American Deserts Research Articles

Intensified dust storm activity and Valley fever infection in the southwestern United States.

Climate models have consistently projected a drying trend in the southwestern United States, aiding speculation of increasing dust storms in this region. Long‐term climatology is essential to documenting the dust trend and its response to climate variability. We have reconstructed long‐term dust climatology in the western United States, based on a comprehensive dust identification method and continuous aerosol observations from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. We report here direct evidence of rapid intensification of dust storm activity over American deserts in the past decades (1988–2011), in contrast to reported decreasing trends in Asia and Africa. The frequency of windblown dust storms has increased 240% from 1990s to 2000s. This dust trend is associated with large‐scale variations of sea surface temperature in the Pacific Ocean, with the strongest correlation with the Pacific Decadal Oscillation. We further investigate the relationship between dust and Valley fever, a fast‐rising infectious disease caused by inhaling soil‐dwelling fungus (Coccidioides immitis and C. posadasii) in the southwestern United States. The frequency of dust storms is found to be correlated with Valley fever incidences, with a coefficient (r) comparable to or stronger than that with other factors believed to control the disease in two endemic centers (Maricopa and Pima County, Arizona).

The effects of urbanization and human disturbance on problem solving in juvenile house finches (Haemorhous mexicanus)

Urbanization exposes wildlife to unfamiliar environments, including novel structures and food sources. Adapting to such anthropogenic conditions may require superior innovation and problem-solving skills (e.g., for navigating, foraging). Human presence in urban areas is a particular biotic challenge that may impact problem-solving capabilities in wildlife. Urban animals may be superior problem solvers in the face of human disturbance, due to familiarity with—and reduced fear of—humans. Alternately, rural animals may be better innovators if heightened fear responses from unfamiliar humans facilitate problem solving. Here, we studied the relationship between human disturbance, urbanization, and the ability to solve a novel foraging problem in wild-caught juvenile house finches (Haemorhous mexicanus). This songbird is a successful urban dweller and native of the American desert southwest. In captivity, juvenile finches from urban and rural populations were presented with a novel foraging task (sliding a lid covering their typical food dish) and then exposed to regular periods of high or low human disturbance over several weeks before again being presented with the task. We found that rural, high-disturbance birds showed reduced task performance compared to highly disturbed urban finches. This is consistent with the hypothesis that urban birds are less affected by human disturbance due to habituation or adaptation. Additionally, we found that the best behavioral predictor of solving success was related to visual inspection of the problem and that urban low-disturbance birds exhibited this behavior more than rural high-disturbance birds. Overall, these findings suggest that urbanization and habituation to humans predict avian response to novel problems. As Earth’s landscapes become more urbanized, wildlife is exposed to many new anthropogenic challenges. In particular, animals in cities may have to locate food in unique ways and/or cope with increased human presence. We tested the effects of urbanization and human disturbance on problem solving in juveniles of a common urban and rural songbird (the house finch, H. mexicanus). We found that, when subjected to high levels of human disturbance, urban birds outperformed rural conspecifics on a novel foraging task (sliding open a lid to uncover food). We also found that task focus (i.e., average bout length of inspecting the dish) was an important predictor of problem solving. Our results suggest that acclimation to humans may be important for the superior behavioral performance of urban house finches and ultimately their colonization of many North American cities.

Ancient twined garments of fur, feather, and fiber: Context and variation across the American Desert West

Ancient perishable textiles, once overlooked in favor of other material classes, now are recognized as revealing indicators of style, function, and technology, importantly shaped by complex social and economic processes. With adequate preservation and concerted dating efforts aimed specifically at textiles, archaeologists are asking new questions about technological and stylistic variations and their ability to highlight once obscure social forces, such as learning modes, idiosyncratic behaviors, social interactions and identities, population mobility, and more. Here, I review potential sources of variation in the production and use of fur, feather and fiber open-twined robes/blankets (and their associated cordage components) by ethnohistoric and prehistoric mobile foragers, and semi-settled and sedentary farmers of the American Desert West. With a qualitative approach, I assess the proposition that patterned variation within and between foraging and farming cultures, each with different mobility patterns and diverse social structures, is observable in the mechanical details of such textiles, and that such variation can inform us at different scales of inquiry. Examining limited samples of preserved textiles from dry caves, cliff dwellings, and pueblos, now housed in western museum collections, I consider evidence for variation and regularity in cordage and twined textile production mechanics at the level of the individual, site, community, and region. Finally, I note the persistence and relevance of ancient textile traditions in contemporary contexts of identity.

Seed removal in a tropical North American desert: an evaluation of pre- and post-dispersal seed removal in Stenocereus stellatus.

To determine seed removal influence on seed populations, we need to quantify pre- and post-dispersal seed removal. Several studies have quantified seed removal in temperate American deserts, but few studies have been performed in tropical deserts. These studies have only quantified pre- or post-dispersal seed removal, thus underestimating the influence of seed removal. We evaluated pre- and post-dispersal seed removal in the columnar cactus Stenocereus stellatus in a Mexican tropical desert. We performed selective exclosure experiments to estimate percentage of seeds removed by ants, birds and rodents during the pre- and post-dispersal phases. We also conducted field samplings to estimate abundance of the most common seed removers. Birds (10-28%) removed a higher percentage of seeds than ants (2%) and rodents (1-4%) during pre-dispersal seed removal. Melanerpes hypopolius was probably the main bird removing seeds from fruits. Ants (62-64%) removed a higher percentage of seeds than birds (34-38%) and rodents (16-30%) during post-dispersal seed removal. Pogonomyrmex barbatus was probably the main ant removing seeds from soil. Birds and ants are the main pre- and post-dispersal seed removers in S.stellatus, respectively. Further studies in other S.stellatus populations and plants with different life forms and fruit types will contribute to evaluate seed removal in tropical American deserts.

Genomic characterization of Ensifer aridi, a proposed new species of nitrogen-fixing rhizobium recovered from Asian, African and American deserts

BackgroundNitrogen fixing bacteria isolated from hot arid areas in Asia, Africa and America but from diverse leguminous plants have been recently identified as belonging to a possible new species of Ensifer (Sinorhizobium). In this study, 6 strains belonging to this new clade were compared with Ensifer species at the genome-wide level. Their capacities to utilize various carbon sources and to establish a symbiotic interaction with several leguminous plants were examined.ResultsDraft genomes of selected strains isolated from Morocco (Merzouga desert), Mexico (Baja California) as well as from India (Thar desert) were produced. Genome based species delineation tools demonstrated that they belong to a new species of Ensifer. Comparison of its core genome with those of E. meliloti, E. medicae and E. fredii enabled the identification of a species conserved gene set. Predicted functions of associated proteins and pathway reconstruction revealed notably the presence of transport systems for octopine/nopaline and inositol phosphates. Phenotypic characterization of this new desert rhizobium species showed that it was capable to utilize malonate, to grow at 48 °C or under high pH while NaCl tolerance levels were comparable to other Ensifer species. Analysis of accessory genomes and plasmid profiling demonstrated the presence of large plasmids that varied in size from strain to strain. As symbiotic functions were found in the accessory genomes, the differences in symbiotic interactions between strains may be well related to the difference in plasmid content that could explain the different legumes with which they can develop the symbiosis.ConclusionsThe genomic analysis performed here confirms that the selected rhizobial strains isolated from desert regions in three continents belong to a new species. As until now only recovered from such harsh environment, we propose to name it Ensifer aridi. The presented genomic data offers a good basis to explore adaptations and functionalities that enable them to adapt to alkalinity, low water potential, salt and high temperature stresses. Finally, given the original phylogeographic distribution and the different hosts with which it can develop a beneficial symbiotic interaction, Ensifer aridi may provide new biotechnological opportunities for degraded land restoration initiatives in the future.

Allergic to the Twentieth Century: Intentional Communities and Therapeutic Landscapes in The Village and Safe

The concept of therapeutic landscapes has been used to explore diverse spaces and places of healing or wellness, from hospitals to gardens, libraries to smoking areas. A central strand of this work considers rural and/or natural landscapes as affording particular healing experiences. In this paper, I draw on this lineage of work alongside research into the formation of intentional communities in rural settings and the body of writing on representations of rural landscapes and country life. The two representations I analyse are films: The Village (M. Night Shyamalan, 2004) and Safe (Todd Haynes, 1995). In the former, an apparent settler village in rural Pennsylvania is revealed, in the film’s denouement, as an intentional community built as a retreat from the violence of contemporary urban life, guarded by Elders and a shared mythology about border-policing creatures. In Safe, the health hazards of modern suburban living, which lead the central character to develop multiple chemical sensitivity (MCS), can only be escaped by a similar retreat to a wilderness commune in the American desert. In both films, the spaces of rural life are constructed as therapeutic landscapes through their nostalgic, anti-modern withdrawal, and their protective boundary keeping.

Assessing Water Use Variation Among the Cytotypes of the Autopolyploid Southwestern Desert Creosotebush (Larrea tridentata[DC.] COVILLE: ZYGOPHYLLACEAE)

Genome duplication, or polyploidy, has played an important role in the diversification of flowering plants, but the ecological and evolutionary consequences of polyploidy still remain unclear. Polyploidy is known to either cause or facilitate phenotypic changes, and ploidy-specific phenotypic differences may lead to the exploitation of novel niche space. Many studies reporting phenotypic variation between cytotypes have been observational, and may not reflect ecological adaptation by diploids and polyploids to different habitat. One such trait, water use, may play an outsized role in survival and population expansion into novel habitat. To test whether water use differs among ploidy levels, we grew field-collected diploid, autotetraploid, and autohexaploid cytotypes of the characteristic North American desert plant, Larrea tridentata (DC.) Coville, under greenhouse conditions. We measured whole plant water use gravimetrically over six wk, standardizing water use with measures of total stomatal area. We found that water use was positively correlated with stomatal area, but the cytotypes had similar total stomatal areas and did not differ significantly in mean water use/total stomatal area. Cytotype-specific water use responses through time were also not significantly different. Taken together, these results suggest that the cytotypes have similar water relationships, and possibly fitness outcomes, with respect to water use in common environments.

Musings on the Southwest as a Region by a Southwesterner

Musings on the Southwest as a Region by a Southwesterner David Yetman (bio) Defining the Southwest is intuitively simple but practically difficult. Inevitably, regional disagreements about inclusion or exclusion erupt, cultural divergences complicate, physiographic boundaries interfere, and political boundaries deceive. Generous definitions include either all of New Mexico or only that portion west of its eastern plains, the Llano Estacado. How much, if any, of California, Colorado, Nevada, Texas, and Utah to include gives rise to xenophobic sentiments. Chihuahua and Sonora, Mexico have strong affinities for the American Southwest, culturally, economically, and physiographically, but just where to draw the southern line is an elusive problem. Arizona alone lies unequivocally and uncontestably within the Southwest. The definition is complicated by a century or so of shameless promotion of a region championed by hucksters, developers, and investors eager to capitalize on regional images they connived to create. But at the same time it is a region that most Americans would view as our most exotic and, perhaps, romantic. For purposes of this essay I will adopt the definition offered and argued by James W. Byrkit in his profound and lengthy 1992 essay The Southwest Defined, published in the Journal of the Southwest.1 Byrkit, after a sustained and nuanced discussion, proposed the boundaries as the area encompassed by 29 degrees to 39 degrees latitude north and 104 degrees to 117 degrees longitude west. It stretches from the east slope of the mountains of eastern New Mexico to slightly beyond the western edge of the deserts of Southern California. It includes portions of Mexico’s northern Baja California, Chihuahua, and Sonora to the northern limits of the Mojave Desert, much of southern Nevada, the Slickrock Country of Utah, and a representative [End Page 101] slice of southern Colorado. This definition is satisfying to many scholars of the Southwest, for it includes virtually all the cultures, contemporary and pre-Columbian, that comprise the more popular conception of the Southwest and the most notable physiographic features, the most prominent landmarks, those basic definitions of place, and the loci of native peoples who provide the history and content to southwestern heritage, are included. Even Texans are afforded their appropriate inclusion—El Paso, historically prominent, is retained, and the Guadalupe Mountains sneak through the eastern boundary. The California inclusion does justice to the Salton Sink and Death Valley, the grandest, hottest, and lowest of drylands. The 39-degree north and 104-degree west coordinates extend well into Colorado. For better for worse, Las Vegas, Nevada, is in. Byrkit’s definition is marginally arbitrary—the limits could be adjusted a few global minutes in any direction without doing violence to the idea of the Southwest. I would advocate setting the western limit at 116.5 degrees west longitude, rather than at 117 degrees, since 117 degrees just barely excludes San Diego and Tijuana and includes too much of Southern California’s urban mass for my liking. But his boundaries seem to satisfy the intuitions of those who study the region in detail. For example, the Anza-Borrego Desert is justifiably southwestern and the Mojave National Preserve clearly belongs to the Southwest, but the Great Basin and the Sierra Nevada do not.2 And Byrkit’s Southwest offers an enormity of topics to the scholar and the essayists, an unending gradient of objects of fascination and complexity, in a background of traditional southwestern imagery. The Southwest is home to the most celebrated geological phenomena, the most prototypical deserts, the greatest concentration of present Indigenous peoples of any region, and offers unmatched heat, drought, and biological diversity. Included, quite logically, are the entirety of the north-south reach of the Río Grande with its gathering of Puebloan peoples and its rift-related lavas and mountain ranges, including the San Luis Basin in Colorado, the river’s headwaters. The Grand Canyon and its associated Colorado Plateau tableau of geological spectacles and revelations lie inside; the massive San Juan and Sangre de Cristo ranges of southwest Colorado are in as well, as are much of the four great American Deserts: the Chihuahuan, Great Basin, Mojave, and Sonoran. The boundaries enclose the most spectacular American archaeological sites. By the inclusion of Sonora...

Quantifying the human influence on fire ignition across the western USA.

Humans have a profound effect on fire regimes by increasing the frequency of ignitions. Although ignition is an integral component of understanding and predicting fire, to date fire models have not been able to isolate the ignition location, leading to inconsistent use of anthropogenic ignition proxies. Here, we identified fire ignitions from the Moderate Resolution Imaging Spectrometer (MODIS) Burned Area Product (2000-2012) to create the first remotely sensed, consistently derived, and regionally comprehensive fire ignition data set for the western United States. We quantified the spatial relationships between several anthropogenic land-use/disturbance features and ignition for ecoregions within the study area and used hierarchical partitioning to test how the anthropogenic predictors of fire ignition vary among ecoregions. The degree to which anthropogenic features predicted ignition varied considerably by ecoregion, with the strongest relationships found in the Marine West Coast Forest and North American Desert ecoregions. Similarly, the contribution of individual anthropogenic predictors varied greatly among ecoregions. Railroad corridors and agricultural presence tended to be the most important predictors of anthropogenic ignition, while population density and roads were generally poor predictors. Although human population has often been used as a proxy for ignitions at global scales, it is less important at regional scales when more specific land uses (e.g., agriculture) can be identified. The variability of ignition predictors among ecoregions suggests that human activities have heterogeneous impacts in altering fire regimes within different vegetation types and geographies.

An ecoregion assessment of projected tree species vulnerabilities in western North America through the 21st century

Forest ecosystems across western North America will likely see shifts in both tree species dominance and composition over the rest of this century in response to climate change. Our objective in this study was to identify which ecological regions might expect the greatest changes to occur. We used the process-based growth model 3-PG, to provide estimates of tree species responses to changes in environmental conditions and to evaluate the extent that species are resilient to shifts in climate over the rest of this century. We assessed the vulnerability of 20 tree species in western North America using the Canadian global circulation model under three different emission scenarios. We provided detailed projections of species shifts by including soil maps that account for the spatial variation in soil water availability and soil fertility as well as by utilizing annual climate projections of monthly changes in air temperature, precipitation, solar radiation, vapor pressure deficit and frost at a spatial resolution of one km. Projected suitable areas for tree species were compared to their current ranges based on observations at >40000 field survey plots. Tree species were classified as vulnerable if environmental conditions projected in the future appear outside that of their current distribution ≥70% of the time. We added a migration constraint that limits species dispersal to <200myr-1 to provide more realistic projections on species distributions. Based on these combinations of constraints, we predicted the greatest changes in the distribution of dominant tree species to occur within the Northwest Forested Mountains and the highest number of tree species stressed will likely be in the North American Deserts. Projected climatic changes appear especially unfavorable for species in the subalpine zone, where major shifts in composition may lead to the emergence of new forest types.

A temporally dynamic approach for cladistic biogeography and the processes underlying the biogeographic patterns of North American deserts

We implemented a temporally dynamic approach to the cladistic biogeographic analysis of 13 areas of North American deserts and several plant and animal taxa. We undertook a parsimony analysis of paralogy-free subtrees based on 43 phylogenetic hypotheses of arthropod, vertebrate and plant taxa, assigning their nodes to three different time slices based on their estimated minimum ages: Early-Mid-Miocene (23−7 Ma), Late Miocene/Pliocene (6.9−2.5 Ma) and Pleistocene (2.4−0.011 Ma). The analyses resulted in three general area cladograms, one for each time slice, showing different area relationships. They allowed us to detect influences of different geological and palaeoclimatological events of the Early-Mid-Miocene, Late Miocene/Pliocene and Pleistocene that might have affected the diversification of the desert biota. Several diversification events in the deserts of North America might have been driven by Neogene uplift, marine incursion and the opening of the California Gulf during the Miocene–Pliocene, whereas climatic fluctuations had the highest impact during the Pleistocene.

Ecosystem Services and Disservices for a Vulnerable Population: Findings from Urban Waterways and Wetlands in an American Desert City

Vulnerable human populations are exposed to social and biophysical stressors, but have limited capacity to mitigate them, and thus may access ecosystem services in unconventional ways. As a result of this access, they may also experience disservices (i.e., functions of ecosystems harmful to human wellbeing) in ways that are not well understood. We use a mixed-method socio-ecological approach to examine how persons experiencing homelessness in Phoenix, Arizona, access ecosystem services and encounter disservices in urban waterways. We find that urban waterways provide users with drinking and bathing water, and cooler, shaded areas, but potentially expose them to pathogens and legal persecution. The wetlands provide cultural services by affording a sense of place and safety; however, these locations can also be associated with restrictive ordinances and aggressive law enforcement. This study explores the role of ecosystem services and disservices in bridging the gap between biophysical and social vulnerability.

Zooming out from archaeological discontinuities: The meaning of mid-Holocene temporal troughs in South American deserts

Building on previous research at smaller scales, in this paper we assemble paleoecological data and archaeological time-series for deserts located in three latitudinal bands along the South American Arid Diagonal (16°–41°S, ∼1,236,000km2 of area). Diverse proxies suggest the existence of arid and extremely arid conditions in large parts of these deserts. Working with a database composed of 914 archaeological dates falling between the first human presence in each region and 3000years BP, which produce a minimum number of 578 occupational events, we identify a series of patterns at a macro-regional scale: a robust increase in the temporal signal at the beginning of the mid-Holocene (8000–7600years BP) followed by two troughs (7600–7200, 6800–6400years BP) during this period. The spatial scope of the data presented provides an opportunity for disentangling processes of spatial re-localization from actual changes in population size. We present a demographic hypothesis at a macro-regional scale, which suggests the existence of mid-Holocene population bottleneck(s). This hypothesis would account not only for the mid-Holocene troughs, but also for the posterior record of an intense and relatively rapid population growth (release) observed in many regions of the arid diagonal. These mid-Holocene events provide the context for independent trajectories of economic intensification based on different sets of resources -marine foods, camelids, and also probably wetland resources-, some of which lead to domestication processes. These cases occur in association with a tendency towards reduced residential mobility in regions that may have acted as refugia during arid periods of the mid-Holocene.The analysis produces testable expectations for future research at different scales and for different research domains, including human DNA and morphometric evidence. We consider that these issues have a fecund comparative potential, since the analysis of the socio-demographic meaning of archaeological discontinuities in different continents shares a similar conceptual structure.

Including Fossils in Phylogenetic Climate Reconstructions: A Deep Time Perspective on the Climatic Niche Evolution and Diversification of Spiny Lizards (Sceloporus).

Fossils and other paleontological information can improve phylogenetic comparative method estimates of phenotypic evolution and generate hypotheses related to species diversification. Here, we use fossil information to calibrate ancestral reconstructions of suitable climate for Sceloporus lizards in North America. Integrating data from the fossil record, general circulation models of paleoclimate during the Miocene, climate envelope modeling, and phylogenetic comparative methods provides a geographically and temporally explicit species distribution model of Sceloporus-suitable habitat through time. We provide evidence to support the historic biogeographic hypothesis of Sceloporus diversification in warm North American deserts and suggest a relatively recent Sceloporus invasion into Mexico around 6 Ma. We use a physiological model to map extinction risk. We suggest that the number of hours of restriction to a thermal refuge limited Sceloporus from inhabiting Mexico until the climate cooled enough to provide suitable habitat at approximately 6 Ma. If the future climate returns to the hotter climates of the past, Mexico, the place of highest modern Sceloporus richness, will no longer provide suitable habitats for Sceloporus to survive and reproduce.

Phylogenetic community structure of North American desert bats: influence of environment at multiple spatial and taxonomic scales.

Numerous processes influence community structure. The relative importance of these processes is thought to vary with spatial, temporal and taxonomic scales: density-dependent interactions are thought to be most important at small scales; at intermediate scales, environmental conditions may be the most influential factor; and biogeographic processes are thought to be of greater importance at larger scales. Additionally, the stress-dominance hypothesis suggests that communities experiencing harsher environmental conditions will be predominantly structured by habitat filtering, whereas communities experiencing more favourable conditions will be structured predominantly by density-dependent interactions such as competition. The aim of this study was to investigate the influence of environmental factors on phylogenetic community structure (PCS) of North American desert bats at multiple spatial and taxonomic scales. We also examined whether the stress-dominance hypothesis is upheld in desert bats across an environmental gradient. Phylogenetic community structure metrics were calculated using species pools that differed in spatial (from all deserts to individual deserts) and taxonomic (all bat taxa, a single family and a single genus) scales. We calculated mean temperature, precipitation and seasonality for each site to determine whether environmental gradients were related to degree of community structure. At the largest spatial and taxonomic scales, communities were significantly phylogenetically clustered while degree of clustering decreased at the smallest spatial and taxonomic scales. Climatic data, particularly mean temperature and temperature seasonality, were important predictors of PCS at larger scales and under harsher conditions, but at smaller scales and in less stressful conditions there was a weaker relationship between PCS and climate. This suggests that North American deserts, while harsh, are not uniform in the challenges they present to the faunas residing in them. Overall, the relationship between PCS and climatic data at large spatial and taxonomic scales, and in harsher conditions, suggests the influence of habitat filtering has been important in North American desert bat community assembly and that other processes have been important at smaller scales.

Local Population Structure and Patterns of Western Hemisphere Dispersal for Coccidioides spp., the Fungal Cause of Valley Fever.

ABSTRACTCoccidioidomycosis (or valley fever) is a fungal disease with high morbidity and mortality that affects tens of thousands of people each year. This infection is caused by two sibling species, Coccidioides immitis and C. posadasii, which are endemic to specific arid locales throughout the Western Hemisphere, particularly the desert southwest of the United States. Recent epidemiological and population genetic data suggest that the geographic range of coccidioidomycosis is expanding, as new endemic clusters have been identified in the state of Washington, well outside the established endemic range. The genetic mechanisms and epidemiological consequences of this expansion are unknown and require better understanding of the population structure and evolutionary history of these pathogens. Here we performed multiple phylogenetic inference and population genomics analyses of 68 new and 18 previously published genomes. The results provide evidence of substantial population structure in C. posadasii and demonstrate the presence of distinct geographic clades in central and southern Arizona as well as dispersed populations in Texas, Mexico, South America, and Central America. Although a smaller number of C. immitis strains were included in the analyses, some evidence of phylogeographic structure was also detected in this species, which has been historically limited to California and Baja, Mexico. Bayesian analyses indicated that C. posadasii is the more ancient of the two species and that Arizona contains the most diverse subpopulations. We propose a southern Arizona-northern Mexico origin for C. posadasii and describe a pathway for dispersal and distribution out of this region.

Reservoirs and water management influence fish mercury concentrations in the western United States and Canada

Anthropogenic manipulation of aquatic habitats can profoundly alter mercury (Hg) cycling and bioaccumulation. The impoundment of fluvial systems is among the most common habitat manipulations and is known to increase fish Hg concentrations immediately following impoundment. However, it is not well understood how Hg concentrations differ between reservoirs and lakes at large spatial and temporal scales or how reservoir management influences fish Hg concentrations. This study evaluated total Hg (THg) concentrations in 64,386 fish from 883 reservoirs and 1387 lakes, across the western United States and Canada, to assess differences between reservoirs and lakes, as well as the influence of reservoir management on fish THg concentrations. Fish THg concentrations were 1.4-fold higher in reservoirs (0.13±0.011μg/g wet weight±standard error) than lakes (0.09±0.006), though this difference varied among ecoregions. Fish THg concentrations were 1.5- to 2.6-fold higher in reservoirs than lakes of the North American Deserts, Northern Forests, and Mediterranean California ecoregions, but did not differ between reservoirs and lakes in four other ecoregions. Fish THg concentrations peaked in three-year-old reservoirs then rapidly declined in 4–12year old reservoirs. Water management was particularly important in influencing fish THg concentrations, which were up to 11-times higher in reservoirs with minimum water storage occurring in May, June, or July compared to reservoirs with minimum storage occurring in other months. Between-year changes in maximum water storage strongly influenced fish THg concentrations, but within-year fluctuations in water levels did not influence fish THg concentrations. Specifically, fish THg concentrations increased up to 3.2-fold over the range of between-year changes in maximum water storage in all ecoregions except Mediterranean California. These data highlight the role of reservoir creation and management in influencing fish THg concentrations and suggest that water management may provide an effective means of mitigating Hg bioaccumulation in some reservoirs.

Evaluating the influence of connectivity and distance on biogeographical patterns in the south‐western deserts of North America

AbstractAimTo examine the role of geological history, connectivity and distance in shaping the biogeographical structure of North American desert clades that are restricted to habitat islands (sand dunes and relictual aquatic habitats), using statistical model choice on old and new probabilistic biogeographical models.LocationNorth America, Mojave, Sonoran and Chihuahuan Deserts.Materials and methodsDated phylogenies were estimated for three field‐sampled insect clades (Trigonoscuta,Rhaphiomidasand sand treader crickets), and five other literature‐sampled clades (the snailsAssiminea,PyrgulopsisandTryonia; the desert fringe‐toed lizardUma; and the desert pupfishCyprinodon).BioGeoBEARSwas used to statistically compare biogeographical models assuming unconstrained or connectivity‐constrained dispersal, with or without founder‐event speciation (jump dispersal) permitted. Finally, we introduce and test a novel distance‐based dispersal model (+x) where dispersal probability is multiplied by distance to the powerx.ResultsWe observed little concordance between biogeographical patterns and timing of geological events. Model comparisons were decidedly in favour of inclusion of founder‐event speciation in the models for most taxa, with only a small taxon,Uma, showing support for the model favouring vicariance. The inclusion of a constrained‐dispersal matrix was favoured by three of the eight taxa examined (Cyprinodon, sand treader crickets, andTrigonoscuta). Surprisingly, tests for distance influencing dispersal probability were mostly negative.Main conclusionsOur results do not show support for any one geological event shaping the biogeographical patterns of these desert taxa. Instead, the histories of desert dune and aquatic taxa are largely products of rare jump dispersal events, and can be considered island‐like systems. Although results are negative for the distance‐based dispersal model, this in itself demonstrates the superiority of explicit statistical model testing overa prioriassumption of fixed models in historical biogeography.

Phylogeographic and population genetic structure of bighorn sheep ( Ovis canadensis ) in North American deserts.

Fossil data are ambiguous regarding the evolutionary origin of contemporary desert bighorn sheep ( Ovis canadensis subspecies). To address this uncertainty, we conducted phylogeographic and population genetic analyses on bighorn sheep subspecies found in southwestern North America. We analyzed 515 base pairs of mtDNA control region sequence and 39 microsatellites in 804 individuals from 58 locations. Phylogenetic analyses revealed 2 highly divergent clades concordant with Sierra Nevada ( O. c. sierrae ) and Rocky Mountain ( O. c. canadensis ) bighorn and showed that these 2 subspecies both diverged from desert bighorn prior to or during the Illinoian glaciation (~315–94 thousand years ago [kya]). Desert bighorn comprised several more recently diverged haplogroups concordant with the putative Nelson ( O. c. nelsoni ), Mexican ( O. c. mexicana ), and Peninsular ( O. c. cremnobates ) subspecies. Corresponding estimates of effective splitting times (~17–3 kya), and haplogroup ages (~85–72 kya) placed the most likely timeframe for divergence among desert bighorn subspecies somewhere within the last glacial maximum. Median-joining haplotype network and Bayesian skyline analyses both indicated that desert bighorn collectively comprised a historically large and haplotype-diverse population, which subsequently lost much of its diversity through demographic decline. Using microsatellite data, discriminant analysis of principle components (DAPC) and Bayesian clustering analyses both indicated genetic structure concordant with the geographic distribution of 3 desert subspecies. Likewise, microsatellite and mitochondrial-based FST comparisons revealed significant fixation indices among the desert bighorn genetic clusters. We conclude these desert subspecies represent ancient lineages likely descended from separate Pleistocene refugial populations and should therefore be managed as distinct taxa to preserve maximal biodiversity. Los datos de fósiles sobre el origen evolutivo de las ovejas del desierto ( Ovis canadensis subespecies) contemporáneas son ambiguos. Para dilucidar esta incertidumbre, llevamos a cabo análisis filogeográficos y de genética de poblaciones entre cinco subespecies de ovejas del suroccidente de Norteamérica. Analizamos 515 pb de secuencia de la región control del ADN mitocondrial y 39 microsatélites en 804 ovejas de 58 localidades. Los análisis filogenéticos revelaron 2 clados altamente divergentes concordantes con ovejas de la Sierra Nevada ( O. c. sierrae ) y de las Montañas Rocosas ( O. c. canadensis ), y demostraron que estas dos subespecies divergieron antes o durante la glaciación de Illinois (315,000–94,000 años). Las ovejas del desierto formaron varios haplogrupos recientemente derivados concordantes con las subespecies de Nelson ( O. c. nelsoni ), México ( O. c. mexicana ) y peninsular ( O. c. cremnobates ). Las estimaciones correspondientes al tiempo de separación efectiva (17,000–3,000 años) y edades de haplogrupos (85,000–72,000 años) son los plazos más probables para las divergencias entre subespecies de ovejas del desierto dentro de la última glaciación máxima. Análisis de redes de haplotipos de unión de medias y análisis bayesianos de líneas de horizonte indicaron que las ovejas del desierto formaron una población históricamente grande y diversa en términos de haplotipos, que luego perdieron gran parte de su diversidad a través de un descenso demográfico. Utilizando datos de microsatélites los análisis DAPC y TESS indicaron agrupamiento genético concordante con la distribución geográfica actual de las tres subespecies. Asimismo, comparaciones de FST con datos de microsatélites y mitocondriales revelaron índices de fijación significativos entre los grupos genéticos de ovejas del desierto. Concluimos que estas subespecies de ovejas del desierto representan linajes antiguos que probablemente descienden de poblaciones de distintos refugios del Pleistoceno, y que por lo tanto deben ser manejadas como taxones distintos para preservar su biodiversidad máxima.

Wood decay in desert riverine environments

Floodplain forests and the woody debris they produce are major components of riverine ecosystems in many arid and semiarid regions (drylands). We monitored breakdown and nitrogen dynamics in wood and bark from a native riparian tree, Fremont cottonwood (Populus deltoides subsp. wislizeni), along four North American desert streams. We placed locally-obtained, fresh, coarse material [disks or cylinders (∼500–2000cm3)] along two cold-desert and two warm-desert rivers in the Colorado River Basin. Material was placed in both floodplain and aquatic environments, and left in situ for up to 12years. We tested the hypothesis that breakdown would be fastest in relatively warm and moist aerobic environments by comparing the time required for 50% loss of initial ash-free dry matter (T50) calculated using exponential decay models incorporating a lag term. In cold-desert sites (Green and Yampa rivers, Colorado), disks of wood with bark attached exposed for up to 12years in locations rarely inundated lost mass at a slower rate (T50=34yr) than in locations inundated during most spring floods (T50=12yr). At the latter locations, bark alone loss mass at a rate initially similar to whole disks (T50=13yr), but which subsequently slowed. In warm-desert sites monitored for 3years, cylinders of wood with bark removed lost mass very slowly (T50=60yr) at a location never inundated (Bill Williams River, Arizona), whereas decay rate varied among aquatic locations (T50=20yr in Bill Williams River; T50=3yr in Las Vegas Wash, an effluent-dominated stream warmed by treated wastewater inflows). Invertebrates had a minor role in wood breakdown except at in-stream locations in Las Vegas Wash. The presence and form of change in nitrogen content during exposure varied among riverine environments. Our results suggest woody debris breakdown in desert riverine ecosystems is primarily a microbial process with rates determined by landscape position, local weather, and especially the regional climate through its effect on the flow regime. The increased warmth and aridity expected to accompany climate change in the North American southwest will likely retard the already slow wood decay process on naturally functioning desert river floodplains. Our results have implications for designing environmental flows to manage floodplain forest wood budgets, carbon storage, and nutrient cycling along regulated dryland rivers.