Hi Paul

> What I am having trouble understanding is the claim
> that natural pollinators aren't abundant and fairly 
> diverse even in and around our most intensive 
> monocultures.

The four photos of pollinators which you gave show only the migrant butterflies painted ladies and monarchs.  They aren't particularly effective pollinators and may not have been raised in the locality.  A healthy ecosystem would deliver a variety of solitary and social bees and wasps, as well as a range of pollinating flies, beetles, moths and butterflies. 

Perhaps there were other pollinators there too that didn't make it into your photographs.  There are bushes in the background which indicate non-cultivated land that could nurture a range of pollinators.

You asked about (US) studies that quantify 'serious yield losses over large areas'.  I can't give you that, but the Canadian study below shows economic effects linked with pollinators from nearby uncultivated land.

all the best

Gavin

Title:  Pollinators provide economic incentive to preserve natural land in agroecosystems. 
Author(s):  Morandin, L. A.; Winston, M. L. 
Source:  Agriculture, Ecosystems & Environment 116 (3/4) : 289-292 2006 
Language:  English 
Abstract:  Natural habitats are considered inherently indispensable to the global economy by conservationists, but few natural ecosystems afford direct and quantifiable economic benefits. Quantification of natural land value can provide compelling evidence favoring preservation over development. Wild bees are important pollinators of many crop plants, and natural patches in agroecosystems enhance pollinator services and crop yield. Bee abundance was greatest in canola fields that had more uncultivated land within 750 m of field edges and seed set was greater in fields with higher bee abundance. A cost-benefit model that estimates profit in canola agroecosystems with different proportions of uncultivated land is presented. Yield and profit could be maximized with 30% of land uncultivated within 750 m of field edges.

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