weren't around to pollinate them. It's the same for the plants in your garden. These trees and plants serve as home to hundreds of small creatures, like birds and squirrels down to the tiniest of insects. None of them could survive if bees were gone because their habitats and food would disappear.