untitled

A population can most easily be stimulated into a burst of growth by increas-

ing the level of a limiting resource or, much more rarely, reducing the level of pre-

dation to which it is subject. The key resource may be nest sites or cover, but in

most cases it is food. For example, red kangaroos increase if the pasture biomass

exceeds about 200 kg / ha dry weight and decline if it is below that level (Bayliss

1987). The easiest way to increase the average amount of food available to an indi-

vidual animal is to reduce the number of animals competing with it for that food.

The average standing crop of food then rises and the amount available to an

individual animal is thereby increased. As a direct consequence the fecundity of

individuals is enhanced, and mortality, particularly juvenile mortality, is reduced. The

population enters a regime of increase as it climbs back towards its unharvested

density.

The trade-off between yield and density is the most important thing to know about

sustained-yield harvesting. In general, the further the density is reduced the higher

is the yield as a percentage of population size. The maximum rate of sustain-

able yield is the population’s intrinsic rate of increase, but that rate of population

growth is obtained only when food (or whatever other resource is limiting) is at a

maximum, which in turn usually occurs only when the population is at minimum

density.

Whereas sustained rate of yield (absolute yield divided by population size) tends

to increase as density is reduced, the same is not true of the absolute sustained yield.

If the population is reduced just a little, the induced rate of increase will be small

and the sustained yield will be a small proportion of a relatively large population.

The absolute yield will be modest. If the population is drastically reduced, the induced

rate of increase will be large and the sustained yield will be a large proportion of

what is now a relatively small population. Again the absolute yield will be modest.

The highest yield is taken from a density at which the induced rate of increase

multiplied by the density is at a maximum. It tends to be at intermediate density

levels. For example, suppose that a population grows in a manner well described by

the Ricker logistic model (see Chapter 6), with projected population size next year,

f(N), predicted from current population density in the following way:

Net recruitment, R(N), will be defined as the difference between population density

in year tand population density the previous year, in the absence of harvesting:

R(N) =f(N) −N

Note that both positive and negative net recruitment values are possible, with

negative values occurring when N> f(N) and positive values occurring when

N<f(N). Fixed harvest policies are predicated on sustained use of the net recruit-

ment, treating it as a surplus that can be safely harvested without harming resource

sustainability in the long term. For example, imagine that we, as population man-

agers, set the harvest at 20 units (Fig. 19.1).

fN N r

N

K

( ) exp=−max

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336 Chapter 19
19.2.1Harvesting
trades off yield
against density