Ecology of Small Mammals on Two Isolated Buttes in Canyonlands National Park, Utah

Abstract

-Fewer mammalian species occurred on summits of sheer-walled buttes than occurred in similar control sites, but density of mammals on buttes equaled or exceeded mainland density. Peromyscus crinitus, the only terrestrial mammal on Jug Butte, occupied a larger habitat niche there than in control areas. No such niche expansion was noted for P. crinitus on Junction Butte, where four other species of mammals (Eutamias quadrivittatus, Neotoma lepida, Neotoma cinerea, Peromyscus truei) and at least one large mammal (Canis latrans) also occurred. At least 18 species of terrestrial mammals inhabit the control sites. It is hypothesized that the species number remains static on each butte and is a function of the unique dispersal abilities of each species and does not represent an equilibrium between recurrent colonization and extinction. The Island in the Sky District of Canyonlands National Park is composed of a large central mesa (approximately 80 km2) with deep canyons radiating down to a surrounding bench called the White Rim. Deep canyons also cut into the White Rim and lead down 300 m to the Green River on the west and the Colorado River on the east. Erosion has gouged deeply into the Island in the Sky and has cut through some peninsulas of the mesa, forming free-standing isolated buttes such as The Jug and Junction Butte. Like islands, these buttes support habitats separated from areas of similar habitat on the mainland mesa by the 150-m high Wingate sandstone cliffs which form the buttes. Also like islands, movement by some taxa to and from these isolated habitats may be limited by the cliffs, resulting in exclusion of some species from the buttes' summits. Little is known about the organisms inhabiting these areas. In 1937, Anthony scrambled up Shiva Temple in the Grand Canyon, and because access to the summit was easy, all mammal species expected in that habitat were found. Mason and West (1970) flew to the summit of Timber Top Mesa, a sheer-walled mesa in Zion National Park, Utah, to study the ecology of naturally occurring fires, but only noted the presence of small rodents, birds and insects. In Canyonlands, the only previous research concerning isolated buttes was a survey of the vegetation of Junction Butte (Loope, 1977). Armstrong (1979a, 1979b) studied mammals of Canyonlands, but did not investigate butte faunas. Since isolated buttes are similar to islands in certain respects, their mammalian faunas may exhibit several of the phenomena typical of islands. Insular faunas generally have a lower species richness than mainland faunas (Case, 1975; Crowell and Pimm, 1976; Lack, 1971; Vaughan and Schwartz, 1980), with larger islands supporting more species than smaller islands (Lowe, 1955; MacArthur and Wilson, 1967; Simberloff, 1976). MacArthur and Wilson (1967) state that the lower species diversity on islands results from the different migratory abilities of the mainland species and that the number of species on an island is an equilibrium between rates of recurrent E SOUTHWESTERN ATURALIST 26(4):395-407 NOVEMBER 20, 1981 This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 05:41:35 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist colonization and extinction. The magnitude of this equilibrium is related directly to the area of the island and inversely to the proximity of the island to the mainland source of colonizers. Because island faunas generally lack some of the species which normally occupy similar habitats on mainlands, many island species exhibit expanded habitat, food, and spatial niche dimensions (Cameron, 1962; Diamond, 1970; Grant, 1965; Lack, 1971). Usually concomitant with expanded niches, islands support a greater number of individuals per species (Grant, 1965) and may have total densities per taxon equaling or exceeding mainland levels (Case, 1975). This density compensation has been recorded for island lizards (Bennett and Gorman, 1979; MacArthur, 1972), birds (Cruz, 1977; Grant, 1965; MacArthur et al., 1972), and mammals (Sullivan, 1977; Webb, 1965). The faunas of many island-like habitats (so-called habitat islands) such as caves (Culver, 1970), lakes and springs (Barbour and Brown, 1974; Brown, 1971; Aho, 1978), rivers (Sepkowski and Rex, 1974), cushion plants (Tepedino and Stanton, 1976), eastern U.S. woodlots (Gottfried, 1979), California oak forests (Opler, 1974), British trees (Strong, 1974), badger mounds (Platt, 1975), Great Basin mountaintops (Brown, 1971, 1978) and Andean paramo habitats (Vuilleumier and Simberloff, 1980) have been examined in light of island biogeography theory. In general, these studies support a positive species-area relation, but often historical or ad hoc explanations are sufficient to account for the species present. Since the isolated buttes on the Colorado Plateau were (and still are) virtually unstudied, the present study was undertaken to (1) determine which, if any, terrestrial mammals inhabit isolated buttes and (2) determine the applicability of island biogeography theory to the mammalian faunas of these buttes. STUDY SITES.-Field work was conducted in Canyonlands National Park, Utah, from 25 September 1975 through 10 December 1975 on four study sites: two isolated buttes and two control areas on the Island in the Sky Mesa. Buttes were chosen on the basis of size, degree of isolation from the surrounding habitat, and habitat similarity. Controls were chosen because habitats were similar to those on buttes. The Jug is an isolated butte located 0.4 km SW of the Island in the Sky Mesa in Holeman Spring Basin (Sec. 34, T27S, R18E, Upheaval Dome Quadrangle, Utah). It rises approximately 400 m from the White Rim to an elevation of 1,761 m. The final 130 m is sheer Wingate sandstone capped with Kayenta sandstone, except at the northeastern end of the butte, where a remnant of Navajo sandstone overlies the Kayenta. The summit of the butte is approximately 5 ha and is divided into two major habitats. The southwestern two-thirds is an open juniper (Juniperus osteosperma) woodland with large areas of slickrock. Soil, where present, is thin, poorly developed, and composed either of sand or gravel. The northeastern third of the butte is a shadscale (Atriplex confertifolia)-blackbrush (Coleogyne ramosissima) community covering the gravelly cone leading to the summit cap of Navajo sandstone. The soil is poorly developed, but deeper than that found on the southwestern part of the butte. Access to the Jug was by helicopter. To the best of my knowledge, the summit had not been visited by man prior to this study. The first control site was located at the head of a canyon in the west-central portion of Red Sea Flat on the Island in the Sky Mesa at an elevation of 1,768 m (Sec. 9, T27S, R19E). Substrate, physiography, and vegetation are similar to those of the Jug, and like the Jug the site supports two vegetation types-open woodland and brushland. The western three-fifths of the site is an open pinyon (Pinus edulis)-juniper woodland with large areas of slickrock and thin, poorly developed, sandy soil. The underlying rock is Kayenta sandstone. The eastern two-fifths of the site is 396 vol. 26, no. 4 This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 05:41:35 UTC All use subject to http://about.jstor.org/terms Johnson-Small Mammals on Isolated Buttes TABLE 1.-Per cent canopy coverage of common plant species in each study area (x = species known to be present but not sampled, o = species known not to be present). Per cent coverage of the two dominant species at each site are italicized. Jug Junction Butte Red Sea Flat Grandview Point Species Woodland Brushland Woodland Woodland Brushland Woodland Juniperus osteosperma Pinus edulis Artemsia bigelovii Atriplex confertifolia Cercocarpus intricatus Cercocarpus montanus Chrysothamnus sp. Coleogyne ramosissima Cowania mexicana Echinocactus sp. Ephedra viridis Fraxinus anomala Gutierrezia sp. Mahonia fremontii Opuntia sp. Rhus trilobata