2013 International Symposium on Ranaviruses by Julia Earl

1. Ranavirus could speed up extinction
for the endangered frog, Rana sevosa
Julia E. Earl, Matthew J. Gray and William B. Sutton

2. The Big Unknowns:
Can Ranavirus affect population dynamics?
Can Ranavirus cause local or global extinction?
Evidence so far:
– About 40-60% of USA amphibian die-offs (1996-
2005) attributable to ranavirus (Green et al. 2002, Muths et
al. 2006)
– Common Frogs- adult populations decline ~80% in
ponds with ranavirus (Teacher et al. 2010)

3. Population Models
• Great tool to examine how changes in survival
might affect populations
• Start with species most likely to be vulnerable
to extinction
– closed populations of an endangered species
– examine effects of ranavirus exposure in one life
stage at a time
– different intervals of exposure

4. Dusky Gopher Frog- Rana sevosa
• One of the most
endangered frogs in the
USA- listed in 2001
• Only one regular, viable
population- Glen’s Pond
(MS)
• Pond breeder- eggs in Dec.
• Metamorphs emerge in
June when the pond dries
• Adults in long leaf pine
often associated with
Gopher Tortoise Burrows
IUCN

5. Model
Juveniles Adults
• Very simple stage-structured matrix model of
females
• Parameterized using data from 1995-2001
monitoring Glen’s pond (Richter et al. 2001, 2003)
• Built in stochasticity in the model- drew random
values for parameters from a normal distribution
each year
• Hydroperiod threshold for metamorph
production- 190 days

6. Calibrating the Model
• Examined correlations between model
generated data and actual data (r = 0.6-0.8)
• Sensitivity analysis- which parameter values
change the model the most?
– Sensitive to survival from eggs to juveniles
• Further- data from 2000s suggests population
decline of 10% each year (Pechmann, unpubl. data)
– Our model output is consistent with this

7. Simulations
• Ran model with 3 hydroperiods: 148 (Glen’s
pond average), 200 (slightly wetter
period), and functionally permanent
• Ranavirus- challenge trials complete on
adults, showed 100% mortality in water bath
– See Bill Sutton’s Poster!
– Die-off concentrations of virus (103 pfu/mL)
– Assuming only adults are exposed
– Examined different exposure intervals

8. Adult Exposure to Ranavirus

9. Quick Extinction with No Intervention
7
8
9
10
11
12
13
14
15
16
17
148 (Glen's Pond Avg.) 200
TimetoExtinction(years)
Average Hydroperiod (days)
No Disease
Adult Ranavirus Yr. 1
Adult Ranavirus Yr. 5
Adult Ranavirus Yrs. 1 & 10
Adult Ranavirus Yrs. 1 & 5

10. Intervention: Hydroperiod
• Experimental water addition in 2001 (Seigel et al.
2006)
• Examined a non- limiting hydroperiod
– Assumes water added to the pond every year

11. Permanent Hydroperiod
40
45
50
55
60
65
70
75
80
85
No
disease
Every 50 every 25 every 10 every 5 every 2
TimetoExtinction(yrs)
Disease Interval (years) **Realistic
intervals

12. Supplemental Rearing
• Water addition not feasible- too expensive
• Rearing all the eggs in cattle tanks beside the
pond is the current strategy
– Producing 200-900 juveniles each year
– Population now at a 10% increase each year
(Joe Pechmann, personal communication)
• Examined this scenario (still adult mortality only)

13. • Previous scenarios- all populations went
extinct within 1000 years
– Not true with this scenario
0
0.05
0.1
0.15
0.2
0.25
No
disease
Every 50 Every 25 Every 10 Every 5 Every 2 Every 1
ExtinctionProbability
Disease Interval (years)

14. Conclusions
• Exposure to ranavirus could speed up extinction and
increase extinction probabilities, depends on exposure
interval and intervention strategy
• Unlikely to have a large effect for Dusky Gopher Frogs if
the current intervention is maintained
– The population dynamics are not actually very sensitive to
adult mortality
– Future data on other life stages will be really important
• However, interactions with other
stressors could make this worse
• Results will need to be reassessed if
other populations become viable
creating a metapopulation

15. Future Directions
• Data to come on other life stages of the Dusky
Gopher Frog- larvae, metamorphs
• Wood frog model- comparing exposure in
different life stages- WDA Thurs.
– Even this common species can go extinct
• Add a transmission component and using
meta-population models

16. Acknowledgements
• Help with Gopher Frog trials: Becky
Hardman, Dr. Becky Wilkes, Dr. Debra Miller
• Funding: