By Venerable Khenchen Thrangu Rinpoche
In general, we study karma, the law of cause and effect, in Buddhism. The principle of karma is that if you commit non-virtuous actions you will have bad results and if you engage in virtuous actions then you will have good results. The ten non-virtuous actions can be classified into ten different categories and the opposite of these can be classified into the ten types of virtue.* The first non-virtuous action is the worst, the strongest, and that is to take life, then after that is to take that which is not given or stealing; committing sexual misconduct is another harmful action. These are the first of the bodily non-virtues.
Of the ten non-virtuous actions, really the worst is killing. And the opposite is saving lives. From among all the things one should abandon, first you should abandon killing, and the most important virtue to practice is to save lives. By saving lives you accomplish great virtue; you can protect beings and you can save them from fear.
So because of this, of the ten virtues - saving lives or protecting lives - we can either save the lives of humans or that of animals. My teacher in Kham, Khenpo Gangshar, taught me that among these two - saving the lives of animals or of humans - saving humans is not as powerful and the reason for this is that man can speak for and help himself, he or she can do things to improve their situation – we have ways to help ourselves. But animals, on the other hand, are really lost for means to help themselves, they cannot say anything to anyone and they cannot protect themselves from being eaten and tormented, and that is why saving the lives of many animals is really very important...........http://www.rinpoche.com/animalrelease.htm