Ecological Distribution of Hydracarina in North Central Colorado

Abstract

Ninety-nine species of Hydracarina were collected in an extensive survey of streams, lakes, ponds, and hot springs of north central Colorado during 1965, 1966, and 1967; 72 are new for Colorado and three are new for continental North America. The faunas of lentic and lotic habitats are distinct, with only four species contributing appreciably to both faunas. The stream fauna is composed of 44 species. When the 29 most common stream species are analyzed on the basis of their altitudinal distributions and their habitat ranges for bound carbonates, ash content, organic content, and temperature, four groups are evident: euryzonaleuryokous species, with wide altitudinal and habitat ranges; euryzonalstenokous species, with wide altitudinal but narrow habitat ranges; altitudinally restricted-euryokous species; and altitudinally restrictedstenokous species. The ecological basis for these groups is the variable adaptability of the water mites coupled with the physical-chemical characters of Rocky Mountain streams. The effects of substrate, stream velocity, and pollution on the distribution of Hydracarina were found to be significant, but highly localized; aquatic plants exert a negligible influence. The maximum number of species was found between 2450 and 2700 m, where faunas from higher and lower elevations overlap and where numerous small streams provide a great diversity of habitats. Zoogeographic affinities of stream species are with faunas to the north and west, suggesting colonization of this region by northern species. The fauna of hot springs consists of only two species: Thermacarus nevadensis, a species common in hot springs; and Tyrrellia circularis, not previously reported from hot springs. Lakes and ponds yielded 66 species; their low frequencies and low abundance reflect their haphazard passive dispersal and their sensitivity to environmental factors. The common lentic species also form four distinct groups: plains species, tolerant of high concentrations of dissolved salts and high temperatures; relatively stenokous montane species; alpine species, largely high-altitude stream forms that have invaded lakes; and euryzonal species, which are highly euryokous. Aquatic plants play a significant but highly localized role in the distribution of lentic Hydracarina. The greatest number of species occurs in nonalkaline plains lakes where a great variety of habitats are available and montane and plains faunas slightly overlap. Most species of lentic Hydracarina have a wide Holarctic and Nearctic distribution; there is a considerable overlap of eastern and western faunas, and a close affinity with the northwestern fauna in some genera. A comparison of lotic and lentic faunas indicates that their major differences are the product of the divergent nature of the two types of habitat, while similarities reflect characters common to the Hydracarina as a group. INTRODUCTION Though the Hydracarina live in a wide variety of aquatic habitats throughout the world, we know few details of their ecological distribu1 Contribution No. 52, Limnology Laboratory, University of, Colorado. 2 Present Address: Biology Department, Southwest Texas State College, San Marcos, Texas 78666.