Before the advent of mass production, there were restaurants and cafes that would buy and prepare only foods grown locally in sustainable ways. History shows us obvious ways for sustainability. Our society is so caught up in technology that we've lost the lessons taught from "the old ways." In order to have a sustainable planet, we need to reduce the population. However, after that happens, we can return to a sustainable planet.
Restaurants have the ability to use sustainable food sources to supply some of the food for their business. There are always going to be sources in areas that use reasonable care in how they grow or maintain their farms or gardens. Restaurants will probably not be able to totally supply all their food needs using sustainable sources, but there will be some that they can choose to obtain from these.
I think that a lot of restaurants today have the ability to use sustainable food sources. While it is clear that supply of some food sources may be in trouble, restaurants have the ability to adapt and change their ways to fix such problems. I think that it is important that restaurants try to minimize food shortages before it hurts their company's profits.
It is really not that hard if restaurants wanted to do this. My guess is most restaurant owners figure they do not have the time to do this or they do not want to pay for the extra labor. The gardens for restaurants are actually getting pretty popular. It does not get much more sustainable than growing your own food.
Restaurants do have the ability to use sustainable food sources, but the real question is how do we, as a society, make sustainable food options more profitable so that restaurants are more willing and motivated to actually use them. Another point, is that now with the Internet it is much easier to connect local food producers with restaurants.