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?