Saguaro National Research Articles

EXPLORING RELATIONSHIPS AMONG CULTURAL ECOSYSTEM SERVICE VALUES, SPIRITUAL WELL-BEING, AND LIFE SATISFACTION: IMPLICATIONS FOR THE POST-PANDEMIC ERA

Cultural ecosystem services significantly affected human well-being in the COVID-19 era. This study investigates visitors’ cultural ecosystem service values at Saguaro National Park (US) and how those values affect tourists’ spiritual well-being and life satisfaction in the post-pandemic era with an exploratory mixed-methods approach. In a qualitative phase using a netnographic analysis of Instagram data (N = 449), seven themes emerged: national environment, learning, socialization, escape and solitude, mental well-being, outdoor recreation during COVID-19, and tourism during COVID-19. Subsequently, these themes were used to develop and test the proposed hypotheses on data collected from 316 participants of an online survey using partial least squares structural equation modeling. The results disclosed that cultural ecosystem service values can significantly affect different dimensions of spiritual well-being. This study emphasizes the mediating role of personal and communal spiritual well-being between cultural ecosystem service values and life satisfaction. Further, it bridges the application of real-time social media surveillance with a content analysis of text and images and a quantitative analysis of qualitative results. The managerial implications provide suggestions for promoting small/medium national parks in the post-pandemic era.

Effectiveness of a decade of treatments to reduce invasive buffelgrass (Pennisetum ciliare)

AbstractThe invasion of nonnative grasses threatens biodiversity and ecosystem function globally through competition with native plant species and increases to wildfire frequency and intensity. Management actions to reduce buffelgrass [Pennisetum ciliare (L.) Link], an invasive warm-season perennial bunchgrass, are widely implemented, with chemical and mechanical treatments extending over two decades within Saguaro National Park in the Sonoran Desert of North America. We assessed how the effectiveness of treatments to reduce P. ciliare cover spanning from 2011 to 2020 were influenced by stage of invasion, treatment type and intensity, and environmental conditions. An increase in treatment effectiveness was largely explained by high initial cover of P. ciliare, an indicator of a late invasion stage and associated with high treatment intensity. Treatments had potential to be effective in patches as small as 0.3-m2P. ciliare canopy per 400-m−2 area (<0.001% canopy cover) across treatment types and environmental gradients. Chemical treatments had higher or equal effectiveness compared with mechanical treatments, and greater reductions in P. ciliare were associated with shorter average years of treatment interruptions, or gaps, and to a lesser degree, total years of treatment. In many cases, P. ciliare was reduced with as little as 2 yr of treatment, but more than 3 average years of treatment gap could result in reduced treatment effectiveness. There was generally higher treatment effectiveness on shallow slopes, north- and east-facing aspects, and on higher elevations within one district of the park. Our findings highlight that resource-intensive treatments in all but the smallest patches of P. ciliare have largely been effective. Further opportunities for improvement include more frequent surveillance, limiting treatment gaps to ≤3 yr in areas of low P. ciliare cover, and comparison of treated with untreated areas.

Asynchronous flowering patterns in saguaro cacti (Carnegiea gigantea)

AbstractThe saguaro cactus (Carnegiea gigantea [Engelm.] Britton & Rose) is a keystone species endemic to the Sonoran Desert of northern Mexico and the southwestern United States. The saguaro produces large white flowers near its stem apex (crown) during April–June, which bloom at night and close the following day. In 1924, Duncan Johnson reported that saguaro floral buds are likely to have an asymmetrical distribution in which buds occur in higher densities on the eastern half of a plant's crown. Using technology not available to Johnson, we tested his observations to determine whether flowers are asymmetrically distributed using repeat photography. We also tested whether there is a seasonal pattern of flowering that may explain Johnson’s observations. We tracked intra‐individual flowering phenology of 20 saguaros and measured 2372 flowers across two reproductive seasons in Saguaro National Park, Tucson, Arizona. Flowers first appeared on the east side of all saguaro crowns at the start of the reproductive season, and then spread radially in a counterclockwise direction as the season progressed. In contrast to previous reports, saguaro flowers were consistently more abundant on the northern part of the crown than in the eastern part. To our knowledge, this study is the first to document a seasonal, counterclockwise pattern of asynchronous flowering in saguaro or any angiosperm. We discuss potential drivers of this phenomenon as well as implications for saguaros responding to climate change.

Climate‐mediated shifts toward bunchgrass dominance 40 years after grazing cessation in Saguaro National Park, Arizona, USA

AbstractQuestionsVegetation transects were surveyed in Saguaro National Park, Arizona, in 1976 and 2007 as part of a grazing study. We resurveyed the same transects in 2018 and accessed fire and climate data in order to examine trends in plant community structure. We asked the question: has the plant community shifted in species’ density, canopy cover and diversity in the past 40 years since grazing cessation? And if so, do these shifts favor a specific species, or plant functional guild? We also asked if climate and time since fire influenced plant community structure?LocationSaguaro National Park East is located in the Sonoran Desert and Madrean Sky Island ecoregion interface in Tucson, Arizona, United States.MethodsPlots (10) were 20 m × 50 m, with 21 20‐m transects within each plot used to gather plant canopy cover and density of perennial species with the line‐intercept method. Data were aggregated by plot and analyzed using generalized linear mixed‐effects modeling and non‐metric multi‐dimensional scaling.ResultsOverall canopy cover, density, and Shannon diversity increased significantly between 1976 and 2018. Ferns, forbs, graminoids, subshrubs and vines increased significantly in cover and density between 1976 and 2018. Wildfire recency was significantly negatively correlated with forb density, and density or cover of five dominant species (mostly woody). Shifts in plant community structure between survey years was correlated with increased temperature, precipitation, and vapor pressure differential.ConclusionAfter 40 years of grazing cessation, significant increases in herbaceous cover, and a lack of significant increase in woody encroachment are signs of recovery. Increases in temperature may be responsible for accelerating shifts to warm‐season species‐dominant communities. Fire was unexpectedly not a significant factor in plant community structure; however, park management will likely need to address increased wildfire risk given the increase in herbaceous cover and density.

Valuing impacts of proximity to Saguaro National Park on house prices

AbstractProximity to public lands is well established within the literature as having a positive effect on nearby house prices. However, little research exists examining the impact of proximity to national parks. We use a hedonic property model with matching methods to estimate the value of proximity to Saguaro National Park West and Tucson Mountain Park. We find positive impacts of proximity to each park, with a larger aggregate effect from proximity to Saguaro National Park West. Our results provide useful insight into the potential impacts of changes in land use designation on nearby house prices.

Review: Elizabeth Scheu Close: A Life in Modern Architecture, Powerhouse: The Life and Work of Judith Chafee

Review: <i>Elizabeth Scheu Close: A Life in Modern Architecture</i>, <i>Powerhouse: The Life and Work of Judith Chafee</i>

Play it Safe

Play it Safe Michelle Ross Jessie once drove through a hurricane because, stupidly, she'd thought the eye of the storm was far enough away that returning to her apartment in Tallahassee—she'd been staying with a friend four hours away in Atlanta—was safe. By the time she realized her mistake, it was one in the morning, and turning back with no place to stay—her two cats were in the car with her, screeching all the way—seemed even more precarious than continuing. Driving to her daughter's school carnival, where she'd have to socialize with the parents of Ellen's schoolmates in the service of raising funds for new playground equipment, felt kind of like that drive through heavy rain and wind and felled trees and the impending threat of more falling trees. She couldn't relate to these people. Take Ava's mom, for instance. Sharon was the PTA committee member in charge of organizing this carnival. The woman ended every email with "Smiles, Sharon." Jessie puzzled over this letter closing for longer than Gabe thought warranted. "You overthink everything," he said from the passenger seat. He was drinking a vodka tonic—out of a regular drinking glass, too, not even trying to conceal that he had an open container of alcohol. "I'm not the one driving," he said when she eyed his drink. Then he said she was just jealous and not to worry, he'd brought a flask and a small bottle of tonic water and she could have a drink in the parking lot when they got there. Their eight-year-old daughter, Ellen, had her headphones on in the backseat. She was watching a cartoon in which the main characters changed size and donned various techy animal suits. In a Sonoran desert episode, one of the few episodes Jessie had watched with Ellen, the men transformed themselves into faux rattlesnakes and then observed a real, albeit cartoon, rattlesnake go after a squirrel. Cornered, the squirrel pumped extra blood to its tail and shook the tail so that in the snake's infrared heat vision, the squirrel's tail appeared to inflate like a balloon, fooling the snake into striking at the tail rather than the vital organs. Jessie had cheered when the snake missed and the squirrel escaped. Ellen had said, "Yeah, but maybe the squirrel living means the snake starves to death." That was Ellen for you. Her capacity for empathy extended even to venomous predators. What was different about the dread Jessie felt for this particular school event versus school events of the past was that three days ago she'd been attacked while out running. She'd been running along a residential [End Page 59] street, not a remote location, such as the trails in Saguaro National Park East, where she was always scanning the desert scrub for rattlesnakes and mountain lions. It hadn't even happened particularly early in the morning. The sun had already started to peak over the Rincon Mountains. When she heard footfalls and turned, even though the man was dressed in jeans and was looking right at her, still her brain reasoned he must have a good reason for seeming to lunge towards her that she had simply failed to decipher. She thought he must be trying to protect her from something. A mountain lion perhaps? Though rare, mountain lion sightings did occasionally occur in town. Or maybe he was running from a mountain lion; he needed her help. But then he crashed into her and one hand went directly to her breast, the other over her mouth, and she understood that this man was the mountain lion she'd imagined. She'd fought him off and other than a few cuts and bruises, and the memory of his hands, she was fine really. But in her post-attack shock, adrenaline high, she'd allowed herself to be interviewed by a local reporter, who'd arrived nearly as soon as the police, and almost an hour before Gabe. He'd inadvertently left his damn cellphone in silent mode again. The only sentence from Jessie's mouth that had not been too...

Assessing ecological uncertainty and simulation model sensitivity to evaluate an invasive plant species’ potential impacts to the landscape

Ecological forecasts of the extent and impacts of invasive species can inform conservation management decisions. Such forecasts are hampered by ecological uncertainties associated with non-analog conditions resulting from the introduction of an invader to an ecosystem. We developed a state-and-transition simulation model tied to a fire behavior model to simulate the spread of buffelgrass (Cenchrus ciliaris) in Saguaro National Park, AZ, USA over a 30-year period. The simulation models forecast the potential extent and impact of a buffelgrass invasion including size and frequency of fire events and displacement of saguaro cacti and other native species. Using simulation models allowed us to evaluate how model uncertainties affected forecasted landscape outcomes. We compared scenarios covering a range of parameter uncertainties including model initialization (landscape susceptibility to invasion) and expert-identified ecological uncertainties (buffelgrass patch infill rates and precipitation). Our simulations showed substantial differences in the amount of buffelgrass on the landscape and the size and frequency of fires for dry years with slow patch infill scenarios compared to wet years with fast patch infill scenarios. We identified uncertainty in buffelgrass patch infill rates as a key area for research to improve forecasts. Our approach could be used to investigate novel processes in other invaded systems.

Buffelgrass invasion and glyphosate effects on desert soil microbiome communities

Buffelgrass (Cenchus ciliaris) is a drought-tolerant invasive grass in the Americas and Australia that significantly impacts native plant communities and ecosystems. Despite the clear need to develop a comprehensive understanding of how buffelgrass is able to invade and rapidly establish in arid ecosystems, there is still a lack of knowledge as to if and how this weed might change the soil microbiome in a way that affects its dominance in the presence of management. We investigated the effect of buffelgrass on soil microbial communities in areas that have either been exposed to or not exposed to glyphosate in Saguaro National Park, Arizona USA. We found that buffelgrass roots in invaded areas are surrounded by a distinct soil community that includes a greater number of nitrifiers than in uninvaded soil. We also observed increases in arbuscular mycorrhizal fungi, ectomycorrhizal fungi and methanotrophs with buffelgrass invasion compared to uninvaded soil. Finally, we found no evidence of glyphosate effects on the soil microbiome. Overall, our study results suggest that buffelgrass can escape the limitation of nutrient availability in arid ecosystems by directly or indirectly modifying the soil microbiome. The competitive dominance of buffelgrass in arid systems might be indirectly enhanced by nitrifiers and fungal symbionts, which are often involved in rapid biomass accumulation. This work highlights the importance of soil microbiome considerations in weed science research.

Nurse rock microclimates significantly buffer exposure to freezing temperature and moderate summer temperature

Nurse tree canopies mitigate exposure to freezing temperatures that could result in injury or mortality to the saguaro cactus (Carnegiea gigantea). Abiotic objects have been hypothesized to provide similar beneficial microclimates. We used data loggers at 11 nurse rock sites to record daily daytime summer maximum and winter nighttime minimum temperatures at Saguaro National Park, Arizona, to examine the effectiveness of rocks to moderate seasonal temperature extremes in the microclimate. Temperatures at rock sites averaged 2 °C warmer than exposed open control sites in winter. We found that the efficiency of rocks to act as insulators significantly increased as temperature at control sites decreased, consistent with studies of tree canopies, and that the insulation effect lasted throughout the night. In summer, rocks reduced exposure to maximum temperatures but did not offer significantly more cooling at higher temperatures. Our results suggest that the protection from freezing temperature offered by rocks in winter is more ecologically beneficial to the saguaro than extreme temperature amelioration during summer in the cold-limited frontiers of the species’ range.

Developing an expert elicited simulation model to evaluate invasive species and fire management alternatives

AbstractInvasive species can alter ecosystem properties and cause state shifts in landscapes. Resource managers charged with maintaining landscapes require tools to understand implications of alternative actions (or inactions) on landscape structure and function. Simulation models can serve as a virtual laboratory to explore these alternatives and their potential impacts on a landscape. To be useful, however, managers need to participate in model development to ensure that model structure can evaluate the response of key resources to plausible actions. Here, we detail development of a state‐and‐transition simulation model (STSM) to evaluate buffelgrass (Cenchrus ciliaris L. syn Pennisetum ciliare (L.) Link) in Saguaro National Park (SNP), Arizona, USA, through collaboration between managers and researchers. We integrate expert knowledge and research to create and parameterize a stochastic, spatially explicit STSM to evaluate specific management objectives. We also develop a dynamic link between the STSM and a fire behavior model to allow exploration of potential novel processes introduced to the ecosystem by buffelgrass invasion. Our projections show that buffelgrass can be expected to increase on the landscape and that the integration of fire into the model accelerates the projected rate of invasion and increases degradation of resources of management concern. We highlight the benefit of engaging end users in the modeling process so that the model is targeted to evaluate management objectives, in this case retention of saguaro cacti (Carnegiea gigantea (Engelm.) Britton &amp; Rose) on the landscape. Being able to integrate an external model that can help address the unique characteristics of a problem such as the introduction of fire into the SNP desert ecosystem increases the ability of simulations to address complex ecological and management questions.

Invasive buffelgrass detection using high‐resolution satellite and UAV imagery on Google Earth Engine

AbstractMethods to detect and monitor the spread of invasive grasses are critical to avoid ecosystem transformations and large economic costs. The rapid spread of non‐native buffelgrass(Pennisetum ciliare) has intensified fire risk and is replacing fire intolerant native vegetation in the Sonoran Desert of the southwestern US. Coarse‐resolution satellite imagery has had limited success in detecting small patches of buffelgrass, whereas ground‐based and aerial survey methods are often cost prohibitive. To improve detection, we trained 2 m resolution DigitalGlobe WorldView‐2 satellite imagery with 12 cm resolution unmanned aerial vehicle (UAV) imagery and classified buffelgrass on Google Earth Engine, a cloud computing platform, using Random Forest (RF) models in Saguaro National Park, Arizona, USA. Our classification models had an average overall accuracy of 93% and producer's accuracies of 94–96% for buffelgrass, although user's accuracies were low. We detected a 2.92 km2 area of buffelgrass in the eastern Rincon Mountain District (1.07% of the total area) and a 0.46 km2 area (0.46% of the total area) in the western Tucson Mountain District of Saguaro National Park. Buffelgrass cover was significantly greater in the Sonoran Paloverde‐Mixed Cacti Desert Scrub vegetation type, on poorly developed Entisols and Inceptisol soils and on south‐facing topographic aspects compared to other areas. Our results demonstrate that high‐resolution imagery improve on previous attempts to detect and classify buffelgrass and indicate potential areas where the invasive grass might spread. The methods demonstrated in this study could be employed by land managers as a low‐cost strategy to identify priority areas for control efforts and continued monitoring.

Mapping saguaro cacti using digital aerial imagery in Saguaro National Park

Population research on the saguaro cactus (Carnegiea gigantea) has historically been limited to repeat monitoring efforts in small plots with areas of 1 to 4 hectares. Large extent saguaro censuses of areas >2.59 km2 have not been undertaken. This research developed an automated shadow detection method for mapping mature saguaros in digital aerial imagery using saguaro shadows as proxies for saguaro locations. The shadow method detected 446,092 saguaro shadows in aerial imagery of 231 km2 of the Saguaro National Park (SNP) near Tucson, Arizona. The results were validated against saguaro location data in 11 small plots provided by SNP staff. The shadow method correctly identified 58% of mature saguaros in these 11 plots. Twenty-one percent of the saguaros in the plots did not produce a shadow signature in the imagery. Fieldwork was conducted in two plot locations to investigate reasons why these mature saguaros failed to produce shadow signatures in the aerial imagery, which showed that vegetation interference, rocky slopes, and shadow alignments were the primary factors that precluded shadow signatures from appearing in the imagery. This research provides a methodology for automated large-area saguaro mapping, which will be useful in long-term population monitoring and saguaro ecology research.

The Iconic Saguaro Cactus Across its Landscape

All photographs in this section are provided by authors of papers in our scientific journals and are used by permission. All copyrights reserved. The saguaro cactus (Carnegiea gigantea) is a long-lived columnar cactus that is among the most well-studied plants in the world. Long-term research indicates that saguaro establishment is generally episodic and strongly influenced by precipitation and temperature. Drought conditions can reduce survivorship of recently germinated saguaros up to 100%. Long-term drought will likely cause saguaro populations to decline as older or vulnerable saguaros die without new cohorts replacing them. However, the relative importance of local habitat variability and the coinciding drought impacts on establishing populations remain largely unexplored. We modeled saguaro establishment in response to drought in various habitat and soil types at Saguaro National Park. Recent, severe drought coincided with drastic declines in saguaro establishment that has seen few saguaros establishing in recent years. Overall, saguaro establishment was best explained by the interaction of drought and habitat type. The best model identified bajada and foothill plots as responding somewhat similarly to drought regardless of severity but foothill plots outperformed bajada plots regardless of drought severity. Furthermore, the predicted number of saguaros to establish in bajada or foothill plots dropped to near zero under the most severe drought but remained higher in slope plots, suggesting that the most suitable habitat type for establishing saguaros has shifted during the recent drought. These results reveal that saguaro establishment strongly correlates with drought but that the impact of drought varies with local habitats and their associated physical characteristics. Overall, our study shows that incorporating the range of local variability in how plants may respond to past climate conditions is important for predicting future response to climate change. The following image gallery is meant to illustrate the demographic complexity of saguaros and how the interactive dynamics of desert climate with landscape position influence population regeneration trends in Saguaro National Park. Photo by D. E. Winkler. Photo by D. E. Swann Photo by J. L. Conver. Photo by D. E. Winkler. Photo by D. E. Winkler. Photo by D. E. Swann. Photo by D. E. Winkler. Photo by D. E. Winkler. Photo by J. L. Conver. Photo by J. L. Conver. These photographs illustrate the article “The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea)” by Winkler et al., published in Ecology 99(3), https://doi.org/10.1002/ecy.2124

Forecasting an invasive species’ distribution with global distribution data, local data, and physiological information

AbstractUnderstanding invasive species distributions and potential invasions often requires broad‐scale information on the environmental tolerances of the species. Further, resource managers are often faced with knowing these broad‐scale relationships as well as nuanced environmental factors related to their landscape that influence where an invasive species occurs and potentially could occur. Using invasive buffelgrass (Cenchrus ciliaris), we developed global models and local models for Saguaro National Park, Arizona, USA, based on location records and literature on physiological tolerances to environmental factors to investigate whether environmental relationships of a species at a global scale are also important at local scales. In addition to correlative models with five commonly used algorithms, we also developed a model using a priori user‐defined relationships between occurrence and environmental characteristics based on a literature review. All correlative models at both scales performed well based on statistical evaluations. The user‐defined curves closely matched those produced by the correlative models, indicating that the correlative models may be capturing mechanisms driving the distribution of buffelgrass. Given climate projections for the region, both global and local models indicate that conditions at Saguaro National Park may become more suitable for buffelgrass. Combining global and local data with correlative models and physiological information provided a holistic approach to forecasting invasive species distributions.

Urban growth and landscape connectivity threats assessment at Saguaro National Park, Arizona, USA

ABSTRACTUrban and exurban expansion results in habitat and biodiversity loss globally. We hypothesize that a coupled-model approach could connect urban planning for future cities with landscape ecology to consider wildland habitat connectivity. Our work combines urban growth simulations with models of wildlife corridors to examine how species will be impacted by development to test this hypothesis. We leverage a land use change model (SLEUTH) with structural and functional landscape-connectivity modeling techniques to ascertain the spatial extent and locations of connectivity related threats to a national park in southern Arizona, USA, and describe how protected areas might be impacted by urban expansion. Results of projected growth significantly altered structural connectivity (80%) when compared to current (baseline) corridor conditions. Moreover, projected growth impacted functional connectivity differently amongst species, indicating resilience of some species and near-complete displacement of others. We propose that implementing a geospatial-design-based model will allow for a better understanding of the impacts management decisions have on wildlife populations. The application provides the potential to understand both human and environmental impacts of land-system dynamics, critical for long-term sustainability.

The interaction of drought and habitat explain space-time patterns of establishment in saguaro (Carnegiea gigantea).

The long-lived columnar saguaro cactus (Carnegiea gigantea) is among the most studied plants in the world. Long-term studies have shown saguaro establishment to be generally episodic and strongly influenced by precipitation and temperature. Water limitation through lower-than-average seasonal rainfall and elevated temperatures increasing evaporative loss can reduce survivorship of recent germinates. Thus, multi-year, extended drought could cause populations to decline as older saguaros die without replacement. Previous studies have related establishment to temporal variation in rainfall, but most studies have been on non-randomized plots in ideal habitat and thus might not have captured the full variability within the local area. We studied how saguaro establishment varied in space and which habitat features may buffer responses to drought on 36 4-ha plots located randomly across an elevation gradient, including substantial replication in landscape position (bajada, foothills, and slopes) in the two disjunct districts of Saguaro National Park in southern Arizona, USA. Recent, severe drought coincided with drastic declines in saguaro establishment across this ~25,000-ha area. Establishment patterns derived from the park-wide data set was strongly correlated with drought, but the Park's two districts and diversity of plots demonstrated substantially different population outcomes. Saguaro establishment was best explained by the interaction of drought and habitat type; establishment in bajada and foothill plots dropped to near-zero under the most severe periods of water limitation but remained higher in slope plots during the same time span. Combined with saguaro density estimates, these data suggest that the most suitable habitat type for saguaro establishment shifted to higher elevations during the time span of the recent drought. These results place into context the extent to which historical patterns of demography provide insight into future population dynamics in a changing climate and reveal the importance of understanding dynamics across the distribution of possible local habitat types with response to variation in weather.

Estimating Annual Vertebrate Mortality on Roads at Saguaro National Park, Arizona

Road-killed vertebrates are a conspicuous effect of roads on animals, particularly in natural preserves where wildlife is protected. Knowledge of the number of vertebrates killed by vehicles in a national park or other natural area is important for managers, but these numbers are difficult to estimate because such mortality patterns vary greatly in space and time and by taxonomic group. Additionally, animals killed by vehicles may be difficult to observe, particularly during driving surveys, and carcasses may not persist between surveys due to scavenging and other factors. We modified an estimator previously developed for determining bird mortality at wind turbines to estimate the average annual number of vertebrates killed by cars on roads within and along the boundary of Saguaro National Park, Tucson, Arizona. Our model incorporates estimates of carcass (hereafter, roadkill) persistence and detectability (determined, respectively, by conducting surveys on 8 consecutive days and by comparing simultaneous walking and driving surveys) with data from regular roadkill surveys conducted throughout the park over a 6-year period. Using this model, we estimated that an average of 29,377 (SE 6,807) vertebrates (approximately 1.1/km/day; SE 0.24) were killed annually during 1994 to 1999 on 76.6 km of roads associated with the park. The majority of killed animals were amphibians and small reptiles, but birds and mammals also were killed in large numbers. The amount of wildlife killed in and near reserves by vehicles may be higher than appreciated by many managers and should be factored into within-park and regional transportation planning.

Demographic changes over >70 yr in a population of saguaro cacti (Carnegiea gigantea) in the northern Sonoran Desert

We studied changes in a population of saguaro (Carnegiea gigantea) cacti on a 259 ha plot in the northern Sonoran Desert (Saguaro National Park) that was first surveyed in 1941. With the help of citizen scientists, we mapped and measured the height of all detected individual saguaros. The total number of detected saguaros was 13,304 in 1941 and 9023 in 2012, a 31.4% decrease. The population in 1941 consisted of mostly larger, older individuals, whereas in 2012 it was dominated by smaller, younger individuals. Based on estimated years of saguaro establishment, a period of high establishment in the mid-late 1800s was followed by many decades of poor establishment. However, establishment surged from the 1970s to early 1990s but has been very low since. Comparisons with other long-term saguaro studies suggest that the dramatic demographic changes in this area have resulted from climatic factors, most notably recent long-term drought and higher temperatures, in combination with land-use changes, especially loss of nurse trees due to wood-cutting early in the twentieth century.

Elusive does not always equal rare: genetic assessment of a protected Gila monster (Heloderma suspectum) population in Saguaro National Park, Arizona

Population genetic baselines for species perceived to be at-risk are crucial for monitoring population trends and making well-informed management decisions. We characterized the genetic status of a population of Gila monsters (Heloderma suspectum), a large venomous lizard native to deserts of the southwestern United States and northern Mexico, by sampling 100 individuals in Sonoran Desert upland habitat at Saguaro National Park, Arizona, USA. We used 18 microsatellite markers, along with 1195 bp of sequence data from the mitochondrial DNA 12S locus, to examine genetic diversity, estimate effective population size, and assess demographic history. Despite suburban development adjacent to the study area, we observed high genetic diversity with uninhibited gene flow within this protected population. We estimated effective population size (Ne) for the total sample area (80 km2) using the linkage disequilibrium method in NeEstimator to be 94 individuals (95% confidence interval: 80.7-111.2). In 2011, we used capture-recapture methods to estimate that 80 adult Gila monsters (95% CI = 37-225) inhabited the area along the 14-km transect that we surveyed most frequently; probability of detecting resident Gila monsters during surveys was &lt;0.01, highlighting the challenges of studying the species. Despite being considered an elusive and thus potentially rare species, these data reveal that in this protected environment the population appears healthy and robust. The results provide an important genetic baseline for future studies and monitoring, and exemplify the success of protective population measures in National Parks and under Arizona state laws.