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Increases in the incidence and severity of drought threaten the viability of rare plants in arid regions. The endangered Nichol's Turk's head cactus (Echinocactus horizonthalonius Lemaire var. nicholii L. Benson) occurs only in four small, isolated populations in the Sonoran Desert of North America. Since 1995 we have monitored a population in southeastern Arizona (USA). Here we report 23 years of observations on abundance, growth, mortality, flowering and recruitment. Abundance of plants decreased from 132 in 1996 to 40 in 2017, with 100 individuals recruited and 203 dying during the study. Individual plants grew slowly, increasing annually by an average of 0.22 cm (95% confidence interval, 0.18–0.26 cm) in diameter and 0.27 cm (0.20–0.33 cm) in height. Growth was slowest when drought was most severe and slowed as plants reached maximum size. Annual mortality increased markedly across the study period and did not vary with plant size. Annual probability of flowering increased as plants increased in diameter but not in height, and varied with precipitation and drought but not with mean annual temperature. Recruitment was higher when average temperature was higher and the number of recruits per capita increased across the study period. The annual rate of change in abundance averaged −6%, but shifted markedly from −1% during 1995–2008 to −11% during 2008–2017. Our results indicate that the population's decline was not a consequence of failed recruitment but of increased mortality, which we discuss in the context of climate and herbivory. more...

Many of the processes that govern the viability of animal populations vary spatially, yet population viability analyses (PVAs) that account explicitly for spatial variation are rare. We develop a PVA model that incorporates autocorrelation into the analysis of local demographic information to produce spatially explicit estimates of demography and viability at relatively fine spatial scales across a large spatial extent. We use a hierarchical, spatial, autoregressive model for capture-recapture data from multiple locations to obtain spatially explicit estimates of adult survival (ϕad ), juvenile survival (ϕjuv ), and juvenile-to-adult transition rates (ψ), and a spatial autoregressive model for recruitment data from multiple locations to obtain spatially explicit estimates of recruitment (R). We combine local estimates of demographic rates in stage-structured population models to estimate the rate of population change (λ), then use estimates of λ (and its uncertainty) to forecast changes in local abundance and produce spatially explicit estimates of viability (probability of extirpation, Pex ). We apply the model to demographic data for the Sonoran desert tortoise (Gopherus morafkai) collected across its geographic range in Arizona. There was modest spatial variation in λ ^ (0.94-1.03), which reflected spatial variation in ϕ ^ ad (0.85-0.95), ϕ ^ juv (0.70-0.89), and ψ ^ (0.07-0.13). Recruitment data were too sparse for spatially explicit estimates; therefore, we used a range-wide estimate ( R ^ = 0.32 1-yr-old females per female per year). Spatial patterns in demographic rates were complex, but ϕ ^ ad , ϕ ^ juv , and λ ^ tended to be lower and ψ ^ higher in the northwestern portion of the range. Spatial patterns in Pex varied with local abundance. For local abundances >500, Pex was near zero ( 0.25). This approach to PVA offers the potential to reveal spatial patterns in demography and viability that can inform conservation and management at multiple spatial scales, provide insight into scale-related investigations in population ecology, and improve basic ecological knowledge of landscape-level phenomena. more...

In grassland ecosystems, invasions by nonnative grasses typically decrease floristic diversity and structural heterogeneity in ways that alter the quantity and quality of habitat for animals. Grassland arthropods that rely directly on herbaceous plants for food, shelter, or as substrates for reproduction are especially vulnerable to these invasions because many have evolved specialized relationships with host plants that might be displaced. We evaluated how invasions by nonnative grasses affected abundance and richness of foliage-dwelling arthropods in semidesert grasslands of Arizona, USA. On 90, 3.1-ha plots established along a gradient of invasion where dominance of nonnative grasses ranged from 0 to nearly 100% of grass cover, we captured > 90,000 arthropods from 11 orders during 270 surveys in 2014 and 2015. Although the invasion by nonnative grasses (primarily Eragrostis lehmanniana and secondarily E. curvula) increased the amount of herbaceous foliage available to arthropods, richness of arthropods decreased by an average of 2% and total abundance by an average of 7% for every 10% increase in nonnative-grass dominance. Responses to the plant invasion, however, varied among taxa and functional groups. As dominance of nonnative grasses increased, abundances of most predators and specialist herbivores decreased, whereas abundances of most generalist herbivores were lowest at intermediate points of the invasion gradient. The changes we observed in the arthropod community have potential to alter broad-scale ecological processes, including energy flow and nutrient cycling, and to reduce food resources for insectivores, which can have adverse, cascading effects on imperiled grassland ecosystems. more...

Understanding demography of vertebrate populations requires quantifying reproductive success. For taxa that are difficult to distinguish, estimates of reproductive success can be biased if species are misidentified or if breeding concludes before identification is confirmed. We surveyed desert grasslands where three species of cryptic sparrows breed: Peucaea botterii, P. cassinii, and Aimophila ruficeps (Emberizidae). Nests, eggs, and nestlings of these species are similar, making it difficult to differentiate nests without observing adults, which can be challenging. We collected seven types of material from nests for DNA analysis: maternal cells from exterior surfaces of unhatched eggs, epithelial cells from the oropharyngeal cavity of nestlings, eggshells, feathers, feather sheaths, feces, and fecal sacs. From these materials, we amplified and sequenced a fragment of the diagnostic locus, mitochondrial cytochrome b (cyt b), and analyzed the data in a phylogenetic framework to classify nests to species. We validated our classification by sequencing the same locus from feathers of museum specimens. Overall, 72% of samples from nests yielded high-quality sequences. We identified to species 44 of 51 nests and identified museum specimens with archival ages of up to 47 years. Our study extends previous research by demonstrating the efficacy of standard kits, inexpensive reagents, low DNA concentrations, and diverse materials in classifying nests of grassland sparrows. Compared to more invasive methods of acquiring DNA, the approaches we describe are less likely to affect nesting behavior and bias estimates of nesting success of birds, issues especially important for species of conservation concern. more...

Lands modified by humans vary widely in their environmental features, from areas that retain much of their natural character to areas in the urban core that retain little natural character. For example, a city that was built on lands that were once covered by a forest might be dominated at its core by pavement and buildings and support only a few trees, many of which are nonnative species. Areas surrounding the urban core usually are residential neighborhoods composed of private homes and small parks, where trees are more common. On the outskirts of the city are exurban areas dominated by natural forest vegetation with only scattered houses and other human structures. Consequently, urban areas often represent a gradient of development1 that spans an array of natural and anthropogenic features, many of which influence the probability of an area being inhabited by a species (figure 4.1). Some raptors are capable of inhabiting urban environments at one or more points along this gradient, provided the areas support their specific habitat resources and match their tolerance of human activity.2,3 more...

Montane riparian areas across the Sky Island region of the southwestern United States and northwestern Mexico contribute disproportionally to biodiversity at local and regional scales. We explored how composition and structure of vegetation influenced species richness and community composition of songbirds breeding in riparian areas of southern Arizona. We surveyed birds and vegetation during two breeding seasons at 45 stations established across three riparian canyons in two mountain ranges. Species richness of songbirds increased as volume of understory vegetation, density of Arizona sycamore (Platanus wrightii), and density of alligator juniper (Juniperus deppeana) increased; species richness decreased as density of Acacia and Mimosa species increased and oak (Quercus) trees increased. Composition of breeding bird communities was structured along an elevation and vegetation gradient, from open-canopy desert scrub to closed-canopy oak woodlands. Richness and composition of breeding bird communi... more...

By altering ecosystem structure and function, invasions by nonnative plants have the potential to alter the quantity and quality of habitat for animals. We examined effects of buffelgrass (Pennisetum ciliare), a nonnative grass that is increasing in distribution markedly throughout the southwestern United States and northern Mexico, on demographic characteristics and condition of Sonoran desert tortoises (Gopherus morafkai). In 2010 and 2011, we established 50 4-ha plots that spanned the gradient of buffelgrass cover in areas with environmental features characteristic of high-quality habitat for tortoises in southern Arizona. We surveyed each plot four times per year to characterize density, population structure, and condition of tortoises. We detected tortoises on 45 of 50 plots (90 %) and ≥1 tortoise during 114 of 200 surveys (57 %). We used a hierarchical model to estimate density of tortoises while accounting for imperfect detection rates. Density of tortoises averaged 0.35 individuals/ha (SE = 0.04) and did not vary appreciably with the amount of buffelgrass cover; similarly, age and sex structure of tortoise populations did not vary with buffelgrass cover. Condition of adult tortoises, however, averaged 10 % lower in areas where cover of buffelgrass was high (>15 %) relative to areas where buffelgrass was absent or cover was low ( more...

The rate of somatic growth determines when individuals transition between life stages, which along with survival and reproduction, are principal factors governing the rate of population change. For short-lived species that inhabit seasonally dynamic environments, accounting for fluctuations in somatic growth is necessary to make reliable inferences about population dynamics. We describe a Bayesian, state-space formulation of a von Bertalanffy growth model that integrates a sinusoidal model to allow for seasonal fluctuations in growth while also accounting for individual heterogeneity and measurement error. We use this model to describe post-metamorphic growth of canyon treefrogs, Hyla arenicolor, based on capture-recapture data from 404 individuals over a two-year period. For simulated data where we assumed growth varies seasonally, our model provides unbiased estimates of growth rate, mean asymptotic length, standard deviation of individual asymptotic lengths, and date of maximum growth. For field data from canyon treefrogs, we found strong evidence of seasonal variation in growth, with maximum growth during the summer monsoon season. Growth rate of females was 19 % lower than males, although on average, females reached asymptotic lengths that were 15 % greater than males. Ignoring systematic intra-annual variation in growth can bias inferences about population dynamics, particularly for fast-growing species that reproduce more than once per year or inhabit environments with strong seasonal signals. We present a straightforward approach for using repeated length measurements from individuals of unknown age to estimate growth while accounting for seasonality and individual heterogeneity, which are sources of variation common in many vertebrate populations. more...

The abundance and distribution of woody plants have increased in grassland ecosystems worldwide. Robust generalizations about the consequences of this transformative process on animal communities have been elusive, especially in semiarid regions where populations of many species have declined. We evaluated how distributions and species richness of breeding birds responded to woody plant encroachment by using spatial variation in woody cover as a proxy for the temporal process by which grasslands transform into shrub savannas. Specifically, we surveyed breeding birds and vegetation on 140 10-ha plots in semiarid grasslands that spanned the gradient of cover by Prosopis (mesquite), a genus of shrubs that has proliferated in semiarid grasslands worldwide. We used a multispecies occupancy model to characterize distributions of breeding bird species along the encroachment gradient. Distributions of 29 of 35 species changed markedly in response to encroachment, with distributions of most obligate grassland species contracting and most facultative grassland species expanding. Species richness increased sharply as cover of woody plants increased and peaked at ∼22% cover; this increase was driven by recruitment of generalist and shrub-associated species, many of which are common at regional scales. Lastly, we identified thresholds of woody cover where distributions contracted or expanded markedly, which provide targets for conservation and restoration efforts. Our results highlight the importance of understanding species-specific responses to woody plant encroachment as the basis for explaining community-level patterns because increases in diversity at local scales might ultimately reduce diversity at broader scales as grassland specialists are displaced. more...

Woody vegetation in grasslands and savannas has increased worldwide over the past 100–200 years. This phenomenon of “woody plant encroachment” (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to woody plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of woody plant expansion. Although probably not a triggering factor, rising atmospheric CO2 levels may have favored C3 woody plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna ecosystems and its endemic biodiversity. Traditional management goals aimed at restoring forage and livestock production after WPE have broadened to support a more diverse portfolio of ecosystem services. Accordingly, we focus on how WPE and management actions aimed at reducing woody plant cover influence carbon sequestration, water yield, and biodiversity, and discuss the trade-offs involved when balancing competing management objectives. more...

Standardized protocols for surveying secretive marsh birds have been implemented across North America, but the efficacy of surveys to detect population trends has not been evaluated. We used survey data collected from populations of marsh birds across North America and simulations to explore how characteristics of bird populations (proportion of survey stations occupied, abundance at occupied stations, and detection probability) and aspects of sampling effort (numbers of survey routes, stations/route, and surveys/station/year) affect statistical power to detect trends in abundance of marsh bird populations. In general, the proportion of survey stations along a route occupied by a species had a greater relative effect on power to detect trends than did the number of birds detected per survey at occupied stations. Uncertainty introduced by imperfect detection during surveys reduced power to detect trends considerably, but across the range of detection probabilities for most species of marsh birds, variation in detection probability had only a minor influence on power. For species that occupy a relatively high proportion of survey stations (0.20), have relatively high abundances at occupied stations (2.0 birds/station), and have high detection probability (0.50), � 40 routes with 10 survey stations per route surveyed 3 times per year would provide an 80% chance of detecting a 3% annual decrease in abundance after 20 years of surveys. Under the same assumptions but for species that are less common, � 100 routes would be needed to achieve the same power. Our results can help inform the design of programs to monitor trends in abundance of marsh bird populations, especially with regards to the amount of sampling effort necessary to meet programmatic goals. 2013 The Wildlife Society more...

Invasions by nonnative plants have changed the structure of many terrestrial ecosystems and altered important ecological processes such as fire, the dominant driver in grassland ecosystems. Reestablishing fire has been proposed as a mechanism to restore dominance of native plants in grasslands invaded by nonnative plants, yet fire may function differently in these altered systems, potentially affecting animals in novel ways. To assess whether invasions by nonnative plants alter the effects of fire on animals, we performed a manipulative experiment in semi-desert grasslands of southeastern Arizona that have been invaded by a perennial, nonnative grass from Africa, Lehmann lovegrass (Eragrostis lehmanniana). We applied fire to 36 of 54 1-ha plots established along an invasion gradient where dominance of E. lehmanniana ranged from 0% to 91% of total live plant biomass. Over the 5-year period from 2000 to 2004, we used mark-recapture methods to assess how population and community attributes of small ... more...

Effective conservation requires strategies to monitor populations efficiently, which can be especially difficult for rare or elusive species where field surveys require high effort and considerable cost. Populations of many reptiles, including Sonoran desert tortoises (Gopherus agassizii), are challenging to monitor effectively because they are cryptic, they occur at low densities, and their activity is limited both seasonally and daily. We compared efficiency and statistical power of 2 survey methods appropriate for tortoises and other rare vertebrates, line-transect distance sampling and site occupancy. In 2005 and 2006 combined, we surveyed 120 1-km transects to estimate density and 40 3-ha plots 5 times each to estimate occupancy of Sonoran desert tortoises in 2 mountain ranges in southern Arizona, USA. For both mountain ranges combined, we estimated density to be 0.30 adult tortoises/ha (95% CI = 0.17–0.43) and occupancy to be 0.72 (95% CI = 0.56–0.89). For the sampling designs we evalua... more...

Many aquatic species in the arid southwestern United States are imperiled, persisting primarily in isolated, low-order streams that are increasingly vulnerable to stochastic disturbances. During 2003 and 2004, we surveyed 39 mountain canyons in southeastern Arizona, USA, for lowland leopard frogs (Rana yavapaiensis), a species that has declined in abundance and distribution across its range in the United States. We quantified habitat features at 2 spatial scales, canyon and pool, to identify features that distinguished sites inhabited by frogs from those uninhabited by frogs. Canyons inhabited by frogs had watersheds that averaged 8.1 km2 larger (SE = 2.52), pools that averaged 37.8 m3 greater (9.30) in volume, gradients that averaged 4.1% (1.40%) less steep, and locations that averaged 3.2 km closer (1.06) to the nearest valley stream than did uninhabited canyons. Plunge pools inhabited by frogs averaged 13.5% (5.66%) more perimeter vegetation, 11.2% (5.34%) more canopy cover, and 1.9 (0.60) m... more...

Because invasions by nonnative plants alter the structure and composition of native plant communities, invasions can alter the function of ecosystems for animals that depend on plants for food and habitat. We quantified effects of an invasion by a nonnative grass on the insect community in grasslands of southeastern Arizona. We sampled insects on 54 1-ha plots established across a gradient of invasion by Lehmann lovegrass (Eragrostis lehmanniana Nees), a perennial species native to southern Africa. Between 2000 and 2004, we captured 94,209 insects representing 13 orders, 91 families, and 698 morphospecies during 2,997 trap nights. Richness of families, richness of morphospecies, and overall abundance of insects decreased as dominance of nonnative grass increased. With every 100 g/m2 increase in biomass of nonnative grass, the average number of insect families decreased by 5%, morphospecies decreased by 6%, and overall abundance decreased by 14%. In areas dominated by nonnative grass, 2 of 8 orders and 6 of 27 families of insects were present less frequently and one family was present more frequently; 5 of 8 orders and 6 of 27 families of insects were less abundant and 3 families were more abundant than in areas dominated by native grasses. As a result, this plant invasion altered the structure of the insect community, which has consequences for animals at higher trophic levels and for ecosystem processes, including decomposition and pollination. Because complete eradication of nonnative plants might be possible only rarely, maintaining stands of native vegetation in invaded areas may be an important practical strategy to foster persistence of animals in grasslands invaded by nonnative plants. more...

Inferences about abundance often are based on unadjusted counts of individuals observed, in part, because of the large amount of data required to generate reliable estimates of abundance. Where capture-recapture data are sparse, aggregating data across multiple sample elements by pooling species, locations, and sampling periods increases the information available for modeling detection probability, a necessary step for estimating abundance reliably. The process of aggregating sample elements involves balancing trade-offs related to the number of aggregated elements; although larger aggregates increase the amount of information available for estimation, they often require more complex models. We describe a heuristic approach for aggregating data for studies with multiple sample elements, use simulated data to evaluate the efficacy of aggregation, and illustrate the approach using data from a field study. Aggregating data systematically improved reliability of model selection and increased accuracy of abundance estimates while still providing estimates of abundance for each original sample unit, an important benefit necessary to maintain the design and sampling structure of a study. Within the framework of capture-recapture sampling, aggregating data improves estimates of abundance and increases the reliability of subsequent inferences made from sparse data. Additional tables and datasets may be found in the online supplements. more...

The distribution of desert tortoises (Gopherus agassizii) spans a wide range of biotic and abiotic conditions in the southwestern United States and northwestern Mexico, with physical and behavioral differences distinguishing tortoises inhabiting the Mojave Desert from those inhabiting the Sonoran Desert. Relative to tortoise populations in the Mojave Desert, populations in the Sonoran Desert have not been well-studied. To assess how habitat use of desert tortoises in the Sonoran Desert was influenced by topography, vegetation, geomorphology, and soil, we surveyed 40 randomly located 3-ha sites for presence of adult tortoises within a site-occupancy framework. We modeled both occupancy and detection probability as a function of environmental features, and compared those results with a logistic regression model that assumed detection probability was equal to 1. Results from both approaches agreed, suggesting that habitat selection of tortoises in the Sonoran Desert was influenced primarily by topographic and geomorphologic features rather than by vegetation. Specifically, tortoises were more likely to occupy sites that were steep (we detected tortoises on 29% of sites with mean slope 15°) and predominantly east-facing (53% of sites with 20% facing E), and less likely to occupy north-facing slopes (100% of sites with 60% facing N). Our results contrast with patterns of habitat use in the Mojave Desert where tortoises primarily occupy valley bottoms. Habitat use of tortoises in Sonoran and Mojave Desert populations differ considerably, contributing to the mounting body of evidence suggesting that these geographically distinct populations may represent separate species. more...

Fire and invasions by nonnative plants can change the structure and function of ecosystems, and independent effects of each of these processes have been well studied. When fire is restored to areas where it has been excluded and the native plant communities have been invaded by nonnative species, changes in vegetation structure and composition are likely to alter the fire regime. These changes, in turn, might alter the effects of fire on wildlife and wildlife habitat. In this paper, we develop a framework to evaluate whether fire and plant invasion act as independent, additive processes, or whether applying fire in invaded areas results in novel effects on wildlife. We explore changes in abundance of three small mammal species in response to experimental fires set along a gradient of dominance by Lehmann lovegrass (Eragrostis lehmanniana), an African bunchgrass that has invaded semidesert grasslands of the southwestern USA. For two of three species of small mammals, the effect of fire on abundance varied with the degree of invasion, suggesting a fire × invasion interaction. In systems dominated by nonnative plants, fire can function differently than it did prior to invasion, especially for animals with habitat requirements that match conditions created by the invading plant species. Consequently, prescriptions for restoration fires will need to consider the novel effects of fires on native plants and animals in areas where the plant community has changed. more...

We studied a population of Cooper’s hawks (Accipiter cooperii) in Tucson, Arizona from 1994 to 2005. High rates of mortality of nestlings from an urban-related disease prompted speculation that the area represented an ecological trap and habitat sink for Cooper’s hawks. In this paper, we used estimates of survival and productivity from 11years of monitoring to develop an estimate of the rate of population change, λ, for Cooper’s hawks in the area. We used a Cormack–Jolly–Seber approach to estimate survival of breeding hawks, and a stochastic, stage-based matrix to estimate λ. Despite the urban-related disease, the estimate of λ indicated that the area does not function as a habitat sink for Cooper’s hawks ( $$\widehat\lambda $$ = 1.11 ± 0.047; P = 0.0073 for the null of λ ≤ 1). Because data required to reliably identify habitat sinks are extensive and difficult to acquire, we suggest that the concept of habitat sinks be applied cautiously until substantiated with reliable empirical evidence. more...

Survey techniques that are both reliable and efficient are necessary to accurately estimate population parameters, especially for rare species. Cactus ferruginous pygmy-owls (Glaucidium brasilianum cactorum; hereafter pygmy-owls) have declined in southwestern North America and are surveyed often to comply with federal law. We studied owl responses to broadcasted calls to quantify how detectability and response rates (owls/station/transect) vary with environmental, spatial, temporal, and weather-related factors. We surveyed owls along 392 transects (1,113 km) throughout Sonora, Mexico, including a subset of 14 transects (47.2 km) that we surveyed repeatedly to assess factors that affected response rates. We challenged 17 adults and 23 juveniles that were radiomarked, adults attending 50 occupied nests, and adults attending 6 groups of radiomarked juveniles to respond to broadcasted calls to assess factors that affected detectability. Across Sonora, response time averaged 2.6 ± 0.1 minutes (𝑥 ± SE... more...

Dynamics of many amphibian populations are governed by the distribution and availability of water. Therefore, understanding the hydrological mechanisms that explain spatial and temporal variation in occupancy and abundance will improve our ability to conserve and recover populations of vulnerable amphibians. We used 16 years of survey data from intermittent mountain streams in the Sonoran Desert to evaluate how availability of surface water affected survival and adult recruitment of a threatened amphibian, the lowland leopard frog (Lithobates yavapaiensis). Across the entire study period, monthly survival of adults ranged from 0.72 to 0.99 during summer and 0.59 to 0.94 during winter and increased with availability of surface water (Z = 7.66; P < 0.01). Recruitment of frogs into the adult age class occurred primarily during winter and ranged from 1.9 to 3.8 individuals/season/pool; like survival, recruitment increased with availability of surface water (Z = 3.67; P < 0.01). Although abundance of frogs varied across seasons and years, we found no evidence of a systematic trend during the 16-year study period. Given the strong influence of surface water on population dynamics of leopard frogs, conservation of many riparian obligates in this and similar arid regions likely depends critically on minimizing threats to structures and ecosystem processes that maintain surface waters. Understanding the influence of surface-water availability on riparian organisms is particularly important because climate change is likely to decrease precipitation and increase ambient temperatures in desert riparian systems, both of which have the potential to alter fundamentally the hydrology of these systems. more...

Although once described as common in lowland central and southern Arizona (Bendire 1888, Fisher 1893, Breninger 1898, Gilman 1909, Bent 1938), cactus ferruginous pygmy owls (Glaucidium brasilianum cactorum; hereafter, pygmy owls) have been extirpated throughout much of their former range in Arizona. As a result, this northernmost subspecies of ferruginous pygmy owls (Van Rossem 1937, Johnsgard 1988) are now listed as endangered by the U.S. Fish and Wildlife Service (USFWS 1997, Hardy et al. 1999). Immediately south of Arizona in northern Sonora, Mexico, pygmy owls occur primarily in desert-scrub and grassland vegetation communities where woodlands are near stands of large saguaro cacti (Carnegia gigantea) (Flesch 2003a). Because pygmy owls are thought to be abundant in northern Sonora, these populations may prove critical for recovery in Arizona as well as for long-term persistence of pygmy owls in the Sonoran Desert. Natural or facilitated dispersal of pygmy owls from Sonora may augment populations in Arizona, especially when combined with habitat management (USFWS 2003). Numerous threats to pygmy-owl habitat exist in northern Sonora, however, including woodcutting, vegetation clearing for agriculture or buffelgrass (Pennisetum ciliare), and overgrazing, and there are few regulatory mechanisms in place to protect habitat. Despite the importance of pygmy-owl populations in northern Sonora, there are currently no data on population trends or on design parameters for population monitoring. Therefore, between 2000 and 2004 we monitored relative abundance of pygmy owls in northern Sonora, estimated temporal variation in relative abundance, and assessed the efficacy of different sampling designs for monitoring population trends. Our goals were to assess population trends, determine environmental factors that explained variation in trends, and evaluate the statistical power of our monitoring program for future monitoring efforts. more...

Understanding how the distribution and abundance of food resources influence space use of organisms is an important element of successful conservation and recovery strategies for endangered species. We investigated interrelationships between space use, activity patterns, and food resources for lesser long-nosed bats (Leptonycteris curasoae), an endangered nectar-feeding bat, during an energetically demanding phase of its annual life cycle. We estimated the size of home ranges (95% kernel areas) and core use-areas (50% kernel areas) of bats and estimated density of their forage plant (Agave palmeri) in and near these use areas. Density (x± SE plants/ha) of flowering agaves within home ranges (3.6 ± 1.04) exceeded that which was available on the landscape (1.8 ± 0.36), indicating that bats selected areas with high food abundance. Although density of agaves within home ranges of bats differed in successive years (1998: 3.6 ± 1.04; 1999: 0.8 ± 0.15), sizes of home ranges and core use-areas of adult ... more...

We examined factors influencing foraging behavior of lesser long-nosed bats, Leptonycteris curasoae, in southeastern Arizona. When L. curasoae are present in this region, their diet is restricted to nectar and pollen from 1 species, Agave palmeri, which has heightened conservation concerns for the plant as forage for this endangered bat. We found that visitation rates of L. curasoae to individual A. palmeri near a roost were high (mean = 273 ± 17 visits per plant per hour) and varied with the spatial distribution and morphological characteristics of individual plants. Specifically, visitation rates varied with time of night, distance and orientation from the bat roost, and number and relative vertical position of flowers along the inflorescence. We suggest that both spatial distribution and temporal variation in flowering chronology be considered when developing strategies to manage A. palmeri to support L. curasoae. more...

On the Colorado Plateau, some environments occupied by Mexican Spotted Owls ( Strix occidentalis lucida ) receive a great deal of recreational use. To assess the effects of hikers on breeding owls, we quantified changes in the activity budgets of owls at nests in response to a controlled amount of hiking in canyons of southern Utah in 1997 and 1998. We examined differences in the duration and frequency of eight behaviors as well as in the type and frequency of owl vocalizations between hiking and control treatments. In general, activity budgets of owls did not change markedly when hikers were near nests, although during hiking treatments, females decreased the amount of time they handled prey by 57% and decreased the amount of time they performed daytime maintenance behaviors by 30%. Further, hikers caused both females and males to increase the frequency of contact vocalizations by 58% and 534%, respectively. The order in which we applied treatments at nests ( control-hiking or hiking-control ) also influenced owl responses to hiking treatments, suggesting that observers near nests may have affected owl behavior. We concluded that the cumulative effects of high levels of short-duration recreational hiking near nests may be detrimental to Mexican Spotted Owls. Given current levels of visitation rates to most remote canyons occupied by owls, however, owl populations on the Colorado Plateau are not likely threatened by hiking. Notable exceptions are those canyons that receive use by ≥50 hikers per day. We recommend monitoring of owl occupancy, nest success, and hiking intensity in these high-use canyons. Resumen: El la Meseta de Colorado, algunos ambientes ocupados por Buhos Manchados Mexicanos ( Strix occidentalis lucida ) reciben altos niveles de actividad recreativa. Para evaluar los efectos de excursionistas sobre buhos reproductores, cuantificamos cambios en presupuestos de actividad de buhos anidantes en respuesta a niveles controlados de excursionistas en canones del sur de Utah en 1997 y 1998. Examinamos diferencias en la duracion y frecuencia de ocho conductas. asi como del tipo y frecuencia de vocalizaciones de buhos entre tratamientos de excursionismo y control. En general, los presupuestos de actividad de los buhos no cambiaron marcadamente cuando habia excursionistas cerca de los nidos, aunque durante los tratamientos de excursionismo las hembras redujeron el tiempo en que manejan presas en 57% y disminuyeron el tiempo en que presentaron conductas de mantenimiento diurno en 30%. Mas aun, los excursionistas causaron que tanto hembras como machos incrementaran la frecuencia de vocalizaciones de contacto en 58% y 534% respectivamente. El orden en que aplicamos los tratamientos en los nidos ( control-excursionismo, excursionismo-control ) tambien influyo en las respuestas de los buhos a los tratamientos de excursionismo, lo que sugiere que observadores cercanos a los nidos pudieron afectar el comportamiento de los buhos. Concluimos que los efectos acumulativos de altos niveles de pequena duracion de excursionismo recreativo cerca de nidos pueden ser perjudiciales para Buhos Manchados Mexicanos. Sin embargo, dados los niveles actuales de tasas de visitacion a los canones mas remotos ocupados por buhos, es probable que las poblaciones de buhos en la Meseta de Colorado no esten amenazadas por el excursionismo. Notables excepciones son aquellos canones que son utilizados por 50 excursionistas/dia. Recomendamos monitorear la ocupacion por buhos, el exito de anidacion y la intensidad de excursionismo en estos canones de alto uso. more...

Some animals modify the environment in ways that can influence the resources available to other species. Because red squirrels (Tamiasciurus hudsonicus) create large piles of conifer-cone debris (middens) in which they store cones, squirrels concentrate resources that might affect biodiversity locally. To determine whether other animals are attracted to midden sites beyond their affinity for the same resources that attract red squirrels, we assessed associations between middens, mammals, and birds at population and community levels. We surveyed 75 middens where residency rates of red squirrels varied during the previous five years; sampling along this residency gradient permitted us to evaluate the influence of resources at middens beyond the influence of a resident squirrel. At each location, we quantified vegetation, landscape structure, abundance of conifer cones, and midden structure, and used capture-recapture, distance sampling, and remote cameras to quantify presence, abundance, and species richness of mammals and birds. Red squirrels and the resources they concentrated at middens influenced mammals and birds at the population scale and to a lesser extent at the community scale. At middens with higher residency rates of red squirrels, richness of medium and large mammals increased markedly and species richness of birds increased slightly. After accounting for local forest characteristics, however, only species richness of medium-to-large mammals was associated with a red squirrel being resident during surveys. In areas where red squirrels were resident during surveys or in areas with greater amounts of resources concentrated by red squirrels, abundances of two of four small mammal species and two of four bird species increased. We conclude that the presence of this ecosystem modifier and the resources it concentrates influence abundance of some mammals and birds, which may have implications for maintaining biodiversity across the wide geographic range inhabited by red squirrels and other larderhoarding animals. more...

Professional biologists assume a variety of roles during their careers. The potential role of advocate has come under increased debate and scrutiny by the public, resource users, legislators, and professional biologists themselves. We believe that biologists should offer their opinions on resource issues if those opinions are based on data, experience, and insight. As much as possible, professionals should resist involving personal philosophies, agendas, and emotions in favor of the most objective stance possible, realizing that there will be instances when they will be asked for their opinion when sufficient data are lacking, but biologists should not always defer to others to make interpretations and construe implications of data without their participation. This approach is best described as professional or expert opinion. Professionals should guard their unique position of providing reliable information and expert opinions while striving for objectivity and impartiality. For many professionals, contro... more...

Vegetation structure and floristic composition strongly influence the structure of bird communities. To assess the influence of vegetation and other environmental characteristics on songbirds, we quantified nest-site characteristics and reproductive success of a riparian songbird community in Arizona. Although we found interspecific variation in characteristics associated with nest sites, we identified two suites of species that chose sites with similar characteristics. These “nest groups” were explained largely by nest height and characteristics of nest trees. Overall, nest success was low for songbirds in this community, and averaged 23%. The most common cause of nest failure was predation (81%), although brood parasitism by Brown-headed Cowbirds (Molothrus ater) was highest at nests of Bell's Vireos (Vireo bellii) (29%). No vegetation or environmental features were associated with the likelihood of cowbird parasitism for any species; nest success for Bell's Vireos was negatively associated with the amount of netleaf hackberry (Celtis reticulata) in the understory. Arizona sycamore (Platanus wrightii) and netleaf hackberry trees contained 41% and 17% of all nests, respectively, and therefore provide critically important nesting substrates for birds in this rare yet diverse vegetation community. more...

To assess the consequences of increased recreational activity in wilderness areas, we studied the effects of human activity on breeding behavior of Bald Eagles (Haliaeetus leucocephalus) in interior Alaska. Activity budgets of breeding eagles changed considerably when humans were camped for 24 h at a distance of 100 m from nests (treatment) compared to when they were camped 500 m from nests (control) (P = 0.0036). With humans near nests, adult eagles decreased the time they preened (percentage change from control to treatment = −53%), slept (−56%), maintained nests (−50%), and fed themselves and their nestlings (−30%) and increased the time they brooded nestlings (+14%). Further, overall activity (total number of behaviors performed by adults at nests per day) decreased by 27% with humans near nests, as did the amount of prey adults consumed (−26%) and fed to nestlings (−29%). In contrast, nest attendance did not change with humans near nests (percentage change = 0.3%, P = 0.9); however, the time adults w... more...

Along narrow rivers, spatial restriction of human use based on wildlife responses can effectively eliminate the entire river corridor from human use. Therefore, if river use by both wildlife and humans is a goal, an alternative management strategy is necessary. We measured flush response rate and flush distance of breeding and nonbreeding Bald Eagles (Haliaeetus leucocephalus) to recreational boating along the Gulkana River in interior Alaska from 1989 to 1992. Eagle responses to our nonmotorized boat were governed by the context within which human-eagle encounters occurred. Flush response rate of nonbreeding eagles decreased as perch height and its distance from the river's edge increased, increased as the season progressed and as eagle group size increased, was lower for juveniles (20%) than other age classes (49-65%), and varied with the existing level of human activity in a geographic location (P < 0.01 for all parameters). Flush distance of nonbreeding eagles increased as the distance a disturbance was first visible to a perched eagle increased, as perch height and its distance from the river's edge increased, and as the season progressed. In contrast to flush response, flush distance was strongly associated with age and was greatest for adults, least for juveniles, and intermediate for subadults. Breeding adults were much less likely to flush than nonbreeding adults, and flushed at lesser distances. We recommended that along narrow wilderness rivers, the impacts of human activity on Bald Eagle populations be regulated with temporal, rather than spatial, restrictions. more...

Determining which vegetation types organisms perceive similarly and classifying these types into groups that function as similar habitats are necessary steps toward expanding the focus of conservation strategies from single species to ecosystems. Therefore, the methods used to determine these habitat classifications are crucial to the successful design and implementation of these conservation strategies. Typically, this process has been accomplished through best professional judgement. We used quantitative techniques to group vegetation types into habitats based on the occurrence of breeding wildlife species (n = 420) in Oregon. After calculating faunal similarities among all regional vegetation types (n = 130), we used cluster analysis to group vegetation types into wildlife habitats. We classified the original 130 vegetation types into 30 wildlife habitat types that we believe function similarly. We tested this classification to assess whether vegetation types could be correctly classified into habitat types based on wildlife species composition. Discriminant analysis correctly classified 95% of the vegetation types into their wildlife habitat types, strengthening our confidence in this approach. This approach for classifying habitat types allows consistent development of conservation strategies at coarse resolutions and aids in identifying vegetation types where additional biodiversity surveys are needed. Finally, this approach can be refined continuously as the precision of vegetation mapping and our understanding of organism-habitat associations improve. Determinar que tipos de vegetacion son percibidos en forma similar por los organismos y clasificar estos tipos en grupos que funcionan como habitats similares, constitutyen pasos necesarios para expandir el foco de las estrategias de conservacion, pasando de especies individuales a ecosistemas. Por consiguiente, los metodos utilizados para determinar la clasificacion de estos habitats resultan cruciales para et diseno e implementacion de estas estrategias de conservacion. Tipicamente, este proceso ha sido llevado a cabo a traves del mejor juicio profesional. Utilizamos tecnicas cuantitativas para agrupar los tipos de vegetacion en habitats, basadas en la ocurrencia de especies silvestres durante el periodo reproductivo (n = 420) en Oregon. Despues de calcular las similitudes faunisticas entre tados los tipos regionales (n = 130), utilizamos analisis de agrupamiento para agrupar los tipos de vegetacion en habitats de vida silvestre. Clasificamos los 130 tipos vegetales originales en 30 tipos de habitats de vida silvestre, que nosotros consideramos funcionan en forma semejante. Probamos esta clasificacion para evaluar si los tipos de vegetacion pueden ser clasificados correctamente en tipos de habitats basados en la composicion de especies silvestres. El analisis discriminante clasifico correctamente un 95% de los tipos de vegetacion en sus tipos de habitat silvestres, aumentando nuestra confianza en este metodo. Este metodo para la clasificacion de tipos de habitat permite un desarrollo consistente de las estrategias de conservacion a resoluciones bajas y ayuda en la identificacion de tipos vegetales en aquellos casos en que son necesarios estudios adicionales de la biodiversidad. Finalmente, este metodo puede ser continuamente mejorado, en la medida en que la precision del mapeo de la vegetacion y nuestra comprension de las asociaciones entre organismos y habitats mejoren. more...

Although many species may be vulnerable to changes in climate, forecasting species-level responses can be challenging given the array of physiological, behavioral, and demographic attributes that might be affected. One strategy to improve forecasts is to evaluate how species responded to climatic variation in the past. We used 22 years of capture-recapture data for Sonoran desert tortoises (Gopherus morafkai) collected from 15 locations across their geographic range in Arizona to evaluate how environmental factors affected spatial and temporal variation in survival. Although rates of annual survival were generally high ([Formula: see text] = 0.92), survival of adults decreased with drought severity, especially in portions of their range that were most arid and nearest to cities. In three locations where large numbers of carcasses from marked tortoises were recovered, survival of adults was markedly lower during periods of severe drought ([Formula: see text] = 0.77-0.81) compared to all other periods ([Formula: see text] = 0.93-0.98). Assuming continued levels of dependency of humans on fossil fuels, survival of adult tortoises is predicted to decrease by an average of 3 % during 2035-2060 relative to survival during 1987-2008 in 14 of the 15 populations we studied. This decrease could reduce persistence of tortoise populations, especially in arid portions of their range. Temporal and spatial variation in drought conditions are important determinants of survival in adult desert tortoises. more...

Strategies to conserve rare species require identifying resources that function as important habitat elements and that promote high demographic performance. Assessing the relative importance of resources, however, can be confounded by natural variation in resource availability and by the hierarchical spatial structure in which resources are organized. Because availability and relative importance of resources often vary across environmental and spatial gradients, we used gradients together with resource selection functions and variance decomposition to assess the relative importance of resources to nest site selection and reproductive performance of Ferruginous Pygmy-Owls (Glaucidium brasilianum). We measured habitat characteristics of 106 nests and paired available sites at five spatial scales across a 220-km gradient of precipitation and vegetation in northwest Mexico, in a region adjacent to the southwestern United States where pygmy-owls have declined to near extinction. Resources explained 76-85% of variation in nest site selection and 21-31% of variation in reproductive performance across all spatial scales combined. Although we found evidence of resource selection at each scale, the magnitude of selection and influence of resources on reproductive performance were greatest where availability of selected resources were low and where temperature extremes and predation risk likely increased the relative importance of these resources. At larger scales, geographic changes in resource use corresponded with changes in availability, whereas at smaller scales, resource use varied little despite changes in availability, suggesting higher specificity and importance of resources at smaller scales. At the smallest scale, owls selected nest cavities with smaller entrances, larger volume, greater height, and orientations that produced cooler microclimates in the hottest regions of the study area; these choices promoted higher reproductive performance. Cavity resources explained more variation in selection and reproductive performance than resources at larger scales, highlighting their importance as conservation targets. High correlation of resource characteristics among spatial scales, however, indicated that selection of resources at small scales depended on characteristics of resources at larger scales. Assessing how resource selection changes in response to underlying variation in resource availability can help prioritize resources most important for conservation and management. more...

By definition, exurban development does not occur in isolation from other environments. Indeed, in many if not most situations, “exurban” is a categorization for a range of development types that occur somewhere between wild or rural lands and cities. Urban environments and the habitats found in cities and suburbs play important roles in the ecological health and biodiversity of adjacent and nearby exurban lands. In this chapter, we review the importance of wildlife and wildlife habitats in metropolitan areas and the influence of these urban habitats on adjacent exurban lands. We also describe how planning and science can work together to develop large-scale land-use plans that advance wildlife conservation goals. The Sonoran Desert Conservation Plan (SDCP), developed for Pima County, Arizona, serves as a case study for this type of conservation planning. more...

Although single-species approaches have played an important role in conservation in the United States, the Endangered Species Act provides a mechanism for conservation at larger scales through Habitat Conservation Plans (HCPs). HCPs not only offer the potential for comprehensive conservation planning for a wide range of species across broader geographic scales but also provide assurances that eliminate risks related to endangered species concerns for nonfederal landowners, developers, and planners. Given their benefits, dozens of municipalities have adopted HCPs to address planning issues related to rare and vulnerable species. The challenge, however, is to develop conservation plans that reliably meet broader-scale conservation and planning objectives while not increasing risks posed to vulnerable species. Consequently, we designed a science-based framework from which to develop regional conservation plans, including HCPs. We designed a rigorous process that classifies areas based on their relative conservation value as part of a conservation strategy for more than 20,000 km2 of Sonoran desert in Pima County, Arizona. This chapter describes our approach including the fundamental planning elements selected, the process used to quantify the relative biological importance of each landscape unit, and how we assembled landscape elements into units that form the framework of the Sonoran Desert Conservation Plan. more...

Context Invasions by non-native plants can alter the abundance and distribution of resources that can affect habitat quality for native animals. Aims We sought to understand the demographic consequences of a plant invasion on a functionally and numerically important rodent in a grassland ecosystem. Specifically, we evaluated how abundance, survival, reproductive activity and population structure of Arizona cotton rats (Sigmodon arizonae) varied across a gradient of invasion by Eragrostis lehmanniana (Lehmann lovegrass), a bunchgrass native to Africa that has invaded grasslands in North America. Methods Over a four-year period, we used capture–recapture methods to survey small mammals on 54 1-ha plots between 10 and 13 times. We used vegetation data collected each autumn to quantify biomass of non-native grass, total biomass and vegetation heterogeneity to characterise vegetation structure on each plot. Key results We captured 1344 individual cotton rats during 106 560 trap-nights across all sampling periods. In areas dominated by non-native grass, abundance of cotton rats increased 7- to 10-fold and survival increased by 117% relative to areas dominated by native grasses. In contrast, reproductive activity of adults decreased by 62% for females and 28% for males, and the proportion of adults in the population decreased by 20% in these same areas. Conclusions Demography of Arizona cotton rats differed markedly in areas invaded by a non-native plant relative to native grasslands, supporting the long-held idea that life histories can reflect local environmental conditions. Because distributions of many non-native plants are predicted to increase in response to future changes in natural and anthropogenic drivers, the potential breadth of these complex effects on communities of native animal is immense. Implications The complex variation in demographic responses across the invasion gradient suggests that it may be necessary to evaluate a suite of vital rates to fully understand the consequences of plant invasions on animals. This is especially important for species of conservation concern because single demographic parameters, which are used frequently as targets to gauge the success of conservation and management activities, could be misleading. more...

Reed and Blaustein (1995) add an important perspective to the debate over amphibian population trends, asserting that if a null hypothesis of no trend is not rejected and the power of the statistical test to detect the trend was low, then the study's results can only be considered inconclusive. Too frequently researchers infer that failure to reject a null hypothesis indicates the null is true, without considering statistical power of these tests and the consequences of committing Type II errors (Peterman 1990). Reed and Blaustein correctly assert that estimating the Type II error rate is essential when evaluating evidence suggesting no decline. Reed and Blaustein attempted to estimate the statistical power of tests using data from studies of eight amphibian populations, none of which demonstrated statistically significant trends with time. Although the authors focused on amphibian populations, they implicitly addressed the more general question of whether it is reasonable to conclude a null hypothesis to be true when it is not rejected (P > cx); in their analysis, the null hypotheses were that no significant correlations existed between amphibian population sizes and time. Reed and Blaustein suggest that it is only reasonable to conclude the null to be true when statistical power is high; we more...

The introduction of nonnative species in the United States has resulted in significant environmental damage and economic losses exceeding $1 billion per year. We assessed the influence of nonnative species on biological diversity in the southwestern United States in systems prone to fire using a rigorous experimental framework. Our specific objectives were to (1) determine %effects of fire season on responses of biotic communities, and (2) quantify% relationships between biological guilds before and after burning and through post-fire recovery. This experiment is taking place within grasslands and Prosopis savannas at the Fort Huachuca Military Reservation. The experiment evaluates the main and interactive effects of dominance by nonnative plants and fire season with three replicates in each of two years. Biomass of Eragrostis lehmanniana declined and persisted following burns for more than 2 years. The degree of the response was dependent on annual precipitation and fire season. Plant species richness did not change with fire treatments but remained lower on plots dominated fire. lehmanniana and higher on native-dominated plots. In general, species richness and relative abundance of small mammals decreased in the first year following fire. However, 2 years after fire, values approached that observed in unburned areas. more...

Model selection based on information-theoretic methods (Burnham and Anderson 2002) has gained prevalence as a strategy for analyzing ecological data, especially among wildlife biologists (Stephens et al. 2005). Development of this alternative strategy has been refreshing because it has encouraged many of us to reexamine the analytical strategies we use and especially to evaluate models built explicitly on a biological foundation. However, some proponents now advocate model selection as the only reasonable strategy for a wide range of analyses. In particular, there is growing sentiment that strategies based on the framework of hypothesis testing are now systematically inferior or inappropriate. Because all analytical strategies have strengths and weaknesses, all can be misused. My goal is to suggest that analysts make an informed choice from all available strategies, employing each in contexts where they are most informative, as the most appropriate strategy will always depend on the specific analytical context. For example, authors of recent texts on distance sampling (Buckland et al. 2001, 2004), an estimation framework rooted firmly in model selection, and authors of a monograph written by acknowledged experts in model selection (Burnham et al. 1987), use hypothesis tests routinely when they are appropriate and informative. Application of a single analytical strategy in all circumstances is inappropriate. Failure to consider important parameters or to obtain sufficient data limits all analytical strategies, including model selection. If sample sizes are small and precision of estimates is low, the ability to distinguish among candidate models will be weak, analogous to having low statistical power for traditional hypothesis tests. Selecting a model solely because it satisfies a fixed criterion such as the lowest Akaike’s Information Criterion (AIC) is potentially as naive an approach as rejecting a null hypothesis of zero effect on the basis of a P-value being marginally less than 0.05. In the absence of additional information such as estimated effect sizes, these approaches are too simplistic to be uniformly reliable as endpoints of an analysis. Model selection can be an elegant and effective strategy to distinguish among candidate models based on data. Reliance on a small, fixed set of candidate models, however, presents an additional potential liability for model selection. If the set of candidate models is incomplete, the resulting inferences will be unreliable (Burnham and Anderson 2002, Johnson and Amland 2004). Therefore, in cases where a strong set of candidate models is not or cannot be developed, model selection may be less effective and less informative than approaches based on hypothesis testing. Consequently, model selection requires more information about the system of interest because inferences are contingent on this set of candidate models. The process of developing candidate models is a strength of model selection when done well and a weakness when done poorly. Although I routinely use both approaches, colleagues and reviewers have suggested increasingly that I use model selection instead of hypothesis testing in contexts where I felt hypothesis testing was more appropriate. Similarly, I have observed authors using model selection when their analyses may have been better informed by hypothesis tests. In general, I find hypothesis testing to be a more informative strategy when there is insufficient information available to formulate a strong set of candidate models. I will use a hypothetical study of habitat associations to illustrate a context where hypothesis testing may be more informative than model selection. Although wildlife ecologists often are chastised for assessing pattern too frequently, these studies are commonplace and can be valuable in the appropriate circumstances. Typically, the short-term goal in these studies is to identify environmental features that explain variation in abundance of a species. The long-term goal often is to identify habitat features that managers can alter to meet a conservation or management objective for a species. On a series of study plots, assume that we estimate abundance, perhaps using a distance-sampling or capture– recapture framework where we choose a model for estimation based on model selection. We then assess associations between abundance and the environmental features measured on each plot. Because we have little a priori information as to which features may be influential, we measure 12 different environmental features that span a range of physical and biological elements. We then seek to identify a subset of features that is closely associated with abundance. To use model selection in its intended spirit, we would need to develop a set of candidate models that potentially link abundance with habitat features that we can differentiate objectively through an information criterion such as AIC. In circumstances where there is a solid foundation for 1 E-mail: steidl@ag.arizona.edu more...

Statistical inference is an important element of science, but these inferences are constrained within the framework established by the objectives and design of a study. The choice of approach to data analysis, while important, has far less consequence on scientific inference than claimed by Sleep et al. (2007). Their principal assertion—that when model selection is used as the approach to data analysis, all studies provide a reliable foundation for distinguishing among mechanistic explanatory hypotheses—is incorrect and encourages faulty inferences. Sleep et al. (2007) overlook the critical distinction between inferences that result from studies designed a priori to discriminate among a set of candidate explanations versus inferences that result from exploring data post hoc from studies designed originally to meet pattern-based objectives. No approach to data analysis, including model selection, has the power to overcome fundamental limitations on inferences imposed by study design. The comments by Sleep et al. (2007) reinforce the need for scientists to understand clearly the inferential basis for their scientific claims, including the roles and limitations of data analysis. more...

Identifying habitat characteristics that influence selection of nest sites can provide information necessary for understanding and managing songbird populations. We quantified patterns of nest-site selection of 7 species in a riparian songbird community (n = 162 nests) at 2 spatial scales in southeastern Arizona, USA: Bell's vireo (Vireo bellii), verdin (Auriparus flaviceps), phainopepla (Phainopepla nitens), summer tanager (Piranga rubra), northern cardinal (Cardinalis cardinalis), blue grosbeak (Guiraca caerulea), and hooded oriole (Icterus cucullatus). We compared vegetation characteristics at nests to points chosen at random both near each nest (nest-patch scale) and within the entire study area (canyon scale). At the nest-patch scale, riparian vegetation-particularly Arizona sycamore (Platanus wrightii) and netleaf hackberry (Celtis reticulata)-was selected strongly by most species. At the canyon scale, most species nested in areas with higher vegetation density and volume than available at random. Managing riparian areas to foster high vegetation density and key structural components, such as sycamore and hackberry trees, is essential for meeting the habitat requirements necessary to maintain abundant and diverse songbird communities in the arid southwestern United States and adjacent Mexico. more...

The Walker Basin Allotment grazing system in central Arizona is designed to allocate resource use under elk (Cervus elaphus L.) and cattle (Bos taurus L.) grazing. The grazing system was designed to promote biologically acceptable levels of forage use on the half of the allotment scheduled for cattle grazing and to rest the other half by attracting elk to pastures recently grazed by cattle. The objectives of our 2-year study were to determine whether the grazing system facilitated proper forage use as defined by recent forage use and residual stubble height guidelines (i.e., 30 to 40% use and an 8- to 10-cm stubble height) and whether the system rested one half of the allotment from elk and cattle grazing. Mean (+/- SEM) total elk and cattle forage use for western wheatgrass (Pascopyrum smithii Rydb.), the key forage species, was 32 and 61% +/- 7 in 1997 and 1998, respectively; corresponding mean (+/- SEM) stubble heights were 11 and 10 cm +/- 0.6. Mean total cattle and elk forage use in 1998 (61%) exceeded the 30 to 40% use guidelines. However, mean end-of-year stubble height was never below 10 cm. The grazing system did not provide half the allotment with complete rest; elk used all study pastures. Elk use was higher in pastures with heavier tree cover and steeper terrain in both years, regardless of where cattle grazing occurred. Elk grazing patterns were apparently more dependent on tree cover and topography than any changes in forage caused by the grazing system. DOI:10.2458/azu_jrm_v55i4_halstead more...