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3 April 2012 Effects of cattle grazing on Platte River caddisflies (Ironoquia plattensis) in central Nebraska
Mary J. Harner
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Abstract

The Platte River caddisfly (Ironoquia plattensis) is a semiterrestrial limnephilid that inhabits sloughs along the Platte River in central Nebraska (USA). The species was discovered in 1997, and little is known about what controls its limited distribution or threatens its existence. We investigated effects of grazing by cattle (Bos taurus) on caddisfly abundance in a grassland slough. In April 2010, we established exclosures to isolate cattle from areas with caddisflies. We measured aquatic larval densities in April 2010 and 2011. We estimated grazing intensity from the normalized difference vegetation index (NDVI) values extracted from aerial images made in autumn 2010. Grazing intensity varied among plots, but ungrazed plots had more vegetation (higher NDVI values) than grazed plots. In April 2011, larval densities were greater in ungrazed than in grazed plots. Larval densities and NDVI values were strongly positively correlated, a result suggesting that reduction in vegetative cover from grazing was associated with decreased densities of caddisflies. Increased vegetative cover may have provided structure needed for adult courtship and inputs of organic matter to support larval feeding. Repeated, season-long grazing may have long-term negative consequences for the Platte River caddisfly in grassland sloughs when vegetation does not recover and other effects of cattle persist year after year. Resting pastures from grazing to permit vegetation to rebound appears to allow cattle and Platte River caddisflies to coexist in sloughs along the Platte River.

In 1997, the Platte River caddisfly (Ironoquia plattensis) was described from an intermittent grassland slough (shallow, linear wetland) along the Platte River in central Nebraska (Alexander and Whiles 2000). In subsequent surveys, it was observed in isolated wetlands along a 320-km reach of the Platte River from Lincoln County to Merrick County (Vivian 2010). The Platte River caddisfly develops in sloughs through 5 larval instars from autumn to spring, but unlike most caddisflies, larval I. plattensis emigrate from aquatic to terrestrial habitats in spring to aestivate for summer and then pupate in autumn (Whiles et al. 1999, Geluso et al. 2011). The terrestrial phase of its life cycle coincides with periods when off-channel aquatic habitats, especially sloughs, dry or have intermittent flow (Whiles et al. 1999). The complete life cycle of I. plattensis occurs within a relatively small area near the slough. Adults emerge from adjacent terrestrial areas in late September and early October and return to the slough channel to deposit eggs. Specific habitat requirements and potential threats to the Platte River caddisfly remain largely unknown. To date, only a few publications have been focused on the species (Whiles et al. 1999, Alexander and Whiles 2000, Geluso et al. 2011).

Off-channel aquatic habitats used by the Platte River caddisfly have been reduced substantially in the Platte River valley by flow regulation, conversion of wet meadows to agricultural fields, and depletion of the regional aquifer (Williams 1978, Eschner et al. 1983, Sidle et al. 1989, Friesen et al. 2000). Where suitable habitat remains, management practices, such as prescribed fire and livestock grazing, are used to mimic natural disturbances that once shaped the prairie landscape (Helzer 2010). However, effects of these management activities on the Platte River caddisfly are unknown. Recently, I. plattensis was petitioned for listing as federally endangered in the USA (Rosmarino and Tutchton 2007, USFWS 2009). Multiple factors, including habitat loss and degradation, were outlined as support for listing the species (USFWS 2009).

Abiotic factors linked to river hydrology and flooding ultimately create and maintain aquatic habitats required by I. plattensis, but other factors, such as fish predation (Whiles and Goldowitz 2001, 2005) and livestock grazing (Goldowitz 2004, Vivian 2010), also may affect population dynamics. Goldowitz (2004) noted that season-long cattle grazing had “visually striking impacts on the habitat” at 1 site with I. plattensis, but it was unclear what effects, if any, such management had on caddisflies. We investigated relationships between cattle grazing and abundance of aquatic larval caddisflies along the only slough known to have high densities of the caddisfly and a long history of cattle grazing.

Methods

Study site

In 2010 and 2011, we studied I. plattensis in a slough on the floodplain of the Platte River on Shoemaker Island, Hall County, Nebraska (lat 40°47.660′N, long 98°26.722′W). The floodplain supports mixed and tallgrass prairie vegetation and is traversed by numerous sloughs that create a ridge/swale structure on the landscape (Henszey et al. 2004). Sloughs remain inundated for various durations depending on precipitation, groundwater elevation, and river flow (Whiles and Goldowitz 2001, 2005, Henszey et al. 2004, Harner and Whited 2011). As a result, the mosaic plant communities in the floodplain are structured largely by availability of soil moisture (Henszey et al. 2004). Dominant upland vegetation includes many native and nonnative grasses and a diversity of forbs (Helzer and Jelinski 1999). Sedges, rushes, grasses, and forbs dominate the vegetation in the low-lying areas (Meyer et al. 2010).

The grassland surrounding our slough was managed with a 4-y rotation of prescribed fire and cattle grazing to promote heterogeneity in habitats and optimal grassland production (Kim et al. 2008, Helzer 2010). This rotation involved spring burning followed by early and late grazing in the season (Year 1), grazing mid-season (Year 2), and no burning or grazing (Years 3 and 4). At our study site, the pasture was rested in 2008 and 2009 and burned on 17 March 2010. Cattle grazed the pasture 14 May to 30 June 2010 and September to October 2010, and cattle were rotated to a neighboring pasture in July and August 2010. Sixty animal units, primarily cow–calf pairs, grazed the 83-acre (34 ha) pasture. This protocol resulted in a stocking rate of 2.5 animal unit months (AUM)/acre for the 2.5 mo that cattle were in this pasture. However, when the adjacent 96-acre (39 ha) pasture, where cattle grazed for 2 mo during mid-summer, was included, the overall stocking rate was 1.8 AUM/acre. Recommended stocking rates are 1.7 to 1.9 AUM/acre for season-long, continuous grazing on rangeland in excellent condition in similar habitats in this region (Waller et al. 1986). At our site, stocking rates were within this recommended range when the mid-season rotation was included. Cattle obtained water directly from the slough and a river channel along the southern border of the pasture.

The slough segment we examined had flowing water all year, except in the uppermost 30 m (Geluso et al. 2011). Its deep-channel morphology and proximity to groundwater caused it to have longer periods of inundation than other sloughs on Shoemaker and adjacent Mormon Island (Harner and Whited 2011). Slough banks were 25 to 50 cm high, the wetted channel perim