Geology topics

Brett L. Walker

Publications and source records attributed to Brett L. Walker.

9 recordsLinked to original sources

USGS2026

Modeling observational uncertainty improves estimation of breeding-season parameters that inform lek-based monitoring for Centrocercus urophasianus (Greater Sage-Grouse)

Counts of males on leks are widely used as abundance indices to monitor populations of lek-breeding species, but they can be influenced by variation in male attendance, inter-lek movement, and mortality. When there is imperfect observability of males on leks, estimates of lek attendance and inter-lek movement are likely to be biased low. We compared traditional multi-state mark–recapture (MSMR) and MSMR models with state uncertainty (MSMR-SU) to determine how accounting for uncertainty in GPS-derived lek attendance improves inference of rates underlying lek-based abundance indices for male Centrocercus urophasianus (Greater Sage-Grouse). We found that modeling observational uncertainty with MSMR-SU models consistently reduced bias in lek attendance and inter-lek movement estimates for adult and yearling males compared to MSMR models. Estimates of daily maximum male lek attendance (adults: 0.79; yearlings: 0.68) and inter-lek movement rates (adults: 0.10; yearlings: 0.07) from MSMR-SU models were higher than daily maximum lek attendance (adults: 0.74; yearlings: 0.55) and inter-lek movement rates (adults: 0.01; yearlings: 0.04) from MSMR models. Consistent with previous studies, adults exhibited higher lek attendance and mortality rates than yearlings. However, unlike earlier studies, inter-lek movement rates were similar between age classes, with adults tending to move earlier in the lekking season than yearlings. Wind speed and precipitation had a weak negative effect on lek attendance. Greater distance to the nearest known lek strongly reduced inter-lek movement. Accounting for lekking-season mortality shifted the estimated date of maximum lek attendance earlier in the season by 8–9 days for adults and 1–3 days for yearlings. Accounting for state uncertainty due to error in state assignment should reduce bias in demographic estimates for use in wildlife conservation and management decisions.

Colorado, Wyoming↗

USGS2025

Greater sage-grouse seasonal habitat associations: A review and considerations for interpretation and management applications

Habitat features needed by wildlife can change in composition throughout the year, particularly in temperate ecosystems, leading to distinct seasonal spatial-use patterns. Studies of species-habitat associations therefore often focus on understanding relationships within discrete seasonal periods with common goals of prediction (e.g., habitat mapping) and inference (e.g., interpreting model coefficients). Across the range of the greater sage-grouse ( Centrocercus urophasianus ) of western North America, the increasing use of high-frequency tracking devices has led to a surge in habitat association studies covering multiple temporal periods and spatial extents. We reviewed the literature for seasonal habitat association studies corresponding to the second and third orders of selection (Johnson 1980). Our objectives were to summarize the methodological approaches used to estimate habitat associations to aid understanding in cross-study comparisons and identify common habitat features reported as selected or avoided within different seasonal periods. We reviewed 71 second- and third-order studies published from 2007–2023 that evaluated covariates collected in a geographic information system (GIS) and modeled probability of selection or intensity of use. The most common study design evaluated a single level of selection (third-order) and was multi-scale (i.e., covariates were measured at varying spatial grains). The most common model approach estimated habitat associations using resource selection functions (RSFs) fit with logistic regression. Studies mostly focused on the breeding periods and winter, but all seasons throughout the annual cycle were covered. There was clear support for selection of sagebrush and avoidance of trees and rugged terrain across seasons, and strong selection of mesic conditions in summer. However, habitat associations for most covariates were mixed, with proportionally equivalent selection and avoidance reported, even within the same seasons. Different factors hampered cross-study comparisons, including variation in study design, but additional contributors likely included important context-dependent habitat associations, such as functional responses to changing habitat availability. We suggest collaborative studies leveraging multiple datasets can help improve seasonal habitat inference by removing the effects of variable study designs.

Journal of Wildlife Management↗

USGS2021

The birth of a Hawaiian fissure eruption

Most basaltic explosive eruptions intensify abruptly, allowing little time to document processes at the start of eruption. One opportunity came with the initiation of activity from fissure 8 (F8) during the 2018 eruption on the lower East Rift Zone of Kīlauea, Hawaii. F8 erupted in four episodes. We recorded 28 min of high‐definition video during a 51‐min period, capturing the onset of the second episode on 5 May. From the videos, we were able to analyze the following in‐flight parameters: frequency and duration of explosions; ejecta heights; pyroclast exit velocities; in‐flight total mass and estimated mass eruption rates; and the in‐flight total grain size distributions. The videos record a transition from initial pulsating outgassing, via spaced, but increasingly rapid, discrete explosions, to quasisustained, unsteady fountaining. This transition accompanied waxing intensity (mass flux) of the F8 eruption. We infer that all activity was driven by a combination of the ascent of a coupled mixture of small bubbles and melt, and the buoyant rise of decoupled gas slugs and/or pockets. The balance between these two types of concurrent flow determined the exact form of the eruptive activity at any point in time, and changes to their relative contributions drove the transition we observed at early F8. Qualitative observations of other Hawaiian fountains at Kīlauea suggest that this physical model may apply more generally. This study demonstrates the value of in‐flight parameters derived from high‐resolution videos, which offer a rapid and highly time‐sensitive alternative to measurements based on sampling of deposits posteruption.

Island of Hawai'i↗

USGS2020

The effects of management practices on grassland birds—Brewer’s Sparrow ( Spizella breweri breweri )

Keys to Brewer’s Sparrow ( Spizella breweri breweri ) management include maintaining extensive, unfragmented patches of suitable breeding habitat; reducing conifer cover and height; preventing the invasion of conifers and nonnative plants, especially cheatgrass (downy brome [ Bromus tectorum ]); minimizing disturbance to soil; and restricting the use of pesticides and herbicides during the breeding season (April–July). Brewer’s Sparrows have been reported to use breeding habitats with 12–170 centimeter (cm) vegetation height, 2–34 cm visual obstruction reading, 1–74 percent grass cover, less than (<) 19 percent forb cover, 1–65 percent shrub cover, 1–75 percent bare ground, 2–61 percent litter cover, and <1 cm litter depth. During post-fledging dispersal in July, Brewer’s Sparrow adults and young may shift habitat use to nearby aspen ( Populus species [spp.]), riparian shrub, or deciduous mountain shrub habitats, so these habitats also may be important for management.

Professional Paper↗

USGS2020

Petrologic insights into rift zone magmatic interactions from the 2011 eruption of Kīlauea Volcano, Hawaiʻi

The high frequency of historical eruptions at Kīlauea Volcano presents an exceptional opportunity to address fundamental questions related to the transport, storage, and interaction of magmas within rift zones. The Nāpau Crater area on Kīlauea’s East Rift Zone (ERZ) experienced nine fissure eruptions within 50 years (1961–2011). Most of the magma intruded during these frequent eruptions remained stored within the rift zone, creating a potential magma mixing depot within the ERZ. The superbly monitored and sampled 2011 eruption (Puʻu ʻŌʻō episode 59) presents an extraordinary opportunity to evaluate magma mixing processes within the ERZ. Whole-rock, glass, and olivine compositions were determined, not only for lava from the 2011 eruption, but also for a new suite of Nāpau Crater area samples from the 1963, 1965, 1968, 1983, and 1997 eruptions, as well as the previously undocumented 1922 eruption. Whole-rock XRF data revealed two geochemically distinct magma batches for episode 59: one less evolved (∼6·6 wt % MgO, 0·46 wt % K2O) than the other (∼6·2 wt % MgO, 0·58 wt % K2O). Episode 59 lava is remarkably aphyric (∼0·1 vol. % phenocrysts), making use of mineralogy to identify parent magma affinities problematic. Linear compositional trends of whole-rock major and trace elements, and reversely zoned olivine crystals indicate episode 59 lavas underwent magma mixing. Least squares regression calculations and plots of major and trace element data, were used to evaluate whether the episode 59 samples are products of mixing summit-derived magma with residual magma from previous Nāpau Crater area eruptions. The regression results and trace element ratios are inconsistent with previously proposed mixing scenarios, but they do support mixing between summit-derived magma and residual magma from the 1983 and 1997 Nāpau Crater area eruptions. These magmas were stored in physically and chemically distinct pods at depths of 1·6–3·0 km prior to mixing with new magma intruded from the summit to produce the episode 59 lava. One pod contained a fractionated equivalent of 1983 lava, and the other a hybrid of compositions similar to 1983 and 1997 lavas. The petrology of episode 59 lava demonstrates that magmas from two previous eruptions (1983 and 1997) were available to mix with magma intruded from the summit region. This study clarifies the pre-eruptive history of the mixed episode 59 lava, and elucidates the evolution of the volcano's magmatic system in a region of frequent eruptions.

Hawaii↗

USGS2014

Habitat prioritization across large landscapes, multiple seasons, and novel areas: an example using greater sage-grouse in Wyoming

Animal habitat selection is an important and expansive area of research in ecology. In particular, the study of habitat selection is critical in habitat prioritization efforts for species of conservation concern. Landscape planning for species is happening at ever-increasing extents because of the appreciation for the role of landscape-scale patterns in species persistence coupled to improved datasets for species and habitats, and the expanding and intensifying footprint of human land uses on the landscape. We present a large-scale collaborative effort to develop habitat selection models across large landscapes and multiple seasons for prioritizing habitat for a species of conservation concern. Greater sage-grouse ( Centrocercus urophasianus , hereafter sage-grouse) occur in western semi-arid landscapes in North America. Range-wide population declines of this species have been documented, and it is currently considered as “warranted but precluded” from listing under the United States Endangered Species Act. Wyoming is predicted to remain a stronghold for sage-grouse populations and contains approximately 37% of remaining birds. We compiled location data from 14 unique radiotelemetry studies (data collected 1994–2010) and habitat data from high-quality, biologically relevant, geographic information system (GIS) layers across Wyoming. We developed habitat selection models for greater sage-grouse across Wyoming for 3 distinct life stages: 1) nesting, 2) summer, and 3) winter. We developed patch and landscape models across 4 extents, producing statewide and regional (southwest, central, northeast) models for Wyoming. Habitat selection varied among regions and seasons, yet preferred habitat attributes generally matched the extensive literature on sage-grouse seasonal habitat requirements. Across seasons and regions, birds preferred areas with greater percentage sagebrush cover and avoided paved roads, agriculture, and forested areas. Birds consistently preferred areas with higher precipitation in the summer and avoided rugged terrain in the winter. Selection for sagebrush cover varied regionally with stronger selection in the Northeast region, likely because of limited availability, whereas avoidance of paved roads was fairly consistent across regions. We chose resource selection function (RSF) thresholds for each model set (seasonal × regional combination) that delineated important seasonal habitats for sage-grouse. Each model set showed good validation and discriminatory capabilities within study-site boundaries. We applied the nesting-season models to a novel area not included in model development. The percentage of independent nest locations that fell directly within identified important habitat was not overly impressive in the novel area (49%); however, including a 500-m buffer around important habitat captured 98% of independent nest locations within the novel area. We also used leks and associated peak male counts as a proxy for nesting habitat outside of the study sites used to develop the models. A 1.5-km buffer around the important nesting habitat boundaries included 77% of males counted at leks in Wyoming outside of the study sites. Data were not available to quantitatively test the performance of the summer and winter models outside our study sites. The collection of models presented here represents large-scale resource-management planning tools that are a significant advancement to previous tools in terms of spatial and temporal resolution.

Wyoming↗

USGS2012

Interseasonal movements of greater sage-grouse, migratory behavior, and an assessment of the core regions concept in Wyoming

Animals can require different habitat types throughout their annual cycles. When considering habitat prioritization, we need to explicitly consider habitat requirements throughout the annual cycle, particularly for species of conservation concern. Understanding annual habitat requirements begins with quantifying how far individuals move across landscapes between key life stages to access required habitats. We quantified individual interseasonal movements for greater sage-grouse (Centrocercus urophasianus; hereafter sage-grouse) using radio-telemetry spanning the majority of the species distribution in Wyoming. Sage-grouse are currently a candidate for listing under the United States Endangered Species Act and Wyoming is predicted to remain a stronghold for the species. Sage-grouse use distinct seasonal habitats throughout their annual cycle for breeding, brood rearing, and wintering. Average movement distances in Wyoming from nest sites to summer-late brood-rearing locations were 8.1 km (SE = 0.3 km; n = 828 individuals) and the average subsequent distances moved from summer sites to winter locations were 17.3 km (SE = 0.5 km; n = 607 individuals). Average nest-to-winter movements were 14.4 km (SE = 0.6 km; n = 434 individuals). We documented remarkable variation in the extent of movement distances both within and among sites across Wyoming, with some individuals remaining year-round in the same vicinity and others moving over 50 km between life stages. Our results suggest defining any of our populations as migratory or non-migratory is innappropriate as individual strategies vary widely. We compared movement distances of birds marked using Global Positioning System (GPS) and very high frequency (VHF) radio marking techniques and found no evidence that the heavier GPS radios limited movement. Furthermore, we examined the capacity of the sage-grouse core regions concept to capture seasonal locations. As expected, we found the core regions approach, which was developed based on lek data, was generally better at capturing the nesting locations than summer or winter locations. However, across Wyoming the sage-grouse breeding core regions still contained a relatively high percentage of summer and winter locations and seem to be a reasonable surrogate for non-breeding habitat when no other information exists. We suggest that conservation efforts for greater sage-grouse implicitly incorporate seasonal habitat needs because of high variation in the amount of overlap among breeding core regions and non-breeding habitat.

Wyoming↗

USGS2011

Conservation of greater sage-grouse- a synthesis of current trends and future management

Recent analyses of Greater Sage-Grouse (Centrocercus urophasianus) populations indicate substantial declines in many areas but relatively stable populations in other portions of the species? range. Sagebrush (Artemisia spp.) habitats neces-sary to support sage-grouse are being burned by large wildfires, invaded by nonnative plants, and developed for energy resources (gas, oil, and wind). Management on public lands, which con-tain 70% of sagebrush habitats, has changed over the last 30 years from large sagebrush control projects directed at enhancing livestock grazing to a greater emphasis on projects that often attempt to improve or restore ecological integrity. Never-theless, the mandate to manage public lands to provide traditional consumptive uses as well as recreation and wilderness values is not likely to change in the near future. Consequently, demand and use of resources contained in sagebrush land-scapes plus the associated infrastructure to sup-port increasing human populations in the western United States will continue to challenge efforts to conserve Greater Sage-Grouse. The continued widespread distribution of sage-grouse, albeit at very low densities in some areas, coupled with large areas of important sagebrush habitat that are relatively unaffected by the human footprint, sug-gest that Greater Sage-Grouse populations may be able to persist into the future. We summarize the status of sage-grouse populations and habitats, provide a synthesis of major threats and chal-lenges to conservation of sage-grouse, and suggest a roadmap to attaining conservation goals.

Book chapter↗

USGS2004

Effects of management practices on grassland birds: Brewer's sparrow

Information on the habitat requirements and effects of habitat management on grassland birds were summarized from information in more than 5,500 published and unpublished papers. A range map is provided to indicate the relative densities of the species in North America, based on Breeding Bird Survey (BBS) data. Although birds frequently are observed outside the breeding range indicated, the maps are intended to show areas where managers might concentrate their attention. It may be ineffectual to manage habitat at a site for a species that rarely occurs in an area. The species account begins with a brief capsule statement, which provides the fundamental components or keys to management for the species. A section on breeding range outlines the current breeding distribution of the species in North America, including areas that could not be mapped using BBS data. The suitable habitat section describes the breeding habitat and occasionally microhabitat characteristics of the species, especially those habitats that occur in the Great Plains. Details on habitat and microhabitat requirements often provide clues to how a species will respond to a particular management practice. A table near the end of the account complements the section on suitable habitat, and lists the specific habitat characteristics for the species by individual studies. A special section on prey habitat is included for those predatory species that have more specific prey requirements. The area requirements section provides details on territory and home range sizes, minimum area requirements, and the effects of patch size, edges, and other landscape and habitat features on abundance and productivity. It may be futile to manage a small block of suitable habitat for a species that has minimum area requirements that are larger than the area being managed. The Brown-headed Cowbird (Molothrus ater) is an obligate brood parasite of many grassland birds. The section on cowbird brood parasitism summarizes rates of cowbird parasitism, host responses to parasitism, and factors that influence parasitism, such as nest concealment and host density. The impact of management depends, in part, upon a species’ nesting phenology and biology. The section on breeding-season phenology and site fidelity includes details on spring arrival and fall departure for migratory populations in the Great Plains, peak breeding periods, the tendency to renest after nest failure or success, and the propensity to return to a previous breeding site. The duration and timing of breeding varies among regions and years. Species’ response to management summarizes the current knowledge and major findings in the literature on the effects of different management practices on the species. The section on management recommendations complements the previous section and summarizes specific recommendations for habitat management provided in the literature. If management recommendations differ in different portions of the species’ breeding range, recommendations are given separately by region. The literature cited contains references to published and unpublished literature on the management effects and habitat requirements of the species. This section is not meant to be a complete bibliography; a searchable, annotated bibliography of published and unpublished papers dealing with habitat needs of grassland birds and their responses to habitat management is posted at the Web site mentioned below.

Report↗