Is Dog Pee Destroying the Ecosystem?

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As you probably know, because I mention it in every other video, I have a dog. His name is Indy and we have been friends for 5 years now, and in that time we’ve had a lot of back and forth while trying to figure out a form of exercise that both of us like. For instance, I enjoy running, which Indy hates because he would much rather either go WAY faster or just stop and eat grass. I like surfing and swimming, while Indy considers water one of his greatest nemeses. I like biking and even got a little trailer to take Indy along, but he spends most of his time in the trailer just screaming. Like “ahhhhhhhhhhh I HATE THIS.”

And it turns out that *I* don’t derive much enjoyment from Indy’s favorite activities, like rolling in cow poop and eating the cotton out of stuffed animals.

But we finally found a hobby we both love: hiking! We go out several times a week to explore the many dog-friendly trails around the Bay Area, and it’s great! Lakes, rivers, waterfalls, black bears (in the zoo), hidden graffiti walls: every hike is a fun new adventure.

So you can understand why I was alarmed to read this Washington Post article headlined “Dogs peeing and pooping in nature reserves disrupt ecosystems, Belgian study finds.” Like, we finally found a healthy activity we both enjoy and it turns out we are DESTROYING our local habitats! Shit!

I already knew that dogs CAN be quite harmful to ecosystems. When I first adopted Indy I lived on a beach right next to a bird sanctuary, and dogs were barred from all the beaches to protect important breeding and nesting areas for shorebirds, including the endangered least tern. Dogs are predators who can scare away, hurt, or even kill birds (purposely or accidentally), so it’s important to keep them on-leash and far away from habitats where they might cause damage.

But that isn’t what this article is about: it’s about a new study from Belgian researchers who are concerned about how dogs impact our environment by peeing and pooping all over it. Now, I have some issues with this study, the biggest of which is that they didn’t actually sample the environment they studies to determine whether or not dogs were, you know, actually causing the damage they claim. It’s all just kind of assumed, and the assumptions they made don’t necessarily reflect reality. So the big takeaway up front is that you should NOT let this study stop you from taking your well-trained and leashed dog out on hikes where dogs are allowed.

But I do want to nip in the bud a few ideas that might occur to the average person at first. For instance, “Nature is FULL of animals pooping and peeing everywhere. Squirrels don’t have sewer systems, do they? So what’s it matter if my dog poops and pees everywhere?”

Ecosystems are, to a point, self-contained and self-correcting: when a bunch of plants and animals spend a few millennia evolving together in the same place, there’s a kind of uneasy balance. The local plants and fungi and insects use up the nutrients that are deposited by the herbivores, omnivores, and carnivores in that area, who feed on those plants or on the animals who are feeding on those plants. When humans come along and introduce a new large amount of nutrients (and bacteria and other stuff) it isn’t necessarily good: we feed our dogs at home outside of the ecosystem, and then bring them into the ecosystem to deposit nutrients. Theoretically, that could really upset that balance, killing certain species, or introducing or attracting new invasive species that could cause additional damage.

So it’s not just “fertilizer, an unqualified good.” If you, like me, used quarantine to get really into houseplants, you may have learned this: MORE is not always BETTER. Hell, if it’s even true of things like water and sunlight, so of course it’s true of nutrients. You probably learned in grade school that nitrogen is important for plant growth, but too much nitrogen can burn your plants’ leaves, cause their stems to weaken, and inhibit the growth of their roots. Some plants love nitrogen, some don’t. It’s the same with phosphorus: too much, and plants tend to die. Both phosphorus and nitrogen are introduced to the soil via animal waste.

It is possible, therefore, for dogs to contribute negatively to an ecosystem if they are depositing enough nitrogen and phosphorus in areas where those nutrients cause harm. So..are they?

To find out, these researchers took a doggy census at 4 different nature preserves situated around Belgium, finding about 1,600 dogs of whom two-thirds were leashed and one-third were off-leash.

They then took averages of how much a dog pees and poops on one walk, without consideration of the dog’s size or diet or the length of the walk. They took the average amount of nitrogen and phosphorus in pee and poop and used all of these averages to estimate how much nitrogen and phosphorus dogs were putting into the ecosystem. Again, they did not take any samples to confirm or deny these estimates.

See an issue? For a start, dog walkers can quite easily pick up their dogs’ poop. They don’t always do it, which is infuriating, but they can, and when they do, they take away 97% of the phosphorus and 56% of the nitrogen their dog might contribute to the environment.

That leaves us with the amount of each nutrient that dogs deposit via their urine: a negligible amount of phosphorus and about 11 pounds of nitrogen deposited per dog per year per hectare, assuming that literally every dog they saw at the park returns to pee there once per day every day for a year, a number that pales in comparison to the amount of nitrogen that enters ecosystems every day due to agricultural runoff. (Being a distant second doesn’t mean that it’s not a substantial amount – I’m just putting it into perspective for you. According to the author of this study, agricultural runoff is nearly 5 times worse).

The authors do admit that if a dog is on a 6-foot leash, they can only deposit these nutrients on and around the main path. Speaking as someone who enjoys taking my dog for walks off-leash so that he can run around and go at his own pace, I can say that he rarely goes off the path by more than two feet and almost exclusively squirts pee on posts, debris piles, and signs that are immediately next to the path. Not all dogs do the same, but if even a quarter of off-leash dogs behave the same way, that takes another huge bite out of the estimated numbers these researchers used. 

Since they didn’t take any samples, we have to look at other studies to determine the real-world results. They actually cite one study that DID take samples: “Dog Urine Has Acute Impacts on Soil Chemistry in Urban Greenspaces.” In 2020, researchers in Finland examined the soil composition in urban parks, forests, and other greenspaces. They confirmed that dogs contributed a significant amount of nitrogen to the soil, but only on trees and signs right next to the walking paths. They found that forests were, contrary to their own hypothesis, the least impacted by dogs’ nitrogen deposits.

The authors of the recent Belgian study suggest that dog owners be further encouraged to pick up their dogs’ poop (obviously) and also that some parks should simply be made off-limits to dogs. While I absolutely agree that dogs aren’t fit to go wherever they want – I certainly don’t want Indy to be responsible for cock-blocking a least tern – I think the Finnish researchers are a bit more pragmatic. Not only did they actually evaluate samples from the field, but their results led them to suggest a better alternative: designing park systems that take advantage of the fact that dogs like to pee on objects near the trail, especially if another dog has peed there before. Set up little pee stations for them with drains! Boom, done.

I mean, maybe: Indy is a male dog, and as such he is cursed with the overwhelming desire to pee just a few precious drops on a stick here, a tree there, a weird rock down that way. It’s known as “marking,” and he can’t really help it. He wants all the other dogs to know that this is HIS forest. I’m never going to get him to fully empty his stupid bladder in one spot, no matter how appealing that spot is. But that’s also yet another complication for the Belgian study: sure, the average dog may pee 184 ml per walk, but surely it matters to the plants and the soil whether that’s all dumped in one spot or if it’s sprinkled along a 2-mile path that winds throughout an entire forest.

While I’m not blown away by this Belgian study, which is really just a mathematical model built on a lot of assumptions, I DO think more research would be fantastic. Does a tiny amount of pee cause a problem? How does the nitrogen move around when it rains? Is the impact less or more than the baseline impact of carving out a dirt or rocky path through an otherwise untouched forest for humans to wander along? And is it worth redesigning our parks or instituting tougher rules, or would we see a greater impact by enforcing existing rules (like making sure people pick up after their dogs, or ensuring that farms aren’t polluting wild areas)?

I’m willing to change my mind if we get more data in, but as of right now I think that the physical and psychological benefits of going out into nature with your best friend are more important than the ecological drawback of your best friend peeing on an unappreciative plant.

Rebecca Watson

Rebecca is a writer, speaker, YouTube personality, and unrepentant science nerd. In addition to founding and continuing to run Skepchick, she hosts Quiz-o-Tron, a monthly science-themed quiz show and podcast that pits comedians against nerds. There is an asteroid named in her honor. Twitter @rebeccawatson Mastodon Instagram @actuallyrebeccawatson TikTok @actuallyrebeccawatson YouTube @rebeccawatson BlueSky

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