As a result, the overfertilisation of the ground with nitrogen and phosphorus by footpaths could reach levels that would be illegal on farmland.
The research stated that dogs are fed at home, then excrete nutrients on walks, leaving an annual average of 11kg of nitrogen a hectare, and 5kg of phosphorous.
This pollution equates to that which is transported through the air from farming, industry and traffic fumes, which amounts to anything from 5kg to 25kg of nitrogen - so the impact of dog poo and urine is substantial.
Yet many dog walkers are under the impression that leaving their pet’s waste products in nature is completely harmless.
Scientists counted dog numbers over eighteen months in four nature reserves on the outskirts of Ghent in Belgium - and said the situation would be similar across Europe, which has about 87 million dogs.
The majority of ecosystems are naturally low nutrient environments, and overfertilisation reduces biodiversity, by allowing several thriving plants, like hogweed or nettles, to dominate, which forces out other plants and the wildlife depending on them.
The research was led by Prof Pieter de Frenne of Ghent University, who told The Guardian: “We were surprised by how high the nutrient inputs from dogs could be. Atmospheric nitrogen inputs from agriculture, industry and traffic rightfully receive a lot of policy attention, but dogs are entirely neglected in this respect.”
Researchers estimated the illegal levels of nitrogen and phosphorus in scenarios where dogs must be kept on leads, and only venture two metres either side of a path.
Dr Frenne said: “Those levels are quite staggering, as our study concerned nature reserves. Of course, there are a lot of beneficial effects [to walks in nature], both physically and psychologically, for owners and their dogs but the drawback is bringing in significant amounts of nutrients.”
As the level of dog ownership does not vary in many countries in Western Europe, Dr Frenne believes the situation in Ghent is applicable elsewhere too.
Meanwhile, the British conservation charity Plantlife has cautioned that nitrogen pollution is “one of the greatest threats to our wild plants, lichens and fungi, yet little is being done to tackle it.”
The research, published in the journal Ecological Solutions and Evidence, counted more than 1,600 dogs during 500 nature reserves visits, at all times of day and all days of the week.
This data was combined with the known levels of nitrogen and phosphorus excreted by dogs to estimate the total nutrients deposited.
Getting rid of dog faeces removed almost all the phosphorus, but only half the nitrogen, due to dog urine.
Dr Frenne explained: “Urine is, of course, difficult to take away.”
He suggested managers of nature reserves with sensitive ecosystems should think about banning dogs, which already happens in certain places, to protect wildlife and birds - especially as previous research found that high nutrient levels can persist even three years after dogs are banned.
“An important first step is making dog owners aware of this fertilisation effect,” De Frenne said. “I think many people will just pick up the faeces.”
Rob Stoneman, director of landscape recovery at The Wildlife Trusts, UK, added: “Nature reserves are special places for nature conservation, where wildlife and fragile habitats are protected.
“Obviously poo is a part of nature, but dog poo contains nutrients which can damage the ecology of vulnerable habitats. Wherever you walk your dog, it is important to pick up, bag and bin poo, to ensure the continued protection of these wild areas for us all to enjoy.”
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