I'm going to echo what everyone else has said.... Leave the markers and graves in place. If those were your animals, that's what you'd want. Yeah, you might be dead and gone and not know, but it's still what you wanted when you still had breath in your body.
One of my dogs is buried at my parents' place in the back yard. I never marked her grave but I know where she's at. There are three cats back there too. My parents are 80 and 78 now and will be selling the place soon. I just hope that the new owners don't dig a post hole for a shed or a swingset where she lays. Another dog is buried at an actual pet cemetery. When she died, the place I lived at (in the 'burbs) had such hard packed clay soil I knew it would be a chore to dig the hole by hand, so it was a lot easier to buy a plot and have them bury her. Again, no marker for her grave, but I know which plot it is and stop by from time to time. When the time comes for my current furry friend I will probably just have him cremated.
Reading through all the responses, it's pretty clear that we're a bunch of soft-hearted folks here on TBN!