when hunting has significant impact on wildlife
DESCRIPTION
Erindi Tomas Willebrand, Hedmark University College, Norway, flutt á ráðstefnu SKOTVÍS í samstarfi við UST, fimmtudaginn 21. mars 2013, Grand Hotel, ReykjavíkTRANSCRIPT
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
0100
200
300
400
500
-0.4 -0.2 0.0 0.2 0.4
0100
200
300
400
500
-0.4 -0.2 0.0 0.2 0.4
0100
200
300
400
500
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
0
2000
4000
6000
8000
10000
12000
0
2000
4000
6000
8000
10000
12000
0
2000
4000
6000
8000
10000
12000
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
02
46
8
Adult Density
Bre
edin
g su
cces
s
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
0.2
0.4
0.6
0.8
1.0
1.2
Harvest
App
aran
t Sur
viva
l
2000 2005 2010
05
1015
År
Antal
2000 2005 2010
05
1015
År
Antal
2000 2005 2010
05
1015
År
Antal
2000 2004 2008 2012
05
1015
År
Antal
Harvest rates. All years, all areas
Rate
Frequency
0.0 0.1 0.2 0.3 0.4 0.5 0.6
05
1015
2025
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
●
2008 2009
−1.5
−0.5
0.5
1.5
Jämtland
2008 2009
−1.5
−0.5
0.5
1.5
Västerbotten
●
2008 2009
−1.5
−0.5
0.5
1.5
Norrbotten
Large scale forces are present
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-1
-0.5
0
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1
1.5
2
2.5
3 R
elat
ive
chan
ge in
den
sity
North Sweden
North Norway
North Finland Distance
North Finland Triangles
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
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