The Relationship Between Wetland Productivity And The Distribution Of Breeding Mallards, Black Ducks, And Their Broods: Historical And Spatial Analysis

During the last 50 years mallards have increased dramatically in southern Ontario and have completely replaced black ducks in many areas. In northern Ontario, black duck densities appear stable at present, however, mallard densities now exceed 60 pairs/100 km{dollar}\sp2{dollar} in many areas. I examined historical and spatial distributions of mallards and black ducks in Ontario in relation to water chemistry and physical habitat characteristics. My objectives were to determine (1) if mallards invaded the most fertile wetlands, and (2) if mallards replaced black ducks on the most productive wetlands.;I used Canadian Wildlife Service survey data (1971-87) to define 7 wetland categories regarding changes in mallard and black duck distributions in southern Ontario. In northern Ontario, I used 1990-1992 breeding pair survey data to define wetlands as used by mallards only, black ducks only, shared, or vacant. Habitat was evaluated based on water chemistry and physical characteristics.;In southern Ontario, wetlands where mallards first appeared were, on average, more fertile than those where mallards later appeared. Wetlands where mallards first replaced black ducks were more fertile than those where black ducks were replaced later. On Canadian Wildlife Service plots in southern Ontario, black ducks persist only on wetlands with extremely low fertility. Major conclusions for southern Ontario: (1) Mallards did not invade southern Ontario randomly with respect to wetland fertility, but invaded the most fertile wetlands first. (2) Mallards replaced black ducks from fertile wetlands, and black ducks are now restricted to the least fertile area of southern Ontario.;In north-central Ontario, mallards occupy the most fertile wetlands, with areas dominated by mallards having the most fertile wetlands. Major conclusions for northern Ontario: (1) Wetland fertility has played a major role in the distribution and abundance of mallards in north-central Ontario. (2) Areas that support the most breeding mallards have the most productive wetlands. (3) Mallards and black ducks appear to select for similar wetland characteristics and likely compete for breeding sites.;Mallards and black ducks are ecological equivalents, therefore competition for breeding sites is likely. Of the many factors suggested as causing the decline of the black duck the mallard may be having the most significant impact