Harvest management - Ecological Applications: Toward a Sustainable World - page 174

details of the resource they are managing - trees are big and don't move - and a
forestry company may not have the challenge of managing a resource used by other
exploiters too.
Sometimes, though, as you heard from Elena, forests are available for exploitation
by a community of exploiters - the resource then is a good that is held in common .
This can lead to what Hardin (1968) called the 'tragedy of the commons' because
when individuals use a public good, they do not necessarily bear the entire cost of
their actions - which may in the long term be borne by others. The story goes that
the best short-term strategy is for individuals to selfi shly try to exploit more than
their share, gaining in the short term but leading inevitably to overexploitation and
fewer resources for later generations. That is what happened in earlier times in
Elena's forest. Given human nature, the selfi sh approach is often to be expected in
an unregulated situation, but the tragedy can be avoided if neighbors agree on
appropriate behaviors and rules (lore) or if competing exploiters are regulated by
government (law) so that they take only their share of a sustainable harvest.
The diffi culties of managing resources are most acute when, in contrast to trees,
the exploited organisms are hidden from view, relatively short lived, and have birth
and death rates that are highly responsive to the vagaries of climate. Determining
what constitutes a sustainable harvest is then fraught with diffi culty. For these
reasons, and because they are also wild and held in common , marine fi sheries are
truly diffi cult to manage. Their importance as a human food supply has meant that
the most sophisticated of management regimes have been developed for these deni-
zens of the deep but, even so, the strategies don't always work, as you will see in
this chapter.

7.1. 2 Killing just
enough - not too
few, not too many

I noted in Chapters 5 and 6 that the aims of conservation and pest control are dia-
metrically opposed - conserve endangered species by reducing mortality but get rid
of pests by killing as many as possible. Harvest management is based on the same
population dynamics theory, but its objective lies somewhere in between. In this
case exploiters aim to kill as many as possible, so that human benefi t from a food
resource or forest timber is maximized, but not too many - which would lead to the
exploited population shrinking to a size that is economically insignifi cant or even
biologically extinct. Simultaneously avoiding both overexploitation and underex-
ploitation is not as easy as it sounds. Box 7.1 presents the essential population theory
that underpins both how to work out the maximum sustainable yield (MSY) and
how to devise ways of achieving it. This extends further my treatment of population
dynamics theory and can be viewed in combination with the boxes in Chapters 5
and 6.
The MSY concept is central to harvest management, and it brings into stark relief
some of the core concepts that managers need to bear in mind. However, there are
a number of critical limitations to the simple application of the MSY approach - I
will highlight these in Section 7.2 by considering examples of harvest management
in practice.
A major simplifi cation in the MSY approach is to treat the population as a number
of identical individuals, ignoring all aspects of population structure such as size
or age classes with their different rates of growth, survival and reproduction.
Alternative approaches that incorporate population structure will be considered in
Section 7.3.