The impacts of dogs on wildlife and water quality: A literature review
Compiled by Lori Hennings, Metro Parks and Nature, April 2016
Metro periodically reviews the science literature behind its natural resource policies to ensure policies
are based on the most current science. Recently staff reviewed the scientific literature regarding the
impacts of dogs on wildlife to inform Metro Regulatory Code Title 10.01, which excludes pets from most
Metro properties. The only exceptions are service dogs, leashed dogs on some regional trails, Broughton
Beach, boat ramps and properties managed by others through intergovernmental agreements that are
integrated into larger parks where leashed dogs are allowed (e.g., Forest Park).
Any human related activity can disturb wildlife. In order to meet Metro's dual goals of protecting natural
resources and providing access to nature, Metro has tried to strategically locate trails in less sensitive
habitat and to ensure that human activity is as non-disruptive as possible. Part of that strategy has been
to allow public access, while limiting certain activities such as bringing dogs into natural areas.
The evidence that dogs negatively impact wildlife is overwhelming. It is clear that people with dogs – on
leash or off – are much more detrimental to wildlife than people without dogs. Dogs (Canis lupus
familiaris) are considered to be a subspecies of wolves (Canis lupus), and wildlife perceive dogs as
predators.(30) Impacts include:
1. Physical and temporal displacement – The presence of dogs causes wildlife to move away,
temporarily or permanently reducing the amount of available habitat in which to feed, breed
and rest. Animals become less active during the day to avoid dog interactions. Furthermore, the
scent of dogs repels wildlife and the effects remain after the dogs are gone.
2. Disturbance and stress response – Animals are alarmed and cease their routine activities. This
increases the amount of energy they use, while simultaneously reducing their opportunities to
feed. Repeated stress causes long-term impacts on wildlife including reduced reproduction and
growth, suppressed immune system and increased vulnerability to disease and parasites.
3. Indirect and direct mortality – Dogs transmit diseases (such as canine distemper and rabies) to
and from wildlife. Loose dogs kill wildlife.
4. Human disease and water quality impacts - Dog waste pollutes water and transmits harmful
parasites and diseases to people.
Metro owns 17,000 acres of parks and natural areas and does not allow dogs or other pets on the vast
majority of these lands. Exceptions include service animals, leashed dogs on some regional trails,
Broughton Beach, boat ramps and certain properties managed by others through intergovernmental
agreements that are integrated into larger parks where leashed dogs are allowed (e.g., Forest Park). The
policy that prohibits visitors from bringing pets to most of Metro’s managed parks and natural areas was
initiated by Multnomah County in the 1980s and continued in practice after Metro assumed
management of those parks in the early 1990s. After a review of the scientific literature and meaningful
public discourse, Metro formally adopted the pets policy into its code in 1997 (Metro Council Regulatory
code Title 10.01 adopted in Ordinance 96-659A).
To ensure this decision reflects the most up-to-date information, Metro staff examined 54 peer-
reviewed scientific journal articles and several research reports relating to the impacts of dogs in natural
areas, including numerous literature reviews on the impacts of various types of recreation on wildlife
and habitat.(10, 28, 42,54,61,63, 65,68,71,73,77) The results of our literature review are summarized below.
PHYSICAL AND TEMPORAL DISPLACEMENT
Displacement may be the most significant impact due to the amount of habitat affected. The presence
of dogs causes most wildlife to move away from an area, which temporarily or permanently reduces the
amount of functionally available habitat to wildlife. The research is clear that people with dogs disturb
wildlife more than humans alone.(5,10,33,38,39,41,44,61,68,69) These effects reduce a natural area’s carrying
capacity for wildlife, and also reduces wildlife viewing experiences for visitors.
Studies on a variety of wildlife in many countries and settings demonstrate that dogs along trails and in
natural areas significantly alter wildlife behavior.(9,33,39,41,49,53,58) A 2011 literature review found negative
dog effects in all 11 papers that examined such effects.(65) Studies demonstrate dog-specific impacts on
reptiles,(29,31,48) shorebirds and waterfowl,(24,32,51,69) songbirds,(5,9,10) small mammals,(33,39,56) deer, elk and
bighorn sheep,(4,36,38,44,49,59,63) and carnivores.(22,33,52,58)
A study in France found that two hikers disturbed an area of 3.7 hectares walking near wild sheep,
whereas two hikers with dogs disturbed 7.5 hectares around the sheep.(41) In Chicago, migratory
songbirds were less abundant in yards with dogs.(9) Dog walking in Australian woodlands led to a 35%
reduction in bird diversity and a 41% reduction in the overall number of birds.(5) The same study showed
some disturbance of birds by humans, but typically less than half that induced by dogs.
Studies in California and Colorado showed that bobcats avoided areas where dogs were present,
including spatial displacement(22,33,52) and temporal displacement in which bobcats switched to night
time for most activities.(22) The Colorado study also demonstrated significantly lower deer activity near
trails specifically in areas that allowed dogs, and this effect extended at least 100 meters off-trail.(33)
This negative effect was also true for small mammals including squirrels, rabbits, chipmunks and mice,
with the impact extending at least 50 meters off-trail.
Evidence suggests that some wildlife species can habituate to certain predictable, non-threatening
disturbances such as people walking on a trail in a natural area; this effectively lowers the stress
response. Part of this adaptation may be due to wildlife learning what is and isn’t a threat, and also
avoidance of hunters.(19,55,63,70) Habituated animals still react, but amount of habitat affected is not as
large.(55,56,63,70) However, dogs – especially off-leash dogs – may prevent wildlife habituation because
wildlife consistently see them as predators. Dog-specific disturbance has been studied for birds, with no
evidence of habituation even with leashed dogs, even where dog-walking was frequent; this effect was
much weaker for people without dogs.(5)
Even the scent of dog urine or feces can trigger wildlife to avoid an area. Therefore, the impacts of dog
presence can linger long after the dog is gone, even days later. One literature review found that
predator odors caused escape, avoidance, freezing, and altered behavior in a large suite of wildlife
species including scores of amphibian, reptile, bird, and mammal species from other studies.(30) The
scent of domestic dogs has been shown to repel American beaver (Castor Canadensis), mountain beaver
(Aplodontia rufa), deer (Odocoileus species), elk (Cerus elaphus), and a wide variety of wildlife native to
other countries.(20,30) Mountain beaver cause economic damage to young tree stands in the Pacific
Northwest, and foresters are considering using dog urine as a repellant.(20) An experimental study
demonstrated that dog feces are an effective repellent for sheep, with no habituation observed over
seven successive days.(1)
One Colorado study showed mixed effects of dogs on wildlife.(44) The study compared effects of
pedestrians alone, pedestrians with leashed dogs and unleashed dogs alone on grassland birds. Vesper
Sparrows (Pooecetes gramineus) and Western Meadowlarks (Sturnella neglecta) waited until dogs were
closest to flush – that is, they fly or run away. This could be an attempt to remain undetected against the
greatest threat, but could also mean that these bird species perceive humans as a greater threat than
dogs. However, the same study found strong dog-specific impacts on mule deer in woodlands. A
literature review found that ungulates (deer, elk and sheep) had stronger flight responses in open
habitats compared to forested habitats.(63) Unlike small ground-nesting songbirds, larger animals would
have no cover and could easily be seen in open habitats.
The disturbance effects of off-leash dogs are stronger than on-leash and substantially expand the
amount of wildlife habitat affected,(32,59,63,69) and the unpredictability of off-leash dogs may prevent
wildlife habituation in large areas of habitat.(5,10,32,61,69) The negative effects are increased even further
when dogs and people venture off-trail, probably because their behavior is less predictable.(44,67) Off-
leash dogs are likely to reduce the number and types of wildlife in large areas of habitat.
A Colorado study found off-leash dogs ventured up to 85 meters from the trail, although this result was
from 1 square meter plots covering a very small percentage of the area. (33) Remote cameras in another
study documented the same dog 1.5 miles apart in the same day.(61) In Utah, mule deer showed a 96%
probability of flushing within 100 meters of recreationists located off trails; their probability of flushing
did not drop to 70% until the deer were 390 meters from the recreationists.(67) A California shorebird
study found that off-leash dogs were a disproportionate source of disturbance, and that plovers did not
habituate to disturbance; birds were disturbed once every 27 minutes on weekends.(32)
To illustrate the potential of dogs to displace wildlife we explored two well-known local park examples
that allow dogs on leash. Forest Park is one of the largest urban parks in the U.S. and was always
intended to connect urban dwellers with nature; people have been walking their dogs there since before
the park’s 1948 dedication. Forest Park covers 5,172 acres of forest, including approximately 80 miles of
trails and service. Using a very conservative 25-meter buffer around mapped trails to represent the
“human + dog on leash” area of disturbance and assuming 100% compliance with leash rules, the area
affected would be 1,406 acres – that’s 28% of the entire park. In 651-acre Tryon Creek Natural Area, 207
acres of land (32%) is within 25 meters of a trail.
DISTURBANCE AND STRESS RESPONSE
Stress response is the functional response of an animal to an external stressor, such as seasonal changes
in temperature and food availability or sudden disturbance.(3) Specific stress hormones are released to
enable the animal to physically respond to the stressor. Acute stress response, when an animal reacts to
an immediate situation, can benefit an animal by triggering it to respond appropriately to a threat.
However, chronic stress such as repeated disturbances over time may reduce wildlife health,
reproduction, growth, impair the immune system and increase vulnerability to parasites and
Dogs cause wildlife to be more alert, which reduces feeding, sleeping, grooming and breeding activities
and wastes vital energy stores that may mean life or death when resources are low, such as during
winter or reproduction.(8,32,40,41,69) Animals release stress hormones and their heart rates elevate in
response.( 3,27,37,38) When stress becomes too high, animals may flush, freeze, or hide.(26,30)
Several studies document that disturbance reduces reproductive success for some wildlife
species.(11,35,40,50,63) Numerous studies found that female deer and elk, and deer and elk groups with
young offspring, show greater flight responses to human disturbances than other groups.(63) Stress
hormones may cause male songbirds to reduce their territorial defense, females to reduce feeding of
their young, nestlings to have reduced weight and poor immune systems, and adult birds to abandon
nests.(11,34,35,76) A Colorado study showed that elk repeatedly approached by humans had fewer young.(50)
Although research is lacking on whether dogs specifically reduce the reproductive success of wildlife, the
fact that humans with dogs create much stronger disturbance effects than without dogs (5,33,38,41,44,61,68,69)
implies that these stress effects would be magnified if people had dogs with them.
INDIRECT AND DIRECT MORTALITY
Dogs chase and kill many wildlife species including reptiles, small mammals, deer and
foxes.(12,13,29,31,48,58,62) A Canadian study found that domestic dogs were one of the top three predators
that killed white-tailed deer fawns.(4) In northern Idaho winter deer grounds, an Idaho Fish and Game
conservation officer witnessed or received reports of 39 incidents of dogs chasing deer, directly resulting
in the deaths of at least 12 animals.(36) A study in southern Chile revealed that domestic dogs preyed on
most of the mammal species present in the study area.(60) A 2014 literature review of dogs in parks
identified 19 studies that investigated the effects of dogs preying on wildlife.(73) Of these, 13 reported
observing or finding strong evidence of dog predation on wildlife. The Audubon Society of Portland’s
Wildlife Care Center took in 1,681 known “dog-caught” injured animals from 1987 through March
Dogs transmit diseases to wildlife and vice versa including rabies, Giardia, distemper and
parvovirus.(18,23,66,74) A Mexico City study concluded that feral dogs continually transmitted parvovirus,
toxoplasmosis and rabies to wildlife including opossums, ringtails, skunks, weasels and squirrels.(66) Large
carnivores such as cougars are especially vulnerable to domestic dog diseases including canine
HUMAN DISEASE AND WATER QUALITY IMPACTS
Under the Oregon Department of Environmental Quality (DEQ), Metro is a Designated Management
Agency to protect water quality in compliance with the federal Clean Water Act. Limiting dog access at
most natural areas is one of Metro’s commitments to DEQ, because dog feces pollute water. Feces are
often delivered to waterways through stormwater.(57) The average dog produces ½ to ¾ pound of fecal
matter each day – a hundred dogs can produce more than 500 pounds of waste per week.(45) The DEQ
identifies pet waste as a significant contributor to one of the region’s most ubiquitous and serious
pollutants, E. coli bacteria. Contact with E. coli-polluted water can make people sick. Because dog waste
can be a relatively simple source to reduce or eliminate exposure to E. coli, DEQ considers reducing or
eliminating dog waste an important action item in jurisdictions’ clean water implementation plans for
the Willamette Basin watershed.(47)
Humans can catch parasites and diseases such as hookworms (causes rash), roundworms (may cause
vision loss in small children, rash, fever, or cough) and salmonella (causes gastrointestinal illness) from
dog waste.(7,57) Aside from potential illnesses, dog waste can negatively affect visitors’ experience in a
natural area. Dog waste left on the ground is a leading complaint in Portland parks, and violators may be
fined up to $150 per incident.(14)
Several examples illustrate local dog impacts. A Clean Water Services DNA study found that dog waste
alone accounts for an average of 13% of fecal bacteria in stream study sites in the Tualatin River
Basin.(17) Off-leash dog walking is documented to cause erosion in Portland’s Marshall Park, creating
sediment problems in stream water.(15) In 2014 Portland school administrators expressed concern
because playgrounds had become “a minefield for animal waste” from people using school grounds as
after hours, off-leash dog parks, threatening the health of school children.(21) The City of Gresham found
extremely high levels of E. coli bacteria in water quality samples of a very specific stretch of a stream,
where dog feces were found along stream banks behind several yards with dogs.1 The city sent letters to
1 Personal communication with Katie Holzer, Watershed Scientist at the City of Gresham, Oregon, 4/11/2016.
residents in the neighborhood about the incident and how to properly dispose of dog feces; the levels
have not been elevated in follow-up sampling.
BELIEF, BEHAVIOR AND REALITY
People do not always take responsibility for their impacts on wildlife. Several studies demonstrate that
natural area visitors, including dog owners, often don’t believe they are having much of an effect on
wildlife, or assign blame to different user groups rather than accepting responsibility themselves.(6,64,67,68)
Some natural area visitors assume that when they see wildlife, it means that they are not disturbing the
animals – or worse, that because they didn’t see any wildlife, they didn’t disturb any.(64)
For example, in Utah, about half of recreational visitors surveyed did not believe that recreation was
having a negative impact on wildlife; of those that did, each user group blamed other groups for the
strongest impacts.(67) In Austria, 56% of people surveyed at a national park agreed that wildlife is in
general disturbed by human activity.(64) However, only 12% believed that they had disturbed wildlife in
their visit that day, and dog-walkers ranked their activities as less disturbing than other user groups’
activities. When asking different user groups to rate the impacts of overall human disturbance on
wildlife, dog-walkers rated the impacts the lowest, at 2.6 out of 5 possible impact points.
Surveys indicate that many dog owners desire fewer restrictions, while non-dog owners often feel the
opposite.(72,73) However dog owners don’t always follow the rules, and some dog owners allow their
dogs to run free in leash-only natural areas.(32,52,73) In a Santa Barbara study, only 21% of dogs were
leashed despite posted leash requirements.(32) And despite regulations and claims to the contrary, dog
owners often don’t pick up their dog’s waste.(6,32) An English study revealed that although 95% of
visitors claimed to pick up their dog’s waste only 19-46% actually did so, depending on location within
In summary, people and their dogs disturb wildlife, and people are not always aware of or willing to
acknowledge the significance of their own impacts. Wildlife perceive dogs as predators. Dogs subject
wildlife to physical and temporal displacement from habitat, and dog scent repels wildlife with lingering
impacts. Dogs disturb wildlife which can induce long-term stress, impact animals’ immune system and
reduce reproduction. Dogs spread disease to and outright kill wildlife. People with dogs are much more
detrimental to wildlife than people alone; off-leash dogs are worse; and off-trail impacts are the highest
Urban wildlife is subjected to many human-induced stressors including habitat loss, degraded and
fragmented habitat, impacts from a variety of user groups, roads, trails, infrastructure, noise and light
pollution.(26) These stressors will increase with population; from July 2014 to 2015 the Portland-
Vancouver metropolitan region added 40,621 new residents.(43) Current population in the region stands
at 2.4 million, with another 400,000 residents expected over the next 20 years.
Figure 1. Conceptual illustration of the relative impacts on
wildlife due to people without and with dogs.
Among medium to high density cities, Portland currently ranks second in the total area covered by parks
at nearly 18%, and also second in the number of park acres per resident.(25) Of 34 park providers in the
Portland region, all but four allow dogs in most or all of their natural areas, typically on-leash; more than
two-thirds also offer dog parks or off-leash dog areas (Table 1 at end of document).
Wildlife conservation is not the only valid reason to preserve natural areas. Park providers must weigh
the trade-offs between wildlife, habitat, water quality and recreational values. But when considering
different types of public access in a natural area, it is important to understand that the research is clear:
people with dogs substantially increase the amount of wildlife habitat affected and are more
detrimental to wildlife than people without dogs.
trail, dogs on
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Table 1. Park providers’ dog policies in the greater Portland, Oregon metropolitan area.
Audubon Society of Portland
City of Beaverton
City of Cornelius
City of Durham
City of Fairview
City of Forest Grove
City of Gladstone
City of Gresham
City of Happy Valley
City of Hillsboro
City of Lake Oswego
City of Milwaukie6
City of Oregon City
City of Portland
City of Sherwood
City of Tigard
City of Troutdale
City of Tualatin
City of West Linn
City of Wilsonville
City of Wood Village
Clean Water Services (Fernhill
2 All parks except fountain provided by Tualatin Hills Parks & Recreation District.
3 Considering off-leash dog area at Water Park.
4 Dogs on leash allowed at all parks except Salish Ponds (no dogs).
5 Dogs on leash except prohibited in playgrounds.
6 All city parks are operated by North Clackamas Parks and Recreation Department.
7 The City of Oregon City is currently testing off-leash areas in three parks.
8 Dogs on-leash except prohibited at Foster Floodplain Natural Area, Tanner Springs Park, Whitaker Ponds Nature
Park, Riverview Natural Area, and the amphitheater at Mt Tabor Park.
9 33 off-leash dog areas.46
10 Most parks: dogs not allowed. Exception: Sunrise Park and large Beaver Creek Greenway, leash only. Considering
two more on-leash dogs allowed parks.
11 Plans for an off-leash area at Sunrise Park.
12 One off-leash dog area: field near parking lot at Mary S. Young Park. Off-leash dogs were identified as an issue by
Federal / State (Sandy River Natural
N. Clackamas Parks & Recreation
OR Department of Fish and Wildlife
OR Parks & Recreation Department
Port of Portland
The Nature Conservancy
The Wetlands Conservancy
Tualatin Hills Park and Rec. District
U.S. Fish & Wildlife Service
U.S. Forest Service19
13 Leashes required only on/near Confluence Trail and in parking area. Leash-off everywhere else. Region’s largest
off-leash area, and heavily used.
14 Metro does not allow dogs except for service dogs, leashed dogs on regional trails, Broughton Beach, boat ramps
and properties managed by others through intergovernmental agreements that are integrated into larger parks
where leashed dogs are allowed (e.g., Forest Park).
15 All dogs must be on leash, except while hunting during seasons authorized on Sauvie Island Wildlife Area, or
pursuant to a valid “Competitive Hunting Dog Trial Permit” or “Sauvie Island Wildlife Area Individual Dog Training
16 Includes Vanport Wetlands and mitigation sites. No dogs allowed except Government Island State Recreation
Area (leased to Oregon Parks Department).
17 No formal policy.
18 Dogs allowed on-leash except Tualatin Hills Nature Park and Cooper Mountain Nature Park.
19 Refers specifically to the Sandy River Delta, owned and administered by the National Forest Service, Columbia
River Gorge National Scenic Area.