Larval odonates as bioindicators of cattle grazing and water quality at prairie wetlands

The grazing regime in the pasture surrounding the wetlands used in this study supported one of three treatments: deferred grazing, from mid-July to mid-August, continuous grazing, approximately May to August, and idle or ungrazed pastures. These ungrazed pastures acted as the control treatment. The study was conducted over two years, 2000 and 2001.

Water quality results

• Wetland pH was significantly higher at deferred grazing sites compared to idle or continuously grazed sites during both years
• Salinity was significantly higher at wetlands with deferred grazing compared to continuously grazed regimes in one of the two years
• Grazing treatment did not significantly affect wetland temperature or dissolved oxygen
• No significant differences in Total Dissolved Solids (TDS) were found between grazing treatments. TDS is used to measure the impact of cattle urine on water quality as a result of cattle grazing.
• No significant differences in fecal coliforms, chlorophyll-a, total dissolved phosphorus, total phosphorus, total Kjeldahl nitrogen (TKN), total nitrogen, including NO₂, NO₃, and TKN, or ammonium were found between grazing treatments.

Odonate results

• Significantly fewer larvae were collected at deferred grazing wetlands rather than continuously grazed wetlands as initially predicted. Deferred grazing regimes involve combining many herds of cattle into a super herd with hundreds of cattle, whereas continuously grazed wetlands involve smaller grazing operations. The number of cattle rather than simply the duration of their presence may be more important than initially recognized, making deferred grazing a more acute grazing disturbance.
• This study only examined the number of odonate larvae, and the proportion of larvae that successfully emerge and reproduce is unknown

Summary

• No significant difference in water quality due to cattle presence was detected.
• The cattle stocking rates were unusually low within the study area compared to other regions, due to low precipitation and carrying capacity of the pastures. Hence, higher stocking rates may have a greater impact.
• Deferred grazing substantially disturbs the wetlands, likely because of the large number of cattle on these pastures. Fewer odonate larvae inhabit these wetlands, presumably due to the acute disturbance resulting from hundreds of cattle focusing their grazing efforts on the littoral zone of one wetland.
• Odonates are a suitable bioindicator because their populations and diversity were affected by cattle and could be measured, even when cattle had no biologically significant effect on wetland water quality or vegetation abundance.

This text has been adapted from Christine's thesis. For further information, Christine's complete thesis is available on Dr. Lee Foote's website.

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