Dave, it's exactly what's happenning to me: When moving an object to another layer, or when hitting the undo button, the available memory falls down to zero. Tom Thir from Tech support suggested to clear ecosystems before doing such operations when you know memory is getting low, and repopulate afterwards. I'll check if it works on a particular scene that gives me problems, but it's not a very nice workaround, since you never know if your eco will look the same after repopulating.