1. Status - Yellowstone Bison Population
August 2008
Abundance
Life History
Movements
Genetics
Brucellosis
IBMP Implications

2. Abundance – 2008 Annual Population Estimate
Park wide
3,000 bison in July 2008
2,500 adults and yearlings from winter
Less winterkill than expected
500 calves of the year
Central Interior
1,500 bison (1,300 adults and yearlings; 200 calves)
Northern Range
1,500 bison (1,200 adults and yearlings; 300 calves)
Next Park-wide Distribution Survey in December 2008

3. Abundance ?

4. Abundance

5. Life History Age structure Park wide 68% 12% 20%
Adult Yearling Calf
Park wide 68% % 20%
Central Interior 70% % 18%
Northern Range % % %

6. Life History
Sex Ratio – males per 100 females

7. Life History
Survival
Annual adult, female survival = 91% (excluding risk management and hunter removals)
Annual adult female survival = 83% (with risk management and hunter removals)
Pregnancy
Depends on age and disease status
90% of sero-negative, pregnant bison
20% lower in sero-positive bison

8. Life History

9. Life History Birth Synchrony
Concurrent with the emergence of spring forage
Maximizes calf growth prior to weaning
Peak calving period = April 25 to May 25 (80% of births)

10. Movements Streams strongly influence bison travel
Roads may facilitate movements in some areas.
Autumn-winter movements on the Northern Range follow river corridors
Autumn-winter movements from the Central Interior are highly variable
West only (50%)
North only (31%)
West early, then north later (8%)
Stay in Central Interior (11%)

11. Movements
Northern Range Winter Movements

12. Movements
Central Interior Ranges Winter Movements

13. Movements
Few bison stay in Pelican/Hayden system all winter

14. Movements
Pelican Creek to Gardiner Basin in 12 days

15. Movements
Madison Junction to Gardiner Basin in 3 days to 3 weeks

16. Movements
Early arrivals in Gardiner Basin (Oct. to Dec.) -Central Interior Bison

17. Movements
February
Peak of movement to boundary ranges
April

18. Population sub-division Removals affects genetic diversity
Genetics
Population sub-division
Removals affects genetic diversity
University of Montana – effects of risk management removals on conserving diversity

19. Brucellosis 2008 population sero-prevalence = 45 %
Long-term estimate = % sero-positive
Extensive data set from winter ( )
* ~ 400 bison sampled for culture
* ~ 1600 bison sampled for serology
Culturing of samples in progress
Sero-prevalence is variable by age

20. Brucellosis Gender Age % sero-positive Male calf 11 yearling 35 > 262
Female 13 2 52 3 67 4 69
> 5

21. IBMP Projections and 2008 Status
Item
IBMP 2000
IBMP 2008
Level of Risk Management Removal
~ approximately 7% or 250 bison removed per year
varies between 0-40%, up to 1700 per year
Population Size and Distribution
population stabilize ~ 3700
* ~ ¾ Central Interior
* ~ ¼ Northern Range
population approach 5000 and varies widely
* ½ Central Interior
* ½ Northern Range
Movement
Northern Range to Gardiner Basin
Central Interior to West Yellowstone
Northern Range to Gardiner Basin and mixing with Central Range
Central Interior to both boundaries
Population Structure
1 male : 1 female
73 adult : 11 yearling : 16 calf
~ 1 male : 1 female population wide
Ratios Variable by Range
Vital Rates
Pregnancy 50 %
Birthing Rate 50 %
Survival ?
Pregnancy 70 – 90 %
Birthing Rate 90%
Survival 91%

22. IBMP Implications Start winter 2008-2009 with 2,900-3,000 bison
Central Interior herd reduced to 1,500 and Northern Range herd increased to 1,500
Natural winterkill and predation mortality average ~10% or 300 bison
MT hunt and treaty harvest expected ~5-7% or bison
Increased probability of large movement from Northern Range
IMBP risk management removal limited to a total <300 bison