Spatial and diel availability of flying insects as potential duckling food in prairie wetlands

Abstract

We examined spatial and diel availability of flying insects that are a critical food resource to young ducklings. We sampled insects in three native prairie wetlands on the Woodworth Study Area of south-central North Dakota. Insects were sampled with floating sticky traps within emergent macrophyte, edge, and open water microhabitat zones. Sampling took place from 12 June to 4 July 1995, a period that coincided with peak dabbling duck (Anas spp.) hatching in this region. Our sticky trans collected 28,527 insects and spiders totaling at least 32 families and 150 species. Chironomidae (Diptera) was the most abundant group, constituting 60% of the total insect count and 32.9% of the biomass (mg dry weight). Mixed-model ANOVA showed that a population of similar undisturbed wetlands may show differences in insect availability when considering both time and space, primarily due to differences in community structure among sites. In spite of these significant random effects, interactions between or among the fixed date, zone, and trap-height effects significantly influenced insect availability. Insects stratified near the water surface in open water areas on all dates except during cool, rainy weather (28 June); on this date, insects were virtually absent from open water. Vertical stratification of insects was less prevalent within and at the edge of stands of emergent vegetation, although most insects were present in the emergent zone near the water surface during inclement weather. ANOVA models from our diel study showed that a significant diel pattern in insect availability existed among zones, but this interaction also depended upon trap-height (chironomid biomass) or date (chironomid counts and biomass). Generally, more insect numbers and biomass were captured in and along stands of emergent macrophytes during the day but chiefly in open water near the surface at night. This diel-zone effect was especially apparent for large chironomids, which were essentially absent during daylight but abundant in open water and edge zones during night. Daytime chironomids were small and predominantly trapped in stands of emergent vegetation. Our results are consistent with previously documented brood foraging behavior and may indicate a trade-off between low energy foraging in the open at night and potentially safer but less productive foraging in stands of emergent vegetation during the day.