This article is from the Birds FAQ, by Lanny Chambers with numerous contributions by others.
Many human activities lead to environmental damage in one degree or
another. We clear, farm, flood, drain, divide, and build upon our
surroundings with alacrity. We have also begun to realize that we
can take steps to minimize the damage we do.
Often, taking steps to preserve the environment is a lot like voting:
it's not clear that any one person's action will have more than a tiny
effect. Nevertheless, like voting, there are many reasons why one should
go ahead and take those steps anyway:
+ Doing so demonstrates that one is a member of a community and
+ Doing so sets an example and provides education to others.
+ One should always act in a way that, if you lived in a world
where EVERYONE acted so, would make that world a good place.
One way human beings damage the environment is by breeding animals to
suit their own purposes. An example of such an "artificial animal" is
the domestic cat, which provides affection and companionship for its
owner and sometimes reduces domestic pests; unfortunately, it also
hunts wild birds with little regard to its own food needs. Some
domestic cats probably do little damage to wild birds. Others have
single-handedly sent entire species (such as the St. Stephen's Island
Wren) into extinction. Regardless, if you own a cat, you can take
steps to diminish its take. You can keep it indoors, or you can bell
it (though the effectiveness of belling cats is often questioned).
Perhaps those steps will have little impact; perhaps your cat will
only kill one fewer bird during its lifetime than it would have
otherwise. Remember that there are billions of cats in the world,
and, for example, only a few hundred Kirtland's Warblers (Dendroica
Invocations of "the survival of the fittest" are not relevant here.
Perhaps many birds are not competent to compete with housecats, or
DDT, or highway construction programs. Nevertheless, we wish to
preserve those birds because they pre-date their human-assisted
competitors, because they represent irreplaceable parts of our world,
and because they are beautiful.
Finally, it's worth mentioning that indoor cats live longer.