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But if they have a long neck that shouldn't limit them to only eating vegitation from the top. It should only give them a greater range to select from. A giraffe does have to drink water on the ground, so he is capable of lowering his neck. So if vegitation wasn't left at the top from other long neck animals eating it, then he could certainly eat near the middle or the bottom.
This is indeed the point about environmental pressures. The giraffe can graze at ground level but is clearly not as well adapted to do so as, for example, antelope.
If the loss of treetop fodder were sustained enough, the giraffes, which in the short term struggled by, would be out-competed long-term by the antelope who were far better able to graze there and would die out. Perhaps a lineage of shorter necked giraffes, better suited to grazing on the ground, would evolve. Perhaps not.
You'll be interested to know, by the way, that giraffes bending down at waterholes are quite vulnerable to attack from predators so they often drink in pairs so one of them can act as a lookout. This is a nice example of how behaviour and physical traits can combine to address selective pressures.
The plight of the red squirrel in Europe and the Douglas squirrel in the States is an example of the kind of grazing selection you discuss with regards to giraffe.
The grey squirrel can outcompete both these species by a number of means - perhaps more intelligence, perhaps aggression, but also because they can eat nuts which are less ripe. As a result, there are fewer nuts left for other species. The red and Douglas squirrels, may evolve behaviourally and physiologically to compete, but at the moment they are losing out.
BTW, nice thread, thanks.