Harvest management - Ecological Applications: Toward a Sustainable World - page 175

You have seen in earlier chapters about applications of population theory (manag-
ing endangered populations and pest control) that a full understanding had to
include an evolutionary dimension. Harvest management is no different. Harvesters
exert a very strong selection pressure on the populations they exploit, and inevitably
this leads to evolutionary changes in life-history features, and particularly to a
reduction in body size of the harvested animals. The implications of evolution for
harvest management will be discussed in Section 7.4.
Finally, and as with other topics in this topic, harvesting needs to be viewed in
a broad context. Thus, harvest managers must combine ecological considerations
(t he wel l-b ei ng of the exploited population) with economic (the profi ts being made
from the operation) and social considerations (local levels of employment and the
maintenance of traditional lifestyles). The economic and sociopolitical dimensions
of harvesting will be discussed in Sections 7.5 and 7.6.

Box 7.1 Population
dynamics theory 3

This is the last of three boxes that set out population dynamics theory. Here I extend ideas presented
in Box 5.1 about population growth and carrying capacity to show, in simple terms, how it is
possible to identify a sustainable harvest.

Logistic growth and the underlying patterns in births and deaths
You saw in Box 5.1 that a combination of density-dependent and density-independent factors
combine to determine a population's carrying capacity ( K ), the population size about which the
population fl uctuates. Every population has the potential to increase exponentially, because parents
have the capacity to produce more offspring than are needed just to replace themselves. However,
real populations do not increase indefi nitely; instead they increase until density-dependent factors
(e.g. too little food per individual) bring birth rate and death rate roughly into balance. Mathemati-
cians call this pattern of population growth logistic (Figure 7.1a; the equation that describes logistic
growth is given in Box 5.1).
It is instructive to consider for different segments of the logistic growth curve what is happening
in terms of births and deaths (Figure 7.1b). The numbers of both births and deaths progressively
increase because as we move to the right in Figure 7.1b there are ever more individuals available
to give birth or to die. But, in addition, density-dependent forces mean that the per capita birth
rate can be expected to fall at higher densities (e.g. because of food limitation) and the per capita
death rate to rise (e.g. because of increased probabilities of starvation or predation or disease). In
the exponential phase of the curve (when the population is still at low density), resource availability
is high, birth rate is high and death rate low. At higher densities, when density-dependent factors
start taking effect, the environmental brakes are applied and the rate of population growth declines,
because of a decrease in birth rate and/or an increase in death rate. Finally, when the population
reaches the carrying capacity, birth rate is matched by death rate and the population stabilizes. A
population with exactly equal birth and death rates must, by defi nition, stay the same.

When is a harvest sustainable?
How do these considerations of birth and death rates help to identify a sustainable harvest? Note
that the net recruitment at any particular density in Figure 7.1c is a surplus of births over deaths
that can be removed without causing the population to decline from that density. Thus, as long as
we know the density of the population to be harvested and understand the relationship between
net recruitment (surplus) and density, we can harvest the surplus without reducing the population
size. This is a sustainable harvest (or yield). It is low when the population is very small and low
also, at the other end of the scale, when competition for resources is intense. And it is crucial to
note that when the population is at carrying capacity there is no net recruitment, so there can be
no sustainable harvest. In other words, you cannot both harvest and maintain a population at car-
rying capacity. Harvesting always involves reducing a population below its carrying capacity. By
so doing, density-dependent factors are relaxed and the population responds by raising its birth