When hunting has significant impact on wildlife
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Population effects of harvesting grouse
Actually, quite a lot! How difficult can it be?
Can harvest reduce the growth of small game populations?
YES!
A poorly formulated question!
When will harvest reduce the population growth below its
normal levels?
-0.4 -0.2 0.0 0.2 0.4
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Without harvest With harvest
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
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Annual cycle • Autumn • Winter • Summer
NO compensation through increased production
§ Large annual variation § Iceland an exception
§ No reports on density dependence § For any grouse species
0 5 10 15
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Adult Density
Bre
edin
g su
cces
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Willow grouse Sweden 1996-2011
Hunted/Closed Areas
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
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.
Smith, A. and Willebrand, T. 1999. Mortality causes and survival rates of hunted and unhunted willow grouse. - J Wildlife Manage 63: 722–730.
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
0.0 0.1 0.2 0.3 0.4
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Harvest
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Antal
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Harvest rates. All years, all areas
Rate
Frequency
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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?
§ Bag statistics is needed § Including effort, numbers § Caution – bag as a proxy for population abundance § Other index or count is needed! § Two different questions:
§ The role of hunting in a population decline § The role of hunting in keeping a declined population at low
abundance
effort
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chick
State land – Sweden
Avoid reaching 30% harvest over a to large area
Closed hunting on state land. One of three counties, 2009
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2008 2009
−1.5
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Jämtland
2008 2009
−1.5
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Västerbotten
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2008 2009
−1.5
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Norrbotten
Large scale forces are present
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3 R
elat
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sity
North Sweden
North Norway
North Finland Distance
North Finland Triangles
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North Sweden
North Norway
North Finland Distance
Rodent index north Sw
Breeding success
In the end …
§ Hunters often care about their wildlife § Can contribute to observations & data § Managers organize & educate § Researchers analyze § Annual feedback § Mutual trust
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