We examined landscape patterns in the physical conditions and vegetative composition of montane riparian zones to identify their most important sources of variation. Information on plant species cover and on physical characteristics that occur at coarse, medium, and fine scales was collected for 144 riparian plots located throughout the Lake Tahoe Basin, which straddles the California-Nevada border in the western United States. Constrained and unconstrained ordination analyses were used to identify the most important correlates of physical form and plant species composition. Through multivariate analysis of environmental variables (principal components analysis), vegetation data (detrended correspondence analysis), and the combined relationship between the environmental and vegetation data (canonical correspondence analysis), we consistently found that the greatest variation occurred along a gradient of decreasing valley width, decreasing stream sinuosity, and increasing stream slope. Although surface characteristics reflected a 2nd important source of variation in physical conditions, plant species distribution was not strongly correlated with riparian surface conditions. Strong correlations among physical variables that occur at different scales, such as between valley form and geofluvial surface and between geofluvial surface and surface conditions, support the use of a physically based hierarchical framework for organizing riparian zones within the landscape. Such a hierarchical framework would be useful for interpreting patterns in riparian structure and process at different scales and could be applied to riparian zones in other mountain landscapes of the western United States and elsewhere. Moreover, our finding that riparian plant species composition is most strongly correlated with environmental variables that occur at coarse to moderate scales, most of which can be derived from existing data, supports the idea that modeling montane riparian community distribution using topographic and remotely sensed data could be useful; however, a large degree of species variation, unexplained by the variables we collected, indicates that other variables, perhaps disturbance regime, should be included in such a venture.

Nomenclature: Hickman (1993).

Twenty-eight sites along the Wasatch Front, north central Utah, representing the range of urban land use intensity for wadeable streams of the area, were sampled in September 2000. Fish communities were assessed by single-pass electrofishing, and physical habitat and water-quality characteristics were measured. On average, nonnative species comprised 54% of species richness and 53% of relative abundance, although only Salmo trutta and Pimephales promelas were very abundant at any 1 site. Salmo trutta and Catostomus platyrhynchus, a native species, were the most widely distributed and abundant species captured. Analysis of fish communities using nonmetric multidimensional scaling (NMDS) revealed a community gradient from sites dominated by Salmo trutta and Cottus species (C. bairdi, C. beldingi) to sites dominated by Catostomus platyrhynchus. Sites dominated by C. platyrhynchus were smaller in size and had less habitat cover, smaller average substrate size, higher concentrations of dissolved constituents, and higher water temperature than sites dominated by S. trutta. Sites dominated by C. platyrhynchus were located in more intensely urbanized watersheds at lower elevations. Stream size and associated instream habitat availability appear to limit S. trutta distribution and abundance, while native species appear more tolerant of decreased water quality and increased water temperatures in more urbanized streams. Most of the study sites are affected by development of water infrastructure for human water use (e.g., dams and diversions), and this infrastructure may play a role in dispersal of species.

There is a paucity of information on the effects of wildfire on carnivores. We studied the effects of a 237-km² catastrophic wildfire in the Mazatzal Mountains, Arizona, on gray foxes (Urocyon cinereoargenteus) and coyotes (Canis latrans). We indexed relative abundance 3 times each year from 1996 to1998 using scat transects in burned and unburned areas. We collected scats to estimate diet and measured small mammal abundance and mast availability in 1997 and 1998. We also measured vegetation cover in burned and unburned sites. Gray fox indices declined 3 months post-fire, but after 30 months, indices returned to preburn levels. Coyote indices did not change. Primary foods for both species in burned and unburned sites were mast (fruits of shrubs) and rodents, and diet comparisons between sites became similar as plant succession continued. Scat indices did not correlate with seasonal small mammal abundance. We suspect that the lower abundance indices of gray foxes were related to reduction in cover and food availability, because indices increased as vertical cover and mast crop increased. These data indicate that the effects of this catastrophic fire were short-term but also highlight the importance of preserving shrub and vegetation diversity for gray fox.

I compared acute tolerance of Gila topminnow, Poeciliopsis occidentalis, and western mosquitofish, Gambusia affinis, to ammonia and high water temperature and also compared diet, food selectivity, and impact on invertebrate populations to evaluate the potential for each species to effectively control mosquitoes (Culicidae) in wetland habitats. Critical thermal maxima differed among species, sexes, and lifestages, but these differences were likely not biologically significant due to their small magnitude and the ability of wild populations of both species to adapt to high temperatures. In 24-hour tests, G. affinis displayed about twice the ammonia tolerance as did P. occidentalis. However, long-term survival in habitats with high ammonia concentrations is questionable, thus reducing the perceived benefit of using G. affinis for mosquito abatement in these habitats. Diet overlap was significant between sexes within species but was not significant between species. However, both species exhibited significant dietary selection for mosquito larvae and pupae under experimental conditions. Shared dietary preference for mosquito larvae and pupae, in addition to similarity in temperature tolerance between species, suggests that P. occidentalis, a native and historically widespread species, can be used in place of nonnative G. affinis for mosquito control. There is no evidence to suggest that nonnative G. affinis is better suited for mosquito abatement, and since G. affinis is replacing P. occidentalis throughout the native species' former range, it should no longer be introduced into the Gila River basin for mosquito control. However, managers who would use P. occidentalis for mosquito abatement must take care not to interfere with conservation efforts by disrupting the genetic structure of natural populations of P. occidentalis.

In the southwestern United States, the nonnative athel pine (Tamarix aphylla) was presumed to be sterile and therefore not as likely to spread as its widely distributed, nonnative congener, T. ramosissima. However, at Lake Mead National Recreation Area (LMNRA) in southern Nevada, populations of T. aphylla have recently spread beyond their limited pre-1990 distribution and now form extensive monospecific stands. Over a 3-year period, we quantified seed production and germination from 60 T. aphylla trees at LMNRA. The annual mean seed production period was 50.6 days, and the mean potential germination (under laboratory conditions) was 22%, indicating that T. aphylla trees at LMNRA are capable of sexual reproduction in southern Nevada. No seeds germinated in field experiments, apparently because of high soil salt levels. However, seedling regeneration is becoming increasingly common at LMNRA. Tamarix aphylla trees occupied a distinct zone along the shoreline of Lake Mead, above T. ramosissima in elevation and below native Larrea tridentata communities, suggesting either competitive exclusion or differential resource utilization. The T. aphylla zone tended to have the highest mean values for total vegetation cover, leaf litter depth, soil salinity, soil moisture, pH, total Kjeldahl nitrogen, and soil organic matter. The capacity for sexual reproduction of this alien plant, combined with a suite of characteristics shared with the invasive T. ramosissima (e.g., drought tolerance and copious saline leaf litter), makes T. aphylla a potentially invasive species along the shores of LMNRA and other mesic areas in the desert Southwest.

Many desert perennials are long-lived, but there are few data on ages or population structures of desert plants. We used 2 methods to estimate ages of a population of Yucca brevifolia (Joshua tree) in southwestern Utah from a 14-year census of plant sizes. Plant height at the 1st census ranged from 0.08 m to 6.0 m, and trees grew in height at a mean rate of 3.75 cm · yr⁻¹. Plants also increased slowly in basal diameter (0.142 cm · yr⁻¹) and branch length (0.024 m · yr⁻¹), but basal diameter varied greatly from year to year, with many plants decreasing in diameter between sequential censuses. Forms of a nonlinear growth equation (the Richards function) did not fit the data on growth in height, but a simple linear relationship fit the data well. Using height as a linear predictor of age, we estimated that ages of a sample of 69 Joshua trees ranged from <20 to >300 years old, with 75% of the trees between 20 and 100 years and 11% over 150 years old. These data support the assumption that Joshua trees are indeed long-lived.

Species richness in Madrean mixed-grass prairies dominated by native or exotic species in southeastern Arizona was characterized at the community and point scales using ten 1-m² quadrats nested within each of eight 1000-m² plots. In the 1000-m² plots average richness was significantly higher in oak savanna (OS, 121.0 species) than in exotic grassland on mesa tops (EMT, 52.0 species), whereas native grassland on mesa slopes (NMS, 92.5 species) and native grassland on mesa tops (NMT, 77.0 species) did not differ significantly in richness from OS or EMT. When richness was partitioned by life form, EMT was notably poorer than other community types in species of perennial grasses, perennial herbs, and summer annuals. In the 1-m² quadrats, OS (21.2 species), NMS (20.9 species), and NMT (20.7 species) were significantly richer than EMT (5.9 species). Cover in 1-m² plots was significantly higher in EMT than in NMT, NMS, or OS. Species richness at the point scale showed a unimodal relation to canopy cover, with cover accounting for 30% of the variation in number of species in 1-m² quadrats. Competitive exclusion and allelopathy have perhaps limited species richness at the point scale in exotic grassland. There was no evidence of a species-pool effect between point and community scales, but such an effect between community and landscape scales was supported. Madrean mixed-grass prairies are landscapes with high species richness in comparison to other grassland types in North America, providing a large pool of potential colonizing species at the community scale. Beta-diversity (between communities) within the landscape of the Appleton-Whittell Research Ranch was consequently high despite a relative lack of habitat diversity.

This study examined the effect of livestock grazing on a common herbivore in semiarid grasslands of the Southwest: the rainbow grasshopper, Dactylotum variegatum. Population attributes and key environmental variables were compared between sites on active cattle ranches and sites on a 3160-ha ungrazed sanctuary. Although density of D. variegatum nymphs did not differ significantly between grazed and ungrazed sites, adult density was significantly lower on grazed sites, and sex ratios differed between grazed and ungrazed sites over time. Grazed sites had higher percentages of bare ground and fewer Baccharis pteronioides, a common shrub. However, only bare ground was highly correlated with adult D. variegatum density; lower adult densities were associated with more bare ground. At a smaller scale, D. variegatum tended to be more abundant in areas with B. pteronioides on grazed sites and less abundant in areas on un-grazed sites. This study suggests that cattle grazing influences ecological variables important in determining density, sex ratio, and spatial distribution of D. variegatum at both large and small spatial scales.

We investigated the temperature tolerance and growth of 2 federally protected freshwater gastropods from southern Idaho: Valvata utahensis and Pyrgulopsis idahoensis. Snails were collected in the Snake River and transported to the laboratory where they were kept under highly controlled conditions. In varying-temperature, short-duration experiments, the temperatures tolerated by both species were between 7°C and 34°C. In constant-temperature, long-duration experiments, growth rates were between 0.004 mm · d⁻¹ and 0.016 mm · d⁻¹, and we created growth curves for both species that were previously lacking. Our results are among the first to report temperature tolerances and growth rates of native Snake River snails, and we discuss implications for the biology and management of both species.

The swift fox (Vulpes velox) was classified in 1995 as a candidate for listing under the Endangered Species Act. Since then, several studies have addressed survey methods for monitoring swift fox populations. The purpose of this paper is to assist field researchers in documenting the presence of swift foxes by identification of recently active den sites. We propose that swift foxes have unique external den characteristics that can be distinguished during aerial surveys. We collected data from 30 swift fox den complexes on both continuous rangeland and landscapes fragmented by cropland in northwest Texas. There were no differences in den characteristics between landscape types (P > 0.10). The number of openings per den ranged from 1 to 8 and had an average opening height of 20 cm. As number of openings increased, the maximum distance between openings also increased (r = 0.88, P < 0.001). One of the most distinguishing characteristics of swift fox dens was their dirt tailing. Of 74 den openings examined, all but 3 had conspicuous dirt tailings that averaged 267.0 cm long by 63.4 cm wide. We found that swift fox den openings were smaller than those of coyotes (Canis latrans) and red foxes (Vulpes vulpes), 2 species that can be sympatric with swift foxes. We suggest that aerial den searching followed by ground verification may be an effective technique for determining presence of swift foxes in areas of short-grass prairies and fragmented habitats.

We observed a breeding Bald Eagle (Haliaeetus leucocephalus) pair nesting in a short-grass prairie and agricultural community on the southern Great Plains of the Texas Panhandle in 2004 and 2005. The nesting eagles produced 1 fledgling in 2004 and 2 fledglings in 2005. Our assessment of landcover types within a 5-km radius of the nest indicated that grasslands accounted for most of the area (90%), followed by agricultural lands (8%). Black-tailed prairie dog (Cynomys ludovicianus) colonies occupied 2.5% of the area, and single human residences with associated structures (i.e., barns) occupied <1%. The nearest source of permanent surface water >2.5 ha in surface area was 51 km from the nest. An analysis of regurgitated castings collected near the nest revealed a mammalian-dominated, breeding-season diet with black-tailed prairie dogs occurring in 80.9% of the castings. Other identified prey included cottontails (Sylvilagus spp., 15.9%), black-tailed jackrabbits (Lepus californicus, 3.2%), pronghorn (Antilocapra americana, 3.2%), and plains pocket gopher (Geomys bursarius, 1.6%). Bird remains were also present in 34.9% of the castings. This is the first reported successful nesting of Bald Eagles in the panhandle region of Texas since 1916; the nest is particularly unique because of its distance from any substantial body of water.