Why No Species Solidarity?

When visiting the grocery store this weekend, I found myself parked next to a mid-size SUV with a back end nearly covered with bumperstickers. This is usually a bad sign in regards to the moderation of the driver's opinions, and indeed this person had chosen to display on their back-end (sometimes repeated several times) thoughts such as "Bush is a Chump-Ass Punk", "I [heart] my [dog]", "I'm Pro-Choice and I Vote" and "Keep Austin Planned".

Why is it that advocating that fewer humans be born so often seems to come with a great devotion to the care of other species, whether endangered ones or domestic? Where's the species loyalty?

I wonder if in part this trends stems from a tendency to think of the person as a free floating individual defined by mind and memory rather than a member of a family, state, species, etc. which constitutes a revolving population in which our place is to grow up, care for others, reproduce, rear the next generation, and eventually die. This "circle of life thing" seems to have been clearly (though perhaps not consciously) understood and accepted in pre-industrial societies, but as it has become easier to think of ourselves as little eternal being in the abstract, primarily an individual rather than part of a great chain on beings, I think people lose track of it.

Thus, animals assume greater importance to the extent to which they form cherished parts of memory and everyday experience. And having offspring is often seen as an obstacle to fulfillment rather than one of the primary purposes of one's life.