The other day, I came home early and caught a bit of "Fresh Air" on NPR. Terry Gross was interviewing Michael Pollan, faculty here at Berkeley and author of the recent book, The Omnivore's Dilemma.
Pollan's book explores four meals, and examines the ecological/commercial/cultural/industrial impact of these meals. One recurring theme is our current dependence on monocultures for food production. In particular, Pollan highlights corn as a food that finds its way into virtually everything we eat (you could eat corn flakes for breakfast, with milk from cows raised on corn; a typical fast-food meal will have high-fructose corn syrup in the soda, more corn syrups and starches in the french fries, corn starch and corn-based binders in the chicken nuggets, plus all the feed corn that went into raising the chickens that eventually became the nuggets, etc.). Pollan also manages to weave it together into a fascinating narrative.
I would encourage you to listen to Pollan's interview on Fresh Air.
He also gave a recent talk here on campus (which of course I attended). He was hosted by Davia Nelson (one-half of NPR's "Kitchen Sisters" and also faculty here at Berkeley), and gave a more extended discussion of the book. One of the take-home messages was that we are strangling biodiversity on farms by growing field after field after field of single species. He pointed out that while this agricultural strategy is wildly successful in terms of feeding people, it is a huge burden on the local ecology. One recommendation is to encourage local farmers to grow a variety of different foods by patronizing farmer's markets or by participating in CSA (community-supported agriculture) groups. This gives farmers a financial incentive to continue growing a variety of foods for a local community, rather than selling one specific food product to large-volume industrial purchasers.
Supporting this type of agricultural (and therefore biological) diversity also strengthens the resilience of our nation's food supply. To put things in perspective, European wine grapes were decimated in the 1850's by phylloxera, and blight destroyed the Irish potato about the same time. While the French didn't need wine to survive, the Irish were overly dependent on the potato, and almost one million died. Many now argue that we're overly dependent on corn. Should we be relying on so few species for the bulk of our nation's food supply? A consumer can vote with his or her wallet— simply avoid highly processed foods and cook with a variety of locally grown produce.
UPDATE: Supporting local agriculture is also an effective way to reduce energy consumption (you really don't need tomatoes shipped all the way from Chile if they can instead be grown 90 miles away).
UPDATE: Although this article in New Scientist sounds a little alarmist to me, I think it's worth considering the dangers of banana monoculture before it's too late.
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In the book and in public talks, Pollan basically suggests that Whole Foods, while an overall improvement on the supermarket experience, sometimes "pretties up" the image of organics without really staying true to what organic foods are all about. In a nutshell, Pollan contended that Whole Foods sells foods that meet the bare minimum standard for organics and yet aren't really what people would think about as organic. As an example, he cited a story in 
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Jason Choy, 1976-2005.
