The North American Waterfowl Management Plan (NAWMP) is built on the premise that the collective effects of regionally-delivered habitat conservation actions will impact waterfowl population growth by influencing survival and recruitment. Availability of food energy was identified as the factor most likely to limit vital rates of ducks during non-breeding periods. Daily ration bioenergetics models based on Ideal Free Distribution (IFD) theory are used to estimate quantitative habitat objectives for target duck populations. These models incorporate assumptions about daily energy demands of waterfowl, energetic carrying capacity of priority habitats, and the manner in which ducks use foraging habitats of varying energy density. The accuracy of these assumptions affects the degree to which habitat objectives are sufficient to support duck populations and contribute to achievement of NAWMP goals.
I measured energetic carrying capacity (ECC), duck use, and wetland- and landscape-scale variables associated with actively and passively managed wetlands in central and northwest Ohio during autumn and spring, 2001-2004, to test assumptions of a bioenergetics model used by the Upper Mississippi River and Great Lakes Region Joint Venture (UMR & GLR JV). Mean ECC was statistically similar between actively and passively managed wetlands (actively managed = 3,446 duck energy-days [DED]/ha; passively managed = 2,047 DED/ha), and exceeded the UMR & GLR JV assumption that 1,236 DED/ha are provided by managed wetland habitats. However, ECC of actively and passively managed wetlands declined by >80% between autumn and spring migration, resulting in low food energy densities during spring (mean = 66 – 242 DED/ha).
The probability of a wetland being used by ducks and cumulative duck use of wetlands were significantly and positively affected by food energy density. However, ducks did not use wetlands in direct proportion to food energy density, which contradicted predictions of daily ration models (DRM) and IFD theory. For example, parameter coefficients from one analysis suggested that for every doubling of ECC, cumulative duck use increased by only 33% during autumn and 13% during spring. Factors other than food energy density affected wetland use by ducks. Based on a subsample of 68 passively managed wetlands, duck use during autumn increased with ECC, wetland area, and area of wetlands within 1 km, but decreased as wetlands became increasingly isolated from other wetlands. Duck use during spring increased with ECC, decreased with increasing distance from rivers, but was unrelated to wetland area.
Although DRMs provide a simplistic and useful method for calculating habitat objectives for wintering and migrating ducks, they likely do not accurately describe the complexity of factors affecting duck use of foraging habitats. My results suggest habitat objectives based on linear predictions of DRMs may underestimate habitat requirements of target duck populations. Lacking bioenergetics models that accurately describe functional carrying capacity of foraging habitats, conservation planners should consider providing habitat in excess of DRM predictions to ensure populations are not limited by undetected constraints on food availability.