1. Harvesting and Control
Photo: J.-D. Lebreton

2. 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

3. Concept of Sustainable Harvest
Consider the logistic model
dN/dt = RN*(K - N)/K
Nt+1 = Nt + RNt(1-Nt)/K

4. Relationship of harvest
size to population size
What’s going on?
The maximum per
capita growth rate
occurs when ….

6. 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

7. 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

8. 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

9. Implications of Fixed-Effort Harvest
Proportion
removed
MSY
A yield
curve with
varying effort
is useful—
On board

10. Environmental variability
Limitations of Logistic-based Harvest Models
Environmental variability
Estimation of carrying capacity
Estimating population parameters
More complex relationships
age/stage structure

11. 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

12. 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

13. Additive versus Compensatory Mortality
Importance of
regulation

14. CONTROL
Two main goals:
Limit populations of desired species
Eradicate unwanted populations
(usually exotic species)

15. Control
Consider the logistic model once again…
How could control work?
dN/dt N

16. Lethal control of animal populations
Issues and Biological Considerations

17. Control of pest plant populations
Considerations?...

18. Fertility Control
Two goals:
eradication
limitation of populations
Mechanism:
prevent reproduction
Population-level implications?

19. Issues with wildlife contraception
Technological
Ethical
Biological

20. 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)

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