Why are some animal populations unaffected or positively affected by roads?
Trina Rytwinski and Lenore Fahrig
Electronic Supplementary Material
Table S1. Definitions of the behavioural scenarios used in the six experiments and how roads and traffic were expected to impact
population size/persistence for each. Traffic mortality = enhanced mortality due to collisions with vehicles; resource inaccessibility =
less access to food, mates, habitat etc. (barrier effect); habitat loss = less habitat available for breeding, foraging etc.); population
subdivision = populations become fragmented into smaller, partially isolated local populations that are more vulnerable to extinction.
Behavioural Responses
1. road attraction + vehicle
avoidance
Definition
Animal are attracted to a road for a resource (e.g., for food,
a nesting site, a mate, or thermoregulation). Once on the
road, animals can avoid oncoming vehicles, resulting in
some individuals returning to the original side of the road
and others crossing the road.
Expected road and traffic impacts
no negative road effects
increased resources
2. road attraction + no vehicle
avoidance
Animal are attracted to a road for a resource (e.g., for food,
a nesting site, a mate, or thermoregulation). Once on the
road, animals cannot avoid oncoming vehicles.
3. vehicle avoidance
Animals can avoid oncoming vehicles, resulting in some
individuals returning to the original side of the road and
others crossing the road.
no negative road effects
4. traffic disturbance avoidance
Animals avoid roads from a distance due to traffic
disturbance (e.g., lights, noise, chemical emissions).
resource inaccessibility
habitat loss
population subdivision
5. road surface avoidance
Animals avoid going onto the road surface itself.
resource inaccessibility
population subdivision
6. no avoidance
Animals move onto roads irrespective of traffic.
traffic mortality
traffic mortality
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Why are some animal populations unaffected or positively affected by roads?
Trina Rytwinski and Lenore Fahrig
Animal’s randomly selected moving trajectory
would take it across a road
T
Probability animal avoids road from
a distance due to traffic disturbance
(noise/emissions)
Probability animal is
attracted to the road
A
R
Probability animal avoids the road
surface
C
M
Probability animal avoids
oncoming vehicles
Probability animal is killed on
road
Probability that the animal crosses the road
Modified from Jaegar & Fahrig 2004
Figure S1. Illustration of traffic disturbance avoidance, T, road surface avoidance, R, road
attraction A, vehicle avoidance C and the probability of animals killed on the road, M.
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Why are some animal populations unaffected or positively affected by roads?
Trina Rytwinski and Lenore Fahrig
(a)
1.10
1.00
Probability of Traffic Avoidance, T
T =kV/1+kV
0.90
k=0
k=0.01
k=0.025
k=0.05
k=0.1
k=0.25
k=0.5
k=1
k=2.5
k=5
k=10
k=100
k=200
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
1
2
3
4
5
6
7
8
9
10
11
Traffic Density (number of vehicles/unit length of road), V
3
Why are some animal populations unaffected or positively affected by roads?
Trina Rytwinski and Lenore Fahrig
1.10
(b)
1.00
Probability of Mortality, p(M )
Pr
ob
ab
ilit
y
of
Tr
aff
ic
m
ort
ali
ty,
M
0.90
p(M
) =dV/1+dV
M =dV/1+dV
0.80
d=0
d=0.01
d=0.025
d=0.05
d=0.1
d=0.25
d=0.5
d=1
d=2.5
d=5
d=10
d=100
d=200
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0
1
2
3
4
5
6
7
8
9
10
11
Traffic Density (number of vehicles/unit length of road), V
Figure S2. a) Probability of an animal avoiding the road from a distance, as a function of traffic density V (held constant at 3 for the
current runs). Probability of traffic disturbance avoidance was estimated by the avoidance parameter k (note: probability of vehicle
avoidance was estimated by the same avoidance parameter k); b) Probability of being killed on a road for a given animal, as a function
of traffic density. Probability of traffic mortality was estimated by varying the mortality parameter d. In the current runs, k was either
0 or 200, corresponding to 0% and 100% probabilities (respectively) and d was either 0 or 2.5, corresponding to 0% and 88%
probabilities (respectively).
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Why are some animal populations unaffected or positively affected by roads?
Trina Rytwinski and Lenore Fahrig
Disturbance zone
Figure S3. Illustration of a traffic disturbance zone. Animals that avoid traffic disturbance due to
vehicle lights, emissions, noise or pollution, avoid the habitat near roads from a certain distance
away from the road(s). In the current simulations we assumed that the large-bodied species
avoided the roads from 45 grid cells away (450m) and the small-bodied species type avoided the
roads from 1 grid cell away (10m).
References
Jaeger JAG, Fahrig L (2004) Effects of road fencing on population persistence. Conserv Biol
18:165-1657
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