It took a lot longer than I thought, and came as something of a surprise. There are over 160 species of plant and animal that I know of that either live in or visit my yard. That's rather more than I expected.
The more diversity, the better, of course. Monocultures are boring, and unhealthy. In this group, we have 16 trees (probably more, I'm iffy on the differences in the various oaks, and there's one or two in there that I haven't identified yet) a dozen shrubs (probably more, since we have multiple types of holly, and there's stuff on the sideyard I can't even begin to guess at) 30 insects, 8 mammals, 24 birds, 3 reptiles, coupla slugs and worms, and the rest are herbaceous plants. (There were also two fungi, but I can't identify them from memory.)
I didn't count annuals that I planted--a dozen petunias does not exactly count as enhancing species diversity. I also didn't count anything I've planted that died already. There are 27 plants that I put in that are alive and either perennials, shrubs, or trees, and they may not prove to be enduring parts of the ecosystem--who knows what'll come back next year. So the number may fluctate a bit, and most of those 27 are probably not contributing notably to the ecosystem at the moment--one stalk of beardtongue isn't exactly a revolution--although some of them are feeding hummingbirds, goldfinches, and bees, and thus earning their keep.
The other thing is that there's potentially a lot more species than I'm listing--I don't know one jumping spider from another, so while I know I've seen multiple jumping spiders, I don't know how many or what species they are, and they all get lumped as "jumping spider." Or "cricket" or "grasshopper" or "mosquito" or "firefly." And let's not even mention "grass"--there's a number of grasses back there, and the only one I know is the hated stiltgrass. And there's a lot that I saw that I couldn't begin to place--what was that bug that looked like a tiny green leaf? or the vividly lavender little one with the bright pink racing stripes? What are those thousand varieties of moths that I had lump together as "moth?"
I'm on more solid ground with the things with backbones, but even then I run into a bit of trouble--I've seen bats flickering by overhead in the evening, but I don't know what kind they may have been.
A surprisingly good number are natives, too--over two thirds. I feel good about that. There's less than a dozen invasive eradicate-for-the-love-of-god species, and that could be a lot worse.
160 species for one yard. I hope that means it's healthy, for a patch of urban ground.
I'll keep this number as a kind of baseline. Undoubtedly, the day will come when I move, and someday, fate being what it is, I may wind up facing a bare patch of exhausted dirt, home to zero species. When that happens, I'll have something to shoot for. Now I know it can be done.
I'm very glad to have a yard this established and diverse to start with--I can blunder around and as long as I don't do something completely nuts, like raze the mature trees or spread pesticides on the lawn, it can handle my trial and error. But at the same time, it's small enough that I can see cause and effect--the stiltgrass ran wild, and now there are no more eastern cottontails visiting the yard. They probably won't be back until spring, although they may come through looking for winter forage. I cringed when they ate my flowers, but now I cringe more at their absence. Rip out the silktree, and the native dogwood that's suddenly getting sun begins growing like mad. Tear down the wisteria, and the sour cherry it throttled explodes with new growth. Plant liatris and echinacea, and watch the goldfinches appear.
As various ecologists have said, some ecosystems will not and cannot be restored where humans are living--we cannot turn pavement back into prairie, or strip mall back into cypress swamp--but what we CAN do is invent a new ecosystem, a mosaic of working gardens that support as many species as can fly, flutter, slither, trundle, or burrow in. It's not a perfect solution, but it beats sterile Weed & Feed lawns. And there, at least, I can do my part. The yard is training me. Some day, thanks to it and the hundred and sixty odd species that make it up, when I do face that bare dirt, I'll hopefully know a little better what I'm doing.