Field notes on things that run themselves
The Less There Is, the More They Get
On the Serengeti in the wet season there are patches of grass cropped to the height of a putting green, and they are the best grazing on the plain. Not despite being eaten flat. Because of it — and a million animals will walk past taller, fuller grass to reach them.
Roughly 1.3 million wildebeest move through the Serengeti–Mara in a year, and in the wet season they crowd onto the southern short-grass plains, which hold less grass than anywhere else they could stand. The ecologist Samuel McNaughton named what they were doing in 1984, in The American Naturalist, in a paper called “Grazing Lawns.” He fenced plots across Tanzania and Kenya and watched both sides of the wire. Some vegetation was “arrested in a short form throughout the wet season” — held there, not stunted, for months. Then he measured the thing that explains it, with a quantity range science ignores. Not grass per hectare. Biomass concentration: foliage per unit of canopy volume. Outside the fences it was, in his words, “consistently and substantially higher.”
That distinction carries the issue. A pasture is measured in area, but a bite is a volume, and no wildebeest harvests a hectare. Tall grass answers how much grass is there with a large number and how much is in this mouthful with a small one, because most of what it has built is stem: scaffolding to hold leaves into the light, and close to indigestible. The lawn has given up on scaffolding, lying flat and packing leaf into the two or three centimetres a muzzle sweeps. Hence McNaughton’s conclusion, about the individual animal: a grazer “obtains a foraging advantage by membership in a herd,” because the yield per bite is greater on a lawn. Each eats better because the others have eaten there. Nobody cooperates. They are exploiting a resource that exists only while it is exploited.
It is tempting — I was tempted, in the paragraph promising you this issue — to say hard grazing pumps nitrogen into the leaf. The careful version stays structural: the standing review of the African literature credits the quality advantage largely to that low stem fraction, and three years of adding nitrogen to a poor South African savanna barely moved leaf nitrogen at all, the fertiliser having bought faster growth that diluted it. A lawn does not deliver richer tissue. It delivers a higher share of the parts worth eating, kept permanently young.
That closes the loop. Grazing holds the lawn down, and being held down keeps off the tall bunch grasses that would shade its flat species to death — couch grass and its relatives, which travel by runners instead of climbing. Around a hundred South African grasses creep like that, and most can be made into lawn by repeated grazing, or a mower.
You can break the loop with a fence, which is the sharpest evidence there is. In a recent three-year trial in the Waterberg, biologists set metre-square exclusion cages on lawn patches. Outside, the lawn grass held its share. Inside, protected, it lost ground to taller neighbours. Fence a grazing lawn to save it and in two seasons you have something else.
McNaughton was careful in a way the retellings are not. The short form, he wrote, was “accentuated by, but was not solely a consequence of” grazing intensity; transplant gardens turned up heritable differences in dwarfing between species. Some of these grasses are simply built low. The mouths are not sculpting an unwilling plant but favouring one that already leans that way, and the two have run together too long for which-came-first to be a good question.
A bolder claim lives next door and should be held at arm’s length. The grazing optimization hypothesis says moderate grazing makes a grassland produce more. Forty years of argument have not settled it — Joy Belsky’s assessment was that the apparent overcompensation is ordinary regrowth read generously — and the lawn does not need it won. It needs only that a grazed sward is worth more per bite and that grazing keeps competitors off — both measured.
Set it beside No. 47, the pines whose cones open only in a fire, and you get opposites that are also rivals. Serotiny needs a rare total destruction that kills the parent to plant the child; a lawn needs a small one arriving constantly and killing nothing. And a lawn will not burn, having no fuel on it — which also means fire elsewhere can steal its animals with a flush of green and leave it to grow out.
Nothing on a lawn is old. The blade cropped this morning was not there last month; the animal cropping it may be two hundred kilometres north by August. The minerals cycle at that pace too, mostly through dung, which is why East Africa’s most durable short-grass patches often sit on abandoned cattle enclosures.
And it is not a switch. You would expect a pattern held up by nothing but appetite to return the moment the appetite did. One recent grazing experiment says otherwise: after a long exclusion, letting the animals back in failed to restore the old structure and settled into something in between — bare soil, no flat grasses. The lawn was never stored anywhere. It was a shape standing in a flow, and a shape you stop can be harder to start again than it looks.
One loop I’m watching
Next: a hill of sand that is alive, and only stands because it is being buried. The first dune ridge behind a beach is built by marram grass, which does something almost no plant does — it grows faster when sand swamps it. Buried, it pushes up through the new layer, knits it with fresh roots, and waits to be buried again, so the ridge climbs for as long as the wind keeps paying it in sand. Stop the burial and the grass declines on ground that ought to suit it perfectly. The same bargain as this issue struck from the plant’s side, with the disturbance arriving as weather instead of teeth. Next time.
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