Even though this question is an obvious attempt at justifying the containment, killing and consumption of non-human animals, I'll answer it anyway because it's very simple: being vegan, or adhering to a plant-based diet, is not about "preserving all life," it's about not inflicting unneccesary cruelty on beings that are capable of having subjective experiences of pain.
Plants do not have a nervous system, nor do they have any other physical components that would allow them to "feel" pain--they would have no use for any such part as they cannot voluntarily move...what good would feeling pain do if they're unable to get away from the source?
Some people living in what's often referred to as "third world countries" simply may not be able to afford to exclude animals from their diet if they depend on foreign aid packages or the market prices in their area have meat artificially cheap. However, almost all people in "developed" countries do not require the consumption of animals to live--in fact, the opposite is true: most people in developed countries OVERconsume.
The beings most commonly killed in the United States are cows, chicken and pigs. All three can, and do, feel just as much pain as any dog or cat, so unless you're also fine with the thought of slitting a cat's throat without anaesthesia and letting it bleed to death (similar to pigs and cows), or shocking a dog just enough to render it paralysed and then boiling its fur off (similar to chickens) then you have an ethical discrepancy which requires remedy.
I would encourage everyone that would like to further educate themselves on the subject to read this pdf document. http://www.veganoutreach.org/EIYLM.pdf It shouldn't take more than five minutes of your time. Lessening your impact on the planet is very easy and doesn't require that you immediately cease everything at once. It can be a very gradual process, and once parts of an animal-based diet are given up for a few months, you wouldn't miss them.
On a different but similar note, the best available research and analysis of oysters, clams and scallops indicates that for all practical purposes they may as well be plants. There's little evidence to support the notion of them being able to experience pain, so, although I am vegan, I could not fault someone for eating any of these three creatures. However, the methods of acquiring them are often very destructive--dragging weighted nets across the bed of the water destroys coral reef, seaweed, and various other plant life that fish and sea creatures may need to stay alive--so the consumption of them still cannot be justified unless you talk to the restaurant's/grocery store's supplier and determine whether they use such methods. If you're unwilling to do this yet still choose to eat them, then you must be willing to accept the fact that you're contributing to the destruction of the planet for the relatively selfish cause of a few seconds physical gratification. This applies to all sea creatures, though almost all of the ones not mentioned can experience pain, according to the best available research. http://news.bbc.co.uk/1/hi/sci/tech/2983045.stm
And, finally, the "study" that Sparks provided on this page is one of the most ridiculous things I've ever read. Quoting The Uncoveror is no different than quoting one of the joke tabloids that you see in the checkout lines at a supermarket. Look at the home page: "RUMSFELD AND WOLFOWITZ ARE VAMPIRES, W KETCHUP CONTAINS BLOOD" and "COCKROACHES ADMIT THEY WILL MISS HUMANITY". Case closed.
Listed in the article.