The Role of Environmental Heterogeneity in the Regulation of Duck Populations

Abstract

Behavioral spacing of breeding pairs and the availability of energy resources have been proposed as major factors that regulate duck populations. To compare the relative importance and interaction of the two mechanisms, the seasonal distribution of duck populations was determined in an edaphically heterogeneous system of beaver (Castor canadensis) ponds west of Ottawa, Ontario, near a boundary of the Ottawa-St. Lawrence Lowland and the Precambrian Shield. Ponds were selected in a number of geological substrates, so that within a relatively small geographical area a variety of water chemical conditions was represented. The seasonal production of submerged vegetation was a function of water hardness and formed the basis of the assumption that water chemistry could be employed quantitatively as a measure of pond fertility. Numbers of breeding pairs of ducks were dependent only on the amount of surface water available, indicating that the major population regulatory mechanism was behavioral spacing. Fledged ducks, on the other hand, selected fertile wetlands regardless of pond size, indicating that populations were regulated by the availability of energy resources. Habitat requirements of broods were intermediate, because both behavioral escape cover and food availability were important. It is postulated that the different environmental requirements of the three life history stages are an evolutionary adaptation to a temporarily unpredictable environment. The adaptations allow duck populations to maintain equilibrium in a temporarily unpredictable environment, and to attain high population size in a spatially heterogeneous environment. J. WILDL. MANAGE. 40(1):22-32 Behavioral spacing mechanisms and the availability of food resources are two factors considered to be of major significance in the regulation of duck populations (Evans and Black 1956, Moyle 1956, Hawkins 1964, Jahn and Hunt 1964:16, Dzubin 1969, Dzubin and Gollop 1972). Although there is a considerable body of knowledge concerning duck population dynamics and the regulatory effects of specific environmental factors, there is a lack of consensus on the relative importance, or relationships, of the various factors known to regulate population size. The question of what overall environmental factors regulate duck populations was asked repeatedly by contributors to the Saskatoon Wetlands Seminar published in 1969. In summarizing the seminar, Jahn (1969:177) stated that for progress to be made in waterfowl research Seasonal needs of the birds must be related to the physical, chemical and biotic features of the ecosystem. Recent developments in theoretical ecology have stressed the importance of the abiotic environment in determining many basic biotic properties (Loucks 1970, Sanders 1969, Slobodkin and Sanders 1969, Smith 1970, Wiegert and Owen 1971). Such fundamental biological phenomena as niche differentiation and mechanisms of population regulation may evolve in response to environmental characteristics. The theoretical evidence that mechanisms of ecosystem organization and population regulation can respond to environmental characteristics provided the conceptual basis for this study. Research emphasis was placed on determining environmental parameters that would quantitatively indicate the seasonal requirements of breeding ducks. To this end, a study area was selected that provided a continuum of wetlands ranging from biologically productive hardwater ponds to less productive soft water ponds, yet was small enough to avoid geographical influences on populations. The presence of numerous beaver ponds J. Wildl. Manage. 40 (1):1976 22 REGULATION OF DUCK POPULATIONS * Patterson overlying a variety of geological substrates provided a natural outdoor laboratory. Ponds of specific chemical regimes and sizes could be selected to provide the treatments necessary for field experiments to determine the environmental requirements of ducks. This work is from a Ph.D. thesis submitted to Carleton University. Financial support was provided by research contracts and graduate scholarships from the Canadian Wildlife Service and Ontario Graduate Fellowships. I thank L. P. Lefkovitch, Director, Statistical Research Service, Agriculture Canada, for performing the statistical analyses and A. T. Prince, Director, Inland Waters Branch, Environment Canada, whose laboratory conducted the water chemical analyses. E. Grinnell's hard work and enthusiasm as my field assistant are greatly appreciated. I also thank H. G. Merriam and J. D. H. Lambert, Department of Biology, Carleton University, and N. G. Perrett and H. Boyd, Canadian Wildlife Service, for advice during the study. H. G. Merriam, H. Boyd, L. P. Lefkovitch, and W. J. D. Stephen critically reviewed the