It was a protest against me. But I sympathised with the demonstrators, who gathered outside the theatre where I was speaking last week to call me a killer. I didn’t dispute their claim. I am a killer.

While making Apocalypse Cow, a film for Channel 4 about how we should feed ourselves without destroying the world, I shot a deer. If it helps (though it didn’t help the deer), I hated every minute of it, from picking up the rifle and learning to use it, to finding and stalking the innocent animal, then shooting it through the chest from 180 metres, watching it rear into the air, stumble, spasm and die. It was a gruesome, horrible experience.

To respect the life of the deer is to disrespect the life of the capercaillie, the red squirrel and the pine marten

I was seeking to demonstrate the realities of ecological restoration. If, I reasoned, we believe something is right, we should be prepared to do it ourselves. But do we really have the right to take another life?

The problem arises in this case because of humanity’s disastrous intervention in the ecology of the Scottish Highlands. By exterminating wolves and lynx, we released the deer from predation, and their numbers exploded. Because tree seedlings are highly nutritious, the deer selectively graze them out. A rich mosaic of habitats becomes a drab monotony of heather and rough grass. The deer I shot was one of thousands killed on the Glenfeshie estate in the Cairngorms. As a result of this cull, the trees are returning. The regenerating forests are full of birds and other mammals.

Surely, as the protesters insisted, there is an alternative? Some of us have campaigned for years for the return of wolves and lynx, but it cannot happen without widespread public consent, and this takes time. In the meantime, what should be done? The protesters’ favoured alternatives are contraception or fencing.

To fire a contraceptive dart into a deer, you have to approach to within 40 metres. But because the deer have wiped out the trees, you can rarely get that close. Even if you could find some other, ecologically safe means of delivering the chemicals (none yet exists), suppressing fertility across a population is extremely difficult, perhaps impossible. A review of the science concluded that “for wild deer populations, contraception is not a substitute for hunting”.

Some landowners seek to fence out the deer. It’s extremely expensive, and difficult on steep mountainsides. When it works, it creates two dysfunctional and unbalanced ecosystems: one with too many herbivores, the other with none. If fencing took place on a large enough scale to make an ecological difference, huge numbers of deer, deprived of subsistence, would starve. Their deaths would be slow and agonising.

Does Johnson 'understand' climate change? – Politics Weekly podcast Read more

I corresponded with a member of the protest group, who raised a much deeper issue: what animal rights advocates call speciesism. “If it were valid to kill deer for their environmental impact, why would it not be valid to kill humans for their far worse environmental impact?” He claimed that there is no coherent argument based on different levels of cognition or sentience. Though his views are clearly influenced by the philosopher Peter Singer, he – with other animal rights activists I’ve met – goes way beyond Singer’s utilitarianism. “Animal rights, like human rights, are individual rights,” my correspondent argued. “It is never acceptable to decide to sacrifice one individual in order to arguably do others good.”

But inaction in this situation is freighted with the same moral problems as action. As a result of prior human intervention (exterminating their predators), refraining from killing deer means killing other wildlife. To respect the life of the deer is to disrespect the life of the capercaillie, the crossbill, the goshawk, the wildcat, the red squirrel and the pine marten. By leaving deer alone, we sacrifice other animals individually and en masse. This conflict is sharpened by the fact that many landowners deliberately keep their deer numbers high, partly because stalking estates are valued for sale by the number of stags. For completely different reasons, like the animal activists, they value the lives of the deer above those of other species.

Were we to apply a universal prohibition on killing other animals, even vegans would starve. Though a plant-based diet requires much less land (including less arable land) than a meat-based diet, it still results in the inevitable death or exclusion of other animals, from the mouse scooped up by the combine harvester to the owl that would have lived in the woods the field replaced. No animal can sustain its existence without privileging itself above other life forms. Even when our minds reject it, our stomachs insist on speciesism.

At the other end of this spectrum the Norwegian philosopher, Ole Martin Moen, contends that because wild mammals and birds endure a great deal of distress, caused by predation, disease and hunger, we should relieve it by “drastically reducing the size of wildlife populations”, and confining the survivors to parks, where humans could look after them. If he had evinced any understanding of ecology, or of the scale and consequences of the intervention he suggests, or had recognised that wild animals feel pleasure as well as pain, his argument might merit a response. Nevertheless, we seem determined to implement this ridiculous idea, if not for the reasons he proposes.

Between these poles – kill nothing and kill almost everything – lies the pragmatic aim of maximising the diversity and abundance of non-human life on Earth, while securing our own survival. But this doesn’t answer the activist’s central and important point. If it’s acceptable to kill wild animals to alleviate environmental damage, why is it not acceptable to kill humans? In other words, why might we see another animal’s right to life as negotiable, but the human right to life as absolute?

A citizens’ assembly on climate is pointless if the government won’t listen | Stephen Buranyi Read more

Because if we did otherwise, society would fall apart. The powerful would decide that the powerless must die for the greater good, as they have done many times before. Our relations would be characterised by extreme distrust and perpetual violence. We could not work together for any purpose, including environmental protection.

Yes, I am a speciesist. Not because I believe human beings are innately superior to other animals, but because I believe we cannot live together (or even alone) without privileging our own existence. We don’t have to see ourselves as the divinely appointed stewards of creation to recognise that we bear responsibility for restoring the magnificent living systems we have harmed. And we don’t have to deny our bias towards ourselves to defend the lives of other beings.

• George Monbiot is a Guardian columnist