A lot of solitary ground nesters just need a little bit of bare dirt. A lot of long-tongued bees want deep trumpet-shaped flowers and have a hard time maneuvering that tongue into shallower spaces, whereas other bees need the shallow flowers. And many need early spring flowers to get them through!
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I get a lot of little pollinator types on my fennel, and the sweet peas tend to get the big bumble bees.
Here's the 1st ed, next one (out in January) has many good updates because, well, science started paying attention to all pollinators.