INCREASING MALLARDS, DECREASING AMERICAN BLACK DUCKS - MORE EVIDENCE FOR CAUSE AND EFFECT

We used Canadian Wildlife Service (CWS) survey data (1971-87) to define 7 wetland categories regarding historical changes in mallard (Anas platyrhynchos) and black duck (A. rubripes) distributions in southern Ontario to examine the hypothesis that mallards first invaded and then replaced black ducks on fertile wetlands. Wetlands were defined as Mallards 1970's-wetlands where mallards invaded in the 1970's but were never recorded as being used by black ducks; Mallards 1980's-wetlands where mallards invaded in the 1980's but were never recorded as being used by black ducks; Black Ducks Replaced 1970's-wetlands where mallards replaced black ducks in the 1970's; Black Ducks Replaced 1980's-wetlands where mallards replaced black ducks in the 1980's; Mallards/Black Ducks-wetlands that mallards and black ducks coinhabited; Black Ducks Only-wetlands used only by black ducks; and Unused. We also evaluated water chemistry (pH, conductivity, alkalinity [CaCO3], calcium, magnesium, sodium, potassium, and total phosphorus) and physical characteristics (shoreline irregularity index [SI], percent emergent cover, wetland size, distance to disturbance, growing degree days, and no. of wetlands in an individual survey plot) for 131 wetlands. Wetlands where mallards first appeared (e.g., Mallards 1970's) were, on average, more fertile than those where mallards later appeared. Wetlands where mallards first replaced black ducks (e.g., Black Ducks Replaced 1970's) were more fertile than those where black ducks were replaced later. On CWS plots, black ducks now persist only on wetlands with extremely low fertility. Mallards and black ducks seem able to coexist, at least temporarily, on wetlands with high amounts of emergent cover and/or SI, or in plots with many wetlands. We conclude that mallards, via competitive exclusion of black ducks from productive wetlands, have greatly contributed to the recent decline of black ducks in southern Ontario.