Peter Singer writes in Animal Liberation:
Among the factors that make it difficult to arouse public concern about animals, perhaps the hardest to overcome is the assumption that "human beings come first" and that any problem about animals cannot be comparable, as a serious moral or political issue, to the problems about humans. A number of things can be said about this assumption. First, it is in itself an indication of speciesism. How can anyone who has not made a thorough study of the topic possibly know that the problem is less serious than problems of human suffering? One can claim to know this only if on assumes that animals really do not matter, and that however much they suffer, their suffering is less important than the suffering of humans. But pain is pain, and the importance of preventing unnecessary pain and suffering does not diminish because the being that suffers is not a member of our species. What would we think of someone who said that "whites come first" and that therefore poverty in Africa does not pose as serious a problem as poverty in Europe?
It is true that many problems in the world deserve our time and energy. Famine and poverty ...all are major issues, and who can say which is the most important? yet once we put aside speciesist biases, we can see that the oppression of nonhumans by humans ranks somewhere along with these issues. The suffering that we inflict on nonhuman beings can be extreme, and the numbers involved are gigantic ... [and] should cause at least as much concern, especially since this suffering is so unnecessary and could easily be stopped if we wanted to stop it. Most reasonable people want to prevent war, racial inequality, poverty, and unemployment; the problem is that we have been trying to prevent these things for years, and now we have to admit that, for the most part, we don't really know how to do it. By comparison, the reduction of the suffering of nonhuman animals at the hands of humans will be relatively easy, once human beings set themselves to do it.
In any case, the idea that "humans come first" is more often used as an excuse for not doing anything about either human or nonhuman animals than as a genuine choice between incompatible alternatives. For the truth is that there is no incompatibility here ... there is nothing to stop those who devote their time an energy to human problems from joining the boycott of the products of agribusiness cruelty. It takes no more time to be a vegetarian than to eat animal flesh. In fact ... those who claim to care about the well-being of human beings and the preservation of our environment should become vegetarians for that reason alone. They would thereby increase the amount of grain available to feed people everywhere, reduce pollution, save water and energy, and cease contributing to the clearing of forests; moreover, since a vegetarian diet is cheaper than one based on meat dishes, they would have more money available to devote to famine relief, population control, or whatever social or political cause they thought most urgent. ... [W]hen nonvegetarians say that "human problems come first," I cannot help wondering what exactly it is that they are doing for human beings that compels them to continue to support the wasteful, ruthless exploitation of farm animals."