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?

• Harvestable surplus (Leopold 1933)

– Originally only the “doomed” surplus

Harvest

• 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

• What if we don’t know enough?

• What if regulations are impractical?

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

• Harvests can stabilize populations– A management tool

N

Time

Harvest

• Economics: Billions of dollars & hundreds of thousands of jobs!

• Costs of not harvesting

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

Harvest

• Differential Vulnerability– Management implications

• Sex & age structure• Mating habits

Harvest

• Federal v. state management

• Public v. private lands– Private lands harvest management initiatives