Multiple Factors Affect a Population of Agassiz's Desert Tortoise (<i>Gopherus agassizii</i>) in the Northwestern Mojave Desert

Berry, Kristin H.; Yee, Julie L.; Coble, Ashley A.; Perry, William M.; Shields, Timothy A.

doi:10.1655/herpmonographs-d-13-00002

Cited by 26 publications

(34 citation statements)

References 33 publications

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“…Corvids prey on tortoises and may have played a role in the reduction of entrants into the study population. Although the effect of corvids on European and North American bird populations was smaller than perceived (Madden et al ), survival of hatchling and juvenile North American desert tortoises ( Gopherus agassizii ) may be affected by predation by common ravens ( Corvus corax ; Kristan and Boarman , Nagy et al ), to the extent that ravens can become a determining factor for desert tortoise population densities and distribution (Berry et al ). In South Africa, pied crows may extensively predate tortoises (Fincham and Lambrechts ).…”

Section: Discussionmentioning

confidence: 99%

Unexpected decline in a population of speckled tortoises

Loehr¹

2017

J Wildl Manag

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Long‐lived, iteroparous species exhibit various mechanisms to overcome temporary unfavorable conditions and promote adult survival and population persistence. Consequently, populations may appear relatively stable while subtle and slow‐progressing threats cause their decline. The speckled tortoise (Homopus signatus) is a long‐lived, iteroparous species in South Africa that is thought to be secure in areas that meet the species’ known requirements. To verify its stability, I monitored a dense speckled tortoise population, integrating 2 mark‐recapture studies (2000–2004 and 2012–2015) in long‐term joint live and dead encounters and POPAN population models. From 2000 to 2015, the study site remained fenced, ungrazed, and was not modified in any way, yet the size‐class frequency distribution, sex ratio, and abundance of speckled tortoises drastically changed. Population numbers decreased 66%, mainly because of the disappearance of juveniles and males. Modeling results identified a reduction in the number of entrants into the population (via births and immigration) as the likely cause of population changes, whereas apparent survival remained high. Observations, in part anecdotal, of increasing numbers of pied crows (Corvus albus) on the study site suggest that increased predation on speckled tortoise hatchlings contributed to the population decline. Examination of rainfall data for 1990–2015 did not reveal evidence of aridification in the study area, a documented cause of reduced reproduction in speckled tortoises. The results of this study caution against the assumption that areas in the range of the speckled tortoise that meet its known requirements can maintain healthy populations. © 2017 The Wildlife Society

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Section: Discussionmentioning

confidence: 99%

Unexpected decline in a population of speckled tortoises

Loehr¹

2017

J Wildl Manag

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“…The indirect effects of transmission lines are not well studied. Of the existing studies that have addressed indirect effects, most have considered grouse (Lammers et al 2007, Coates et al 2008, Coates and Delehanty 2010 or desert tortoises (Gopherus agassizii; Boarman 2003, Berry et al 2013, species of conservation concern potentially preyed upon by corvids and raptors using utility structures as hunting perches. As power lines have proliferated, at least some corvid species appear to have expanded their breeding ranges (Jerzak 2001, Marzluff and Neatherlin 2006, Dwyer et al 2013a or increased their breeding densities ) through utilizing power poles for nesting (Fleischer et al 2008, possibly leading to indirect effects on their prey.…”

Section: Power Linesmentioning

confidence: 99%

Boreal bird abundance estimates within different energy sector disturbances vary with point count radius

Bayne

Leston

Mahon³

et al. 2016

The Condor

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“…, Berry et al. ). While ravens are long known to prey on young tortoises, historically low raven abundances in tortoise habitat meant that the population‐scale impact of this predator was likely minimal (USFWS ).…”

Section: Introductionmentioning

confidence: 99%

“…In the Mojave Desert of the southwestern United States, recent increases in raven abundances add to the list of threats against Mojave Desert tortoise (Gopherus agassizii; hereafter "tortoise") persistence. Tortoise population declines consequent to disease, collection, habitat loss, vehicle strikes, and over-predation led to the listing of tortoise as threatened under California State and U.S. Federal laws (Luckenbach 1982, Esque et al 2010, Berry et al 2013. While ravens are long known to prey on young tortoises, historically low raven abundances in tortoise habitat meant that the population-scale impact of this predator was likely minimal (USFWS 2008).…”

Section: Introductionmentioning

confidence: 99%

Novel management tools for subsidized avian predators and a case study in the conservation of a threatened species

Shields¹,

Currylow

Hanley

et al. 2019

Ecosphere

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Wildlife managers face complex problems in attempting to achieve contemporary conservation goals while maintaining ecosystem function. Anthropogenically altered environments can subsidize certain species, causing them to become overabundant and straining the remaining natural system. Such is the case with the widespread avian generalist and subsidized predator, the common raven (Corvus corax). Ravens depredate nests and prey on small animals, with locally focused high impacts to the ecosystem during rearing of their nestlings. Limiting predator abundance reduces the spatially concentrated predation rates by eliminating nestling provisioning needs. We investigated avenues to limit the reproduction of common raven using customized theoretical and practical tools. Specifically, we developed an online interactive tool, a mathematical model, that calculates the predator's population growth rate under any given proportion of reproductive management. This quantitative tool models an assessment of ravens in a defined area and results in the degree to which raven fitness would need to be reduced to control their populations. To implement the model's results in a field‐practical way and manage reproduction with minimal disturbance, we developed novel technology, a Remote Fluid Application System (RFAS). The RFAS applies the egg addling technique of egg oiling to high (usually out of reach) nesting situations. We tested these tools in the Mojave Desert, CA, where the raven is a heavily human‐subsidized predator. We mounted our RFAS on telescoping poles and unmanned aerial vehicles (UAVs; drones) and found that we were able to aerially, and precisely, spray‐oil eggs high in trees or cliffs/rock faces, halting 100% (n = 46) of raven nesting events. With these tools, we enable managers to calculate the intensity of, and the ability to carry out, egg addling needed to stem or halt predator population growth. We conclude that the combination of the interactive software (StallPOPd Interactive Tool, https://cwhl.vet.cornell.edu/tools/stallpopd; publicly available at https://ecommons.cornell.edu/handle/1813/65718) and the RFAS is the most effective wildlife management strategy to date for the offset or control of undesirable population growth in high‐nesting avian predators.

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Multiple Factors Affect a Population of Agassiz's Desert Tortoise (Gopherus agassizii) in the Northwestern Mojave Desert

Cited by 26 publications

References 33 publications

Unexpected decline in a population of speckled tortoises

Unexpected decline in a population of speckled tortoises

Boreal bird abundance estimates within different energy sector disturbances vary with point count radius

Novel management tools for subsidized avian predators and a case study in the conservation of a threatened species

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