title: Mojave desert tortoise (Gopherus agassizii) thermal ecology and reproductive success along a rainfall cline.
abstract: Desert resource environments (e.g. microclimates, food) are tied to limited, highly localized rainfall regimes which generate microgeographic variation in the life histories of inhabitants. Typically, enhanced growth rates, reproduction and survivorship are observed in response to increased resource availability in a variety of desert plants and short-lived animals. We examined the thermal ecology and reproduction of US federally threatened Mojave desert tortoises (Gopherus agassizii), long-lived and large-bodied ectotherms, at opposite ends of a 250-m elevation-related rainfall cline within Ivanpah Valley in the eastern Mojave Desert, California, USA. Biophysical operative environments in both the upper-elevation, "Cima," and the lower-elevation, "Pumphouse," plots corresponded with daily and seasonal patterns of incident solar radiation. Cima received 22% more rainfall and contained greater perennial vegetative cover, which conferred 5°C-cooler daytime shaded temperatures. In a monitored average rainfall year, Cima tortoises had longer potential activity periods by up to several hours and greater ephemeral forage. Enhanced resource availability in Cima was associated with larger-bodied females producing larger eggs, while still producing the same number of eggs as Pumphouse females. However, reproductive success was lower in Cima because 90% of eggs were depredated versus 11% in Pumphouse, indicating that predatory interactions produced counter-gradient variation in reproductive success across the rainfall cline. Land-use impacts on deserts (e.g. solar energy generation) are increasing rapidly, and conservation strategies designed to protect and recover threatened desert inhabitants, such as desert tortoises, should incorporate these strong ecosystem-level responses to regional resource variation in assessments of habitat for prospective development and mitigation efforts.
authors.affiliation: Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan, USA.
authors.forename: Annette E
authors.initials: AE
authors.lastname: Sieg
authors.affiliation: HDR, Plymouth Meeting, Pennsylvania, USA.
authors.forename: Megan M
authors.initials: MM
authors.lastname: Gambone
authors.affiliation: Nicholas School of the Environment, Duke University Marine Laboratory, Beaufort, North Carolina, USA.|Stratus Consulting, Boulder, Colorado, USA.
authors.forename: Bryan P
authors.initials: BP
authors.lastname: Wallace
authors.affiliation: Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.
authors.forename: Susana
authors.initials: S
authors.lastname: Clusella-Trullas
authors.affiliation: Department of Biodiversity, Earth and Environmental Science, Drexel University, Philadelphia, Pennsylvania, USA.
authors.forename: James R
authors.initials: JR
authors.lastname: Spotila
authors.affiliation: Department of Biology, The College of New Jersey, Ewing, New Jersey, USA.
authors.forename: Harold W
authors.initials: HW
authors.lastname: Avery
mesh_terms: D000818:Animals; D049628:Body Size; D002140:California; D003889:Desert Climate; D017753:Ecosystem; D005260:Female; D010063:Ovum / Q000502:physiology; D011235:Predatory Behavior; D011891:Rain; D012098:Reproduction; D013696:Temperature; D014426:Turtles / Q000502:physiology*
keywords: Gopherus agassizii; operative environments; reproductive ecology; resource gradient