Marsilea mollis (Marsileaceae) sporocarps and associated insect parasitism in southern Arizona

We report observations on reproductive phenology, fecundity, and sporocarp morphology for a population of the semiaquatic fern Marsilea mollis B.L. Rob. & Fernald in southern Arizona and document the parasitic relationships of 2 insects collected with the plants. The recently described weevil Notiodes sporocarpicus O’Brien (Coleoptera: Curculionidae) utilizes the fern’s sporocarp as a brood chamber, only the second documented instance of this form of parasitism in the United States. The wasp Lyrcus maculatus (Gahan) (Hymenoptera: Pteromalidae) is a parasitoid of the weevil in this system.     

Habitat features and predictive habitat modeling for the Colorado chipmunk in southern New Mexico

Two subspecies of Colorado chipmunk (state threatened and federal species of concern) occur in southern New MeXico: Tamias quadrivittatus australis in the Organ Mountains and T. q. oscuraensis in the Oscura Mountains. We developed a GIS model of potentially suitable habitat based on vegetation and elevation features, evaluated site classifications of the GIS model, and determined vegetation and terrain features associated with chipmunk occurrence. We compared GIS model classifications with actual vegetation and elevation features measured at 37 sites. At 60 sites we measured 18 habitat variables regarding slope, aspect, tree species, shrub species, and ground cover. We used logistic regression to analyze habitat variables associated with chipmunk presence/absence. All (100%) 37 sample sites (28 predicted suitable, 9 predicted unsuitable) were classified correctly by the GIS model regarding elevation and vegetation. For 28 sites predicted suitable by the GIS model, 18 sites (64%) appeared visually suitable based on habitat variables selected from logistic regression analyses, of which 10 sites (36%) were specifically predicted as suitable habitat via logistic regression. We detected chipmunks at 70% of sites deemed suitable via the logistic regression models. Shrub cover, tree density, plant proximity, presence of logs, and presence of rock outcrop were retained in the logistic model for the Oscura Mountains; litter, shrub cover, and grass cover were retained in the logistic model for the Organ Mountains. Evaluation of predictive models illustrates the need for multi-stage analyses to best judge performance. Microhabitat analyses indicate prospective needs for different management strategies between the subspecies. Sensitivities of each population of the Colorado chipmunk to natural and prescribed fire suggest that partial burnings of areas inhabited by Colorado chipmunks in southern New Mexico may be beneficial. These partial burnings may later help avoid a fire that could substantially reduce habitat of chipmunks over a mountain range.     

Sperm viability in the reproductive tract of females in a population of Sceloporus mucronatus exhibiting asynchronous reproduction

Asynchronous reproduction is a common phenomenon in high-elevation populations of lizards from Central México. Sperm storage in the reproductive tract of females is the mechanism for making oocyte fertilization possible. Our study addresses questions related to functional oviductal sperm storage of females mating on different dates throughout the reproductive season. A population of Sceloporus mucronatus with copulation in the summer and ovulation in the fall was chosen for this experiment. Eleven females that copulated in the field during June and 13 females that copulated in captivity during August were maintained in the laboratory until parturition. The number of pregnant females and the litter sizes produced in each experimental group were indicative of the viability of the stored sperm. Sperm stored in the reproductive tract of females were able to fertilize eggs after 4 months. No significant differences were found in the number of pregnant females between the 2 experimental groups nor in the litter sizes that they produced. We found that the amount of time sperm were held in the female reproductive tract (ca. 3 months) had no effect on the capacity of sperm to fertilize eggs. Histological examination of 8 oviducts collected before the mating season eliminated the possibility of sperm storage from one year to the next. In this system, sperm retention could have evolved as a response mechanism to deal with the asynchrony between sexes in the reproductive cycles. However, we cannot rule out alternative hypotheses.     

Carbon isotope discrimination in the C 4 shrub Atriplex confertifolia along a salinity gradient

Carbon isotope discrimination (Δ) was measured for leaves of Atriplex confertifolia along a salinity gradient in northern Utah. Over this gradient, the variation of Δ values was high for a C 4 species, and the Δ values were positively correlated with salinity in both years of study. Of the possible explanations for this patter, the Δ results are consistent with the notion that salinity induces an increase in the bundle sheath leakiness of these C 4 plants.     

Shorebird predation on benthic macroinvertebrates in an irrigation reservoir

American Falls Reservoir in southeastern Idaho is an irrigation reservoir used as an inland feeding stopover by many shorebird species. Six exclosure experiments were conducted during the 1990 drawdown period to investigate shorebird predation impact on benthic macroinvertebrate populations. The study sites differed in sediment composition, sediment slope, invertebrate densities, and shorebird abundance. Shorebird predation significantly affected invertebrate densities in only 1 of 6 experiments (Aberdeen Mouth). This site had higher sediment slope and a lower water recedence than other study sites, resulting in concentration of shorebird predation on a small area of newly exposed sediment. Shorebird predation had the greatest impact on medium size class chironomid larvae at Aberdeen Mouth. Our results suggest that inland sites such as American Falls Reservoir represent viable shorebird habitat and may be managed to insure consistent prey availability. Drawdown rate, sediment slope, invertebrate densities, and shorebird abundance are all important factors influencing shorebird predation. Monitoring shorebird abundance and predation impact on invertebrate densities may help in manipulating drawdown rate to provide adequate shorebird prey. Management of inland sites for shorebird use may become more important in the future as human encroachment in coastal areas continues.     

Biotic and abiotic factors influencing the distribution of Coleogyne communities in southern Nevada

Woody plant community composition was analyzed throughout the range of Coleogyne ramosissima in the Spring and Sheep Mountain ranges of southern Nevada. The lower Coleogyne elevational boundary was analyzed in detail in Lucky Strike Canyon, on the eastern edge of the Spring Mountains. TWINSPAN (2-way indicator species analysis) identified 4 primary species and stand groups from the 2 mountain ranges (extensive survey). Analysis of DECORANA (detrended correspondence analysis) results indicated that elevation and soil depth were the environmental factors most significantly associated with distribution of species and stand groups in the extensive survey. Elevation was the only significant physical factor associated with distribution of species and stand groups in the intensive survey. Five vegetation zones form the 2 mountain ranges were identified based on their dominant species in 15 extensive transects. Coleogyne were subdivided into pure stands and upper and lower ecotones for further investigation of species distribution and environmental factors. Selected environmental factors appear to play an important role in structuring the Mojave Desert vegetation zones in southern Nevada.     

Reproductive ecology of bison on Antelope Island, Utah

Autumn musters of bison ( Bison bison ) on Antelope Island State Park, Utah, conducted annually since 1987, provided data on temporal and age-specific reproductive patterns and a basis to evaluate the efficacy of management measures implemented to elevate reproductive performance in the herd. Pregnancy rates were variable and low ( x = 46.2%) in comparison to other free-ranging, noncommercial bison herds in North America. Cows in the 3- and > 6-yr range age classes exhibited lower-than-expected pregnancy rates ( P r = 0.64, P = 0.047) linear decline of 2.5% per annum. Variance in distribution of fetal ages observed in 6 yr indicates substantial temporal fluctuation. Long-term reproductive performance of cohorts born prior to implementation of management measures did not differ from that of cohorts born subsequent to these changes.     

Understory patterns in cut western juniper ( Juniperus occidentalis spp. occidentalis Hook.) woodlands

Western juniper ( Juniperus occidentalis spp. occidentalis ) has rapidly expanded into shrub steppe communities in the intermountain Northwest during the past 120 yr. Cutting juniper is a management tool used to restore shrub steppe communities. Response of the understory after cutting is strongly influenced by plant species composition existing prior to treatment. This study assessed distribution patterns of understory plants over 2 growing seasons after tree cutting in a western juniper woodland. Cover, density, and diversity of understory species were compared among 3 locations: interspaces, duff zones (previously under tree canopies), and debris zones (beneath cut trees). Plant cover density increased in all zones following tree cutting. Understory vegetation in cut woodlands exhibited strong zonal distribution. Cover and density of Poa sandbergii and Sitanion hystrix and canopy cover of annual forbs were greatest in duff zones ( P P < 0.05). Debris zones tended to have the lowest overall understory cover and plant density values. Under juniper debris many species common to interspaces were reduced in density, although plants that survived or established beneath debris grew larger than their counterparts in interspaces. Species increased in density and cover under debris were plants characteristic of duff zones and whose seeds are typically wind dispersed.     

Centric diatoms of Lake Tahoe

An understanding of the mechanisms of phytoplankton species interaction is dependent on a precise knowledge of what species exist within the community. The centric diatoms of Lake Tahoe, California-Nevada, which are often the dominant component of the phytoplankton community, are presented in both light and scanning electron microscopy (SEM) photographs. Specific attention has been given to initial cell forms of Cyclotella stelligera Cleve and Grunow and C. comta (Ehrenberg) K&uuml;tzing through the aid of the SEM.     

Grassland bird associations with introduced and native grass Conservation Reserve Program fields in the Southern High Plains

We examined relative abundances of grassland birds among Conservation Reserve Program (CRP) fields seeded with 2 monocultures of introduced grass species and 2 mixes of native grasses in the Southern High Plains of Texas. We assessed bird compositions among these 4 cover types and between the cover types pooled into categories of introduced and native fields. Breeding season bird diversity and total abundance did not differ among cover types or between introduced and native fields. Grasshopper Sparrows ( Ammodramus savannarum ), Cassin&rsquo;s Sparrows ( Aimophila cassinii ), and Western Meadowlarks ( Sturnella neglecta ) accounted for more than 90% of breeding season detections. Grasshopper Sparrows were the most abundant and found in all cover types. Cassin&rsquo;s Sparrows were 38% to 170% more abundant among the native seed mix without buffalograss ( Buchlo&euml; dactyloides ) compared to 3 other cover types. Although this association was statistically lost when cover types were pooled into introduced or native fields ( U = 93.5, P = 0.91), the species was still 50% more abundant among native CRP than introduced CRP fields. Meadowlarks occurred ubiquitously but at very low numbers during the breeding season. During winter, avian abundance was 44% greater among native CRP than introduced CRP fields. Meadowlarks, Horned Larks ( Eremophila alpestris ), and Savannah Sparrows ( Passerculus sandwichensis ) accounted for 94% of all winter detections. Meadowlarks occurred ubiquitously, but Horned Larks and Savannah Sparrows were 157% and 96% more abundant, respectively, among native CRP than introduced CRP fields. Our data suggest that monocultures of introduced grasses may benefit some bird species but also that native seed mixes may have a more positive influence through increased diversity and abundance of grassland birds. However, pooling cover types into the broader categories of introduced or native grasses may dampen or occlude biologically meaningful results. It may be prudent to avoid broad categorization of CRP fields based solely on native or introduced grass cover when assessing habitat associations of grassland birds.