Dog Droppings Add Unwanted Nutrients to the Environment

You're hiking through a nature preserve and you see a fresh deposit that wasn’t made by a deer or raccoon. Dog feces and urine aren’t just unpleasant when left in nature; they also can negatively impact biodiversity.

Researchers in Belgium recently set out to study the effect that dogs can have when they are walked in nature preserves. Specifically, they were interested in how the animals have an impact on the nutrients in the environment when they relieve themselves outdoors and no one cleans it up.

“Our lab works on the effects of enhanced nutrient availability (both nitrogen and phosphorus) on forest and grassland biodiversity,” Pieter De Frenne of Ghent University in Belgium and lead author of the research, tells Treehugger.

“Our own work, and that of many others from other countries working on similar topics, shows that enhanced inputs of nutrients induces vegetation change and biodiversity loss. As we noticed that there are many visitors with dogs in nature reserves close to Ghent, we then simply wanted to know how much nutrients they bring in to estimate their potential effect.”

For their study, researchers counted the number of dogs visiting four nature reserves then modeled four different scenarios including if the dogs were on or off leashes and if owners picked up after their pets. The counts were conducted on 487 occasions over 18 months.

They searched scientific literature for information on the nutrients in dog feces and urine. They used that information along with the number of dogs, to estimate the average volume of urine and feces, as well as the amount of nitrogen and phosphorus concentrations.

In the scenarios when all dogs were kept on leashes, which is legally required in the reserves, they found that the fertilization rates in the largest part of the reserves dropped, but it increased significantly in the areas around the paths where people walked their dogs.

In the period of one year, the input reached as high as 386 pounds (175 kilograms) of nitrogen and 161 pounds (73 kilograms) of phosphorus per hectare.

“In our scenario where all dogs were kept on leashes, we found that in these concentrated areas around paths, nutrient inputs of both nitrogen and phosphorus exceeded legal limits for fertilization of agricultural land,” De Frenne says. “Which is quite staggering as our study concerned nature reserves!”

In the modeling scenarios where dogs were kept on leashes, but all owners picked up their dogs’ feces, the researchers found that the fertilization rate for nitrogen was reduced by 56% and the rate for phosphorus dropped 97%. That’s because dog feces account for nearly all phosphorus deposits, while nitrogen comes equally from both feces and urine.

“So that is already a very substantial decrease indeed,” De Frenne says.

The results were published in the journal Ecological Solutions and Evidence.

Why Nutrients Matter

Nitrogen and phosphorus are key nutrients that occur naturally in the aquatic ecosystems and in the atmosphere. Organisms need these nutrients to flourish, but too much of them can be harmful.

Nutrient pollution refers to too much nitrogen and phosphorus in the environment. It can come from chemical fertilizer runoff, sewage treatment plants, or from the burning of fossil fuels.

Researchers believe these previously unrecorded sources of nutrients could negatively affect how the ecosystem functions.

“We were surprised by how high 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,” De Frenne says.

“It is difficult to separate the effects of enhanced inputs from dogs from, for instance, the nitrogen coming in via rainfall from the atmosphere (the latter is a key input of nitrogen in many ecosystems in Europe and the UK; the source of the nitrogen here is mostly from agriculture and traffic). And previous research shows that extra nitrogen and phosphorus often leads to lower biodiversity.”

The results would likely be similar in other locations where dog ownership is similar. One big variable could be the rate at which dog feces are cleaned up in that area.

The researchers suggest that those who manager these natural areas emphasize the impact that dogs can have on the environment, encouraging owners to remove their dogs’ deposits and enforcing leash ordinances.

“How natural areas can be best protected is up to forest managers and policy makers to decide,” De Frenne says.

“But our data show that dog faeces and urine can be an important fertiliser in ecosystems, and thus it can indeed be a useful management action to not allow dogs in the most sensitive (parts of) nature reserves (e.g. where sensitive plants occur and/or soils are low in nutrients), but in the same time establish more nearby dog parks or parts of nature reserves with less sensitive vegetation where dogs are allowed.”