Northern Prairie Wildlife Research Center
R = HZB ÷ 2 where R = young females fledged per female in the breeding population, H = hen success, Z = survival of at least 1 brood member to age IIc, and B = average brood size at fledging.
If we enter empirical estimates for H and Z obtained in this study and 4.9 for B from Cowardin and Johnson (1979) we obtain
To assess the implications of this statistic we require a model relating recruitment and survival to annual rate of population change. We will use the model presented by Cowardin and Johnson (1979:24). This model is for a closed population and does not incorporate compensatory mortality. We suspect that a compensatory mechanism may operate to an unknown degree (Anderson and Burnham 1976), but there are insufficient data to construct an appropriate model.
If, in fact, recruitment is averaging 0.27, the model predicts a population declining at 20%/year (Table 21), and hen success would have to reach 31% to obtain a stable population. Obviously, if an annual 20% decline were taking place throughout the breeding range there would soon be no mallards. Such a generally severe decline in mallard populations has not been demonstrated.
|Source||Nest success||Hen success||Predicted annual change (%)|
|Central North Dakota||8.3a||15.2a||-20|
|Needed for stability||15.2||31.0||0|
|aReal measurements; other numbers were derived from the model.|
We believe that 2 factors temper this result:
Management to correct the predicted decline would have to improve either the annual survival rate or the recruitment rate. Usually, harvest management is the tool by which managers attempt to change annual survival rate, but there is considerable argument as to whether survival rates can be altered through harvest management (Patterson 1979). We will not enter this argument here, but will discuss importance of recruitment rate and the implications of the low rates derived for our study area.
Recruitment rate may be the most important factor affecting population size. Hochbaum and Caswell (1978) used a simulation model to predict changes in mallard populations of the Canadian prairies. From a local perspective, they concluded that harvest management within their region would have minor impact on population, but that substantial gains might be achieved through increased recruitment. Their result is not in agreement with the findings of Couch and Boyd (1984) who concluded that harvest on the breeding grounds is an important factor explaining a serious decline in the export of ducks from the breeding grounds of Canada and the northern United States. Martin et al. (1979) used continental estimates of recruitment and survival to predict populations the following spring. To have their modeled predictions track actual population counts, they had to either reduce recruitment by 42% or the survival by 21%.
Our data and predictions of a declining mallard population can be explained by low recruitment on our study area and high mortality during the summer. These predictions cannot be verified by examining population counts on the same area. In fact, the situation there is similar to what Dzubin (1969b) called lethal brood areas where pairs were attracted to an area but failed to recruit young because of lack of brood habitat, except that in our area failure of nests was probably the most important factor causing low recruitment.
This low recruitment rate is probably the result of degradation of nesting habitat through intensification of agriculture. This situation now prevails over much of the most important breeding range of the mallard. The problem manifests itself in low nest success resulting from predation on eggs and ducks. Sargeant et al. (1984) demonstrated that predation not only lowers nest survival rate but also has a major impact on survival through loss of nesting hens. Our data on loss of hens corroborate his findings. Bailey (1981:67) used a theoretical approach to problems in waterfowl management and concluded that ". . . the results of my theoretical concoctions suggest that for the mallard, it would be very optimistic to expect that population sizes will continue unaffected by degradation of habitat much into the future." We concur with his conclusion.
The telemetry study area, though large in area compared to other study areas, is small in comparison to the entire breeding range of the mallard. We cannot assess the geographic extent of the problem, but there is some evidence that it may be widespread and growing. During the summer of 1982, the Central Flyway Technical Committee conducted a nesting study on 3 transects each in North Dakota, South Dakota, and Montana. Nest success for the mallard was only 6.6% (M. Johnson, unpubl. rep. presented to the Central Flyway Technical Committee, March 7, 1984) compared to 8.3% estimated in this study. In addition, the area where grassland habitat is being lost to agriculture is large and extends into much of the Canadian prairie as well as the United States.
Additional field studies will be required to determine the extent and magnitude of the problem identified in our work. Such studies must further clarify the complex interrelations among habitat, land use, mallard populations, and predator populations. Understanding of these long-term problems can only be gained through carefully conducted long-term research. Even though there are some data available on nest survival rates in parts of the mallard breeding range, there are important areas and habitats for which no data are available. In addition, methodology for determining the cause of nest loss is often inadequate. Data on the survival of young from hatching to fledging is a key component of recruitment for which data are extremely limited.