Large herbivores and ungulates

Brown bison grazing in a wide grassy valley. — Bison grazing in the Lamar Valley, in Yellowstone Park, in 2021 (photograph by Jacob W. Frank, US National Park Service). YellowstoneNPS, Public domain, source

A landscape deprived of its large herbivores does not become wild again. It becomes another landscape, and a poorer one.

In Europe, the end of grazing has a name and can be measured — scrub encroachment. Habitats close up, and the plants and insects of open spaces die out locally.

Eating grass is a technical problem

Grass is everywhere, nobody competes for it, and yet very few animals can live on it. The reason comes down to one word: cellulose. This material, which forms the walls of plant cells, makes up most of a blade of grass — and no mammal possesses the enzyme capable of breaking it down.¹

The solution adopted by evolution is always the same: outsourcing. Herbivores host in their digestive tract billions of bacteria and protozoa which, unlike the animal itself, can digest cellulose. The animal does not digest its grass; it feeds a population that digests it on its behalf, and recovers what that population produces.²

There are two ways of going about it, which have led to two distinct body plans.

Ruminants — cows, sheep, deer, giraffes, antelopes — have a stomach with several compartments. The grass ferments for a long time in the first one, the rumen, then comes back up into the mouth to be chewed again: this is rumination. The process is slow but extracts almost everything.
Hindgut fermenters — horses, zebras, rhinoceroses, tapirs, and also elephants, even though they are not ungulates — ferment their food in an enlarged portion of the intestine, after the stomach. The process is faster and less complete, but it allows zebras to use taller and more fibrous grass than wildebeest usually select. This is one of the reasons why zebras and wildebeest can share the same savanna.³

What “ungulate” means

An ungulate is a mammal that walks on hooves — that is, on the tips of its digits, protected by thick horn. The word comes from the Latin ungula, nail.

The group divides according to a simple, visible criterion: the number of digits bearing the weight.

Artiodactyls, with an even number: two digits for cattle, sheep, deer, giraffes and wild boar.
Perissodactyls, with an odd number: a single digit for the horse, three for the rhinoceros and the tapir.

This anatomical detail matches the mode of digestion: ruminants are all artiodactyls, and all perissodactyls are hindgut fermenters. The converse is false, and that is precisely what makes the classification instructive: the elephant is also a hindgut fermenter without being a perissodactyl — it is not even an ungulate, and its closest relative is the manatee. Two characters that apparently have nothing to do with each other — a foot and a stomach — tell the same story of kinship. This is the kind of cross-check on which the whole classification of living things rests.

A clarification is needed, because modern classification holds surprises: cetaceans — whales and dolphins — are, genetically, artiodactyls. Their closest living relative is the hippopotamus.

Animals that shape landscapes

A large herbivore does more than eat:

It browses, which prevents young trees from becoming established.
It tramples, creating paths and bare ground.
It sometimes fells branches or whole trees.
It carries seeds in its coat and its droppings, sometimes over several kilometres.⁴

See also: our page on grasslands and savannas details the other key factor maintaining these habitats: fire, deliberately lit by humans for millennia.

The two mechanisms are inseparable: an open habitat is the direct result of the species that live in it, herbivores and human communities alike.

Population figures: methodological rigour required

One might expect to find elephant numbers here: how many there are, how many there were, what bans on the ivory trade have changed.

We do not give these figures, and that is a choice.

Counting elephants on a continental scale involves divergent methodologies (aerial surveys, ground counts, statistical modelling) over different time frames, producing considerable discrepancies.⁵

It is the same situation as for sharks, where simplified estimates repeated in the media differ by a factor of more than eight from scientific studies.

Giving a single figure without saying how it was obtained, for which year and over what area, would amount to producing a figure that looks solid and is worth nothing — the flaw that this site keeps taking apart elsewhere.

A page may refuse to answer. It must then say why, and that is what this one does.

The ecological impact of large herbivores

Received wisdom has it that large herbivores “open up” habitats by eating tree shoots. The accurate term is “vary”.

Scientific research has identified two major mechanisms:

The creation of vegetation heterogeneity: Feeding takes place unevenly, producing a mosaic of closely cropped areas, untouched patches, trampled corridors and deposits of organic matter. The result is not simply an open habitat, but a diversified landscape matrix.
Zoochorous dispersal: The passive or digestive transport of seeds allows many plant species to colonise areas beyond their natural reach.⁶

Consequences of their disappearance

In Europe, the decline of pastoralism and natural grazing has led to a measurable phenomenon: scrub encroachment.⁷

Without herbivore pressure, habitats close up, which disadvantages species that prefer open areas.

An abandoned landscape does not spontaneously become “wild” again: it turns into a different ecosystem, hosting a distinct biodiversity.

The complex interaction with fire

Elephants break trees; fire destroys them. Far from adding up in a linear way, their impacts reinforce each other: the presence of elephants increases tree mortality during fires compared with the action of each factor alone — this is what a study in Kruger National Park, South Africa, on trees stripped of their bark by elephants showed.⁸

Conversely, intense grazing produces the opposite effect. By consuming dry biomass and reducing ground vegetation cover, herbivores deprive fire of fuel, thus limiting the spread of fires.⁹

So the same biological category can promote or suppress fire depending on grazing intensity and the ecosystem context.

Impact on carbon storage

Research on forest elephants reveals a counter-intuitive mechanism, which is worth describing step by step.

As they move about and browse, elephants reduce the number of trunks in the forest. Fewer trunks means less competition for light, water and space. The remaining trees therefore grow fewer, but larger — and with denser wood.¹⁰

And dense wood stores more carbon. The result: in the long term, a forest with elephants contains more biomass than a forest without them.¹¹

Put the other way round by the researchers, the question becomes striking: if forest elephants disappeared, the forests of Central Africa would lose about 7% of their aboveground biomass.¹²

Finally, the study has the honesty not to hide the downside: these dense-wood forests produce less — growth there is slower. What is gained in stock is lost in production. The authors themselves describe it as a trade-off.¹³

An animal that damages vegetation can therefore increase the carbon stored in a forest.

Methodological limits and caveats

⚠ Scientific caveat: Exclusion experiments, often small and short-lived, capture only part of the effects of large herbivores on vegetation at the landscape scale. The clearest impacts are observed at large scales, where establishing strict causal links remains complex because of confounding variables.
Moreover, some plant species historically dependent on megafauna have survived its disappearance, contradicting simple predictive models.¹⁴

This question remains open. This article documents measured states without making management recommendations. The question of reintroducing large herbivores is a matter of political and societal choices that it is not an encyclopaedia’s role to settle.

The impact of domestication

The history of domestication highlights two established facts:

The emergence of at least eleven independent centres of domestication across the globe, without intercontinental contact.
A repeated osteological finding: the skeletons of the first farmers and herders frequently show a smaller stature and lower weight compared with their hunter-gatherer predecessors.¹⁵

Besides their labour, these species have been sources of essential raw materials: wool, leather, sinew, horn and bone.

Note 1

This represents an enormous source of energy, yet vertebrate cells do not produce the cellulases necessary to break down this abundant material.

Colorado State University, VIVO Pathophysiology, “Digestive Physiology of Herbivores”