Arid Southwestern United States Research Articles

Genetic connectivity in the Arizona toad (Anaxyrus microscaphus): implications for conservation of a stream dwelling amphibian in the arid Southwestern United States

The Arizona Toad (Anaxyrus microscaphus) is restricted to riverine corridors and adjacent uplands in the arid southwestern United States. As with numerous amphibians worldwide, populations are declining and face various known or suspected threats, from disease to habitat modification resulting from climate change. The Arizona Toad has been petitioned to be listed under the U.S. Endangered Species Act and was considered “warranted but precluded” citing the need for additional information – particularly regarding natural history (e.g., connectivity and dispersal ability). The objectives of this study were to characterize population structure and genetic diversity across the species’ range. We used reduced-representation genomic sequencing to genotype 3,601 single nucleotide polymorphisms in 99 Arizona Toads from ten drainages across its range. Multiple analytical methods revealed two distinct genetic groups bisected by the Colorado River; one in the northwestern portion of the range in southwestern Utah and eastern Nevada and the other in the southeastern portion of the range in central and eastern Arizona and New Mexico. We also found subtle substructure within both groups, particularly in central Arizona where toads at lower elevations were less connected than those at higher elevations. The northern and southern parts of the Arizona Toad range are not well connected genetically and could be managed as separate units. Further, these data could be used to identify source populations for assisted migration or translocations to support small or potentially declining populations.

Mesquite-associated soil and phyllosphere microbial communities differ across land-use types in drylands

Plant-associated microbiomes play prominent roles in maintaining plant health and productivity. Here, we characterized the soil and phyllosphere microbiomes associated with mesquite trees in grazing and urban areas compared to natural areas in the arid Southwestern United States. Our results showed that grazing areas were associated with higher phyllosphere fungal richness, while urban areas had higher phyllosphere richness for both fungi and bacteria/archaea, and additionally, urban soils had lower fungal richness. Specifically, grazing areas were characterized by larger proportions of nitrogen-fixing bacteria in the soil and fungal plant pathogens in the phyllosphere, while urban areas presented higher proportions of fungal plant pathogens in both the soil and phyllosphere as well as nitrifying and denitrifying bacteria in the phyllosphere, but a lower proportion of cellulolytic bacteria in the soil. Furthermore, in urban areas, more phyllosphere microorganisms were sourced from the soil. Collectively, these results suggest that plant-associated microbiomes change significantly across land-use types, and these patterns are different between aboveground and belowground parts of plants, as well as between bacteria/archaea and fungi. These changes in plant-associated microbiomes across land-use types might have important implications for nutrient cycling, plant health, and ecosystem restoration.

Increased bat activity at urban water sources: implications for cross-species transmission of bat rabies to mesocarnivores

AbstractWe examined the potential for urban water sources, specifically golf course ponds, to act as centers for rabies transmission from bats to mesocarnivores in the arid southwestern United States where surface water is often limited. Because residential housing can act as den and roost sites for both mesocarnivores and bats, we also examined the effect of housing density around water sources on activity. Using ultrasonic acoustic recorders to assess bat activity and camera traps to estimate mesocarnivore activity, we compared 14 pairs of wet and dry locations over two years by surveying twice during the summer, once prior to summer monsoons and once during the monsoon season, when surface waters were more available. Number of calls for all bat species combined were greater at wet sites compared to dry sites and calls of two bat species often associated with rabies, big brown bat (Eptesicus fuscus) and silver-haired bat (Lasionycteris noctivagans), were recorded more at wet sites than dry sites in the monsoon season. In both years, raccoons (Procyon lotor) were photographed more often at wet sites while striped skunks (Mephitis mephitis) and gray foxes (Urocyon cinereoargenteus) were less likely to be detected at wet sites. Bat, fox and raccoon activity was not associated with housing density while striped skunks showed a positive correlation. Finally, we examined potential for contact between mesocarnivores and big brown bats, a species implicated in cross-species rabies transmission in our area, by combining call activity of this bat species and photo detections of mesocarnivores during individual hours of the night into a Potential Contact Index (PCI) and found no significant effect of season (pre-monsoon vs. monsoon), species, or treatment (dry versus wet) but did find a significant species by treatment interaction, with raccoon PCI 3–30 times higher at wet sites and no effect on the other two mesocarnivores’ PCI. Overall, we found higher activity of bats at urban waters could increase potential for cross-species transmission of rabies from bats to raccoons but not for gray foxes and striped skunks.

Over half a century record of limnology data from Lake Powell, desert southwest United States: From reservoir filling to present day (1964–2021)

AbstractLake Powell is a large water storage reservoir in the arid southwestern United States. Here, we present a 58‐yr limnology dataset that captures water quality parameters from reservoir filling to present day (temperature, salinity, major ions, total suspended solids), as well as a 38‐yr record of Secchi depth, and a ~ 30‐yr record of nutrients, phytoplankton, and zooplankton assemblages. The dataset includes 5208 unique site visits spanning 258 unique sites of which 9 have been consistently visited. It also spans the establishment of an invasive bivalve (Dreissena bugensis, i.e. Quagga mussel) which was first detected in 2012. Given the general lack of long‐term data from lakes or reservoirs in arid regions, this dataset represents a unique contribution to regional, continental, and global‐scale limnology studies. As the hot drought in the desert southwest continues, we expect this dataset will inform water management decision‐making for this largest reservoir in the Upper Colorado River Basin.

Neurotoxic respiratory failure absent following Arizona rattlesnake bites

Warnings of neurotoxic respiratory paralysis following envenomation by rattlesnakes (Crotalus sp.) have been included in numerous scholarly publications over the past 60 years, resulting in fear and anxiety in the public and among clinicians. We examine the validity of the widespread belief that rattlesnakes in the arid southwestern United States, and particularly the Mohave Rattlesnake (Crotalus scutulatus), pose a significant risk of medically relevant respiratory weakness and paralysis in humans. A retrospective review of 3440 suspected rattlesnake bites reported to the Arizona Poison and Drug Information Center between 1999 and 2020 produced no evidence of respiratory weakness in a region with three species known to express significant amounts of neurotoxin in their venoms: Crotalus concolor, C. tigris, and the more widely distributed C. scutulatus. A literature review produced numerous warnings regarding respiratory paralysis following envenomation by rattlesnakes in our region that either lacked references or cited sources that did not contain strong supportive data. We found no case reports of neurotoxic respiratory weakness following Arizona rattlesnake bites in the literature and such reports in surrounding states were scant. We conclude that neurotoxic respiratory failure in this region following rattlesnake envenomation is extraordinarily rare. All rattlesnake bites should receive the same consideration and critical care, and warnings about significant risk of respiratory failure are unwarranted, regardless of species involved.

Macroinvertebrate Community Composition in Wetlands of the Desert Southwest is Driven by wastewater-associated Nutrient Loading Despite Differences in Salinity.

The online version contains supplementary material available at 10.1007/s13157-022-01647-2.

Five-Year Effects of Introduced Mountain Goats and Recreation on Plant Communities and Species of Conservation Concern in an Alpine Sky Island

Alpine ecosystems in the arid southwestern United States are vulnerable to climate change. Many of these ecosystems are experiencing increased recreational use and introductions of nonindigenous ungulates. In the La Sal Mountains of southeastern Utah, alpine plant communities support an endemic plant species, Erigeron mancus, 11 plant and two animal species of conservation concern, and a United States Department of Agriculture (USDA) Forest Service Research Natural Area. Nonindigenous mountain goats (Oreamnos americanus) were introduced by the Utah Division of Wildlife Resources (UDWR) in 2013 and 2014, and recreational use has increased in the intervening period. To evaluate potential effects of mountain goat and recreational use, vegetation monitoring was initiated by the USDA Forest Service in 2016. We used 5 y monitoring data (2016–2020) and generalized linear mixed models to analyze the separate and interacting effects of year, recreational use, and mountain goat use on: (1) populations of plant species of conservation concern; (2) vascular and nonvascular plant cover; and (3) ground cover. Our analyses revealed decreases in proportional frequency of E. mancus at the end of the monitoring. Proportional cover of dominant plant growth forms, especially forbs, declined, while proportional cover of litter and ineffective ground cover (bare soil and pavement) increased. Recreational or goat use were a factor in several of the observed changes, although weather and climate likely also influenced the results. Management complexity arises because of the different missions of USDA Forest Service and UDWR, but adjustments in mountain goat numbers and recreational use may be needed to maintain ecologically resilient ecosystems. Continued monitoring can provide the basis for adaptive management.

Mohave Rattlesnake (Crotalus scutulatus) Identification Revisited.

Crotalus scutulatus (Mohave rattlesnake) is a clinically significant pit viper broadly distributed across much of the arid southwestern United States and mainland Mexico. Identification of C scutulatus is a concern among emergency medical service and emergency department personnel owing to its reputation for severe envenomations and difficulty in visually differentiating between C scutulatus and other species, primarily Crotalus atrox (western diamond-backed rattlesnake). We contrast distinctive characteristics of C scutulatus, C atrox, and 3 other sympatric species: Crotalus molossus, Crotalus ornatus, and Crotalus viridis (western and eastern black-tailed rattlesnakes and prairie rattlesnake, respectively). Greenish coloration eliminates C atrox but does not confirm C scutulatus. Obvious coarse and fine speckling of the dorsal pattern and a pale postocular stripe intersecting the mouth characterize C atrox. Dorsal speckling is insignificant or absent in the other species, whereas the pale postocular stripe passes above the mouth in C scutulatus and C viridis and is absent in C molossus and C ornatus. Tails boldly ringed with alternating black and white or contrasting shades of gray are shared by C atrox and C scutulatus, respectively, but a lack of boldly ringed tails characterizes the other species. The proximal rattle segment is yellow and black, or entirely yellow, in C scutulatus but black in the others. The most reliable visual identifications are based on evaluations of multiple traits, all of which are variable to some extent. Traits such as tail ring width and the size and number of crown scales have frequently been overemphasized in the past.

Seed Menus: An integrated decision-support framework for native plant restoration in the Mojave Desert.

The combination of ecosystem stressors, rapid climate change, and increasing landscape‐scale development has necessitated active restoration across large tracts of disturbed habitats in the arid southwestern United States. In this context, programmatic directives such as the National Seed Strategy for Rehabilitation and Restoration have increasingly emphasized improved restoration practices that promote resilient, diverse plant communities, and enhance native seed reserves. While decision‐support tools have been implemented to support genetic diversity by guiding seed transfer decisions based on patterns in local adaptation, less emphasis has been placed on identifying priority seed mixes composed of native species assemblages. Well‐designed seed mixes can provide foundational ecosystem services including resilience to disturbance, resistance to invasive species, plant canopy structure to facilitate natural seedling recruitment, and habitat to support wildlife and pollinator communities. Drawing from a newly developed dataset of species distribution models for priority native plant taxa in the Mojave Desert, we created a novel decision support tool by pairing spatial predictions of species habitat with a database of key species traits including life history, flowering characteristics, pollinator relationships, and propagation methods. This publicly available web application, Mojave Seed Menus, helps restoration practitioners generate customized seed mixes for native plant restoration in the Mojave Desert based on project locations. Our application forms part of an integrated Mojave Desert restoration program designed to help practitioners identify species to include in local seed mixes and nursery stock development while accounting for local adaptation by identifying appropriate seed source locations from key restoration species.

Late Quaternary geochronologic record of soil formation and erosion: Effects of climate change on Mojave Desert hillslopes (Nevada, USA)

AbstractClimate change is an often-cited control on geomorphic processes in the arid southwestern United States, but links to direct climatic factors and vegetation change remain under debate. Hillslopes at a site in the eastern Mojave Desert in southern Nevada are mantled by 0–1.5 m of colluvial deposits. Accumulation of weathered bedrock combined with eolian inputs of fine sand and silt led to the formation of well-developed soil profiles. Surface sediments from both sources were incorporated into colluvium, allowing both processes to be dated with optically stimulated luminescence (OSL). OSL ages indicate a period of increased colluviation in the Late Pleistocene facilitated by enhanced bedrock weathering and dust deposition. Hillslope aspect strongly controls predominant soil environments and associated vegetation. Well-developed soils with dense grass cover extensively mantle the mesic north-aspect hillslopes, while more xeric south-aspect hillslopes are dominated by thin colluvium with minimal soil development, extensive bedrock exposure, and desertscrub vegetation. Remnants of older colluvium with moderately developed soils on south aspects, however, indicate they were once more extensively mantled by thicker colluvial deposits. The transition to drier conditions in the Holocene diminished vegetation cover on more xeric south aspects, triggering widespread erosion, whereas the more mesic north aspects retained denser grass cover that minimized erosion. The transition to drier conditions in the Holocene altered the vegetation; however, persistent perennial grass cover minimized erosion into the middle Holocene. Increasing aridity during the middle Holocene significantly reduced grass cover on more xeric south aspects, triggering erosion and alluvial deposition. OSL dates of dust incorporated into terrace sediments indicate late Middle Holocene aggradation and soil development in the Late Holocene. In contrast, maintenance of substantial perennial grass cover on mesic north aspects minimized erosion from those hillslopes throughout the Holocene.

Improving water scarcity footprint capabilities in arid regions through expansion of characterization factor methods

Water scarcity footprint (WSF) is a recent addition to life cycle assessment methodology that has advanced the understanding of freshwater environmental impact. The Available Water Remaining (AWARE) method is one approach that has gained significant traction in WSF applications. While an effective method for determining WSF, the methodology has limitations that constrain capabilities for determining freshwater environmental impact in arid regions. The primary limitation is the inability to compare regions when more water demand exists than what is available which typically occurs in arid regions. This limitation reduces resolution and therefore decision-making capabilities. This work proposes a novel method for determining WSF in arid regions by capturing and quantifying scarcity when water demand is greater than availability. The approach presented here, called the demand to availability (DTA) method, is intended to be used for small-scale, or subregion analyses in areas where truncation occurs using standard AWARE methods. With the regional specificity, unique characterization factors can be developed to enhance deterministic resolution and ultimately improve decision-making abilities. The DTA methods are presented universally, allowing for application and implementation to any region. A case study was developed to demonstrate the effectiveness of the DTA method by analyzing characterization factors (CFs) and alfalfa WSFs in the arid Southwestern United States. Using the standard AWARE methods, this region originally truncated 38% of counties resulting in zero resolution or decision-making abilities. Results of the case study that used the proposed DTA method show an improved resolution in 100% of these counties, both within CF and alfalfa WSF. Although the proposed method is an improvement for understanding WSFs in arid regions, limitations and constraints still exist and are discussed.

Interannual Variability and Seasonal Dynamics of Evapotranspiration of Arundo donax L. and Populations of its Biological Control Agent (Tetramesa romana)

Giant reed (Arundo donax L.), a woody grass native to the Mediterranean, has become a cause of concern for national water security in its invaded range of the arid southwestern United States, Australia, New Zealand, and South Africa. The main objective of this study was to provide the first, landscape-level estimates of water use by giant reed. The study utilized the eddy covariance method to quantify evapotranspiration (ET) throughout the 2014 and 2015 growing seasons along the Rio Grande River in Eagle Pass, Texas. We monitored ET concurrently with the implementation of a biological control program targeting giant reed. Daily ET rates were strongly associated with the phenology of giant reed, as estimated by degree day accumulation. Monthly ET was higher in each month of the active growing season (May–October) of giant reed in 2014 than in 2015. Consequently, seasonal (DOY 121-304) ET was substantially higher in 2014 (842 mm) than in 2015 (625 mm) even though the 2015 growing season was wetter. Cumulative rainfall from May to October was 188 and 264 mm in 2014 and 2015, respectively. Our study indicated that the reduction in ET could be attributed to the presence of specialist insect biological control agent, the stem-galling arundo wasp (Tetramesa romana). In 2015, we observed a 28.6% (May), 53.5% (June), and 44% (July) increase in the number of arundo wasps captured in sticky traps from the previous year. Additional studies are needed to quantify conservation and economic benefits of the biological control program in the Rio Grande Basin.

Assessing Long‐Term Changes in Regional Groundwater Recharge Using a Water Balance Model for New Mexico

AbstractIn New Mexico in the arid southwestern United States, groundwater recharge is crucial to sustain groundwater (GW), which is vitally important to life, agriculture, industry, and ecosystems. To better understand the changes in recharge statewide, we explored the changes in groundwater recharge (RE), precipitation (P), surface water inflow (SWin), outflow (SWout), diversions (SWdiv), returns (SWret), and surface water and GW evapotranspiration (SWE and GWET) in five New Mexico counties: Taos, Torrance, Doña Ana, Eddy, and Lea during 1975–2015. The results show that the change‐point of RE was in the 1990s, leading to contrasting tendencies in RE before and after the change‐point with a decreasing rate during the latter one. There was a significant positive relationship between RE and P for Taos. The highest contribution rate (CR) to RE was SWout for Taos, whereas GWET ranked as the top CR for the other counties. Furthermore, the annual CR of GWET to RE increased significantly in all counties except Lea. The above results reveal that P and surface water flows played the dominant role in impacting RE in northern New Mexico where surface water is the primary local water resources, whereas GWET had the highest and increasingly continuous influence in central and southern New Mexico where the surface water is much less than that in northern New Mexico. This study shows that water budgets are important to identify differences in regional hydrological regimes that affect planning to maintain RE to groundwater.

Chemical and hydrological controls on salt accumulation in irrigated soils of southwestern U.S

Soil salinization is a global problem affecting approximately 10% of agricultural soils, particularly in irrigated aridlands. This study quantified salt-loading by flood irrigation and soil fertilizers/amendments versus atmospheric deposition, studied controls of solute transport and salt buildup, and evaluated the effectiveness of gypsum application in improving soil sodicity in the arid southwestern United States. Study sites include one natural site and two agricultural sites in fields of dominant crops of the region, a pecan orchard and an alfalfa field near El Paso, Texas. The salt-loading rate in agricultural soils was dominated by the quantity and quality of irrigation waters rather than by dust. Salt loadings by irrigation waters were estimated ~ 306 g Na+ m−2 yr−1, 129 g Ca2+ m−2 yr−1, 361 g Cl− m−2 yr−1, 419 g SO42− m−2 yr−1, and 284 g HCO3− m−2 yr−1, followed by soil amendments. Whereas dust and fertilizer loadings were negligible in agricultural soils. Soil texture variability physically governs water movement and solute transport; coarser soils retained significantly less water than finer soils upon irrigation (p < 0.005) facilitating salt leaching. More salts accumulated around low-permeability layers. Some soils have approached salinity thresholds after only 90 years of cultivation. The Rio Grande river flow is projected to decrease due to reduced snowfall in Colorado, leading to more groundwater of higher salinity, to be used. If ground water were to be the sole water source, the salt loading rate would almost double. Soil amendments temporarily reduce soil sodicity induced by high Na+ concentrations in irrigation water. Their application is needed annually to prevent soil dispersion, to improve infiltration, and to stop even faster salt accumulation. This study highlighted the challenges that the Rio Grande valley in southwestern United States and other irrigated drylands are facing.

Changing climate drives future streamflow declines and challenges in meeting water demand across the southwestern United States

Society and the environment in the arid southwestern United States depend on reliable water availability, yet current water use outpaces supply. Water demand is projected to grow in the future and climate change is expected to reduce supply. To adapt, water managers need robust estimates of future regional water supply to support management decisions. To address this need, we estimate future streamflow in seven water resource regions in the southwestern U.S. using a new SPAtially Referenced Regressions On Watershed attributes (SPARROW) streamflow model. We present streamflow projections corresponding to input data from seven climate models and two greenhouse gas Representative Concentration Pathways (RCP4.5 and 8.5) for three, thirty-year intervals centered on the 2030s, 2050s, and 2080s, and for a historical thirty year interval centered on the 1990s. Across water resource regions, about half of the RCP4.5 models (51%) and two thirds of the RCP8.5 models (67%) indicate decreases in streamflow in the 2080s relative to the historical period. Models project maximum decreases in streamflow of 36–80% in all water resource regions for all periods and RCPs relative to historical streamflow, and maximum streamflow decreases of up to 20–45% in the 2080s at sites along the Colorado River used for measuring compliance with interstate and international water agreements. Headwaters are projected to experience the greatest declines, with substantial downstream implications. Among these estimates, the streamflows from models forced with RCP8.5 tend to be lower than those forced with RCP4.5. Not all climate models, times, and RCPs project widespread streamflow declines. The most ubiquitous streamflow increases are projected to occur in the 2030s under RCP4.5. Later time periods and enhanced greenhouse gas forcings indicate smaller regions of streamflow increase and lower accumulated streamflows, suggesting that limiting or reducing greenhouse gas concentrations could support future water availability. Although some possible streamflow increases are promising, the modest and spatially limited increases in streamflow projected for later time periods are still unlikely to be sufficient to meet the projected water demand. These results inform the likelihood of future water agreement compliance, and support developing strategies to balance water supply and demand.

Great Basin Paleoclimate and Aridity Linked to Arctic Warming and Tropical Pacific Sea Surface Temperatures

AbstractThe arid southwestern United States is susceptible to sustained droughts that impact water resources and economic activity for millions of residents. Previous work has not systematically investigated the structure, timing, and possible forcings of Holocene Great Basin sub‐orbital hydroclimate changes, impeding our ability to understand the potential future controls on Southwestern aridity. The objective of this paper is to constrain the potential forcings on Holocene aridity and temperature, via comparison of new high‐resolution speleothem data, an Aridity Index synthesizing hydroclimate records, and linkages of Southwestern paleoclimate to other regions. The high‐resolution data from Leviathan Cave provide a paleoclimate record since 13,400 yr ago: A cool Younger Dryas was followed by two pronounced Middle Holocene aridity intervals between 9,850 and 5,310 yr B2k characterized by low growth rates and high δ18O and δ13C values. Subsequently, stalagmite δ18O values show near‐modern levels for the last four millennia during which time growth rates were high and δ13C values were low in response to wetter conditions. The regional Aridity Index documents that Middle Holocene drying coincided with a warm Arctic and decreased sea ice extent, a warm western tropical Pacific, and a large sea surface temperature gradient across the tropical Pacific, all of which likely responded to northern hemisphere summer insolation forcing. Our data suggest that extreme Middle Holocene aridity is more severe than the short medieval droughts evident in the tree ring record, and such extreme aridity may represent a worst‐case analog for future climate.

Tillage and Nitrogen Rate Effects on Corn Production and Residual Soil Characteristics

Core Ideas Strip tillage had relative production advantage over other tillage practices in corn production. Strip tillage showed greater water use efficiency than conventional tillage. No‐tillage showed higher residual carryover soil moisture, NO3–N and total N. ABSTRACTSuccessful corn (Zea mays L.) production in semiarid and arid southwestern United States is limited by available resources, especially water and N, and management of such limited resources need efficient on‐farm tillage and N management decisions. A 3‐yr study was conducted at New Mexico State University Agricultural Science Center, Tucumcari, NM, to evaluate the effectiveness of three tillage strategies (conventional, strip, and no‐tillage) and two N rates (196 and 308 kg ha−1) on corn yield, water use, and carry‐over residual water and N characteristics. Tillage × N‐rate interaction had no significant impact on corn yield, water use, and residual soil characteristics (P &gt; 0.05). No‐tillage showed consistent lower grain and stover yield in all three years. Strip tillage (ST) had a relative grain yield advantage over other tillage practices in 2 out of 3 yrs. Strip tillage showed higher harvesting index and greater water use efficiency (WUE) than conventional tillage (CT). No‐tillage (NT) had 20 to 45% greater residual carry‐over soil moisture than CT during the 3‐yr study period. Percent change (draw‐down) in soil profile moisture and NO3–N in each site‐year were higher in CT than other tillage treatments at both 0 to 30 and 30‐ to 60‐cm depth increments. No‐tillage resulted in higher residual carry‐over NO3–N and total N at both incremental depths in each tested year, without exception. Overall, implementation of ST in the semiarid regions has significant advantage over other tillage practices in improving the resource use efficiency in corn production.

The trophic ecology of a desert river fish assemblage: influence of season and hydrologic variability

AbstractFoodweb analyses can provide insight as to how species persist in naturally dynamic environments and how these communities may respond to anthropogenic alteration. In the remaining undammed rivers of the arid southwestern United States, it is thought that plasticity in feeding behavior allows native fishes to thrive in the context of natural fluctuations in temperature, sediment, and resource availability associated with extreme swings in season and hydrology. There have been few evaluations of this hypothesis, in part owing to the rarity of rivers that retain their natural environmental dynamics and complement of native species. We investigated the feeding ecology of the fish assemblage of one such river, the Little Colorado River, Arizona, focusing on four native species: bluehead sucker (Catostomus discobolus), flannelmouth sucker (Catostomus latipinnis), speckled dace (Rhinichthys osculus), and endangered humpback chub (Gila cypha). Over two years, we sampled fish diets in July, September, January, March, and June, encompassing turbid and clear‐water conditions across seasons. We quantified food resource contributions to fish diets in fish‐centered food webs, comparing diversity of trophic linkages and linkage‐strength distribution across time. We used ordination analyses to investigate differences in diets among species and changes in diet composition across seasons and conducted seasonal comparisons of gut fullness. We observed a high diversity of dietary items (as absolute numbers and consumer–resource ratios) within a fish assemblage that was apparently resistant to extreme and recurrent environmental variability. Fish diet diversity was ~2× that of the nearby dammed Colorado River's tailwater, and over 50% of fish‐resource linkages were weak (&lt;1% of species’ diet). Native fishes consumed a mix of terrestrial and aquatic resources. Notably, the water column‐feeding humpback chub consumed the largest percentage of terrestrially derived food items and appeared to capitalize on food resources made available by a summer flash flood, consuming twice as much during this flood than during other seasons (based on gut fullness, corrected by estimates of food quality). Other fishes exhibited similar dietary variation. Such opportunist/generalist strategies, coupled with diverse food resources, may be important to the persistence of these and other native fishes in dynamic environments.

Disproportionate magnitude of climate change in United States national parks

Anthropogenic climate change is altering ecological and human systems globally, including in United States (US) national parks, which conserve unique biodiversity and resources. Yet, the magnitude and spatial patterns of climate change across all the parks have been unknown. Here, in the first spatial analysis of historical and projected temperature and precipitation across all 417 US national parks, we show that climate change exposes the national park area more than the US as a whole. This occurs because extensive parts of the national park area are in the Arctic, at high elevations, or in the arid southwestern US. Between 1895 and 2010, mean annual temperature of the national park area increased 1.0 °C ± 0.2 °C century−1 (mean ± standard error), double the US rate. Temperature has increased most in Alaska and its extensive national parks. Annual precipitation of the national park area declined significantly on 12% of national park area, compared to 3% of the US. Higher temperatures due to climate change have coincided with low precipitation in the southwestern US, intensifying droughts in the region. Physical and ecological changes have been detected and attributed mainly to anthropogenic climate change in areas of significant temperature increases in US national parks. From 2000 to 2100, under the highest emissions scenario (representative concentration pathway [RCP] 8.5), park temperatures would increase 3 °C–9 °C, with climate velocities outpacing dispersal capabilities of many plant and animal species. Even under the scenario of reduced emissions (RCP2.6), temperature increases could exceed 2 °C for 58% of national park area, compared to 22% of the US. Nevertheless, greenhouse gas emissions reductions could reduce projected temperature increases in national parks by one-half to two-thirds.

Synchronous species responses reveal phenological guilds: implications for management

AbstractPhenological studies are critical for understanding the ability of terrestrial ecosystems to respond to changes in climate. Monitoring seasonal transitions at the species or community level across large areas is challenging and expensive. One approach for lowering costs is to identify phenological guilds—groups of species that exhibit similar timing of seasonal transitions—and limit monitoring to a smaller number of species within a guild. In this study, we evaluated 23 consecutive years of monthly observations of individual species at 15 long‐term study sites at the Jornada Basin USDA‐Long‐Term Ecological Research site to identify patterns in the onset of three phenophases—leaf‐out, flower, and fruit—of 16 widely occurring species in the arid southwestern United States and to investigate the existence of phenological guilds. We conducted univariate analyses of distributions in the timing of leaf, flower, and fruit production across time and space and multivariate cluster analysis of the time series to identify coherent groups of species–site instances that exhibit coherence in timing of phenophase onsets (i.e., guilds). The six species of C3 shrubs demonstrated greater consistency in timing of all phenophases relative to C4 grasses. Further, we found that in all species, leaf‐out occurred prior to the onset of the summer monsoon rains. Cluster analysis revealed six groups of species–site observations demonstrating high within‐year concordance in timing of leaf‐out and first fruit across variable site conditions and rainfall years. The six groups for timing of first fruit differed from those for first leaf in that they exhibited greater multi‐species membership and within‐year variability in timing. We propose that use of phenological guilds can improve the efficiency of ecosystem monitoring, predictive models of ecosystem cues driving phenological events, and land management outcomes.