Geology topics

Kathleen D. Brundige

Publications and source records attributed to Kathleen D. Brundige.

5 recordsLinked to original sources

USGS2023

High female desert tortoise mortality in the western Sonoran Desert during California’s epic 2012–2016 drought

We conducted population surveys for desert tortoises Gopherus agassizii at 2 nearby sites in the western Sonoran Desert of California, USA, from 2015-2018, during the driest ongoing 22 yr period (2000-2021) in the southwestern USA in over 1200 yr. We hypothesized that drought-induced mortality would be female-biased due to water and energy losses attributable to egg production during protracted periods of resource limitation. At the higher-elevation, cooler, wetter Cottonwood site from 2015-2016, the sex ratio of live adult tortoises was biased toward males and the sex ratio of tortoises estimated to have died during the intensified drought conditions from 2012-2016 was essentially even. At the lower-elevation, warmer, drier Orocopia site from 2017-2018, the sex ratio of live adult tortoises was biased toward males and the sex ratio of tortoises with estimated times of death from 2012-2016 was biased toward females. High female mortality at the Orocopia site may have resulted from the interaction of drought effects and the bet-hedging reproductive strategy of tortoises wherein they continue to produce clutches of eggs in drought years. Annual reproductive output results in an estimated loss of up to 13.5% of female tortoise body mass including over 0.20 l of water. Combined with dehydration during severe droughts, these losses may compromise their ability to survive droughts lasting more than 2 yr. The low tortoise density and high mortality of females observed may reflect reduced survival of tortoises near the southern edge of their range due to climate change, including protracted and intensified droughts.

Western Sonoran Desert↗

USGS2020

Micro-geographic variation in burrow use of Agassiz’s desert tortoises in the Sonoran Desert of California

Little has been published regarding the burrowing habits of Agassiz’s desert tortoises ( Gopherus agassizii ) in the Sonoran Desert of California. We monitored the interactions of tortoises with their burrows, and other tortoises, via radio-telemetry at two nearby sites between the Cottonwood and Orocopia Mountains, from 2015-2018. We examined how annual cycles of drought and non-drought years, behaviourally affected how tortoises use their burrows (i.e., burrow fidelity, cohabitation, and location), including the timing of the tortoise brumation period. Burrow locations were strongly dependent on local geology and topography, with a tendency to orientate in conformance with the general aspect of the landscape. The timing of brumation was similar to records for G. agassizii throughout their range (with a few exceptions). There was no difference in the estimated number of burrows used per 30 days between the active seasons (2017 and 2018) at the Orocopia site, despite the occurrence of drought in 2018.

California↗

USGS2020

Refining genetic boundaries for Agassiz’s desert tortoise (Gopherus agassizii) in the western Sonoran Desert: The influence of the Coachella Valley on gene flow among populations in southern California

Understanding the influence of geographic features on the evolutionary history and population structure of a species can assist wildlife managers in delimiting genetic units (GUs) for conservation and management. Landscape features including mountains, low elevation depressions, and even roads can influence connectivity and gene flow among Agassiz’s desert tortoise (Gopherus agassizii) populations. Substantial changes in the landscape of the American Southwest occurred during the last six million years (including the formation of the Gulf of California and the lower Colorado River), which shaped the distribution and genetic structuring of tortoise populations. The area northwest of the Gulf of California is occupied by the Salton Trough, including the Coachella Valley at its northern end. Much of this area is below sea level and unsuitable as tortoise habitat, thus forming a potential barrier for gene flow. We assessed genetic relationships among three tortoise populations separated by the Coachella Valley. Two adjacent populations were on the east side of the valley in the foothills of the Cottonwood and Orocopia mountains separated by Interstate 10. The third population, Mesa, was located about 87 km away in the foothills of the San Bernardino Mountains at the far northwestern tip of the valley. The Cottonwood and Orocopia localities showed genetic affiliation with the adjacent Colorado Desert GU immediately to the east, and the Mesa population exhibited affiliation with both the Southern Mojave and Colorado Desert GUs, despite having a greater geographic distance (0.5x–1.5x greater) to the Colorado Desert GU. The genetic affiliation with the Colorado Desert GU suggests that the boundary for that GU needs to be substantially extended to the west to include the desert tortoise populations around the Coachella Valley. Their inclusion in the Colorado Desert GU may benefit these often overlooked populations when recovery actions are considered.

California↗

USGS2018

Reproductive output and clutch phenology of female Agassiz's desert tortoises (Gopherus agassizii) in the Sonoran Desert region of Joshua Tree National Park

Agassiz's desert tortoise ( Gopherus agassizii ) reaches the southern edge of its range in the Sonoran Desert of California. The reproductive ecology of this wideranging species is understudied here compared to populations in the adjacent Mojave Desert. Understanding potential geographic variation in reproductive ecology is important for effective management of conservation-reliant species like G. agassizii . We studied the fecundity and clutch phenology of female G. agassizii at two study sites in the Sonoran Desert region of Joshua Tree National Park over five years (1997–1999, 2015–2016) spanning two of the strongest El Niño events on record and an epic drought. Across all years, mean clutch size was 4.3±1.5 eggs, mean clutch frequency was 1.78 clutches/female/year, and mean X-ray egg width was 36.51±1.56 mm, all of which are comparable to other published studies both in the Sonoran and Mojave deserts of California. Our results generally support earlier published findings that G. agassizii utilize a bet-hedging strategy of consistently producing small clutches almost every year, even during times of low annual primary productivity. A regionally warmer climate in the Sonoran Desert of California appears to have an effect on the timing of egg production, as the earliest dates of egg visibility in our study (April 6) were approximately two weeks earlier than the earliest dates reported for G. agassizii in the Mojave Desert. Shelled eggs were no longer visible in tortoises after mid-June in all years but the El Niño year 1998, when eggs were visible until mid-July.

California↗

USGS2016

A potential predator-prey interaction of an American badger and an Agassiz's desert tortoise with a review of badger predation on turtles

The federally threatened Agassiz’s desert tortoise ( Gopherus agassizii ) was listed under the U.S. Endangered Species Act in 1990, but thus far, recovery efforts have been unsuccessful (U.S. Fish and Wildlife Service [USFWS] 2015). Predation has been identified as a contributing factor to declining G. agassizii populations range-wide (e.g., Esque et al. 2010, Lovich et al. 2014). Understanding and managing for predator-prey dynamics is thus an important part of the recovery and conservation of this threatened species (USFWS 2011). Desert tortoises have a host of predators at all stages of their life cycle. Over 20 species of birds, mammals, and reptiles have been recorded as known or suspected predators (Woodbury and Hardy 1948, Luckenbach 1982, Ernst and Lovich 2009). American badgers ( Taxidea taxus , family: Mustelidae) are confirmed excavators of desert tortoise nests (Turner and Berry 1984). They are also suspected predators of adult desert tortoises, a possibility which has been presented in some studies but without empirical verification (Luckenbach 1982, Turner and Berry 1984). Active mostly at night, badgers are solitary, secretive predators (Lindzey 1978, 1982; Armitage 2004) that are extremely difficult to observe in predatory encounters. Recently, strong circumstantial evidence presented by Emblidge et al. (2015) suggests that badgers do prey on adult Agassiz’s desert tortoises based on observations of more than two dozen dead tortoises in the Western Mojave Desert of California. In this note, we present another case of potential badger predation on a large adult desert tortoise in the Sonoran Desert of California. Collectively, these recent two cases potentially indicate that badger predation may be more common and widespread than previously thought. In addition, we review the worldwide literature of badger predation on turtles in general and summarize reported badger observations in Joshua Tree National Park, where our observation occurred, over a period of 55 years.

California↗