in the beginning, all were vegetarian:
1:30 And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein there is life, I have given every green herb for meat: and it was so.
at some point, some of the beasts became carnivores. even answersingenesis can only guess how that happened:
Scripture simply does not provide enough information for Christians to insist dogmatically that one or another of these possible explanations is totally right or wrong. Several of them may apply together.
As fallen creatures in a fallen world, we have difficulty imagining what a pre-Fall world was really like. We are also finite creatures lacking all the information. We therefore need to be particularly careful about arguing from the present to the past.
source: http://answersingenesis.org/home/area/faq/bad_things.asp
rather atypically tentative for a creationist tract in my experience. in fact, this seems to be a can of worms for ID advocates, since all those marvelously designed attack & defense mechanisms of animals must have had a different function prior to the fall. for instance, consider the bombardier beetle:
The tiny bombardier beetle could not possibly have evolved. His defence mechanism is amazingly complicated, and could only have been created with all the parts working together perfectly. From twin ‘exhaust tubes’ at his tail, this beetle fires into the face of his enemies boiling-hot noxious gases with a loud pop.
source: http://www.answersingenesis.org/creation/v12/i1/bombardier.asp
what could be the function of this elaborate mechanism in the absence of a predatory threat? or if there was none, then God anticipated the fall, and that does not square in my mind with Adam & Eve being punished for disobeying a commandment God knew would be disobeyed.
in my next installment, i might discuss John Woodmorappe, creationist author of a Noah's Ark Feasibility Study. the impatient reader can jump ahead to this synopsis of his work, here: http://emporium.turnpike.net/C/cs/ark/index.htm
{an apology for not providing links, i still can't get them to work}