Todd> No, the optimal foraging rule doesn't assume scarcity.  It stipulates
that the preferred foods will be those with the highest ratio of caloric
return for calories used up in obtaining them.

That's not a rule, that's a theory.  The only hard and fast rule you can
get from that statement is that you must get more calories from your food
than you expend in obtaining it.  Once you've met that caloric threshold,
other considerations such as nutrient composition, etc. come into play.

An illustration from nature.  Trout need basically four things to survive:
proper water conditions, protection from predators, protection from the
water current, and food.  A pecking order exists with the largest, most
dominant trout getting the best holding spots down to the smallest, least
dominant trout getting the leftovers.  The choice of holding spot is key,
and there is a sort of calculus going on when the decision is made about
where that holding spot will be.  It has to be sufficiently out of the
current, behind a big rock for instance, that they don't have to swim hard
all day.  But it also has to be near enough to that current because the
flow is like the old Lucille Ball gag where she's got the assembly line of
chocolates coming at her.

But a couple of interesting things happen once a certain caloric threshold
is easily met, such as in a rich trout stream.  The trout no longer cares
about getting what are to the fly fisherman's eyes the best spots in the
river, areas behind rocks, convergences of fast and slow water seams.  And
no longer does he choose the most calorically dense foods.  It's maddening
to the fly fisherman because you can no longer just throw the biggest fly
you have behind the most likely looking spot like you can in a less rich
stream.  You have to go find the fish and then figure out what he's eating.
And although they might be plentiful in the stream, a meaty two to three
inch stonefly nymph can pass by untouched because once he's met his caloric
threshold (which is only made possible by the fact that this is a rich
stream) the trout favors clearly less calorically endowed bugs.  A mayfly,
for instance,  might be one twentieth the size of the stonefly.  But there
the trout is, choosing to eat mayflies instead of stoneflies.  Unless
you've got a mayfly in your box, you're not catching anything.  Again, once
the fish has met that caloric threshold, other considerations than caloric
density come into play.