west nile virus: ecology and impacts on greater sage

Dave NaugleBrett WalkerJason Tack

West Nile Virus: Ecology and Impacts on Greater

Sage-grouse Populations

• Arthropod-borne flavivirus

West Nile virus

Primaryhosts

Vector

Alternatehosts

• Outbreaks more common during drought- Epstein and deFilippo 2001, Shaman et al. 2005

• High temperatures = faster virus amplification, mosquito development, and population growth

- Reisen et al. 2006, Zou et al. 2006

• More surface water = more mosquitos for longer- Zou et al. 2006, Doherty 2007

Ecology in sagebrush habitats

Hen survival July-August 2003, PRB

76%(no WNV)

20%(WNV)

Walker et al. 2004, Wildlife Society Bulletin

0.4

0.5

0.6

0.7

0.8

0.9

1

Upper GreenRiver Basin

Southern PhillipsCounty

Alberta SouthernPowder River

Basin

Northern PowderRiver Basin

Cu

mu

lativ

e P

rop

ort

ion

Su

rviv

ing

(1

Ju

ly to

31

Au

gu

st)

WNV Not DetectedWNV Detected

44

20022000-02 1998-02 2001-022003 2003 20032003 2003

110 237155 10985 1118

** *

WNV reduced survival by 25% in 2003

Naugle et al. 2004,Ecology Letters

-1.0

-0.5

0

0.5

1.0

Pro

port

iona

l Cha

nge Non-WNV leks WNV leks

Only one hen seen at one lek in 2004

Lek Attendance Declined ~85% in 2004

Walker et al. 2004, Wildlife Society Bulletin

West Nile virusDetected

Not detected

Sage-grouse range

Current

Potential pre-settlement

Greater

Gunnison

Greater

Gunnison

Reported WNv mortality in sage-grouse

Recent example from western Idaho

• WNv affects both sexes, all age classes- Aldridge 2005, Kaczor 2008, Walker 2008

• Lab tests confirm that all birds that contract disease die – Clark et al. 2006

• WNv mortality varies temporally, geographically- Naugle et al. 2005, Walker and Naugle 2008

• Laboratory and field data suggest very low resistance - Clark et al. 2006, Walker et al. 2007

What we know so far

Population Modeling

How much does WNv mortality affect population growth?

Will increasing resistance offset impacts?

Simulations

• Three simulation scenarios:

1) No WNv

2) Simulated WNv (4% resistance)- Walker et al. 2007

3) Simulated WNv (increasing resistance)

• 1000 replicates per simulation

0

20

40

60

80

100

120

140

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

WNv infection rate

Fre

qu

ency

.

Simulated WNv infection rates• Typically low, but with extreme values (~50%)

- Walker et al. 2007

Disease reduced population growth by 6 – 9% per year

ScenarioChange in Population

Growth

WNv 6 – 9 %

WNv with increasing resistance

6 – 8 %

Resistance increased marginally

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 5 10 15 20

No. years

Res

ista

nce

to

WN

v

.

Higher infection rates would cause resistance to spread faster, but increase impacts

6 ways to build a pond that doesn’t grow mosquitoes

1) Overbuild CBNG ponds to accommodate water

2) Build steep shorelines to reduce shallow water and

vegetation and increase wave action

3) Lower water levels to maintain muddy shorelines

4) Dig ponds in flat areas instead of damming natural

draws to restrict slope seepage

5) Line channel and overflow spillway with crushed rock

to limit sediment and vegetation growth

6) Fence pond site to restrict access by livestock

Effects of West Nile in Valley County:Survival Estimates

0

0.2

0.4

0.6

0.8

1

1.2

1 3 5 7 9 11 13 15 17 19 21 23

Week

Su

rviv

alWNv 1st Detected in Valley County

0 10 205Miles

0 10 205Miles

Milk River Basin North Dakota

Population trends in healthy vs impacted landscapes

Milk River Basin North Dakota

Population trends in healthy vs impacted landscapes

• WNv is an endemic stressor that is here to stay

• Locally we can design water projects to help reduce impacts

• Continue to monitor resistance to see if rates increase to buffer

against losses

• Small populations will experience inordinately large impacts

• A call for conservation of large intact landscapes

Take Homes

Thanks for listening, questions or comments?

west nile virus: ecology and impacts on greater sage

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