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Population Dynamics of Breeding Waterfowl

VIII. Discussion

C. Management Considerations

Many species of birds are of interest to wildlife managers, either because they are rare and their populations must be carefully protected, or because they are game species for which hunters desire greater numbers. Fortunately, few waterfowl species are threatened (exceptions include White-winged Wood Duck and Hawaiian Goose), but many of them are hunted, including virtually all species for which we have good information on population dynamics. For many of the game species, however, numbers are no longer adequate to satisfy the demands of hunters. Wildlife management agencies seeking to increase the population size of some species can try to increase survival rates, reproduction rates, or both. Altering hunting mortality rates without antagonizing the hunting public is often politically difficult and may be biologically ineffective as well (Anderson and Burnham 1976). The alternative is to manage reproduction rates. This review has indicated in a general way the potentially important and manageable components of production: those that are influential in the dynamics and those that can be altered. For the three populations we modeled, different sets of components were most influential. Not only do the influential components vary from one population to the next, but also the manageability of these components is likely to differ.

One approach to managing breeding ground population dynamics would be to identify the reproductive component that explained the greatest proportion of the total variation in reproductive rate and try to influence that component in a cost-effective manner. For example, if reproductive output is closely tied to nest success for a particular species, a natural management tactic would be to increase nest success rates. This may be a reasonable action, but we caution that results may not be as profitable as anticipated. If nest success rates increased, say, from 10% to 50%, our simplified model might predict a very large increase in reproductive output and a substantial increase in the population. However, the hypothesized increase in nest success would greatly increase the number and density of broods. Any density-dependent brood mortality might erase much of the potential gain caused by higher nest success. That one component of reproductive rate is most important under one set of conditions and another is the most influential under other conditions is analogous to Hestbeck's (1987) recent discussion of multiple regulation states for populations. This possibility enhances the value of studies and management programs that manipulate values of reproductive components.

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