Veganism has rocketed in the UK over the past couple of years – from an estimated half a million people in 2016 to more than 3.5 million – 5% of our population – today. Influential documentaries such as Cowspiracy and What the Health have thrown a spotlight on the intensive meat and dairy industry, exposing the impacts on animal and human health and the wider environment.

Our soils were almost dead. Now we have ​19 types of worm, and 23 species of dung beetle in a single cowpat

But calls for us all to switch entirely to plant-based foods ignore one of the most powerful tools we have to mitigate these ills: grazing and browsing animals.

Rather than being seduced by exhortations to eat more products made from industrially grown soya, maize and grains, we should be encouraging sustainable forms of meat and dairy production based on traditional rotational systems, permanent pasture and conservation grazing. We should, at the very least, question the ethics of driving up demand for crops that require high inputs of fertiliser, fungicides, pesticides and herbicides, while demonising sustainable forms of livestock farming that can restore soils and biodiversity, and sequester carbon.

In 2000, my husband and I turned our 1,400-hectare (3,500-acre) farm in West Sussex over to extensive grazing using free-roaming herds of old English longhorn cattle, Tamworth pigs, Exmoor ponies and red and fallow deer as part of a rewilding project. For 17 years we had struggled to make our conventional arable and dairy business profitable, but on heavy Low Weald clay, we could never compete with farms on lighter soils. The decision turned our fortunes around. Now eco-tourism, rental of post-agricultural buildings, and 75 tonnes a year of organic, pasture-fed meat contribute to a profitable business. And since the animals live outside all year round, with plenty to eat, they do not require supplementary feeding and rarely need to see the vet.

The animals live in natural herds and wander wherever they please. They wallow in streams and water-meadows. They rest where they like (they disdain the open barns left for them as shelter) and eat what they like. The cattle and deer graze on wildflowers and grasses but they also browse among shrubs and trees. The pigs rootle for rhizomes and even dive for swan mussels in ponds. The way they graze, puddle and trample stimulates vegetation in different ways, which in turn creates opportunities for other species, including small mammals and birds.

Facebook Twitter Pinterest Little owls - one of five owl species found at Knepp – feast on the burgeoning dung beetle population. Photograph: Ned Burrell

Crucially, because we don’t dose them with avermectins (the anti-worming agents routinely fed to livestock in intensive systems) or antibiotics, their dung feeds earthworms, bacteria, fungi and invertebrates such as dung beetles, which pull the manure down into the earth. This is a vital process of ecosystem restoration, returning nutrients and structure to the soil. Soil loss is one of the greatest catastrophes facing the world today. A 2015 report from the UN Food and Agriculture Organization states that, globally, 25 to 40bn tonnes of topsoil are lost annually to erosion, thanks mainly to ploughing and intensive cropping. In the UK topsoil depletion is so severe that in 2014 the trade magazine Farmers Weekly announced we may have only 100 harvests left. Letting arable land lie fallow and returning it to grazed pasture for a period – as farmers used to, before artificial fertilisers and mechanisation made continuous cropping possible – is the only way to reverse that process, halt erosion and rebuild soil, according to the UN Food and Agriculture Organisation. The grazing livestock not only provide farmers with an income, but the animals’ dung, urine and even the way they graze, accelerates soil restoration. The key is to be organic, and keep livestock numbers low to prevent over-grazing.

Twenty years ago, our soils at the farm – severely degraded after decades of ploughing and chemical inputs – were almost biologically dead. Now we have fruiting fungi and orchids appearing in our former arable fields: an indication that subterranean networks of mycorrhizal fungi are spreading. We have 19 types of earthworm – keystone species responsible for aerating, rotavating, fertilising, hydrating and even detoxifying the soil. We’ve found 23 species of dung beetle in a single cowpat, one of which – the violet dor beetle – hasn’t been seen in Sussex for 50 years. Birds that feed on insects attracted by this nutritious dung are rocketing. The rootling of the pigs provides opportunities for native flora and shrubs to germinate, including sallow, and this has given rise to the biggest colony of purple emperors in Britain, one of our rarest butterflies, which lays its eggs on sallow leaves.

Not only does this system of natural grazing aid the environment in terms of soil restoration, biodiversity, pollinating insects, water quality and flood mitigation – but it also it guarantees healthy lives for the animals, and they in turn produce meat that is healthy for us. In direct contrast to grain-fed and grain-finished meat from intensive systems, wholly pasture-fed meat is high in beta carotene, calcium, selenium, magnesium and potassium and vitamins E and B, and conjugated linoleic acid (CLA) – a powerful anti-carcinogen. It is also high in the long-chain omega-3 fatty acid DHA, which is vital for human brain development but extremely difficult for vegans to obtain.

Much has been made of the methane emissions of livestock, but these are lower in biodiverse pasture systems that include wild plants such as angelica, common fumitory, shepherd’s purse and bird’s-foot trefoil because they contain fumaric acid – a compound that, when added to the diet of lambs at the Rowett Institute in Aberdeen, reduced emissions of methane by 70%.

In the vegan equation, by contrast, the carbon cost of ploughing is rarely considered. Since the industrial revolution, according to a 2017 report in the science journal Nature, up to 70% of the carbon in our cultivated soils has been lost to the atmosphere.

So there’s a huge responsibility here: unless you’re sourcing your vegan products specifically from organic, “no-dig” systems, you are actively participating in the destruction of soil biota, promoting a system that deprives other species, including small mammals, birds and reptiles, of the conditions for life, and significantly contributing to climate change.

Our ecology evolved with large herbivores – with free-roaming herds of aurochs (the ancestral cow), tarpan (the original horse), elk, bear, bison, red deer, roe deer, wild boar and millions of beavers. They are species whose interactions with the environment sustain and promote life. Using herbivores as part of the farming cycle can go a long way towards making agriculture sustainable.

There’s no question we should all be eating far less meat, and calls for an end to high-carbon, polluting, unethical, intensive forms of grain-fed meat production are commendable. But if your concerns as a vegan are the environment, animal welfare and your own health, then it’s no longer possible to pretend that these are all met simply by giving up meat and dairy. Counterintuitive as it may seem, adding the occasional organic, pasture-fed steak to your diet could be the right way to square the circle.

• Isabella Tree runs Knepp Castle Estate with her husband, the conservationist Charlie Burrell, and is the author of Wilding: The Return of Nature to a British Farm