Harvesting and Control Photo: J.-D. Lebreton

Harvest: Control: Number removed is the parameter of interest How many deer can be taken from management unit? Control: Number remaining is the parameter of interest Maintaining 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 harvest size to population size What’s going on? The maximum per capita growth rate occurs when ….

The Maximum Sustained Yield “The largest harvest rate that can be imposed without causing population to decline” For the logistic model: MSY occurs at 1/2 K. The value is RK/4

Logistic-based Harvest Models dN/dt = rN(1-N/K) – L L=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 What’s happening here Q 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 yield curve with varying effort is useful— On board

Environmental variability 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 done on invertebrates

Effects of Age/Stage on Harvest Harvest of youngest age class: 1. Reduced total popn size 2. Altered popn structure Before harvest Total yield inc to 90% harvest rate -yield/ind increased (compensation) After Total yield Ind. yield 4. Increase in life expectancy

Additive versus Compensatory Mortality Importance of regulation

CONTROL Two 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: eradication limitation of populations Mechanism: prevent reproduction Population-level implications?

Issues with wildlife contraception Technological Ethical Biological

Conclusions Both harvest and control draw heavily on population ecology of target species Harvest and control primarily differ in objective not in scientific principles Ethical/societal issues of major importance (this is as much about policy as it is about science)

Ungraded Homework Assignment How do the goals of control and harvest differ? What biological principles do they both rely upon?