This document represents a second year of duck energy day (DED) monitoring on Noxubee NWR’s moist soil unit, based on 128 1-m2 plots spread across 11 moist soil sub-units in the study area. The plot locations were determined in 2015 by a GRTS draw stratified based on the area of each unit.

The specific objectives of this document are:

  1. Estimate actual yields (kg seed/ha) of targeted plants in lieu of using typical yield constants based on the “dot-stem” method of Gray et al. (2009; J Wildl Mgmt 73:1229-1232; pdf); and

  2. Convert plant community information into DED estimates at the unit level, and aggregate to the entire study area.

Study area

Figure 1 delineates the moist soil unit and identifies individual sub-units and final plot locations. Hover the mouse over a polygon for the unit name or a point for the plot number. Units 1 and 5A were surveyed by the University of Tennessee. Contact the refuge for more information. Unit 11N was not surveyed in 2016.

Figure 1.

Community composition

Here we summarize the proportion of cover of the 10 plant species recorded on the moist soil unit, plus a category for no available food. They are ordered in decreasing order of total cover on all plots.

Figure 2 makes apparent that there was very little food to go around in 2016.

Figure 2.

Estimated seed yields

Estimates of DEDs require estimates of seed production (yield per unit area) for relevant plant species. Previously, we used yield constants provided by ???, which lumped plant species into one of three yield classes for waterfowl forage: non-food (0 kg/ha), moderate (496 kg/ha), and good (1100 kg/ha). Rather than use these constants, however, we used the “dot-grid” method of Gray et al. (2009) to estimate seed production for relevant plant species across the study area.

On each unit of the study area, we visited the first three plots of the GRTS sampling and collected seed heads of all individuals of palatable plant species. In combination with counts of the number of stems of each plant on these plots, we generate estimated seed production/yields for each species across the study area using Gray et al.’s (2009) methodology.

Table 1. Assumed yields and energy content of relevant species.
Cover species # stems sampled Gray (2009) yield(kg/ha) USFWS yield (kg/ha) Total metabolizable energy (kCal/g)
TealGrass 1 1794 1100 2.65
Smartweed 5 251.5 1100 2.36
Sprangletop 9 133 1100 2.65
Cyperus 2 71.12 1100 1.96
Millet 8 68 1100 2.75
Foxtail 4 26.17 1100 2.88
YellowNutsedge 1 4.11 1100 1.96
Flatsedge 3 13.78 1000 1.96
PanicGrass 5 18.77 496 2.65
Beakrush 1 7.68 496 1.86

Duck energy days (DEDs) per acre

We can use the estimated energy density and per area yield for each cover species to estimate the duck energy days per acre on each sub-unit (Figure ??). In short, we do this by estimating the DEDs on each individual sample plot and then summarizing by sub-unit. These calculations assume a daily energetic requirement of 292 kCal per duck and the yields and metabolizable energies in Table 1. NOTE: We dropped from consideration any plants of the genus Juncus (rushes) since we did not have a estimate of seed yield; rushes were noted as only a small contributor on 1 of 128 plots.

Figure 5.

Now we can spatially visualize the estimated (median) DEDs per acre by sub-unit (Figure 6), in this case based on the constant yields used in 2015.

Figure 5.

From this data, we can derive estimates (and some measure of uncertainty) for the total DEDs available on each sub-unit (Figure 6).

First, we estimate the median total DEDs on each sub-unit (and bootstrapped 95% confidence interval of this median). We use the median as DED estimates from plots in some sub-units are quite skewed (see Figure 5). To generate these estimates, we draw a large number (1000) of bootstrapped samples, with replacement, of the same size (i.e., number of plots) as measured in the field. For each of these samples, we calculate the median DEDs/ac and multiply times the acreage of the given sub-unit. This produces a distribution of total DEDs for a sub-unit from which Figure 6 is derived; the numbers indicate the median total DEDs (in thousands):

Figure 6.

From this bootstrapping exercise we can also generate an estimate, rounded to the nearest thousand, for the total duck energy days present on the entire moist soil unit, or at least for those units on which surveys were conducted/available. This is calculated by summing the total DEDs for each sub-unit in each of the 1000 iterations. The median (and 95% CI) of total DEDs on the moist soil units was 13000 (5000 - 35000) DEDs.