Least Sandpiper, photo by Michael Gray.

Unraveling the Truth about Knotgrass at Bosque del Apache National Wildlife Refuge

Maribel E. Glass, Bonnie Warnock, Ryan S. Luna, and Dan Collins (USFWS)

At Bosque del Apache National Wildlife Refuge, staff have reported an increasing density of knotgrass within wetland units. In some years, knotgrass has even taken over entire wetland units. Knotgrass is known to become weedy or invasive in some regions or habitats and may displace desirable vegetation if not properly managed. Species within the same genus, such as seashore paspalum (P. vaginatum), have been known to reduce native biodiversity and affect ecosystem function in wetlands. It is still uncertain whether or not this grass is essential to migrating avifauna and furthermore, how management prescriptions affect its abundance. However, the genus has also been known to provide a viable food source to ducks, geese, swans, and other water birds. Due to the increasing density of knotgrass within the refuge this study was introduced to determine the effects of knotgrass on waterfowl.

The objectives of this study are to (1) assess differences, if any, between the refuge’s seed yield survey methodology and core samples, and to assess differences in estimated duck-use-days derived from food use estimates; (2) determine if distinctive management practices such as mechanical treatments, flood duration and water depth influence knotgrass production in wetlands; (3) determine if bird abundance in knotgrass-dominated wetlands is a result of knotgrass’ possible high energetic production value, and if any distinctive management practices have an influence on bird abundance in wetlands.

We found seed productivity and duck-use-days estimates from core samples were significantly higher than the refuge estimates (N = 140; Mann-Whitney U = 1145; SE = 239.7; P < 0.001). Seed productivity, as estimated using core samples, was not significantly influenced by water depth, flood duration, flood initiation date, draw-down date, or the type of disturbances. None of the collected focal species showed evidence of consuming knotgrass.

Funding source: U.S. Fish and Wildlife Service and Houston Safari Club Foundation.