The authors of a provocative new book have bad news for animal-lovers: pets are bad for the planet. They consume vast amounts of precious resources, produce mountains of noxious waste – and they can be a disaster for wildlife

Henrietta Morrison confidently plunges her spoon into a tin of slow-cooked lamb hotpot and lifts out a mouthful for inspection. She passes her nostrils over the meat chunks and accompanying sauce, smiles, then places the whole lot into her mouth. "Delicious," she remarks, as a small crowd of onlookers gathers round to watch the spectacle.

Someone eating their lunch doesn't usually elicit such attention, but, then again, most people aren't prepared to tuck into a tin of dog food for sustenance. Morrison has a point to prove, though: she is at Europe's largest pet trade show, PetIndex, at the Birmingham NEC, and her company, Lily's Kitchen, sells the most expensive pet food on the market. Her dog food, for example, retails in places such as Harrods for more than £2 a tin, with the promise that the contents are "proper food".

A quick inspection of the ingredients ("organic and certified holistic") shows why Morrison is prepared to put her pet food where her mouth is. Lamb ("60%"), rice, pearl barley, broccoli, spinach, blueberries, flaxseed, marigold petals, burdock root and alfalfa are just some of the ingredients contained within a tin of slow-cooked lamb hotpot. It really does look and sound good enough to eat – that's the whole point.

"I eat my pet food regularly to test batches," says Morrison. "My personal favourite is goose and duck feast with fruits, but chicken and turkey casserole is our bestseller."

Lily's Kitchen and its range of anthropomorphised pet "recipes" represent the somewhat rarefied summit of the UK's pet food industry, which is now said to be worth close to £2bn a year. Just like us humans, the nation's 8 million dogs and 8 million cats – as well as our collective menagerie of rabbits, horses, lizards, tropical fish et al – consume a wide variety of foodstuffs. In recent years, and despite the economic downturn, the pet food industry has witnessed a move towards "premium products", but the market is still dominated by products made with ingredients that, frankly, can send a shudder down any owner's spine. "Hydrolysed feather meal", "derivatives of vegetable origin", "ash" and "animal derivatives" are just some of the delights routinely found in pet food.

The industry has been the recipient of both jibes and brickbats about the true origin of its ingredients for decades. Horse meat, whale, kangaroo – before strict legislation tightened up the rules following the BSE scandal, we were used to hearing all sorts of hypotheses and rumours. But now it faces a new source of criticism: just what is the environmental impact of feeding the huge quantity of "companion" animals around the world? A new book with the somewhat provocative title of Time to Eat the Dog? The Real Guide to Sustainable Living has triggered a highly charged debate about the environmental efficacy of our pet-owning habits. If we are to examine the environmental impacts of all our lifestyle choices, the book argues, then we must also include pets in the discussion, no matter how unsettling the answers. The various environmental impacts attributed to the human food chain are well documented, so isn't it right, for example, that we should now be questioning whether it is sensible to be feeding slow-cooked lamb hotpot to our dogs, too?

The New Scientist, in a recent editorial entitled "Cute, fluffy and horribly greedy", largely agreed with the book's findings that some pets, due to the food they eat, have a surprisingly high "ecological footprint" (a way of quantifying human demand on the planet's ecosystems using a measure called "global hectares"). "According to the authors . . . it takes 0.84 hectares [2.07 acres] of land to keep a medium-sized dog fed. In contrast, running a 4.6-litre Toyota Land Cruiser, including the energy required to construct the thing and drive it 10,000km a year, requires 0.41 hectares. Dogs are not the only environmental sinners. The eco-footprint of a cat equates to that of a Volkswagen Golf. If that's troubling, there is an even more shocking comparison. In 2004, the average citizen of Vietnam had an ecological footprint of 0.76 hectares. For an Ethiopian, it was just 0.67 hectares. In a world where scarce resources are already hogged by the rich, can we really justify keeping pets that take more than some people?"

Speaking from his university office in Christchurch, New Zealand, Robert Vale, who co-wrote the book with his partner Brenda Vale, admits that he has received a "few unpleasant emails" from irate pet owners since the New Scientist article was published, but insists that he still stands by his central point. "We need to know what we're doing when it comes to the environment," he says. "We can't go blind into this debate. Nothing should be off limits no matter how uncomfortable it is to discuss it. Human population growth is a huge issue, too. We have to recognise that we live in a world of finite resources."

Vale says he was "genuinely surprised" when calculating quite how large the environmental impact was of some of our most popular pet species. "Of all the areas we researched for the book, the subject of pets was by far the biggest surprise for us. But all we are arguing in the book is that we should be making sensible, informed choices. For example, it's not really going to be that much of a problem if you have a big dog but also take the bus everywhere, never fly and live in a small home. It's when everyone starts to have a big car, big house, big family and a big dog that the problems start."

Vale does not – as some of his critics seem to assume – advocate a mass cull of the world's pets. But some of his proposed solutions are still likely to shock some pet owners. For example, the book suggests catching vermin such as rats and processing them into a "natural" cat food. Equally, the book proposes a return to the days when families would – hence the book's title – have edible pets. For example, a pair of rabbits would be kept as pets and their offspring would be eaten. It's hard to see that one gaining much traction.

When feeding a pet, however, the advice is to "think feathers and long ears, not horns and fins". In other words, favour pet foods made from chicken and rabbit meat and avoid those containing red meat and fish which, by comparison, have a much higher environmental impact. Last and, perhaps, most obvious: the smaller the pet, the better.

Back among the avenues of stalls at the PetIndex show, vendors jostle for attention with their impressive and sometimes baffling range of pet foods and accessories. One woman proudly tells me why her pet food containing yucca extract makes "her dog's poo stink less".

Another tells me why, when you use her hair-grooming tool, you must aim to "never expose a dog's testicles". Two saleswomen from Shanghai try to explain to me the fashion vagaries of doggie handbags (let's be clear: that's handbags to carry around your dog, rather than handbags made from dogs). I also spot car seats for dogs, a "pet fountain" that allows your cat to drink from a constantly flowing source of water, and a "memory foam" mattress for "senior" pooches with bad backs. I even come across a treadmill for obese dogs – the "Fit Fur Life" with its attendant price tag of £1,865.

Amid this paradise of pet paraphernalia, I meet Ben Helm, the sales and marketing director of Golden Acres, the UK's largest manufacturer of own-brand pet foods. The company owns Lancashire's largest arable farm and its on-site factory produces 70,000 tonnes of pet food a year, exporting to 37 countries around the world. By most measures, it's a huge operation, but it's a doggie biscuit in scale compared with the four leading pet-food manufacturers – P&G, Nestlé, Mars and Colgate-Palmolive – which, between them, are thought to account for more than 80% of the world's pet-food market.

"Some people now spend more on feeding their pets than they do feeding their children," says Helm, with a hint of awe. "It's a huge industry. Our bestselling 'kibble' [dried composite biscuit] is lamb and rice. Until about a year ago, we were importing three shipping containers of lamb meal [the labelling term for dry rendered lamb derivative] from New Zealand every week to make our kibbles, but now we try to source more of it locally as people are worried about food miles." As for the rice, he says: "The pet-food industry is now probably the biggest single importer of rice in the country." (I later check this factoid with the Pet Food Manufacturers' Association. It says: "So far as we can tell, our industry uses 50,000-150,000 tonnes of mostly 'broken rice' [a byproduct of the milling process] a year. This compares with rice imports for human consumption of around 450,000 tonnes in the UK.")

Helm picks up a handful of salmon kibbles for cats and rolls one around in his fingers. "Cat food actually requires more processing than dog food because it makes it easier to digest. We also add high-quality fats to the surface of cat kibbles to aid palatability. They say that you can't fool a cat as they will detect bad fats. We source our chicken fat from the UK."

Helm says that pet-food trends follow human food trends by about a year. He says that "no carb" pet food is currently the "big thing" largely because pet obesity – it is now estimated that between a quarter and half of the cats and dogs in the UK are obese – has become such a big talking point for the industry. Hypo-allergenic ranges are also popular, with many pet owners reporting that their pets are displaying signs of intolerance to the wheat found in many pet foods. It is one of the reasons why many owners are scaling up to the premium ranges offered by the likes of Lily's Kitchen.

When viewing the sheer scale of the pet-food industry from on high, it can be tempting to agree with Vale's conclusion that we must urgently consider the associated environmental impacts of owning a pet. But the industry, as you might expect, puts up a spirited defence, arguing instead that the pet-food industry is actually a highly efficient processor of what would otherwise largely be waste material from the human food chain.

"Far from being unsustainable, pet-food manufacturing uses material from animals which are inspected by vets as fit for human consumption but which are surplus to the requirements of the human food industry," says Michael Bellingham, the chief executive of the Pet Food Manufacturers' Association. "These byproducts must meet the very high safety and quality criteria laid down in European legislation. Without us adding value annually to around 630,000 tonnes of animal byproducts in the UK, it might otherwise have to be disposed of via landfill or incineration. Not very green. Furthermore, a recent report by the Waste and Resources Action Programme [more commonly known as Wrap] is rightly damning of the enormous amounts of food – around 30% – that goes to waste each year. Compare that with the 1% of pet food they found went to disposal."

Bellingham says that the "vast majority" of meat used in pet foods is byproduct from the human food chain, as opposed to "human-grade ingredients" or meat specifically reared for the purpose. The situation with fish, he says, is similar: "The vast majority of the fish ingredients used by industry are the surplus from fish filleting plants, or fishmeal produced from such surpluses. Some 'super premium' products may use human-grade ingredients but, for the very small amount of fish used and the tiny market share, this will have a negligible impact on fish stocks."

But Bellingham also argues that the benefits of pets need to be viewed more holistically, rather than just through the prism of their "carbon pawprint". "Our environment, far from being threatened by pets, is greatly enriched by the part they play in our lives," he says. "Pets in the home instil responsibility, encourage social as well as environmental awareness and have positive health benefits on children. Furthermore, children from households with pets are found to have stronger immune systems and take fewer days off school. People with pets make fewer visits to the doctor – 21% less for elderly people. What large polluting car improves your health and gets you out for a walk every day?"

Archaeology has shown that we have been living with companion animals for at least 12,000 years. For example, in what is now northern Israel, a dig at the remains of the Natufian settlement called Ain Mallaha revealed the grave of an elderly man who is tenderly cradling a puppy in his arms.

That we greatly benefit from the presence of pets isn't really disputed. But in order to reduce their impact on the environment, should there possibly be a limit to the number of pets we have? Because, of course, it's not just the food they eat that's the problem. Some conservationists, for example, have long been saying that the population of domesticated cats is having a detrimental impact on native fauna. As obligate carnivores, cats are, by instinct, opportunistic predators. A 2005 study in Bristol, for example, showed that 131 cats returned home 358 animals – birds, small mammals and amphibians – over the course of a year. It didn't record the prey the cats failed to return home.

Professor Stephen Harris, based at the school of biological sciences at the University of Bristol, was one of the study's authors and he believes that the impact of some pets on local ecology needs to be debated much more widely.

"Compulsory neutering of cats is not really practical," he says. "But people really should weigh up the environmental cost of owning a pet. We each need to ask ourselves if we really need a pet? In the US, animal welfare groups strongly recommend that cats are kept indoors. And in Australia, some states are now discussing making it compulsory to neuter cats, as well as introducing 'feline-free' zones where, if found, cats can be trapped and humanely destroyed by the local authority. But here the British attitude to cats is that they should be left to roam as this is natural." (In an earlier study that Harris co-authored, it was calculated that the UK's 7.7 million cats kill around 188 million wild animals a year.)

But Harris says dogs aren't exactly guilt-free, given that an estimated 250,000 tonnes of dog faeces are deposited on our streets and in our parks each year: "It is calculated that 100 tonnes of dog shit is left on Richmond Park in London each year alone. This has a huge impact on the local ecology. If you see aerial photographs of the area, you can see how yellow the grass is around the car park where all the dogs rush out of the owners' cars to urinate. Pets such as dogs and cats can have lots of these little impacts, which really do add up. Ecologically, pets are very demanding and this is a lifestyle choice that is difficult to justify for most people." (In their book, the Vales make the observation that, in San Francisco, city officials say that dog faeces now account for 4% of the municipal waste sent to landfill each year – the same level as used nappies.)

Marina Pacheco, chief executive of the Mammal Society – who owns a cat herself – says education, rather than legislation, is the answer: "We are aware of the huge impact cats have on small mammals. Yes, we probably have too many cats in the UK, but it's too hard to work out the optimum carrying capacity. We have to be pragmatic about the fact that millions of people do own cats. There are things cat owners can do, though. Keeping cats in during dusk and dawn, which is their natural hunting time, is a good idea, as are collar bells. Not owning too many cats is also sensible. One or two is enough and get them neutered, too. It must become socially unacceptable to be an irresponsible pet owner."

Anyone who owns a pet will keenly testify how much joy and companionship they can bring. But they will also acknowledge just how much time, effort and money they can require, too: a tortoise needs its heat and lighting; a horse needs shoeing and a regular supply of straw; an iguana needs its supply of insects; a chicken needs grit and corn; a dog needs its delousing powder; a cat needs a scratch tower. And then there's the insurance, the vet's fees and the annual cost of food and bedding. It's little wonder that some pets are described as being as big a commitment as having a child in the home. So it shouldn't really come as a surprise that some are now viewing pets as having a similar environmental impact to that of a small person. After all, in many owners' eyes, their pets are very much part of the family.

Back at PetIndex, Morrison is handing out samples of her luxury pet food to passersby. "No, we don't use any animal meal in our pet foods," she says proudly to one interested woman. "It's the devil's work. They strip everything that's good off a chicken, even the fat, then they grind it into a powder for pet food. People are fixated on price – most pet food is cheaper than a tin of baked beans. But more and more people are coming round to the view, just as they are doing with their own diet, that quality counts. We've only been going since last November and we've already turned over half a million pounds. We have to start asking more questions about the food we feed our pets."

For altogether different reasons, Morrison is right.