Equal Development of Both Wisdom and Compassion
Walpola Rahula, a prominent monk in the Theravadan school of Buddhism, once observed that maturity in the Buddhist way of life requires an equal development of both wisdom and compassion. Neglect of wisdom leads to
foolishness; neglect of compassion leads to a kind of mental sterility. That's why in Buddhism, he argued, "wisdom and compassion are inseparably linked together." A clear, healthy mind should manifest both of these qualities in abundance. (Walpola Rahula, What the Buddha Taught, 2nd ed. [New York: Grove Press, 1974], 46.)
Compassion may begin as a concern for other people, but if you cultivate it thoughtfully, there are two reasons why it will tend to expand beyond the bounds of our own species. First, the welfare of people is dependent upon the welfare of their environment, and that environment is composed in large measure of other living things. So even if the primary focus of compassion is humanity, it must still include the network of life that supports humanity.
And second, people, like any other form of life, are a part of the environment. (See the health connection.) Your physical being is highly volatile. Much of your physical substance actually resides in you for only a short time; all of it will eventually be pulled elsewhere by the great biological, chemical, and physical processes that govern all life. In a sense, people are just one of the ways in which those processes manifest themselves.
So caring for other people can become a way of caring for the environment (if it's done in an environmentally sensitive way). And caring for the environment can be a way of caring for people (if it's done in a socially sensitive way).