A new study claims dogs cause substantial damage to ecosystems, but don’t stop walking your dogs just yet as the study has severe limitations.
By Sam Johnson
Photo courtesy of Holly Ferguson.
A recent study titled “Nutrient fertilisation by dogs in peri-urban ecosystems” investigated the effects that dogs have on a Belgian nature reserve. The researchers estimated dog prevalence of 1600 individual dogs visiting these nature reserves (which are open to dogs and their owners for public walks) with about 2/3 being on a lead. Then the study modelled the amount of phosphorus and nitrogen deposited through an average dog’s urination and defecation while in the area. The researchers then tested the effects of such quantities on a variety of plant species. The authors of the research found substantial impairment of biodiversity was caused by such deposits. Ecosystems are often self-contained and balance in a fragile equilibrium with the chemical composition, nutrients, and bacteria available in the ecosystem. Disruptions to this mechanism can disturb metabolic and chemical processes as well as change abiotic factors such as pH which can result in many species struggling to survive changes in the ecosystem. This can kill existing species important to an ecosystem, bring in invasive species and feedforward into spreading damage by upsetting the fine balance. Even chemicals which are usually good for plant growth such as nitrogen, at high doses can damage plants leaves, stems, and roots.
However, before we start villainising good dogs for their dirty deeds there are a few issues to point out with this study. Firstly, this model comes with several assumptions or premises necessary for it to apply and several of these assumptions do not reflect reality. This also means they did not actually sample ecosystems and look at actual impacts to see if dogs cause the damage they suggest, leaving their claims empirically unsupported.
The first flawed assumption is that all dog poop was left in the ecosystem. While dog walkers not picking up after their pooches is a perennial problem, evidence suggests the majority of dog walkers do remove their dog’s waste. As most of the chemical deposits come from dog poo, 97% of the phosphorus and 56% of the nitrogen that the study assumes is deposited on plants is dependent on no dog owner doing the good deed of picking up their dog’s poo before it degrades into the ecosystem.
Therefore the 11 pounds of nitrogen per dog, per year, per hectare estimate is likely many times over the actual amount as trips to nature reserves is not something every dog is treated to every single day.
The researchers also assumed that each dog returned to the park, to go number one and two, once per day, every day, for an entire year… suffice to say this is a significant overestimate. Therefore the 11 pounds of nitrogen per dog, per year, per hectare estimate is likely many times over the actual amount as trips to nature reserves is not something every dog is treated to every single day.
The third main flaw is the assumption that this waste is spread equally over all plants in the area. In reality dogs primarily follow their owners along main paths whether on or off the lead. Therefore, these deposits will be concentrated in the already highly disturbed walking paths and have far less impact on more fragile and distant areas of the ecosystem. Additionally, given canine behaviour, it is likely deposits further cluster around bins and signposts where other dogs have already been active, drains or path areas around these signs therefore soak up the majority of these chemicals, lessening the harm across the rest of the ecosystem.
Finally, it is important to compare this to other sources of chemical inception. Agricultural run off for example, is a much larger contributor at approximately five times worse than the highest estimates for dogs in this study. Therefore, there may be better targets if the goal is a reduction in chemicals being introduced to fragile ecosystems.
Another house pet which can damage ecosystems is the cat. These feline predators originally self-domesticated alongside humans due to their talents at keeping pest species, such as rats away from grain supplies. However, these adorable internet celebrities have decimated many bird populations. It is estimated that cats kill between 1.3 and 4 billion birds annually - so watch out for these killers despite their cute playful façade.
If this needs re-stating, you should pick up after your dog. Also, to protect our ecosystems, you should try to keep them relatively clean when travelling through ecosystems (especially long distance via car) as seeds and pods can traverse distances attached to a dog’s fur and paws. Such transfer can bring disease or invasive and problematic species, though the amount that this is due to dogs compared to humans, birds or other long-distance animals is unclear. You should also be careful around wildlife such as sheep and coastal birds as dogs and other predators can scare, disrupt, or kill important and protected species.
While protecting ecosystems is highly important, it is also vital that we critically analyse sources to ensure that actions we take are supported by sound scientific evidence to actually help or prevent harm to nature. As far as this study is concerned you don’t need to stop walking your dog, in fact there are plenty of reasons why getting out in nature with a canine companion could be helpful for many right now.
