Riding the wetland wave: Can ducks locate macroinvertebrate resources across the breeding season?

Abstract

Food availability varies considerably over space and time in wetland systems, and consumers must be able to track those changes during energetically-demanding points in the life cycle like breeding. Resource tracking has been studied frequently among herbivores, but receives less attention among consumers of macroinvertebrates. We evaluated the change in resource availability across habitat types and time and the simultaneous density of waterfowl consumers throughout their breeding season in a high-elevation, flood-irrigated system. We also assessed whether the macroinvertebrate resource density better predicted waterfowl density across habitats, compared to consistency (i.e., temporal evenness) of the invertebrate resource or taxonomic richness. Resource density varied marginally across wetland types but was highest in basin wetlands (i.e., ponds) and peaked early in the breeding season, whereas it remained relatively low and stable in other wetland habitats. Breeding duck density was positively related to resource density, more so than temporal resource stability, for all species. Resource density was negatively related to duckling density, however. These results have the potential to not only elucidate mechanisms of habitat selection among breeding ducks in flood-irrigated landscapes but also suggest there is not a consequential trade-off to selecting wetland sites based on energy density versus temporal resource stability and that good-quality wetland sites provide both.

Keywords: aquatic macroinvertebrates; drought; resource tracking; waterfowl; wetlands.

FIGURE 1

Map of wetlands and sampling locations across the study area in North Park, Colorado, USA from 2020 to 2021. We selected plots from four wetland types (basin wetland, riparian, irrigation ditch, and flooded hay meadow) on each of three properties to which we had access in addition to three public reservoirs in 2020 and 2021. The dashed line is the Wyoming‐Colorado border. We selected three random wetlands of each type on each of the three properties, and three random sampling points within each selected wetland with the exception of reservoirs. We randomly selected two 200‐m sections of shoreline in each reservoir and three random points within those “plots” to sample. The panel on the right shows an example of four different wetlands sampled on one of the properties. From bottom left to top right, the sampled points are in a flooded hay meadow, riparian wetland, basin wetland, and irrigation ditch. The three additional sampling points in the flooded hay meadow indicate that we moved those points in the second sampling year (2021) because the original locations were not flooded, which we accounted for in the analysis by treating them as different points nested in the same site.

FIGURE 2

Model‐predicted estimates of invertebrate energy density (J/cm³) across wetland habitats throughout the breeding season (May–July) in North Park, CO, 2020–2021.

FIGURE 3

Time‐averaged invertebrate energy density (J/cm³) available in each sampled wetland habitat in North Park, CO from 2020 to 2021. Points represent posterior means of the habitat‐specific energy density, bars represent 95% credible intervals, and violin shapes represent the posterior distribution to provide a reference of the amount of sampling variability in each habitat type.

FIGURE 4

Model‐predicted energy density (J/cm³) as a function of observed taxonomic richness (number of taxa) of macroinvertebrates sampled in North Park, CO, USA.

FIGURE 5

Temporal rank abundance curves created by ranking the energy density (J/cm³) of a given wetland type across six sampling occasions in 2020 and 2021, North Park, CO, USA. E _Q provides a metric of the evenness of a resource as measured by the slope of a rank abundance curve (high E _Q indicates higher evenness of the resource over time).

FIGURE 6

Relationship between breeding duck density and invertebrate energy density (J/cm³) for each duck species across wetlands in North Park, CO, USA from 2020 to 2021.

FIGURE 7

Relationship between duckling density and the invertebrate predictor in the top model as measured by WAIC across wetlands in North Park, CO, USA from 2020 to 2021. The left panel is the relationship between pooled mallard and cinnamon teal duckling density with energy density (J/cm³), and the right panel is the relationship between gadwall duckling density and taxonomic richness.