untitled

resource users, should several models continue to make similar predictions. For another,

passive management makes it difficult to discriminate between good management and

good luck. High harvests could accrue by chance during a series of good years, despite

application of a wrong model.

As an example of the potential utility of active adaptive management, let us

consider waterfowl harvesting, specifically mallard ducks, in more detail. Harvest

quotas for a variety of ducks are determined in part using a sophisticated system of

stratified aerial surveys criss-crossing the extensive area of breeding habitat on the

North American prairies (Nichols et al. 1995). Density levels and pond availability

are used to predict stochastic variability in duck recruitment rates, and these recruit-

ment rates are interpreted as a harvestable surplus (Anderson 1975). Much of the

stochastic variability in demographic parameters stems from variation in rainfall on

the prairies. Wet weather generates large numbers of small ponds and pothole lakes

on the prairies, which in turn generates increased success in offspring recruitment.

Banding records, obtained from the recovery of identification bands (in the hunting

season), allow an estimation of mortality rates. This information on offspring recruit-

ment and mortality is then used in quantitative population models to predict safe

harvesting levels year-to-year. The remarkable consistency in duck numbers over time

attests to the robustness of this program (Nichols et al. 1995).

There are indications, nonetheless, that the harvest allocation for some water-

fowl species could be considerably improved. A key uncertainty in the harvest

evaluation procedure is whether mortality is compensatory or not (Anderson 1975;

Williams et al. 1996). In this context, perfect compensation means that increased

duck mortality due to harvest has no effect on overall duck mortality, at least

over some range of harvest rates, because survival in the wild adjusts perfectly

to losses imposed by man (Fig. 15.7). The alternative hypothesis is that there is no

compensation, hunting mortality is in addition to natural mortality, and so total

mortality is linearly related to harvest rates (Fig. 15.7). Current data are inadequate

to discriminate between these two hypotheses, yet they have critical implications

with respect to both the risk of over-harvesting, particularly in poor years, and the

optimal harvest policy (Anderson 1975). Simulation models have been used to show

that by far the most efficient way to decide which of these alternative models is

266 Chapter 15

0.6

0.4

0.2

(^00102030405060)
Harvest rate
Survival rate
Additive model
Compensatory model
Fig. 15.7Schematic
representation of the
compensatory (broken
line) and additive (solid
line) models for survival
in waterfowl in relation
to harvest levels.