Pollination talk, Part II Bees, Master Pollinators and Pollen Pigs

Carpenter Bee with loaded scopa

Most female bees, solitary as well as social species, collect pollen for their offspring. Great fliers, fury, intend on their task, they zip from flower to flower as fast as possible while filling their pollen collection areas on legs or bellies. 

Leaf-cutter bee, loaded with pollen on her underside

They usually concentrate on just one species of flowers at a time - it makes them most efficient. They gather as much and as fast as they can - to provide for as much offspring as possible. Pollen that actually reaches the next flowers stigma is wasted - from the bee's point of view. 

From the plant's point of view, bees may be the fastest and most reliable transporters of pollen between plants of the same species, but they also carry off most of the pollen for their own purposes.
Again, adaptive evolution on the plant's side: 
-we already talked about narrowed access to many flowers that at least excludes the biggest bullies. 
-Some flowers offer nectar, but their pollen is toxic or unpalatable, so the bees quickly get rid of it. 
-Have you seen cherry trees covered in blossoms, buzzing with bees? There are many 'blank' flowers without pollen or nectar among the ones loaded with both. So busy bees will happen upon those, and after a couple of encounters will be encouraged to move on to another tree - which also keeps cross pollination going.  
-Although pollen is usually sticky or statically charged to cling to an insects 'fur' many pollen granules have spikes that prevent the bees from packing it too tightly into their scopae, so hopefully some pollen will fall off to actually pollinate a target stigma.

Tiny fairy bee Perdita sp and giant Carpenter bee Xylocopa sp (photo Joe Wilson)

Of course, the dance of co-adaptation made bees evolve into  tiny fairy bees (Perdita) and gigantic Carpenter bees, and 4000 north American bee species in between. So bees slither or bite through the narrowest throats, became scent-hounds that can tell 'blanks' from a distance, adjust the density of transport hairs to carry the spikier grains and some even specialize in and digest the undigestable pollen. (after: The Accidental Pollinator by Joseph Wilson)

Poster by Joseph Wilson (available on his website)

Honey Bees, Apis mellifera, the imported competition

As an agricultural tool, Honey bees outrank most other pollinating insects. There are quite a few endemic bee species that have the right behavior and physical attributes to be as efficient, but, being highly social, honey bees are available in masses, active throughout the vegetation period, and, most importantly, transportable,  They are also not specialized, they harvest honey and pollen as opportunists. Honey bees can service most crop plants that are grown in huge mono-culture. We don't try to mass produce orchids in an agricultural setting.  So honey Bees are still the most useful species (group) from the human perspective. Their role in the natural ecosystem, however, might be much less positive and can probably be even detrimental where they are brought in by humans and become invasive, as here in the Southwest US. Honey bees can bully endemic bees and out-compete them. They may harvest pollen so thoroughly that not much is left for endemic species once a big hive of HBs has found a pollen source. Remember that this is mostly desert and flowing plants can be few. 
The fact that hives can be moved in and out of an area also allows for the intensive use of pesticides when the bee keepers have moved on. This heavy use of pesticides indiscriminately kills off local pests and pollinators alike.

New study shows strawberry plants 🍓 pollinated by wild bees result in bigger fruits than those pollinated by honey bees. Andrena (mining bees) are some of the most frequent visitors.

As for the question if HBs actively compete with native bees - to the detriment of the latter - see this video of HBs stealing collected pollen from the 'baskets' of a Sonoran Bumble Bee https://www.facebook.com/philipp.wickey/videos/10157772746482921/