There are some spots in the lawn, or along its ambiguous borders, where the good old Kentucky bluegrass has clearly lost its turf war. In the front yard, dandelions, plantains, and crabweed have established their enclaves; in one of the side areas, there is a pebbly place, hardly mown, which yields almost entirely to speedwell and some half-wild asteraceaeous species (very gaunt and not worth picking); and in the back strip, in addition to one corner in the thralls of poison ivy, there are patches subject to outbreaks of red and white clovers, and a border firmly in the grip of ground ivy. Without bothering to count the groundhogs, sparrows, earthworms, springtails, nematodes, mushrooms, and soil bacteria, the lawn is quite a diverse ecosystem.
Regarding turf, Charles Darwin writes in his Origin of Species, as an illustration of the struggle for existence, the following:
If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants; thus out of twenty species grown on a little plot of mown turf (three feet by four) nine species perished, from the other species being allowed to grow up freely.
Fortunately, it seems that when the reverse occurs—when a wild plot is mown—it can be subdued into something like tolerable turf (whether with an increase or decrease of species diversity, I can’t say, but at least with a considerable change). I was struck the other day, mowing the family lawn and comparing notes with Darwin, how much effect a weekly shortening of the grass stems has on the community of life under out feet. There is, for instance, a portion of the backyard, designated for play, which had once been annually smothered in wood mulch, with the plant life kept under control by the unrelenting children’s footsteps, but which now is annually smothered only in snow, and thus is subject to rank grasses, thistles, Velcro weed (id est, Galium aparine), foxtails, smartweed, ragweed, cherry and tuliptree sprouts, and a number of other undesirable species which my guilty disgust has caused me to overlook. I have, however, for the last month or so, included convenient parts of this overgrown area in my mower route, and the effect has been noticeable. Species like thistle and anonymous prickly weeds, which clear a radius of a few inches with their large leaves and require many weeks of growth to flower and reproduce, are going extinct in the mown tract, and the evacuated space is filled by smaller plants with means of asexual propagation. The clover and coarser grass which had grown there before are changing shapes from large, spaced clumps to a continuous low carpet, which I hope will eventually be washed over by the advance of runners and rhizomes from the more civilized bluegrasses a few yards to the west.
Darwin is expansive on the subject of how mowing or grazing grass applies selective pressure to an ecosystem. I excerpt at length an account of the cows and fir trees on the Staffordshire heath:
Many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. … In Staffordshire, on the estate of a relation, where I had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with Scotch fir. The change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath. The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds. Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception of the land having been enclosed, so that cattle could not enter.
But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hill-tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. When I ascertained that these young trees had not been sown or planted I was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted clumps. But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees; and one of them, with twenty-six rings of growth, had, during many years tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch fir… .
As an even more extreme example of species interdependence, Darwin notes how cats determine the frequency (even existence, perhaps) of red clover and heartsease: Only bumblebees have long enough probisci to pollinate these flowers, and bumblebees are only safe from the villainies of field mice in the presence of a sufficient cat population.
I really have to wonder what will happen as a result of mowing the backyard under the swing set.
Image Credit: Albrecht Dürer, Great Piece of Turf