Harvesting and Control
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Transcript of Harvesting and Control
Harvesting and Control
Photo: J.-D. Lebreton
Harvest:
Number removed is the parameterof interest
How many deer can be takenfrom management unit?
Control:Number remaining is the parameter
of interestMaintaining herd size of wild horses
Concept of Sustainable Harvest
Consider the logistic model
dN/dt = RN*(K - N)/K
Nt+1 = Nt + RNt(1-Nt)/K
Relationship of harvestsize to population size
What’s going on?
The maximum percapita growth rateoccurs when ….
The Maximum Sustained Yield
For the logistic model:MSY occurs at 1/2 K.The value is RK/4
“The largest harvest rate that can beimposed without causing populationto decline”
Logistic-based Harvest Models
dN/dt = rN(1-N/K) – LL=loss from harvest
Fixed-Quota Harvest Model:
L = some constant
Fixed-Effort Harvest Model:
dN/dt = rN(1-N/K)-(E*C*N)where E = effort, C = catchability, E*C*N = L
Implications of Fixed-Quota Harvest
Q
What’s happening here
N2 N1
Note: trend of pop declining to extinction is indistinguishable from pop decline to ½ K
Implications of Fixed-Effort Harvest
Proportion removed
MSY
A yieldcurve withvarying effortis useful—On board
Limitations of Logistic-based Harvest Models
•Environmental variability
•Estimation of carrying capacity
•Estimating population parameters
•More complex relationships•age/stage structure
Structured Population Models for Harvest
•Motivation: Many harvest strategies select specific age/stage class
•Implications of life history strategy
•Experimental work frequently doneon invertebrates
Effects of Age/Stage on Harvest
Before harvest
After Total yield
Ind. yield
Harvest of youngest age class:
4. Increase in life expectancy
3. Total yield inc to 90%harvest rate-yield/ind increased
(compensation)
2. Altered popn structure
1. Reduced total popn size
Additive versus Compensatory Mortality
Importance of regulation
CONTROLTwo main goals:
•Limit populations of desired species
•Eradicate unwanted populations(usually exotic species)
Control
Consider the logistic model once again…
How could control work?
dN/dt
N
Lethal control of animal populations
Issues and Biological Considerations
Control of pest plant populations
Considerations?...
Fertility Control
Two goals:eradicationlimitation of populations
Mechanism:prevent reproduction
Population-level implications?
Issues with wildlife contraception
Technological
Ethical
Biological
Conclusions
•Both harvest and control draw heavilyon population ecology of target species
•Ethical/societal issues of major importance(this is as much about policy as it isabout science)
•Harvest and control primarily differ inobjective not in scientific principles
Ungraded Homework Assignment
How do the goals of control and harvest differ?
What biological principles do theyboth rely upon?