Wondering through the forest or grassland of a nature reserve may be an ideal afternoon for dogs, but the urine and feces they leave behind can cause serious harm to these fragile ecosystems, a new study by Belgian researchers finds.

Dog excretions are high in nitrogen and phosphorus, a dangerous mix for ecosystems in nature reserves in Belgium and across Western Europe and parts of the United States that rely on low-nutrient soil. Overnutrition harms the wildlife and biodiversity by attracting nitrogen-loving plants such as nettles, hogweed and hemlock, which are known to spread fast, according to the study.

One main takeaway, said Pieter De Frenne of Ghent University, the lead author, is that owners should pick up their dog’s feces and keep their animal leashed to limit the possible spread of damage.

“Picking up poo would be an important message to dog owners based on this study,” he said. If dogs are leashed and kept to paths, he said, “95% of the other spots are spared.”

Scientists have documented how nitrogen pollution given off by cars or fertilizers can negatively affect an area’s biodiversity by upsetting the soil’s nutrient balance. The high pH levels in dog urine have also been found to potentially harm grass, trees and other plants.

But how dog excretion affects nature reserves, which increasingly abut urban or populated areas, had yet to be analyzed.


In the study, published Sunday in Ecological Solutions and Evidence, a British journal, researchers found that in the four Belgian nature reserves they observed, domesticated dogs excreted an extra 24 pounds of nitrogen and 11 pounds of kilograms of phosphorous yearly per hectare. In comparison, De Frenne said, northern Belgium sees a yearly average of 48.5 pounds of non-naturally occurring nitrogen pollution – just about double the amount from dogs.

“This is really substantial,” he said.

In cases where dogs are leashed and walking along a specific path, the nitrogen and phosphorous levels spike up along the track to levels that are “higher than legal limits allowed on agricultural lands,” De Frenne said. Elsewhere in the reserve, however, nutrient levels are far lower.

Similarly, De Frenne said, when dog feces is removed, nitrogen levels fall by about a half and 97% less phosphorous is added to the soil.

Unlike a small park in a city, nature reserves in Belgium and across Western Europe are managed in a way to reduce levels of nutrients in soil to protect the area’s biodiversity and wildlife. Some of these techniques include mowing, removing hay and top soil, and phytoextraction, or the use of plants to remove impurities in the soil.

“By also allowing dogs in them, especially if the poo is not picked up, these management efforts, which a lot of effort and money goes to, are to some extent counteracted,” De Frenne said.

The Ghent University team spent 18 months counting the number of dogs in four nature reserves around Ghent. They classified each dog according to whether it was wearing a leash and monitored how often people accompanying the animals picked up their feces.


In total, the team observed 1,629 dogs during 487 visits to the nature reserves.

“It is clear that the levels of fertilization by dogs estimated here can potentially exert negative effects on biodiversity and ecosystem functioning of species-rich vegetation that are often pursued in forest and nature management,” the study concluded.

The ecosystems of the reserves studied are similar to environments across Europe, where there are around 87 million dogs.