SELKIRK GRIZZLY BEARDNA MARK/RECAPTURE ANALYSIS

MARK/RECAPTURE EXPLAINED

• 1ST you catch some animals in a popn and “mark” them

• Later on you catch animals in the same popn• Some of those will be new animals, some will

be recaptures• You use the ratio of “marked” to “recapture”

to get a popn estimate

MARK/RECAPTURE

N = Popn sizeM = # initially markedn = # caught in 2nd effortm = # recaptured

CRITICAL ASSUMPTIONS

• Marks must be permanent

• All animals have an equal probability of getting caught

• No changes in population between initial mark and later recapture

MARK/RECAPTURE example

• 1st mark: Trap 20 mice, paint their toenails red

• Recapture: 1 week later, trap again; catch 16 mice, 4 have red toenails

So, N = (20 x 16) / 4, which equals 80

What does this have to do with grizzly bears?

USE DNA as a MARK

50 cm

Barbed Wire

Scent Lure

50 cm

Barbed Wire

Scent Lure

50 cm

Barbed Wire

Scent Lure

• 50 barbed-wire enclosures • 470 sq miles• Visited each site 4 times• Collected over 1,200 hair samples

RESULTS

• ID 15 different grizzly bears• 9 females, 6 males

• 15 bears captured a total of 50 times

• Multiple capture sessions allows for more complicated analysis

• Test for diff male/female capture probability, diff capture prob among sessions, trap happy, trap shy

RESULTS

• Best model estimated 18 grizzly bears – 11 females, 7 males• Density14.1 bears / 1,000 sq km (naïve estimate)9.7 bears / 1,000 sq km (SECR estimate)

3.6 bears / 100 sq miles2.5 bears / 100 sq miles

COMPARISON: PROCTOR 2005

US BC

# CELLS 50 77

POPN EST 18 33

DENSITY (#/1000 sq km) 14.1 16.5

CAUTIONS about mark/recap

• Point Estimate – says nothing about trend

• Density cannot be extrapolated to the entire ecosystem

• No info on reproduction

THE END