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Impact of Red Fox Predation on the Sex Ratio of Prairie Mallards


The impact of predators on prey species has been a matter of conjecture since scientists began to study these relationships. Paul Errington in his pioneering work on predator-prey relationships noted that predation belongs in the equation of life. Other prominent scientists during the 1940's through the 1960's examined both the available data and the related philosophical expressions in an attempt to establish more acceptable biological bases for their recommendations and actions. Confusion over the interpretation of existing information and strong personal emotion often prevailed. In reality many of these earlier studies dealt primarily with the natural history of these creatures. Furthermore, during the 1930's and early 1940's predator population levels were affected by intensive trapping of valuable furbearing species for supplemental income, and by the initiation of bounty systems in many states.

Durward Allen and David Mech, in their studies of wolf-moose relationships on Isle Royale, introduced new concepts based on nearly singular interactions between the two species. Other students of predator-prey relationships began to recognize the added complexity whenever several different species became involved in the same ecosystem. Little attention was given to the impact of mammalian predation on ground-nesting birds, especially the direct effect on nesting females and/or their eggs.

With the initiation of a new waterfowl research program in 1965 at the Northern Prairie Wildlife Research Center at Jamestown, North Dakota, it became apparent that the role of predation in the annual waterfowl production cycle must be better understood. Earlier investigations in the prairie region revealed that red fox, raccoon, striped skunk, and badger were principally responsible for high egg loss among ground-nesting birds, especially dabbling ducks. There was also preliminary evidence from the bird remains at red fox den sites that a substantial number of nesting females was also being taken. Initial research centered on the feeding habits of the red fox in the prairie region of North and South Dakota, with emphasis on the impact on ground-nesting ducks in various physiographic regions.

Alan Sargeant had the unique opportunity to work first in operational animal damage control programs where he learned the basic behavior of predator species, and later with a team of researchers at the Cedar Creek Telemetry Laboratory, University of Minnesota, who had just developed the first automated radio-tracking system. Here he specialized in studying the territorial behavior and daily activity patterns of the red fox. This work provided him with an excellent background to assume the role of project leader for further research on the red fox in the Dakotas at the Northern Prairie Wildlife Research Center. After the first 5 years of data collection, it became obvious that red foxes were taking a significant number of nesting female ducks, including mallards, perhaps enough to influence the sex ratio of the species in a local geographic area. At that stage it was decided to develop a simulation model to help determine what this impact might be. Douglas Johnson, a mathematical statistician at the Center, teamed up with Alan Sargeant to analyze these relationships and develop a simulation model to fit the diverse field data into a coherent whole. The results of their work presented in this publication represent, I believe, a significant milestone in wildlife research. Extensive biological data on red fox population trends mallard breeding population trends and population dynamics, feeding habits of the red fox, and changes in mallard sex ratios have been programmed into the simulation model in various combinations to permit analyses of these interrelationships, which heretofore were impossible. The predictive capabilities of this model tax the imagination of most waterfowl biologists and mammalogists. This effort represents an important first step in better understanding such intricate relationships and suggests important management applications.

There is no question that greater emphasis must be given to these kinds of problems in the years ahead. Continual shrinkage of the habitat base for ground-nesting birds tends to concentrate all remaining creatures mammals, birds, and man. Competition for space and food becomes greater each year. This research may well be the vehicle that will give us new insight into the causes of distorted sex ratios in certain species of ducks and the role of excess drakes in populations. This work also provides a sound base of scientific data that will be vital in developing management alternatives, whether they be animal control, habitat development, hunting regulations, or other species management considerations.

More intensive management in the years ahead will require new and imaginative approaches. Further innovative research can provide such guidelines. Douglas Johnson and Alan Sargeant are to be commended for this significant contribution.

Harvey K. Nelson
Associate Director
Fish and Wildlife Management
U.S. Fish and Wildlife Service

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