Estimating Waterfowl Carrying Capacity at Local Scales: A Case Study From Edwin B. Forsythe National Wildlife Refuge, New Jersey1

Abstract

The management of wintering North American waterfowl is based on the premise that the amount of foraging habitat can limit populations. To estimate carrying capacity of winter habitats, managers use bioenergetic models to quantify energy (food) availability and energy demand, and use results as planning tools to meet regional conservation objectives. Regional models provide only coarse estimates of carrying capacity because habitat area, habitat energy values, and temporal trends in population-level demand are difficult to quantify precisely at large scales. We took advantage of detailed data previously collected on wintering waterfowl at Edwin B. Forsythe National Wildlife Refuge and surrounding marsh, New Jersey, and created a well-constrained local model of carrying capacity. We used 1,223 core samples collected between 2006 and 2015 to estimate food availability. We used species-specific 24-h time–activity data collected between 2011 and 2013 to estimate daily energy expenditure, morphometrically corrected for site- and day-specific thermoregulatory costs. To estimate population-level energy demand, we used standardized monthly ground surveys (2005–2014) to create a migration curve, and proportionally scaled that to fit aerial survey data (2005–2014). Crucially, we also explicitly incorporated estimates of variance in all of these parameters and conducted a sensitivity analysis to diagnose the most important sources of variation in the model. Our results from an outlier-removed, a strict depletion model indicated that at estimated mean levels of supply (923 million kcal) and cumulative demand (3.4 billion kcal), refuge food resources were depleted before November. However, a constant-supply model that represented tidal replenishment of resources indicated that just enough energy was present to sustain peak winter populations. Variation in model output appeared to be driven primarily by uncertainty in population abundance during peak periods of use, emphasizing a new management focus on studying migration chronologies of waterfowl. This model allows for relative assessment of biases and uncertainties in carrying-capacity modeling, and serves as a framework identifying critical science needs to improve local and regional waterfowl management planning.

Does Invasive Common Reed in Coastal Salt Marshes Affect Dabbling Duck Food Availability?

“…Rejecting the null hypothesis that common reed invasion per hectare into high marsh would not affect DEDs, we found that common reed would produce 509-1,743 more DEDs/ha (232-525% increase) across the five species over the 212 d of the nonbreeding season (on the basis of dabbling duck migration chronology at EBFNWR, 1 September-31 March; Ringelman et al 2018; Table 2). Further, areas converted from freshwater impoundment to common reed would produce 297-1,925 more DEDs/ha (145-654% increase) across the five species.…”

“…Because common reed primarily invades high marsh and freshwater impoundments, we tested the null hypothesis that common reed-invaded wetlands would not affect the energetic carrying capacity via available DEDs (per ha) as compared with noninvaded salt-marsh dabbling duck habitat. To inform DEDs, we first assumed average daily energy expenditures calculated by Livolsi (2015) (Ringelman et al 2018).…”

“…Over the past decade, a priority area of research to understand this decline has focused on wintering black duck habitat quantity and food availability (Devers and Collins 2011). There are several studies quantifying food abundance in Atlantic Coast salt marshes (Plattner et al 2010;Cramer et al 2012;Livolsi 2015;Ringelman et al 2018;Goldstein et al 2016) and studies that determine the effects of anthropomorphism and sea-level rise in coastal wetlands (Livolsi 2015;Ringelman et al 2015); however, there has been no directed research that we are aware of that addresses the effects of invasive common reed on black duck habitat and food availability. If such a relationship occurs, it is reasonable to ask whether common reed could also affect other dabbling duck species such as mallard Anas platyrhynchos, green-winged teal Anas crecca, northern shoveler Spatula clypeata, and northern pintail Anas acuta.…”

The Effect of Sea Level Rise on Dabbling Duck Energetic Carrying Capacity

“…Table 2. Mean used food energy (millions of kcal), duck energy days (millions), and extrapolated number of ducks that could be supported for 212 days of the wintering period (based on regional dabbling duck migration chronology, 1 Sep-31 Mar; Ringelman et al 2018) Our estimates of energy supply (kcal) by wetland community type suggest that for most dabbling duck species, energy density of used foods was greater in impoundments than low marsh, high marsh, mudflat, subtidal, or quasi-tidal pool communities. Freshwater impoundments in particular tended to have the greatest energy density because they contain a large volume of energy-dense seeds.…”

The American Black Duck: Three Decades of Science-Based Adaptive Management

“…Ringelman et al [52] used detailed data previously collected on wintering waterfowl at Edwin B. Forsythe National Wildlife Refuge and created a local model of carrying capacity. They used 1,223 core samples collected between 2006 and 2015 to estimate food energy supply and species-specific 24-h time-activity data collected between 2011 and 2013 to estimate daily energy expenditure.…”