Aaron D. Flesch

Potential Effects of the United States-Mexico Border Fence on Wildlife

Security infrastructure along international boundaries threatens to degrade connectivity for wildlife. To explore potential effects of a fence under construction along the U.S.-Mexico border on wildlife, we assessed movement behavior of two species with different life histories whose regional persistence may depend on transboundary movements. We used radiotelemetry to assess how vegetation and landscape structure affect flight and natal dispersal behaviors of Ferruginous Pygmy-Owls (Glaucidium brasilianum), and satellite telemetry, gene-flow estimates, and least-cost path models to assess movement behavior and interpopulation connectivity of desert bighorn sheep (Ovis canadensis mexicana). Flight height of Pygmy-Owls averaged only 1.4 m (SE 0.1) above ground, and only 23% of flights exceeded 4 m. Juvenile Pygmy-Owls dispersed at slower speeds, changed direction more, and had lower colonization success in landscapes with larger vegetation openings or higher levels of disturbance (p < or = 0.047), which suggests large vegetation gaps coupled with tall fences may limit transboundary movements. Female bighorn sheep crossed valleys up to 4.9 km wide, and microsatellite analyses indicated relatively high levels of gene flow and migration (95% CI for F(ST)=0.010-0.115, Nm = 1.9-24.8, M =10.4-15.4) between populations divided by an 11-km valley. Models of gene flow based on regional topography and movement barriers suggested that nine populations of bighorn sheep in northwestern Sonora are linked by dispersal with those in neighboring Arizona. Disruption of transboundary movement corridors by impermeable fencing would isolate some populations on the Arizona side. Connectivity for other species with similar movement abilities and spatial distributions may be affected by border development, yet mitigation strategies could address needs of wildlife and humans.

Importance of environmental and spatial gradients on patterns and consequences of resource selection

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.

Population Trends and Implications for Monitoring Cactus Ferruginous Pygmy Owls in Northern Mexico

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.

Detectability and Response Rates of Ferruginous Pygmy-Owls

Abstract 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 (𝑥̄ ± SE, n = 520), with 99 ± 0.4% of owls detected in ≤8 minutes; response distance averaged 251 ± 7 m, with 91 ± 1% of owls detected at ≤400 m. Response time decreased by an average of 4 ± 2% and response distance decreased by 12 ± 3 m with each half-month period from early courtship through brooding (P ≤ 0.035). Response time averaged 39 ± 24% faster during morning than midday at occupied nests. Detectability was 1.0 ± 0.0 when surveyors were 100 m from occupied nests and decreased to 0.78 ± 0.10 when surveyors were 500 m from occupied nests. Detectability was higher during incubation, brooding, and natal dispersal (0.89 ± 0.05–1.0 ± 0.0) than during fledgling-dependency (0.50 ± 0.20–0.67 ± 0.19). Response rates of males did not vary from early courtship to brooding (P = 0.84), yet those of females decreased systematically to zero across the same period (P < 0.001). Because detectability of pygmy-owls remains consistently high during nesting, response rates generated from carefully designed surveys can provide reliable estimates of occupancy and abundance.

Herpetofauna of the Rincon Mountains, Arizona

Abstract We estimated species richness, distribution, and relative abundance of herpetofauna across a 1,800-m elevation gradient in the Rincon Mountains and compared patterns of occurrence of species with other mountain ranges in southeastern Arizona. We detected 2,378 individuals of 40 species during 596 h of visual-encounter surveys, and 1,793 individuals including six additional species incidentally during 2001 and 2002. Based on probabilistic methods, we estimate that as many as 57 species likely are present in the Rincon Mountains; past observations and expert opinion also suggest as many as 57 species likely are present. Species richness declined with increasing elevation. Relative abundance declined with increasing elevation for seven species, increased with elevation for three species, and was greatest at middle elevations for three species. The Rincon Mountains lack some species that occur in nearby mountain ranges of similar size and elevation, possibly due to climatic or historical factors. Our study was the first detailed inventory of herpetofauna in the Rincon Mountains at a time when these communities may be undergoing significant change.

Spatial, Temporal, and Density-Dependent Components of Habitat Quality for a Desert Owl

Spatial variation in resources is a fundamental driver of habitat quality but the realized value of resources at any point in space may depend on the effects of conspecifics and stochastic factors, such as weather, which vary through time. We evaluated the relative and combined effects of habitat resources, weather, and conspecifics on habitat quality for ferruginous pygmy-owls (Glaucidium brasilianum) in the Sonoran Desert of northwest Mexico by monitoring reproductive output and conspecific abundance over 10 years in and around 107 territory patches. Variation in reproductive output was much greater across space than time, and although habitat resources explained a much greater proportion of that variation (0.70) than weather (0.17) or conspecifics (0.13), evidence for interactions among each of these components of the environment was strong. Relative to habitat that was persistently low in quality, high-quality habitat buffered the negative effects of conspecifics and amplified the benefits of favorable weather, but did not buffer the disadvantages of harsh weather. Moreover, the positive effects of favorable weather at low conspecific densities were offset by intraspecific competition at high densities. Although realized habitat quality declined with increasing conspecific density suggesting interference mechanisms associated with an Ideal Free Distribution, broad spatial heterogeneity in habitat quality persisted. Factors linked to food resources had positive effects on reproductive output but only where nest cavities were sufficiently abundant to mitigate the negative effects of heterospecific enemies. Annual precipitation and brooding-season temperature had strong multiplicative effects on reproductive output, which declined at increasing rates as drought and temperature increased, reflecting conditions predicted to become more frequent with climate change. Because the collective environment influences habitat quality in complex ways, integrated approaches that consider habitat resources, stochastic factors, and conspecifics are necessary to accurately assess habitat quality.

Influence of local and landscape factors on distributional dynamics: a species-centred, fitness-based approach

In spatially structured populations, distributional dynamics are driven by the quantity, connectivity and quality of habitat. Because these drivers are rarely measured directly and simultaneously at relevant scales, information on their relative importance remains unclear. I assessed the influence of both direct and indirect measures of local habitat quality, and of landscape habitat amount and connectivity on long-term territory occupancy dynamics of non-migratory pygmy owls. Direct measures of local habitat quality based on territory-specific reproductive output had greater effects on distribution than landscape factors, but only when spatio-temporal fluxes in performance linked to environmental stochasticity and intraspecific competition were considered. When habitat quality was measured indirectly based on habitat structure, however, landscape factors had greater effects. Although all landscape factors were important, measures of landscape connectivity that were uncorrelated with habitat amount and based on attributes of matrix structure and habitat configuration that influence dispersal movements had greater effects than habitat effective area (amount weighted by quality). Moreover, the influence of connectivity (but not habitat effective area) depended on local habitat quality. Such results suggest the relative importance of local habitat quality in driving distribution has been underestimated and that conservation strategies should vary spatially depending on both local and landscape contexts.

Avian Richness, Status, and Conservation in the Northwestern Neotropics in Sonora, Mexico

ABSTRACT We assessed the status and richness of birds in a remote region of northwest Mexico and provided the first description of a bird community in foothills thornscrub vegetation across the full annual cycle of seasons. Our efforts were part of a broad program to describe biodiversity and conservation value of the Northern Jaguar Reserve. This large private reserve was recently proposed for federal protection, in part, because it supports the northernmost breeding population of jaguars (Panthera onca) and extensive riparian woodlands along the largest free-flowing river in western Mexico. The reserve and surrounding region are dominated by foothills thornscrub, which is a vegetation community that covers a vast transition zone between tropical forest and desert-scrub in northwest Mexico but is not represented in the federal system of Natural Protected Areas. Bird species richness was similar to that found in other protected areas in western Mexico. Over seven years, we observed 214 species of birds and estimate that 241 species are present (95% CI = 225–257). Community composition was temporally dynamic; 49% of species were winter residents or passage migrants, 31% were permanent residents, and 15% were summer residents. We observed 10 species that were breeding or wintering north of the previously described limits of their geographic ranges. We observed 13 species of conservation concern in Mexico, including the Bald Eagle (Haliaeetus leucocephalus) and Military Macaw (Ara militaris) that occur at the extreme southern and northern edges of their breeding ranges, respectively. High environmental variation and unique biogeographic and physiographic settings in this region promote high bird species richness and high conservation value.

Correction: Spatial, Temporal, and Density-Dependent Components of Habitat Quality for a Desert Owl

The captions for Figs ​Figs22 and ​and33 are incorrectly switched. The caption for Fig 3 should be the caption for Fig 2, and the caption for Fig 3 should be the caption for Fig 2. Please see the corrected captions here. Fig 2 Effect of habitat factors on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010. Fig 3 Interactive effects of abundance of potential nest sites and other habitat factors on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.