Joseph R. McAuliffe

Competition for Space, Disturbance, and the Structure of a Benthic Stream Community

Interspecific competition for space limits the distribution and abundance of many ben- thic insects on stones in a western Montana stream. The sessile caddisfly larva Leucotrichia pictipes is territorial and aggressively eliminates conspecifics and other species from its foraging territories. Territoriality produces an intraspecific pattern of regular spacing and negative spatial associations between Leucotrichia and several other sessile insects: Parargyractis confusalis, Rheotanytarsus sp. and Eukiefferiella sp. Experimental removals of Leucotrichia resulted in higher densities of other sessile species and several mobile insects, including Baetis, Glossosoma, and Simulium. These insects show broad microhabitat overlap with Leucotrichia; competition with Leucotrichia limits their dis- tributions and abundances within otherwise suitable microhabitats. Leucotrichia is the only species that monopolizes large areas of space. Physical disturbances interrupt the formation of competitive monopolies by Leucotrichia. During seasonal reduced flows, Leucotrichia is eliminated from stones and boulders in shallow water. Greater densities of a short-lived sessile species with multiple annual generations occupy this ephemeral spatial resource. On permanently submerged stones, Leucotrichia competitively affects a large part of the benthic fauna. On small stones that overturn with higher frequencies during flooding, Leucotrichia densities are reduced and species abundances are highly equitable. Large, more stable substrates with greater densities of Leucotrichia are characterized by lower species evenness.

ECOLOGICAL RESPONSES OF TWO MOJAVE DESERT SHRUBS TO SOIL HORIZON DEVELOPMENT AND SOIL WATER DYNAMICS

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, ψpd, and midday net photosynthetic assimilation, Anet) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics. Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, <4000 yr BP; Intermediate Alluvial, ∼12u200a000 yr BP; Old Alluvial, ∼40u200a000 yr BP), total canopy volume of Larrea (c...

Lithologically influenced geomorphic responses to Holocene climatic changes in the Southern Colorado Plateau, Arizona: A soil-geomorphic and ecologic perspective

Abstract The semiarid landscape occupied by the Navajo and Hopi peoples on the Colorado Plateau in northeastern Arizona in the southwestern United States is characterized by an extensive network of deeply incised arroyos. Since the early 20th century, many researchers have proposed that the recent formation of arroyos in this region and also many attributes of modern vegetation communities were caused directly by overgrazing of vegetation by domestic livestock of the Navajo. Other researchers, however, have proposed other causes for such features, such as climatic change. We believe that the landforms, soils and vegetation of a small area located on Antelope Mesa in this region, underlain by the highly erodible materials of the Miocene Bidahochi Formation, may have been more sensitive to minor climatic changes of the Holocene than landscapes of massive Mesozoic sandstones that dominate the Colorado Plateau. In the Antelope Mesa area, the presence of actively filling channels rather than arroyos in the upper parts of many drainage basins and associated soils and ecologic patterns indicate that the aggradation (1) was initiated in downstream reaches and within the past two centuries, (2) may be linked to recently accelerated slope erosion, and (3) is unrelated to past or ongoing grazing. This suggests the ongoing aggradation may be related to recent minor climatic changes. Geochronologic and soil-geomorphic evidence indicate that the most recent cycle of arroyo incision and filling may be a small-scale analogue for larger-magnitude, older cycles that produced regionally recognizable, paired terraces that are attributable to previous Holocene climate changes. We propose that climatic change, and more specifically, increases in precipitation, caused an acceleration in the erosion of the steep, typically minimally vegetated slopes of the Bidahochi Formation. The beginning of the ‘Neoglacial Period’ (ca. 2–3 ka), effects of which are documented by other proxy records in this region, may be the climatic change that triggered widespread deposition of a large, mid-Holocene alluvial unit in this region. Alternating episodes of eolian activity and soil formation in the study area and in this general region provide independent evidence of Holocene climatic changes. Our interpretations of the origin of the cut-and-fill cycles that emphasize the role of drainage basin lithology and differ significantly from previous interpretations emphasize the linkage of climatic changes, groundwater levels and arroyo incision. This study reveals the importance of integrating soil and ecologic studies with geomorphologic research; such an approach may be critical in helping understand how anthropogenically induced climatic changes of the next century could impact geomorphic processes and the ecology of arid and semiarid regions.

A 22,000-year record of vegetation change in the north-central Sonoran Desert

Abstract Plant macrofossils from eleven packrat (Neotoma sp.) middens provide a history of vegetation change over the last 22,140 years at Eagle Eye Mountain, near the northern border of the Sonoran Desert in Arizona. This site is located in the Arizona Upland subdivision. Departures and arrivals of various perennial plant species during the Holocene were similar to other records from more southerly sites and support earlier generalizations of Van Devender [Van Devender, T.R., 1990. Late Quaternary vegetation and climate of the Sonoran Desert, United States and Mexico. In: Betancourt, J.L., Van Devender, T.R., Martin, P.S. (Eds.), Packrat Middens: The Last 40,000 Years of Biotic Change. University of Arizona Press, Tucson, pp. 134–165.] regarding the tempo and mode of late Quaternary changes in the Arizona Upland. Evergreen woodland containing Pinus monophylla (singleleaf pinyon), Juniperus osteosperma (Utah juniper), and Quercus turbinella (shrub live oak) occupied the site during full glacial times. The latter two species, but not P. monophylla, persisted into the early Holocene and were joined by a limited number of desertscrub species, including Acacia greggii (catclaw), Carnegiea gigantea (saguaro), and several other succulents. With the arrival of many other species during the middle Holocene, a desertscrub community that differed considerably from the modern vegetation developed. Acacia greggii and Cercidium floridum (blue paloverde) were the principal large woody species on exposed hillslopes. Assemblages similar in composition to the modern desertscrub did not appear until approximately 4000 years ago, when the more xerophytic Cercidium microphyllum (foothills paloverde) arrived and C. floridum disappeared from hillslopes. The delayed arrival of C. microphyllum in comparison to other Sonoran Desertscrub species with similar climatic tolerances is attributed to the limited dispersal distance of its seeds by seed-eating heteromyid rodents. At 4540 and 6425 yr B.P., the cold-intolerant shrub Encelia farinosa was present on northeasterly aspect exposures where it is absent today, suggesting slightly warmer winter conditions during the middle Holocene. The presence of the tree Cercidium floridum and the perennial, C-4 grass Setaria leucopila on exposed slopes during the middle Holocene indicates substantially moister conditions during that time than in the late Holocene or today. At sites on the xeric limits of the modern distributions of both species, average annual precipitation is from 50% to 100% greater than that presently received at Eagle Eye Mountain. In particular, the responses of S. leucopila and cacti during the middle Holocene indicate substantial increases in warm-season precipitation.

A rapid survey method for the estimation of density and cover in desert plant communities

. The Log-series survey method allows rapid estimates of density and cover and is applicable for studies of perennial vegetation in arid environments. An optical rangefinder is used to determine boundaries of large circular plots. Numbers of individuals of each species within a plot are assessed; this information is used to assign species to logarithmic density classes equivalent to the logarithm base 2 of actual abundances. Each species is then assigned to a logarithmic canopy cover class, equivalent to the logarithmbase 2 of average cover per individual. Log total cover per species per plot is obtained by the addition of logarithmic density and cover classes. Percent cover per species is rapidly computed by taking the antilog of the difference between log total cover per species and log total plot area.

Carbon isotope discrimination and foliar nutrient status of Larrea tridentata (creosote bush) in contrasting Mojave Desert soils

We investigated the relationships between foliar stable carbon isotope discrimination (Δ), % foliar N, and predawn water potentials (ψpd) and midday stomatal conductance (gs) of Larrea tridentata across five Mojave Desert soils with different age-specific surface and sub-surface horizon development and soil hydrologies. We wished to elucidate how this long-lived evergreen shrub optimizes leaf-level physiological performance across soils with physicochemical characteristics that affect the distribution of limiting water and nitrogen resources. We found that in young, coarse alluvial soils that permit water infiltration to deeper soil horizons, % foliar N was highest and Δ, gs and ψpd were lowest, while %N was lowest and Δ, gs and ψpd were highest in fine sandy soils; Larrea growing in older soils with well-developed surface and sub-surface horizons exhibited intermediate values for these parameters. Δ showed negative linear relationships with % N (R2=0.54) and a positive relationship with ψpd (R2=0.14). Multiple regression analyses showed a strong degree of multicolinearity of gs and Δ with ψpd and N, suggesting that soil-mediated distribution of co-limiting water and nitrogen resources was the primary determinant of stomatal behavior, which is the primary limitation to productivity in this shrub. These findings show that subtle changes in the soil medium plays a strong role in the spatial and temporal distribution and utilization of limiting water and nitrogen resources by this long-lived desert evergreen, and that this role can be detected through carbon isotope ratios.

Determinants of the Vertical Distributions of Woodpecker Nest Cavities in the Sahuaro Cactus

In the Sonoran Desert, Gilded (Northern) Flickers (Colaptesauratuschrysoides) and Gila Woodpeckers (Melanerpes uropygialis) excavate nest cavities within the giant sahuaro cactus. Vertical distributions of nest cavities excavated by the two woodpecker species differ in height and breadth. Flicker cavities are usually restricted to within 3 m of the stem apex. Nest cavities excavated by Gila Woodpeckers are found over a considerably broader vertical range of heights; their average height is also lower. Competitive preemption of potential excavation sites cannot fully explain this differential utilization of vertical space. Rather, the initial positioning of nest cavities is constrained by cactus anatomy. Different nest-cavity sizes and wood-chiseling capabilities of the two woodpecker species limit the vertical range over which nest cavities can be excavated. In addition, an excavation by a flicker often results in death of the sahuaro. With this mortality, a long-running record of flicker nest-cavity heights is not preserved. In sahuaros containing only Gila Woodpecker cavities, the original heights of old excavations are preserved as the sahuaro continues to grow, thus contributing to the greater vertical range of observed cavity heights.

On communal roosting and associated winter social behavior of gray-breasted woodpeckers

The Gray-breasted Woodpecker (Melanerpes hypopolius) is a relatively common species in southwestern Mexico (Guerrero, Mexico, Morelos, Puebla), especially where large columnar cacti are found. However, almost nothing is known about the behavior of this woodpecker species (Short 1982). Here, we describe Gray-breasted Woodpecker communal roosting and winter social behavior and speculate on the possibility of cooperative breeding by this species. Our observations were made between 27 December