be compared directly with that of a wild-
caught population. Instead, the captive
strain could be tested alone and its perfor-
mance compared with some previously
established ‘wild’ standard. To establish the
standard, replicated trials (preferably run at
different times and at different sites) must
be conducted using a wild-caught popula-
tion. These trials would establish the stan-
dard in terms of some appropriate measure
of quality. The measure should reflect the
success of an individual over the whole of
its useful life, and include the necessity for
active searching beyond the immediate
release site. For example, for a parasitoid,
this standard could be per cent parasitism
per parasitoid per unit area measured a set
number of days after release and given some
typical host density.
I have been unable to find any experi-
ments that measure both quantity and qual-
ity in strains with different degrees of
domestication. Indeed, there are relatively
few studies that document the consequences
for biological control of a decline in quality
associated with mass rearing (although see
Ito, 1988; Calkins and Ashley, 1989). This is
probably due to an understandable reluc-
tance of those responsible for captive rearing
to document any such decline, since the effort
is likely to provoke criticism of their captive-
rearing strategy. However, this attitude is
misguided. The important question is not
whether quality has declined, but whether
the product of quality and quantity has been
maximized. The theoretical expectation is
that maximizing this product will almost
inevitably involve some decline in quality.
Theoretical expectationsIn general, quantity and quality are inter-
changeable, i.e. a loss of quality can be com-
pensated for by increased quantity (Nunney,
2002). This assumption leads to the conclu-
sion that the optimum strategy maximizes
the product of these two parameters. Only if
there is an interaction between quantity and
quality is it necessary to maximize a more
complex function. For example, if density-
dependent interference among individuals
caused individual field performance (qual-
ity) to decline at high density, then field den-
sity would need to be factored into the
maximization. Except under these condi-
tions, the effectiveness of mass-rearing pro-
grammes can be evaluated along the two
dimensions of the quantity produced and the
quality of the individuals released.
Mathematically, the effectiveness (E) of a
captive-rearing programme is defined by:
E= Pw (1)
where Pand ware quantity (productivity)
and quality (individual field performance),
respectively. Furthermore, we expect that
adaptation to the rearing environment
(increasing P) will change (and generally
decrease) waccording to some function f:w= f(P)(2)
It follows from (1) and (2) that maximizing E
requires:
dln f(P)
dln P=1(3)The interpretation of equations (2) and (3)
is shown in Fig. 6.1. In both the upper and
lower graphs, the solid curve defines f(P),
the relationship defining how field quality
(w) changes as an initially wild-caught popu-
lation adapts to captive rearing. The popula-
tion is expected to gradually shift from its
initial state (‘wild population’) to a relatively
stable ‘domesticated stock’. This transition is
marked by some decrease in field perfor-
mance. Since effectiveness is the product Pw
(equation 1), points of equal effectiveness are
linked on a log scale by a line of slope 1
(see Fig. 6.1, dashed lines). The maximum
effectiveness is usually defined by equation
(3), i.e. where one of these lines is tangential
to f(P). This can be seen in the upper graph
of Fig. 6.1. The ‘optimum strategy’ shown on
the graph maintains a population that is par-
tially domesticated – at this point the gain in
quantity far outweighs the loss in quality.
The lower graph differs in the shape of
f(P), a difference that alters the optimum
strategy. In this graph, there is a local maxi-
mum near to the point of full domestication;
however, the overall optimum strategy is to
minimize domestication, because of the largeManaging Captive Populations 75