Background

Background on project objectives and questions to be asked

Thank you for agreeing to take part in this online expert elicitation to help inform the development of a national recovery strategy for red squirrels in England. In this exercise, we will seek your expert judgment on a number of parameters that influence red squirrel population dynamics. These parameters span a range of topics, including the life history characteristics of red and grey squirrels, the epidemiology of squirrelpox virus, and the effectiveness of grey squirrel management strategies. Your answers will directly feed into a large-scale population model that aims to predict the recovery of red squirrels under different biological and management scenarios. Essentially, your expertise will help parameterise the model and include uncertainty, ensuring that it reflects the best available knowledge about these species and their interactions.

A - Part I. Squirrel life history

Breeding

Question A.2

How many litters do you expect an average female squirrel of each species to have per year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?


Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 0 2.0 4 85
D 0 6.0 7 60
E 1 2.0 3 98
H 0 1.0 2 80
I 2 2.5 3 100
J 0 3.0 3 85
K 0 3.0 4 100
L 1 2.0 3 80
O 0 1.2 3 100
Q 1 2.0 3 100
Red B 0 2.0 2 100
C 0 1.3 2 90
D 0 1.5 3 70
E 1 2.0 2 95
G 0 2.0 2 100
H 0 0.0 1 100
I 1 2.0 3 100
J 0 2.0 2 100
K 0 2.0 3 100
L 1 2.0 2 90
N 2 2.0 4 100
O 0 1.3 3 100
Q 1 1.0 2 80

Question A.3

How many individuals do you expect to survive after weaning from a single litter produced by an average female of each species of squirrel, under ideal conditions (i.e., in a woodland where inter- and intra-specific competition is negligible)?


Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 0 4 8 80
D 1 4 5 55
E 2 3 8 80
H 1 2 3 80
I 2 3 5 100
J 0 2 3 96
K 0 5 6 100
L 0 2 4 61
N 1 6 6 100
O 0 3 9 100
Red B 1 2 4 100
C 0 3 6 90
D 0 3 4 60
E 1 3 6 80
G 1 3 4 100
I 1 2 3 87
J 0 2 3 98
K 0 3 5 100
L 0 1 4 80
N 1 2 3 100
O 1 2 6 100
Q 0 0 0 50

C. Squirrelpox

Squirrelpox virus (SQPV) plays a crucial role in shaping the population dynamics of red and grey squirrels in the UK. The impact of the disease differs significantly between the two species. For red squirrels, SQPV infection is invariably fatal, with no assumed recovery under any circumstances. Grey squirrels, on the other hand, are assumed to survive SQPV infection without mortality, acting primarily as asymptomatic carriers. Additionally, we recognize that both the likelihood of mortality (in red squirrels) and the recovery process (in grey squirrels) may vary between juveniles and adults of each species. In this section, we ask for your expert judgment on key parameters of SQPV epidemiology, including the time it takes for red squirrels to succumb to the virus, the recovery time for grey squirrels, and the likelihood of reinfection in previously exposed grey squirrels.

Disease progression

Question C.1

Given that either a juvenile or adult red squirrel becomes infected with squirrelpox virus (SQPV), what is the expected time it will take until the individual dies, in days?

Age Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Adult A 12 18 35 75
D 2 14 15 50
E 7 10 15 85
I 8 10 14 100
K 0 10 14 72
L 7 10 14 81
M 3 7 21 90
O 7 14 21 100
P 7 14 28 81
Q 1 1 14 80
B 0 0 0 100
J 0 14 0 100
N 0 0 0 100
Juvenile A 12 18 35 75
B 3 12 15 95
D 2 14 15 50
E 7 10 15 85
I 5 8 14 100
K 0 10 14 69
L 5 10 14 70
M 2 4 15 75
N 0 10 21 100
O 5 10 14 100
Q 1 14 14 90
P 7 28 14 80

Question C.2

Given that either a juvenile or adult grey squirrel becomes infected with squirrelpox virus (SQPV), what is the expected time it will take until the individual recovers, in days?


If we truncated the plot to show a greater resolutions on the lower estimates:

Age Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Adult A 0 0 12 80
O 0 2 7 100
P 7 30 200 100
B 0 0 0 100
I 0 0 0 100
K 0 0 0 89
L 0 0 0 100
M 0 0 0 100
N 0 0 0 100
Q 0 0 0 100
Juvenile A 0 0 12 80
M 1 3 14 65
O 0 3 7 100
P 10 30 180 51
I 0 0 0 100
K 0 0 0 87
L 0 0 0 100
N 0 0 0 100
Q 0 0 0 100

Immune response

Howell et al. 2004 has proposed a new mechanism for squirrepolx epidemiology, where grey squirrels infected with squirrelpox virus (SQPV) can become reinfected even after developing antibodies, indicating that immunity is partial or waning rather than lifelong. Field data showed that seropositive grey squirrels often experience repeated cycles of infection and recovery, with a high prevalence of viraemic individuals in the population. Experimental findings further suggest that seroconversion may require multiple infection challenges, and once antibodies are developed, they persist but do not fully prevent reinfection. These results underscore the complexity of immune responses in grey squirrels and the role of partial immunity in sustaining SQPV dynamics within their populations. For the purposes of this exercise, we assume that grey squirrels develop partial immunity from SQPV.

Question C.3

If a grey squirrel has previously been infected with SQPV and has subsequently recovered (i.e., it has developed an immune response to the disease), what is the relative likelihood that it will become reinfected if exposed under the same epidemiological conditions as its previous initial infection? Assume all other factors remain constant, including habitat, the density of infected individuals in the area, and environmental conditions. NOTE: This is a relative comparison. If you believe the likelihood of reinfection for a seropositive grey squirrel is identical to its initial infection risk, your answer should be 100%. If you believe that a recovered grey squirrel would never be reinfected, your answer should be 0%.

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
A 60 90 100 75
M 50 60 90 60
O 20 50 90 100
P 40 60 80 80
I 0 0 0 100
N 100 100 100 100
Q 0 0 0 100

Vaccination

Vaccination has the potential to be a transformative tool in mitigating the impacts of squirrelpox virus (SQPV) on red squirrel populations. By conferring immunity to individuals, a successful vaccination program could reduce mortality rates and increase the viability of red squirrel populations. However, the effectiveness of such a program depends on several factors, including the likelihood of vaccine development, the expected timeline for its availability, and its efficacy.

Question C.4

What is the probability that an effective vaccine for squirrelpox virus (SQPV), specifically for use in red squirrel populations, will be successfully developed and made widely available for use by landowners, government organizations, and conservation groups?



Some experts seem to have expressed the value in a 0%-100% scale instead of 0-1. If we correct for that, the new plot is

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
C 0.2 0.40 0.5 75
D 0.0 0.20 0.8 50
E 0.5 0.70 0.9 55
J 0.0 0.30 1.0 81
K 0.0 0.50 1.0 88
M 0.0 0.01 0.5 80
N 0.0 1.00 1.0 100
O 0.0 0.50 1.0 100
Q 0.1 0.30 0.5 50
A 20.0 50.00 70.0 75
B 0.0 0.00 0.0 100
H 0.0 0.00 0.0 100
I 1.0 1.00 3.0 80
P 5.0 20.00 30.0 71

Question C.5

If an SQPV vaccine is developed and approved as a management tool for red squirrel populations, by what year is it expected to become widely available?



Some experts seem to have expressed the value in “years from now” instead of calendar years. If we correct for that, the new plot is

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
A 2028 2034 2050 75
B 2030 2050 2099 100
C 2030 2034 2040 80
E 2025 2030 2035 55
M 2030 2045 2060 50
O 2025 2033 2040 100
K 0 5 10 88
N 0 1 1 100
P 5 15 30 65
Q 0 0 0 100

Question C.6

For an individual red squirrel vaccinated against SQPV, what efficacy would you expect such a vaccine to have (i.e., the probability that the vaccinated individual will successfully seroconvert and develop immunity to the virus)?



Some experts seem to have expressed the value in a 0%-100% scale instead of 0-1. If we correct for that, the new plot is

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
C 0.70 0.85 0.95 55
E 0.50 0.80 0.90 55
M 0.20 0.50 0.80 50
O 0.40 0.80 1.00 100
P 0.01 0.40 0.70 50
A 15.00 75.00 85.00 60
N 1.00 1.00 1.00 100

A - Part II. Squirrel life history

Understanding the life history traits of red and grey squirrels is fundamental to predicting their population dynamics and informing management strategies. These traits shape how these species respond to environmental conditions and management interventions. In this section, we will ask for your expert judgment on parameters such as breeding season length, litter size, annual survival probabilities, and home range sizes.

Breeding

Question A.1

Under current conditions, what is the average length, in months, of the breeding season of each species of squirrel (i.e. the window during the year when each species engages in reproductive activities, including mating, gestation, and giving birth to offspring)?

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 1 6 12 100
D 4 10 12 55
E 5 6 9 99
H 6 7 9 90
I 9 10 12 100
J 6 9 12 92
K 6 12 12 100
L 8 11 12 80
O 5 9 10 100
Q 8 9 9 100
Red B 0 1 2 100
C 10 10 12 96
D 6 8 10 70
E 5 6 9 99
I 6 8 9 76
J 4 5 6 100
K 3 9 10 100
L 6 6 7 81
N 0 1 1 100
O 5 9 10 100
Q 7 8 9 100
G 10 10 10 100

Survival

Question A.4

What is the average probability that a given juvenile squirrel (of both species) will survive until adulthood, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible, there is no squirrelpox virus, no pine martens in the landscape and no lethal control being conducted)? Assume a juvenile period of 9 months.


Two experts expressed values above 1 (but too low to be reasonably interpreted as a percentage), so if we remove those:

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 0.00 0.70 1.0 90
D 0.00 0.60 1.0 60
E 0.15 0.25 0.6 70
H 0.20 0.40 0.6 75
K 0.00 0.90 1.0 100
L 0.00 1.00 1.0 50
O 0.20 0.70 0.8 100
Q 0.30 0.40 0.5 60
N 0.00 6.00 6.0 100
Red B 0.00 0.50 1.0 100
C 0.20 0.40 0.7 72
D 0.00 0.30 1.0 60
E 0.15 0.25 0.6 70
K 0.00 0.80 1.0 100
L 0.00 1.00 1.0 50
O 0.20 0.70 0.8 100
Q 0.30 0.40 0.5 100
I 1.00 2.00 3.0 97
N 0.00 1.00 3.0 100

Question A.5

What is the average probability that a given adult squirrel (of both species) will survive a full calendar year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible, there is no squirrelpox virus, no pine martens in the landscape and no lethal control being conducted)?


Two experts expressed values above 1 (but too low to be reasonably interpreted as a percentage), so if we remove those:

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey D 0.00 0.70 1.000 50
E 0.50 0.70 0.800 80
H 0.40 0.70 0.800 70
K 0.00 0.80 1.000 100
L 0.00 1.00 1.000 50
O 0.20 0.70 0.800 100
Q 0.30 0.40 0.500 50
Red B 0.00 0.80 1.000 100
C 0.30 0.60 0.900 73
D 0.00 0.50 1.000 60
E 0.50 0.70 0.800 80
G 0.31 0.77 0.959 100
L 0.00 1.00 1.000 50
O 0.20 0.70 0.800 100
Q 0.30 0.50 0.500 60
K 0.00 7.00 1.000 100
N 0.00 1.00 3.000 100

Movement

Question A.6

What is the average home range size of each species of squirrels (in hectares), under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 1.0 10 50 100
E 0.5 5 20 80
F 0.1 10 12 90
H 2.0 5 10 90
K 1.0 5 10 88
O 3.0 5 20 100
Red B 0.0 10 50 93
C 3.0 4 7 69
E 2.0 7 20 90
K 1.0 2 10 91
N 2.0 7 23 100
O 3.0 8 20 100
Q 1.0 1 2 50

B. Red squirrel translocation

Post-release effects are a critical consideration in translocation efforts, as they can significantly influence the survival and breeding success of released squirrels. After being released into the wild, squirrels may experience a period of acclimation during which their survival and reproduction rates differ from those of established wild populations. These effects may vary depending on whether the squirrels were sourced from captivity or the wild. In this section, we seek your expert judgment on key aspects of post-release effects.

Question B.1

How many months do you believe it will take for released squirrels (from captive or wild sources) to become acclimated to the release site (i.e. have survival and breeding rates expected from the release habitat and not influenced by the translocation itself)?

Source Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Captivity A 3 14 24 65
B 1 3 12 76
C 2 3 4 64
D 2 3 6 50
E 2 5 8 60
K 0 6 12 100
O 1 1 2 100
N 1 1 1 100
Q 0 0 0 100
Wild D 2 3 6 50
E 1 4 6 60
K 0 4 12 100
O 1 1 2 100
A 1 12 4 65
B 0 0 0 100
C 0 0 0 100
N 0 0 0 100
Q 0 0 0 100

Question B.2

After a red squirrel is released into the wild (from captive or wild sources), how many litters do you expect it to produce, while it is still acclimating to its new environment?
NOTE: Take note that this question asks for the total number of litters over the acclimation period. This is dependent on the previous question you answered, not a fixed yearly rate.

Source Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Captivity B 0 1.0 2 76
C 0 0.1 1 60
D 0 1.0 1 50
E 0 0.0 1 60
O 0 1.0 2 100
K 0 0.0 0 100
N 0 0.0 0 100
Q 0 0.0 0 100
Wild D 0 1.0 1 50
E 0 1.0 1 60
B 0 0.0 0 100
C 0 0.0 0 100
K 0 0.0 0 100
N 0 0.0 0 100
O 0 0.0 0 100
Q 0 0.0 0 100

Question B.3

After a red squirrel is released into the wild (from captive or wild sources), what is the probability it will survive until the next month, while it is still acclimating to its new environment?

Source Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Captivity B 0.0 0.9 1.0 81
C 0.3 0.5 0.8 61
D 0.0 0.5 1.0 100
K 0.0 0.5 1.0 100
N 0.0 1.0 1.0 100
O 0.0 0.6 1.0 100
Wild B 0.0 0.6 1.0 100
D 0.0 0.5 1.0 50
K 0.0 0.5 1.0 100
O 0.0 0.7 1.0 100

D. Grey squirrel management

Management of grey squirrel populations is a crucial component in supporting the conservation of red squirrels. Grey squirrels, being an invasive species, have a significant negative impact on red squirrel populations, largely due to competition for resources and the transmission of squirrelpox. To mitigate these effects, various management approaches are utilized, including trapping and shooting. The effectiveness of these approaches are assessed below.

Trapping

The following traps are being evaluated on this elicitation:
- Single-capture live traps: Cage traps designed to capture one grey squirrel at a time without causing harm. Once trapped, the squirrel can be humanely dispatched.
- Multi-capture live traps: Larger cage traps capable of capturing multiple grey squirrels simultaneously, in different subsections of the trap
- Snap traps: Lethal traps that quickly kill grey squirrels using a spring-loaded mechanism. Designed to ensure a swift and humane death.
Goodnature traps: Automated, self-resetting traps powered by CO2 canisters. They humanely kill grey squirrels upon activation and automatically reset after each use, allowing continuous operation with minimal maintenance.

Question D.1

Given a density of 1.25 grey squirrels per hectare in a woodland, what is the average probability that a given trap will catch a squirrel over one day of deployment. Assume all squirrels are within range of the trap.
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).

Trap Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Goodnature traps B 0.0 0.20 1.0 100
C 0.0 0.05 0.1 69
F 0.0 0.01 0.1 80
H 0.4 0.60 0.8 80
K 0.0 0.20 0.5 100
Q 0.1 0.10 0.3 80
E 0.0 0.00 0.0 100
I 0.0 0.00 0.0 100
N 0.0 0.00 0.0 100
O 0.0 0.00 0.0 100
Single capture live traps B 0.0 0.20 1.0 95
C 0.3 0.50 0.6 60
E 0.5 0.70 0.9 65
F 0.0 0.70 1.0 100
H 0.8 0.90 0.9 95
I 0.0 0.75 1.0 100
K 0.0 1.00 1.0 100
N 0.0 0.00 1.0 100
O 0.1 0.50 0.8 100
Q 0.1 0.10 0.3 80
Snap traps B 0.0 0.20 1.0 100
C 0.0 0.30 0.4 65
F 0.0 0.30 1.0 80
K 0.0 0.50 1.0 100
Q 0.1 0.10 0.3 80
E 0.0 0.00 0.0 100
I 0.0 0.00 0.0 100
N 0.0 0.00 0.0 100
O 0.0 0.00 0.0 100

Question D.2

Given a density of 1.25 grey squirrels per hectare in a woodland, how many squirrels will one multi-catch live trap catch over one day of deployment. Assume all squirrels are within range of the trap.
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
B 0 0.2 5.0 95
C 0 0.0 0.1 90
I 0 1.0 1.0 100
K 0 1.0 1.0 100
N 0 0.0 1.0 100
O 0 2.0 6.0 100

Shooting

The following shooting strategies are being evaluated on this elicitation:
- Free shooting: This method involves tracking and shooting individual grey squirrels as they are spotted in the field.
- Drey-poking: This technique targets dreys (nests) by disturbing them, causing the squirrels to flee, making them easier to shoot.
- Shooting over bait stations: In this approach, bait stations are used to attract squirrels to a specific location, where they can be shot.

Question D.3

How much ground does an average shooter cover one day’s fieldwork, in hectares?
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).

Trap Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Drey-poking B 0.25 5.00 10.0 50
E 500.00 700.00 1000.0 55
F 0.00 100.00 120.0 60
I 2.00 3.00 5.0 100
K 0.00 30.00 50.0 100
Q 10.00 15.00 20.0 100
N 0.00 0.00 0.0 100
Free shooting B 0.25 10.00 100.0 75
E 500.00 700.00 1000.0 80
F 20.00 100.00 120.0 70
H 5.00 15.00 25.0 75
I 2.00 6.00 10.0 100
K 0.00 30.00 50.0 100
N 0.00 1.00 1.0 100
Q 10.00 15.00 20.0 90
Shooting over bait-stations B 0.10 0.25 0.5 74
E 200.00 500.00 700.0 55
F 20.00 200.00 240.0 70
H 5.00 15.00 25.0 100
I 2.00 10.00 10.0 100
K 0.00 30.00 50.0 100
Q 1.00 1.00 2.0 100
N 1.00 1.00 1.0 100

Question D.4

Given an average density of 1.25 grey squirrels per hectare in a woodland, how many grey squirrels would a shooter dispatch in a day’s fieldwork, on average?
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).

Type Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Drey-poking F 0 2 8 60
K 0 20 50 100
Q 0 3 6 51
E 0 0 0 100
N 0 0 0 100
Free shooting E 5 15 30 70
F 0 2 4 70
K 0 30 50 100
N 0 1 1 100
Q 0 0 1 100
Shooting over bait-stations F 0 4 10 70
K 0 30 50 100
N 0 1 1 100
Q 0 3 6 100
E 0 0 0 100

E. Pine marten

Pine martens are an important natural predator influencing squirrel populations in the UK. Their presence can have differential effects on red and grey squirrels, potentially benefiting red squirrel populations indirectly - an effect described by Sheehy et al, 2018 and tested in simulations by Slade et al, 2023. In this section, we seek your expert judgment on the role of pine martens in squirrel predation as well as life history traits of pine martens.

Predation

Question E.1

Given an average density of 2 pine martens per km2 on a woodland, for a given individual red or grey squirrel living in such a woodland, what is the average probability of it being preyed by a pine marten over a given month?

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey E 0 0.15 0.2 65
F 0 0.10 0.2 80
K 0 0.80 1.0 100
Q 0 0.20 0.3 50
N 1 1.00 1.0 100
Red E 0 0.05 0.2 65
F 0 0.05 0.1 60
K 0 0.20 1.0 100
Q 0 0.20 0.3 50
N 1 1.00 1.0 100

Life History

Question E.2

Under current conditions, what is the average length, in months, of the pine marten breeding season (i.e. the window during the year when the species engages in reproductive activities, including mating, gestation, and giving birth to offspring)?


Expert Minimum Plausible Most Likely Maximum Plausible Confidence
D 8 9 11 70
E 2 2 3 75
F 6 8 10 90
L 10 11 11 85
O 7 8 10 100
N 1 1 1 100

Question E.3

How many litters do you expect an average female pine marten to have per year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?


Expert Minimum Plausible Most Likely Maximum Plausible Confidence
D 0 1.0 1 70
E 0 1.0 1 90
F 0 0.5 1 100
K 0 1.0 2 86
L 0 1.0 1 70
Q 0 1.0 1 75
N 1 1.0 1 100
O 1 1.0 1 100

Question E.4

How many individuals do you expect to survive after weaning from a single litter produced by an average female pine marten, under ideal conditions (i.e., in a woodland where inter- and intra-specific competition is negligible)?


Expert Minimum Plausible Most Likely Maximum Plausible Confidence
D 0 2.0 4 60
E 1 3.0 5 70
F 0 2.0 4 90
K 0 1.5 2 100
L 0 1.0 6 70
O 1 2.0 4 100
N 1 1.0 1 100

Question E.5

What is the average probability that a given pine marten (either a juvenile or adult) will survive a full calendar year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?

Age Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Adult D 0.0 0.80 1.0 70
E 0.2 0.50 0.8 65
F 0.5 0.85 1.0 85
K 0.0 0.90 1.0 90
L 0.0 1.00 1.0 50
N 1.0 1.00 1.0 100
Juvenile D 0.0 0.60 1.0 55
F 0.0 0.45 1.0 80
L 0.0 1.00 1.0 50
N 1.0 1.00 1.0 100

Training question

The question below is a training question to familiarise you with the survey interface. Make sure to play around with the values so you get a sense of how your answers shape the outcome.

What is the probability that, when dropping a piece of toast, said toast will land on the floor with the spread side down, given the following spreads:

  1. Butter
  2. Jam

Spread Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Butter B 0.0 0.5 1.0 100
C 0.0 0.5 1.0 100
D 0.1 0.8 0.9 80
E 0.5 0.7 1.0 95
I 0.0 0.0 1.0 51
J 0.0 0.5 1.0 94
M 0.0 0.5 1.0 66
N 0.0 1.0 1.0 100
O 0.0 1.0 1.0 100
Q 0.1 0.3 0.6 100
Jam B 0.0 0.5 1.0 100
E 0.6 0.9 1.0 95