The insect fauna of spring habitats in semiarid rangelands in central Oregon

In semiarid rangelands, a variety of permanently wet habitats can be classified within the general category of springs. They may grade imperceptibly to either typical streams or to wetlands; habitats differ in water transit time and nature of aquatic and riparian vegetation. Emergence traps were used to collect aquatic insects from helocrene habitats in the Bridge Creek watershed in central Oregon. Water chemistry analyses showed high conductivity, pH, and cation concentration values at all sites. Nematocerous Diptera typically comprised greater than 90% of the aquatic insects in emergence traps. Other aquatic orders were uncommon and low in species richness and abundance. Plecoptera were common at only one site, while the damselfly, Argia vivida occurred at all sample sites. Terrestrial insects associated with emergent vegetation also were present in the sam ples. Many of the insect taxa that were collected do not occur in larger streams and thus insects of springs provide a significant contribution to the diversity of aquatic fauna in semiarid landscapes. In semiarid landscapes, such as those dominating the Great Basin and much of the Columbia River Basin, water is a critical resource for the biota as well as for human endeavors. Land-use issues relating to utilization of aquatic resources have been exacerbated by drought conditions in the West for at least the past six years. Overgrazing by domestic livestock, particularly when they have access to the stream and riparian areas, has detrimental effects on habitat quality as well as on associated biota (Kauffman and Krueger, 1984; Skovlin, 1984). Irrigation poses another threat to aquatic resources, particularly in time of drought. These issues may be quite volatile when fishery resources are jeopardized. Clearly, de cisions regarding use of aquatic resources will influence biodiversity in semiarid lands. However, such decisions are often based on information that is limited in scope due to the tendency of research and public interest to focus on a limited, albeit visible, portion of the ecosystem. Such is the case in semiarid landscapes where the large lotie habitats, especially those supporting salmonid fish, have received the most attention from aquatic scientists and environmentalists. In the headwaters of perennial streams there is a variety of aquatic habitats that can be classified under the broad category of springs. Little is known about the faunal composition or ecological role of these habitats (though see Cushing and Gaines, 1989). In this paper we report on the habitat characteristics and aquatic insect com position of a range of springs that occur in the Bridge Creek drainage in central Oregon. We also discuss the contribution of springs to the biodiversity in these landscapes and the role of isolated aquatic habitats in semiarid rangelands. Faunal surveys, such as we present, are meant to stimulate interest in these habitats and to provide a basis for further ecological studies. 1 This is contribution No. 10,561 from the Oregon Agricultural Experiment Station. This content downloaded from 207.46.13.158 on Wed, 11 May 2016 05:51:57 UTC All use subject to http://about.jstor.org/terms