policy should always be evaluated in the light of stochastic variation in population
growth. In a stochastic world, harvesting a fixed proportion of the population is much
less likely to lead to population collapse than is fixed quota harvesting. Perhaps the
safest option of all is to strive to maintain populations above a critical threshold (known
as a fixed escapement). Although from an ecological viewpoint recreational harvest-
ing and commercial harvesting do not differ in principle, in practice commercial
harvesting sometimes has greater potential to exceed sustainable levels of harvests.
This is particularly likely when resources are a common property, for which multi-
ple users compete. Under this circumstance, the economically rational behavior will
be for resource users to continue to enter the industry until resources are reduced
to dangerously low levels. The “discount rate” of economic analysis can also encour-
age overharvesting by imparting a greater value to present yields than to future yields,
particularly when the maximum annual rate of resource growth is less than the dis-
count rate. Risk due to discounted valuation once again tends to be most pronounced
when resources are common, rather than private property. Particularly stringent con-
servation measures are called for under these circumstances.354 Chapter 19