Harvest Harvested v. unharvested populations –Why are some species not harvested? –Why are some...
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Transcript of Harvest Harvested v. unharvested populations –Why are some species not harvested? –Why are some...
Harvest
• Harvested v. unharvested populations– Why are some species not harvested?
– Why are some species harvested at different rates?
– Why does harvest not seem to affect some species?
• Assuming a surplus– Overharvest
• Extinction– Regulated hunting has never led to an extinction!
– Underharvest• Environmental degradation (Think like a mountain!)• Loss of recreational opportunities (value!)
Harvest
• Characteristics of harvested populations– Population size– Population stability– Fecundity– Life span– Mortality from other causes
• Unknown effects– Trophy’s smaller?– Genetics?
Harvest
Harvest
• Principles– Yield*– Diminishing returns– Compensatory harvest mortality
– So which one happens?
• History, Trial & Error, and Human Dimensions
Harvest
• When populations are limited by some resource (density-dependence/logistic growth; from Caughley
& Sinclair 1994)
– A population is harvested at its growth rate
– A population must be stimulated to produce a yield
– Harvesting trades off yield against population size
Harvest
• Sustained yield, harvest rate, & population sizeN
Time
Accelerating Phase
K
Inflection Point (½ K)
DeceleratingPhase
Nd
N/d
t
Inflection Point (½ K)
K
HarvestY
ield
200
100
0
Population Size
0 1000750500250
• Sustained yield, harvest rate, & population size
N
dN
/dt
Inflection Point (½ K)
K
Harvest50
150
100
200
0
Surplus or Yield
MSY
potentially sustainable harvestno harvest sustainable
HarvestY
ield
200
100
0
Population Size
0 1000750500250
MSY
Lower SY Upper SY
• Harvestable surplus– Sustained yield (SY)– Maximum sustained yield (MSY)– Optimal sustained yield (OSY)
Harvest
Pop
ulat
ion
Siz
e +
Rec
ruits
(Y
ield
)
750
450
300
150
0
Population Size
0 1000750500250
600
900
1050
1200
75
100
75
Yield
MSY
Lower SY
Upper SY
Harvest
• When populations are not limited by resources (exponential growth)?
N
Time
N
dN
/dt
Yie
ld200
150
100
50
0
Population Size
0 1000750500250
Harvest
200
150
100
50
0
Harvest
• Do we need strict regulations or is the harvest self-regulating?– The Law of Diminishing Returns
Abundance
Tot
al H
arve
st
Abundance
Hun
ter
Effo
rt
Abundance
Har
vest
Rat
e
Harvest
• Additive v. Compensatory Mortality
0
20
40
60
80
100
120
# D
yin
g/y
r
Unhun
ted
Hunte
d A
Hunte
d B
Hunte
d C
Harvest
Predation
Starvation
Exposure
Accidents
Disease
Harvest
• Additive v. Compensatory Mortality– Temporal studies
Harvest Mortality Rate
Ann
ual S
urvi
val R
ate
A
Harvest Mortality Rate
Ann
ual S
urvi
val R
ate
B
C
c = critical threshold
Harvest
• Additive v. Compensatory Mortality– Large scale experiments
– SHunting = SNo Hunting
– Late season harvest
Harvest
• Additive v. Compensatory Mortality– Management implications
• Harvest as compensatory mortality– Doomed surplus– Harvestable surplus
Murray and Frye (1964)
Harvest
• When density independent factors are controlling populations– Harvestable surplus?
• K
N
Time
Harvest
• Harvest Management (Ideal)– Determine the status of the resource– Determine the objectives & goals– Establish management strategies– Determine how closely the management
strategy achieved the objectives & goals– Adjust management strategies
• Adaptive harvest management
Harvest
• Harvest Management– Determine the status of the resource
• Research & monitoring– Mandatory reporting– Check stations– Surveys– Large scale studies
Harvest
• Harvest Management– Determine the objectives & goals
• Biological, social, economic, & enforcement considerations– Increase, decrease, maintain, or stabilize population size*
» Wildlife viewing, hunting, fishing, etc.» Wildlife damage» Wildlife disease (e.g., CWD, rabies)» Habitat & community effects (e.g., brain worm)» MSY or OSY» Quality v. quantity
Harvest
• Managing for the hunter, fisher, or trapper– Quality v. quantity
• E.g., QDM– Deer, habitat, & experience
(U.S Fish & Wildlife Service)
(U.S Fish & Wildlife Service)
(U.S Fish & Wildlife Service)
Harvest
• Harvest Management– Establish management strategies
• Seasons– Opening day phenomenon
• Bag limits• Methods• Sex & age taken• Permits• Areas• Effects on and of farming, timber, & non-consumptive
wildlife uses • Safety• Enforcement
Harvest
• Harvest Management– Determine how closely the management
strategy achieved the objectives & goals• Research & monitoring
– Mandatory reporting– Check stations– Surveys– Large scale studies
Harvest
• Harvest Management– Adjust management strategies & start again
• Adaptive management
• History, Trial & Error, and Human Dimensions
Harvest
• Illegal taking (poaching)– Equal to the legal harvest?– Fair chase?
– This is not hunting!
– Implications• Population & habitat management
– SY considerations
– Estimate illegal take
Harvest
• Differential Vulnerability– Sex– Age
• Mating habits
– Effects on sex ratios & age structure• Productivity
Harvest
• Differential Vulnerability– Flook (1970): Unhunted elk
• Sex ratio of newborn calves: 100:100• Sex ratio of yearlings: 131:100• Sex ratio of 2 year olds: 31:100
• Why?– Fat, teeth, mating system
Harvest• Differential Vulnerability
– Hunted black bears • Sex ratio of population: 72:100• Sex ratio of harvest:145:100
• Why?– Movements: male home range size = 30.8 km2 female
home range size = 5.2 km2
– Harvest technique: 4% of harvest using bait is female 40% of harvest using dogs is female
– Harvest timing: denning
• Management