Ranavirus could speed up extinction for the endangered frog, Rana sevosa

Julia E. Earl, Matthew J. Gray and William B. Sutton

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)

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

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

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

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

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

Adult Exposure to Ranavirus

Quick Extinction with No Intervention

7

8

9

10

11

12

13

14

15

16

17

148 (Glen's Pond Avg.) 200

Tim

e t

o E

xtin

ctio

n (

year

s)

Average Hydroperiod (days)

No Disease

Adult Ranavirus Yr. 1

Adult Ranavirus Yr. 5

Adult Ranavirus Yrs. 1 & 10

Adult Ranavirus Yrs. 1 & 5

Intervention: Hydroperiod

• Experimental water addition in 2001 (Seigel et al.

2006)

• Examined a non- limiting hydroperiod

– Assumes water added to the pond every year

Permanent Hydroperiod

40

45

50

55

60

65

70

75

80

85

No disease

Every 50 every 25 every 10 every 5 every 2

Tim

e t

o E

xtin

ctio

n (

yrs)

Disease Interval (years) **Realistic

intervals

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)

• 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

Exti

nct

ion

Pro

bab

ility

Disease Interval (years)

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

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

Acknowledgements

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

• Funding: