Q:

What is the meaning of 'the food chain'?

The OED confirms my long-held suspicion that the original use of the term 'food chain' is becoming supplanted by an altogether different meaning. 
The term 'food chain' was used extensively in the 1950s when scientists first became aware of the dangers associated with the pesticide DDT. The poison built up in the food chain until it reached humans at the top. 
The OED says: under Ia. A series of organisms, each of which is dependent on the next for food, esp. by direct consumption or predation.
1920   Sci. Monthly Oct. 295   Each such fish has its ‘food-chain’..leading back from the food of man to invertebrates upon which it preys and then to the food of these.
1980   R. Mabey Common Ground i. 32   The accumulation in the birds of toxic agricultural pesticides, passed down the food chain from dressed grain to seed-eater to bird-of-prey.
2005   New Scientist 12 Nov. 44/1   Food chains here [sc. at deep ocean vents] are based not on photosynthesis, but on a process called chemosynthesis.
Nowadays the term 'food chain' seems most frequently to be used to describe the various events  necessary to bring food to the table, from agriculture, to distribution, to packaging, to preparation and sale. 
OED 2.
The system or sequence of events by which food comes to be consumed by human beings.
1951   Ecology 32 351   The base of the human food chain is the production of plant organic matter.
1993   Independent 23 Oct. (Weekend section) 35/1   An elongated food chain involving numerous intermediate links—processors, packers, hauliers.
2003   Observer 5 Jan. i. 8/1   Defences against a bioterrorist attack involving animal diseases such as foot and mouth are to be stepped up to prevent rogue groups targeting Britain's food chain.
We have thus arrived at a confusing situation. How, in the view of subscribers to this site, should 'food chain' be used. If both are to continue how do we make it clear to readers and listeners which one we are talking about?  

A:

I would say the second definition is a metaphoric subdivision of the first: the meaning of the chain is extended to include steps that aren't one organism eating another, but rather one group of people processing organic material to pass it on to another group, and to the next, etc.
As to how to distinguish between the two, I think context will usually make the intended meaning clear: if you're talking about processed food for humans, it will usually be the second definition; if it is about a wild animal, it will be the first definition.

