While everyone is, hopefully, aware that letting your dog defecate freely on the street is a big no-no, we tend to be much more lenient when this happens on the bare ground, particularly outside cities. Poop is a fertilizer, so your pooch’s number two is actually doing nature a favor. Well… not really.

According to researchers at Ghent University in Belgium, dogs defecating and urinating unhindered in natural parks close to cities and towns have a dramatic negative impact on biodiversity due to overfertilization. As a result of nitrogen pollution, the composition of plants in the ecosystem can change substantially, with more nutrient-demanding species outcompeting specialists.

For their study, the researchers studied four peri-urban nature reserves located less than five kilometers from Ghent, a medium-sized city in Belgium. These nature reserves refer to any forests, grasslands, wetlands, and heathlands in populated areas. The areas selected for the study were specifically managed for biodiversity conservation and contained vulnerable species-rich vegetation.

For a year and a half, the researchers identified and pinpointed hundreds of trails where dogs regularly poop and urinate. They collected soil and plant samples to determine the inputs of nitrogen and phosphorus inputs. The nutrients are essential to the growth of all living things, but when in excess can pollute natural areas.

Dog poop is made of three-quarters water plus undigested food, including carbohydrates, fiber, proteins, and fats from the canine’s digestive system, along with a range of digestive bacteria, some of which can be harmful. Since dogs are fed in households, their feces and urine count as net inputs in natural ecosystems, in contrast to grazing cattle and sheep that feed off the land and recycle nutrients within the ecosystem.

The researchers found that dogs contributed 11 kilograms of nitrogen and 5 kilograms of phosphorus per hectare. For comparison, in ecosystems free of dogs (or humans for that matter), each hectare of protected natural land sees about 0.5 kilograms of nitrogen deposited each year.

Since the vegetation is used to much lower nutrient inputs, this extra waste leads to overfertilization that actually hinders the growth of more sensitive plants. More resilient flora like nettles and hogweed flourish though, driving out other plants and reducing biodiversity in the process. This happens particularly around trails since the nutrients released by dogs are not deposited uniformly but in patches.

The solution is rather simple: bag your dog’s poop just as you would in urban parks. The researchers found that disposal bags and pooper-scooper stations can reduce extra phosphorus by 97% and nitrogen by 56%. Keeping dogs on a leash can also restrict the deposited nutrients to significantly smaller areas, concentrated in the near vicinity of trails. In particularly sensitive areas with plants adapted to nutrient-poor soils, park rangers could ban dogs altogether.

Besides protecting vulnerable plant species, restricting the dispersal of dog feces in peri-urban areas protects grazing animals from zoonotic diseases and parasites.

Although our beloved pooches are innocent, they can nevertheless be a nuisance to the environment and the responsibility for protecting natural areas falls on us. In Europe and the United States, about 25% and 49%% of households own at least one dog, respectively. That’s a lot of dogs and consequently a lot of poop that needs to be disposed of properly.

The findings were reported in the journal Ecological Solutions and Evidence.