Grazing and grassland richness
A Socratic walk-through of grazing and grassland richness — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why can a grassland support more kinds of plant when grazing animals are allowed onto it?
Fence off a species-rich meadow, remove the sheep, and leave it alone. The intuition is that the plants, relieved of being eaten, will flourish. What often happens instead is that within a few years the meadow has fewer kinds of plant than when the animals were on it — a dense sward of two or three tall grasses where there had been thirty species.
That is peculiar. Being eaten is a straightforward cost to a plant. How can adding a cost to every species in a community end up with more species present?
Reasoning it through
REASONING #Try the question in the other direction. What happens in an ungrazed, reasonably fertile grassland? A handful of tall, fast-growing grasses grow up, close their canopy, and take the light. Everything shorter is left in shade, and shade is not a partial handicap — for a small rosette plant it is close to fatal. Litter accumulates and blocks seedlings from establishing. Over a few seasons the community collapses toward whoever wins the race upward.
That is competitive exclusion, and notice what makes it possible: an uninterrupted stretch of time. The dominant needs several undisturbed seasons to convert its advantage into a monopoly.
So what does a grazing animal do? It removes biomass, and it removes it disproportionately from whatever is tallest and most abundant, because that is what its mouth encounters. Light reaches the ground again. Hooves and dung open small bare patches where a seed can land and germinate. The dominant is repeatedly knocked back before it can close the canopy, and the subordinates — shorter, slower, more shade-vulnerable — persist because the race they would lose keeps being interrupted.
Now push it the other way, because that is where the shape of the answer lies. Suppose the grazing is very heavy. Now every plant is eaten to the ground repeatedly, and only species that tolerate that survive: prostrate rosettes, rapid annuals, plants defended by spines or toxins. Richness falls again, for a different reason — not competitive exclusion but the disturbance itself.
Low grazing, few species. Heavy grazing, few species. Somewhere in between, many. The relationship is humped, and that shape is the whole content of the answer.
There is a second condition that is easy to miss. If light is not what limits the small plants — if the system is dry or infertile, so that no dominant can build a closed canopy in the first place — then removing biomass is not doing them a favour. There is nothing to release them from, and grazing is simply a cost. The best evidence for this is the tidy result that herbivores tend to increase plant richness precisely where they increase the light reaching the ground, and not otherwise.
A third condition is historical. Where large grazers have been present for a very long time, the flora is stocked with species that evolved under grazing and do well with it. Where livestock arrived recently — much of the interior western United States, parts of Australia — the resident flora has no such background, and grazing at the same intensity does considerably more harm. The plants are not responding to grazing in the abstract; they are responding to a pressure their ancestors did or did not experience.
The analogy
THE ANALOGY #Think of a public square with no rules about stalls. Left alone, a few large traders take the best pitches, expand, and physically crowd out everyone else; by the end of the year the square sells three things. Now impose a rule that every stall is cleared each week. The big traders can never entrench, and small specialised sellers keep finding room, so the square offers far more. Clear the stalls every hour instead and only the very fastest survive — variety collapses again, this time from the clearing rather than from the crowding.
Stalls are cleared indiscriminately, whereas a grazing animal chooses — it prefers palatable species and avoids spiny or toxic ones, so grazing simultaneously suppresses the dominant and selectively favours a particular set of plants, which is why a long-grazed sward has a distinctive composition rather than simply a longer species list.
Clarifying the model
THE MODEL #This reasoning is the intermediate disturbance hypothesis, and here honesty matters more than tidiness. The general theory has been seriously criticised — most pointedly by Fox in 2013, who argued it should be abandoned, on the grounds that the coexistence mechanism it invokes does not follow from the theory as usually stated, and that surveys of empirical studies find the humped pattern in only a minority of cases.
Where does that leave us? With a distinction worth holding onto. The specific result — that moderate grazing raises plant richness in productive grasslands by relieving light competition — is well supported, and it has an identified mechanism that can be measured directly. The general claim — that intermediate disturbance of any kind reliably maximises diversity in any community — is not. The grassland case is not evidence for the sweeping version; it is one system where a particular mechanism happens to operate.
Two further refinements. "Intermediate" is not a fixed dose: what counts as moderate depends on the site's productivity, the species of grazer, and whether the animals are on the land continuously or in short intense bouts. And richness is not the only thing to care about — a regime that maximises the species count can still be wrong for soil carbon, for ground-nesting birds, or for the rarity a site was designated to protect.
A picture of it
THE PICTURE #How to readEach point is a grassland type, not a grazing intensity — read across to the vertical axis for how productive the system is, and down to the horizontal for how long large grazers have been part of it. The upper right is where both conditions for benefit are met: there is a canopy for grazing to open, and a flora with the evolutionary background to cope, so moderate grazing tends to add species. The upper left is the awkward corner, productive but naive, where the same pressure tends to subtract them. The lower band is dry country, where light was never the limiting factor, so grazing changes richness little and damage is slow to heal.
What became clearer
WHAT CLEARED #Grazing does not help plants; it hurts the plants that were winning. In a grassland productive enough for a few tall species to close the canopy, the animals repeatedly interrupt the process by which one dominant would exclude the rest, and the interruption is what the subordinate species live in. Push the interruption too hard and only the tolerant remain, which is why the relationship humps rather than rising. And the two conditions on it — enough productivity for light to be the limiting resource, and a flora with a history of being grazed — explain why the same herd that enriches one grassland degrades another.
Where to go next
ONWARD #- Why cutting hay and grazing produce differently composed meadows despite both removing biomass.
- How rewilding projects choose stocking densities when the "right" intermediate level is site-specific and unknown in advance.
Key terms
TERMS #| Term | What it means |
|---|---|
| Competitive exclusion | the elimination of weaker competitors by a dominant that monopolises a limiting resource, here light. |
| Intermediate disturbance hypothesis | the proposal that diversity peaks at moderate disturbance; influential, and substantially contested as a general theory. |
| Light limitation | the condition, common in productive grasslands, in which access to light rather than water or nutrients determines which plants persist. |
| Grazing tolerance | the set of traits, such as low growth form or rapid regrowth from the base, that lets a plant survive repeated defoliation. |
Every term the collection defines is gathered in the glossary.