Summer Emergence of Mayflies, Stoneflies, and Caddisflies from a Colorado Mountain Stream

Abstract

The summer emergence patterns of mayfly, stonefly, and caddisfly species are described for a second order, southern Rocky Mountain Colorado stream. Frequent, standardized, sweepnetting and emergence trap samples provided 1,779 adults of 45 taxa consisting of 11 mayfly, 15 stonefly, and 19 caddisfly species. The five most dominant species were, in order of importance, Suwallia nr. lineosa, Sweltsa coloradensis, Oligophlebodes minutus, Paraleuctra vershina, and Baetis bicaudatus. Stoneflies contributed 70% of all adults collected. Peak species richness of all orders occurred near the summer solstice and maximum water temperatures. The range of slope values generated by simple linear regression of cumulative percentage catch revealed that emergence was extended (slopes < 4%/day) for five of the seven most abundant species. Two caddisflies, 0. minutus and Rhyacophila pellisa, displayed a synchronous emergence pattern. This study adds 35 new records for the Rio Conejos drainage and Conejos County and provides a baseline of comparison against future changes in species richness among these orders. Mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera) are among the dominant insect orders in Rocky Mountain stream ecosystems, yet there are no published accounts of the species-level diversity or emergence phenology of all three groups in any stream of the region. Ward and Kondratieff (1992) state that most of the comprehensive knowledge of aquatic insects in Colorado has been amassed only recently, and that much work on insect life histories and distribution patterns remains to be conducted. Emergence is an important aspect of the life histories of aquatic insects in that it sets the stage for the next generation (Corbet, 1964; Sheldon and Jewett, 1967). Patterns of emergence may affect the allocation of resources and avoidance of competition through temporal segregation of morphologically or functionally similar species (Grant and Mackay, 1969). Corbet (1964) described four types of emergence patterns: continuous, rhythmic, sporadic, and seasonal. Most aquatic insects of high latitudes and altitudes exhibit a seasonal emergence pattern. The interaction of temperature, photoperiod, and local climate conditions helps to determine aquatic insect seasonal phenology in temperate regions (Masteller, 1983, 1993). Our objectives were to characterize the summer emergence patterns and emergence synchrony of the adult mayflies, stoneflies, and caddisflies in Massey Gulch, a small tributary of the Rio Conejos of south-central Colorado. There are no published reports of previous collections or studies of this stream. Few species records are available for the Rio Grande, Rio Conejos, and adjacent streams of the San Juan Mountains (Stark et al., 1973a, 1973b; Baumann et al., 1977; and Szczytko and Stewart, 1979 for stoneflies; Herrman et al., 1986 for caddisflies; Ward and Kondratieff, 1992; McCafferty et al., 1993 for mayflies). MATERIALS AND METHODS-Massey Gulch is a second-order stream located at 106015'W longitude and 37*03'N latitude. Emergence was monitored along an approximate 100 m stretch of the stream about 1.5 km upstream from its confluence with the Rio Conejos, 23 km west of Antonito, Colorado. The stream was no more than 3 m wide, and it had a permanent flow at this 2,700 m elevation, mixed-coniferous forest site. A Ryan@ model J, continuous recording thermograph was placed in the stream on 21 May 1989 and This content downloaded from 157.55.39.92 on Wed, 22 Jun 2016 06:59:02 UTC All use subject to http://about.jstor.org/terms 250 The Southwestern Naturalist vol. 39, no. 3 ou 14 , S12 _/^// I V'