Lecture Outline: Population Harvesting

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Lecture Outline Population Harvesting

• Why harvest?
• Scope of harvesting

• Additive versus compensatory mortality

• Empirical evaluation mallards

• Empirical evaluation mule deer

• Maximum sustainable yield (MSY)

• Sustainable harvest strategies

• Age- and sex-structured harvesting

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Why harvest?

• Meat

• Income

• Recreation

• Maintain ties to nature

• Management toolmaintain wildlife populations at
  levels acceptable to society

• Avoid overexploitation and underexploitation

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Harvesting as a management tool

• Example Whitetail deer at Allerton Park
• Intolerable densities
• Severe destruction to plant community
• High disease risk
• Initiated archery hunt to reduce herd

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Scope of Harvesting Waterfowl

• 1.5 million hunters in U.S.
• 13 million waterfowl harvested/yr
• 1.6 billion/yr spent locally
• 25 million/yr for habitat from duck stamps

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Scope of Harvesting Illinois Deer

• 2007-2008 Season 198,544 deer harvested
• Data from 2001
• 310,000 hunting licenses sold (238,000 for deer)
• 451,000,000 spent on hunting (232,000,000 for
  deer)
• 4.5 million hunting days (3.1 million for deer)
• 1.5 million Illinois residents (16) are
  consumptive users

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Scope of Harvesting Illinois Furbearers

• 2006-2007 Illinois Fur Harvest Summary (IDNR)
• Total number of pelts sold 213,057
• Total value of pelts sold 1,757,789
• No. 1 species - raccoon (136,883 pelts)
• No. 2 species - muskrat (50,483 pelts)
• No. 3 species - coyote (8,218 pelts)

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Additive vs. Compensatory Mortality
Does harvesting increase the overall mortality
rate for a population?
Does harvesting simply remove a surplus of
individuals that would otherwise die from other
causes?
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Compensatory mortality requires density-dependent
survival
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Additive
Natural survival rate
0.5
Compensatory
0
0
100
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Population size
Hence, all of the issues that we have talked
about in regards to measuring and understanding
density dependence apply to the questions of
whether hunting mortality is compensatory.
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An example

• Assume that harvest mortality takes place first
  and then natural mortality occurs rest of year in
  density-dependent fashion.

Sn B0 B1N
where Sn is natural survival rate outside of
hunting season.
(Borrowed from Gary Whites Lecture Notes, CSU)
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An example

• Now, we remove hunting so that 90 individuals
  undergo natural mortality.

Sn 0.8333 0.005556 (90) 0.333
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Types of compensatory mortality
1. Complete hunting mortality is completely
compensated for by increase in survival outside
of hunting season
2. Partial hunting mortality is partially
compensated for by increase in survival outside
of hunting season
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Types of compensatory mortality
3. Threshold hunting mortality is compensated
for by an increase in survival outside of
hunting season to a threshold harvest value (c).
Beyond threshold, population cannot compensate
for harvest and overall survival rate decreases.
Additive
Compensatory
Annual survival rate (S)
Annual survival rate (S)
c
Hunting mortality rate (K)
Hunting mortality rate (K)
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Empirical evaluations compensatory vs. additive
mortality

• Band recovery data from 410,000 adult mallards

• Multiple studies in N. America from 1950 to 1979

• Alternative hypotheses complete compensatory
  and totally additive

• Rejected the hypothesis of total additivity and
  concluded that it appears that hunting
  mortalities are largely compensated for by other
  forms of mortality.

(Burnham, KP and DR Anderson. 1984. Ecology
65105-112)
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Empirical evaluations compensatory vs. additive
mortality

• Band recovery data for mallards from 1979-1989.

• Strongly rejected the complete compensatory
  hypothesis.

• Concluded that under certain conditions,
  restrictive regulations can successfully increase
  survival rate of mallards.

(Smith, GW, and RE Reynolds. 1992. J. Wildlife
Manage. 56306-316.)
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Empirical evaluations compensatory vs. additive
mortality
(Poysa H et al. 2004. Oikos 104612-615)
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Empirical evaluations compensatory vs. additive
mortality

• Used three experimental manipulations to test the
  hypothesis of compensatory mortality in a
  Colorado mule deer population.

• Focused on survival of fawns and used
  radio-collared deer.

Bartmann, RM et al. 1992. Wildlife Monographs No.
121.
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Experiment I Density reduction in field

• Hunting mortality was simulated by live trapping
  and removing 20 of the population on half of
  the Ridge study area in November-December.

• Estimated mortality rates of fawns on treatment
  and control areas until June.

• No spatial replication

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Experiment II Controlled density in pastures

• Stocked three pastures with different deer
  densities in winter and estimated overwinter fawn
  mortality rates

• Simulated situation in which all pastures stocked
  as same high density (133 deer/km2) and then
  different harvest levels imposed (67, 33, 0)

• No spatial replication

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Experiment II Controlled density in pastures

• Fawn survival was related negatively to density.

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Experiment III Predator removal

• Tested null hypothesis that decreased predation
  rate on mule deer fawns does not affect overall
  survival rate

• Predation rates decreased and starvation rates
  increased, but no change in overall fawn survival
  was detected. Concluded that results again
  supported compensatory mortality.

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Maximum sustainable yield (MSY)

• The largest average harvest that can be
  continuously taken from a population under
  existing environmental conditions (without
  driving population toward extinction)

• MSY equals the maximum rate of recruitment, and
  it is obtained by depressing population to
  density at which the recruitment curve peaks
  (always below K)

• MSY was dominant concept in harvesting for many
  years in fisheries, wildlife, forestry.

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MSY for logistic growth model
Peak recruitment
½ K

• Nu is a stable equilibrium (values Nu will
  decrease to Nu values between NL and Nu will
  increase to Nu)

• NL is an unstable equilibrium (population moves
  away from it, up or down)

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Potential problems with simple MSY harvesting
strategy

• Assumes that managers know K and current
  population size so they can thus harvest exactly
  the number of individuals to maintain population
  at MSY.

• Real populations are not deterministic.
  Environmental variation is common.

• Age- or stage-structure can be important.

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Harvesting Strategies

• Used long-term data set to model fluctuations of
  willow ptarmigan population in Sweden (using
  stochastic version of theta-logistic model).

• One of most important game species in
  Fennoscandia (100,000 hunters harvest
  300,000-500,000 ptarmigan each year in Norway)

• Examined how different harvest strategies affect
  mean annual yields, and how uncertainties in
  population estimates affect choice of strategy.

Annes, S. et al. 2002. Ecological Applications
12281-290.
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Harvesting Strategies
1. Constant harvesting

• Provides stable yield
• Models suggest can drive population to extinction
  (especially populations with low growth rates and
  large stochastic fluctuations)

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Harvesting Strategies
4. Threshold harvesting

• Requires estimate of population size (N) and
  threshold (c)
• Harvest estimated number of birds greater than
  threshold population
• Harvest N c for N c. Otherwise, no harvest.

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Age-structured Harvest

• Hunters select certain ages
• Different ages contribute differently to
  population growth
• Example Elk in Yellowstone
• Hunters select middle-aged cows with highest
  reproductive value
• Wolves select young and old
• Population can support more predation from wolves

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Sex-structured Harvest

• Most models used to predict effects of harvesting
  on wildlife populations disregard males.

• But killing of adult males can reduce population
  growth if immigrant males that replace a removed
  male kill young.

• Sexually selective infanticide hypothesis
  predicts that survival of cubs will be lower
  after resident bear is killed.

• Population consequences of harvesting one adult
  male brown bear equals that of harvesting 0.5 -
  1.0 adult females.

(Swenson et al. 1997. Nature 386450-451)
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Harvest as a selective force

• Example African elephants
• Exploited for illegal ivory market
• Increased proportion of tuskless females
• Sex-linked, heritable trait

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A few summary points

• More field experiments are required to understand
  better the mechanisms underlying the responses of
  wildlife populations to harvesting, especially
  compensatory mortality.

• Harvesting should be conducted as adaptive
  management within a flexible framework that
  allows for changes in regulations as new data and
  insights are obtained.