How Dog Excrement Harms The Environment

Much research on the environmental impact of dogs and cats focuses on their predation of wild animals. But what about the impacts of their feces and urine? Given how many companion animals exist, their waste can add significant nutrients into the environment and disrupt ecosystem dynamics.

Two main macronutrients in dog urine and feces are nitrogen and phosphorus. An oversupply of these nutrients can be detrimental to the environment because they favor a small group of plant species, which grow faster and take the sun away from other plants. In turn, the loss of any plant species can harm wild animals who rely on them.

At which point an ecosystem becomes negatively affected by nitrogen inputs — called the critical deposition loads — depends on several factors, such as the vegetation type. For the areas studied, this threshold was between 20 and 34 kg per hectare (ha) per year. In Europe, the atmosphere already adds 5 to 25 kg/ha/year.

Meanwhile, phosphorus can take a very long time to disappear. Even with phosphorus-removing practices such as mowing, the disappearance rate can be as low as 2 to 20 kg/ha/year in some places. Too much phosphorus can lead to a process called eutrophication, which is the excessive accumulation of a nutrient in an environment. In the most severe cases, animals and other organisms may die as oxygen gets depleted (especially in aquatic environments).

The aim of this study was to estimate nitrogen and phosphorus inputs of dog feces and urine, focusing on wilderness areas near cities and towns where many people walk their dogs. The authors focused on four nature reserves near Ghent, Belgium, with different leash requirements and management strategies. To determine nutrient concentrations, the authors relied on previous studies showing that the average concentration of nitrogen and phosphorus in dog urine is 18.7 g/L and 484.6 mg/L, respectively. For feces, the concentration is 44.3 mg/g and 32.0 mg/g respectively.

The authors found that an average of 4.2 dogs/ha/day visited the nature reserves. However, there was a large variation between areas, with one area averaging 11 dogs/ha/day. About two-thirds of dogs were kept on a leash. 

The researchers assumed that each dog relieved itself once per walk and estimated an average amount of feces and urine per visit based on previous studies. Taking urine and feces together, the authors estimated an average nitrogen and phosphorus deposition rate of 11.5 kg/ha/year and 4.8  kg/ha/year respectively. There was also a large variation from one area to another, with one site having an input of 31.3 kg/ha/year for nitrogen and 13.1 kg/ha/year for phosphorus.

The authors concluded that dogs are an underestimated source of nitrogen and phosphorus in natural areas near cities. The authors give several solutions to this problem. First, advocates should encourage people to pick up after their dogs with the appropriate bags, even in natural environments such as forests. Indeed, in the sites studied, the removal of all excrement reduced nitrogen and phosphorus deposition by 56% and 97%, respectively.

Second, local authorities should require the use of leashes. The nitrogen and phosphorus inputs in that area would be much higher, so guardians would have to pick up after their dogs while governments would need to enact strict environmental management policies to clean the remaining excrements. Finally, the authors call for more off-leash parks in designated areas as well as banning dogs from especially sensitive ecosystems that wouldn’t be able to handle the nutrient oversupply.