It is very possible that plants have sensitivities that we do not yet understand. Because plants do not have nervous systems and cannot run away from predators, it has generally been assumed that they do not experience pain and suffering. Recent scientific evidence suggests that this assumption may be incorrect. However, we do know that birds and other nonhuman vertebrates have well-developed nervous systems and pain receptors the same as humans. Like us, they show pleasure and pain and they present comparable evidence of fear and well-being. Animals cry out in pain, they nurse wounded body parts, and they seek to avoid those who have hurt them in the past.
In order to live, one has to eat. However, when we eat animal products, we consume many more plants indirectly than if we ate those plants directly, because the animals we eat are fed huge quantities of grasses, grains, and seeds to be converted into meat, milk, and eggs. As a vegan (one who eats no animal products) you cause fewer beings to suffer and die for you.
2. What will we do with all the animals if we stop eating them? Won't they overrun the earth?
Farm animals will not overrun the earth if we stop eating them because we will no longer intentionally breed them as we do now. Parent flocks and herds are deliberately maintained by artificial insemination, genetic selection, bizarre lighting schedules and other manipulations to force them to produce billions of offspring each year. This inflated population will fade as people stop eating animal products. In time, as David Gabbe states in Why Do Vegetarians Eat Like That?, "farm animals could be left to fend for themselves; some would make out fine, others would struggle to keep from becoming extinct. But, like all animals (except humans), they would adjust their numbers in accordance with the conditions around them."
In the meantime, we have to remember that we, not they, are responsible for their predicament. We have an obligation to find ways to ease the transitional period for these animals.