Population effects of harvesting grouse
How difficult can it be?
Actually, quite a lot!
Can A harvest reduce the growth poorly formulated of small game populations?
question!
When will harvest reduce the population growth below its YES!levels? normal
500 500 400 400 300 300 200 200 100 100 0 0
-0.4 -0.4
-0.2 -0.2
0.0 0.0
0.2
Without harvest With harvest
0.4
Looking for effects 1. Increased production of young 2. Decreased predation § Additive – Compensatory
3. Population change § Reference population § Before after
In the following: - Ruffed grouse in Wisconsin 1985-1995 - Willow grouse in Sweden, 1992-1995 - Willow grouse in Norway, 1996-1999 - Willow grouse on state land in Sweden, 1996-2011
Possible grouse dynamics
12000 12000 10000 10000 8000 8000 6000 6000 4000 4000
Annual cycle • Autumn • Winter • Summer
2000 2000 00
8
NO compensation through increased production
6
§ Large annual variation § Iceland an exception
Willow grouse Sweden 1996-2011 Hunted/Closed Areas
2
4
§ For any grouse species
0
Breeding success
§ No reports on density dependence
0
5
10 Adult Density
15
From Errington to Erlinge & Marcström § Doomed surplus have to die § Predators as garbage cans § “If we fail to take a human harvest – nature will do it for us” § Predators can regulate their prey § Predator control increase prey abundance § But if predators can reduce numbers of prey … § What about hunters and predators together?
Additive mortality using radio-marked grouse § Additive in hunting season § Weak, if any, compensation after hunting season Smith, A. and Willebrand, T. 1999. Mortality causes and survival rates of hunted and unhunted willow grouse. - J Wildlife Manage 63: 722–730. Is hunting mortality additive or compensatory to natural mortality? Effects of experimental harvest on the survival and cause-specific mortality of willow ptarmigan Brett K. Sandercock1,2*, Erlend B. Nilsen2, Henrik Brøseth2 and Hans C. Pedersen2 J.Anim.Ecol. 2011.
Small, R. J. et al. 1991. Predation and Hunting Mortality of Ruffed Grouse in Central Wisconsin. - The Journal of Wildlife Management 55: 512–520.
Harvest and Population trends Sum of compensation § Ruffed Grouse 91% harvest § No change in population development § Immigration necessary
Pedersen, H. C. et al. 2004. Weak compensation of harvest despite strong density-dependent growth in willow ptarmigan. - Proc. Biol. Sci. 271: 381–385.
Small, R. J. et al. 1991. Predation and Hunting Mortality of Ruffed Grouse in Central Wisconsin. - The Journal of Wildlife Management 55: 512–520.
Swedish long-term investigation 1996 – 2011
0.8
15 10 0
5
Antal
15
2005
2010
2000
2005
År
2010
År
0.0
0.1
0.1
0.2
15 10 5
0.2
0.3
0.3
Harvest2000
Rate
0.4 2005 År
0.4 2010
0.5
0
0.0
0
0
5
Antal
Antal
5
10
0.4
15
10
0.6
Frequency
2000
0.2
Apparant Survival
0
5
20
1.0
Antal
10
1.2
25
15
Harvest rates. All years, all areas
0.6 2000
2004
2008 År
2012
Dispersal+ Migration Females
Scale and connectivity
1. High annual natural mortality 2. Large recruitment required 3. From where?
Summarize so far … § Additive mortality during harvest season using radio-marked grouse. § Seasonal migration & dispersal change local population size § Recruitment is a LARGE scale process § Dispersal 1. emigration 2. patch-movement 3. immigration
§ Landscape quality – Breeding density
Changes in population Changes in harvest? 2
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§ Bag statistics is needed § Including effort, numbers § Caution – bag as a proxy for population abundance § Other index or count is needed! bag § Two different questions: ●
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§ The role of hunting in a population decline § The role of hunting in keeping a declined population at low abundance density ●
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chick
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State land – Sweden
Avoid reaching 30% harvest over a to large area
Closed hunting on state land. One of three counties, 2009 1.5
Västerbotten
1.5
Jämtland
0.5 −0.5 −1.5
−1.5
−0.5
0.5
●
2008
2009
1.5
Norrbotten
−1.5
−0.5
0.5
●
2008
2009
2008
2009
Large scale forces are present 3.5 3
3
2.5 2.5 2 2
Relative change in density
1.5 1.5 1
0.5
1
00.5
-0.5 0 -1 -0.5
-1.5
Breeding success
-1
-2 -1.5 -2
North Sweden North Norway North Finland Distance North Sweden Rodent index north Sw North Norway North Finland Distance North Finland Triangles
In the end … § Hunters often care about their wildlife § Can contribute to observations & data § Managers organize & educate § Researchers analyze § Annual feedback § Mutual trust