8 research outputs found
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Effects of artificial rain on survivorship, body condition, and growth of head-started desert tortoises (Gopherus agassizii) released to the open desert
We subjected neonate Desert Tortoises (Gopherus agassizii) that hatched inside fenced, predator-resistant field enclosures containing natural vegetation to either a natural rainfall regime or a regime of natural rainfall plus irrigation (supplemental precipitation) over a five-year period, to test the hypothesis that mimicking an above-average rainfall regime in years of average or low natural rainfall will improve rates of survival and growth. We also tested the hypothesis that survivorship of released 1-yr olds will be high, due to a decline in predation susceptibility once the vulnerable nesting and hatchling phases are completed. Survivorship inside the enclosures during the first year of life was high (averaging 90%) in both groups, even during a record low rainfall year, but growth rates were always substantially higher (2 to 16-[1] greater) in rain-supplemented juveniles. Body condition index (CI) measurements indicated that first-year juveniles without added rain were able to maintain body conditions similar to rain-supplemented juveniles during two average rainfall years, but not during a drought year. Older juveniles without added rain died during the latter part of the 16 mo drought, suggesting that the high drought survivorship of first-year non-supplemented juveniles may be related to the yolk they carried after hatching, along with possible behavioral and physiological diffrerences. Nearly all yearlings that were set free (in autumn) were dead within 6 mo, regardless of whether they had supplemental rain or not during their first year inside enclosures, and regardless of whether they were released near the head-start enclosures or a kilometer away. The main cause of mortality was predation, primarily by ravens. The poor survival of released yearling tortoises and the drought-induced death of nearly all older captive juveniles raised without added precipitation lead us to recommend that rain supplementation and delayed release be incorporated in the protocol for head-starting Desert Tortoises
Recommended from our members
Effects of artificial rain on survivorship, body condition, and growth of head-started desert tortoises (Gopherus agassizii) released to the open desert
© 2015 Kenneth A. Nagy. All Rights Reserved. We subjected neonate Desert Tortoises (Gopherus agassizii) that hatched inside fenced, predator-resistant field enclosures containing natural vegetation to either a natural rainfall regime or a regime of natural rainfall plus irrigation (supplemental precipitation) over a five-year period, to test the hypothesis that mimicking an above-average rainfall regime in years of average or low natural rainfall will improve rates of survival and growth. We also tested the hypothesis that survivorship of released 1-yr olds will be high, due to a decline in predation susceptibility once the vulnerable nesting and hatchling phases are completed. Survivorship inside the enclosures during the first year of life was high (averaging 90%) in both groups, even during a record low rainfall year, but growth rates were always substantially higher (2 to 16-[1] greater) in rain-supplemented juveniles. Body condition index (CI) measurements indicated that first-year juveniles without added rain were able to maintain body conditions similar to rain-supplemented juveniles during two average rainfall years, but not during a drought year. Older juveniles without added rain died during the latter part of the 16 mo drought, suggesting that the high drought survivorship of first-year non-supplemented juveniles may be related to the yolk they carried after hatching, along with possible behavioral and physiological diffrerences. Nearly all yearlings that were set free (in autumn) were dead within 6 mo, regardless of whether they had supplemental rain or not during their first year inside enclosures, and regardless of whether they were released near the head-start enclosures or a kilometer away. The main cause of mortality was predation, primarily by ravens. The poor survival of released yearling tortoises and the drought-induced death of nearly all older captive juveniles raised without added precipitation lead us to recommend that rain supplementation and delayed release be incorporated in the protocol for head-starting Desert Tortoises
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Head-started desert tortoises (Gopherus agassizii): Movements, survivorship and mortality causes following their release
We released and monitored 53 juvenile Agassiz’s Desert Tortoises (Gopherus agassizii), aged two to 15 y, that were hatched and head-started inside predator-resistant field enclosures. We set free these tortoises under a variety of conditions to evaluate effects of release distance, season of release, and age/body size on homing behavior and survivorship. Some juveniles moved large distances following release, but homing itself was undetectable. The lack of homing behaviors was likely due to release distance and selection of release sites out of the line-of-sight of natal enclosures. The use of small halfway-house enclosures to accustom some relocated juveniles to release sites for four months before release (i.e. “soft release”) had no effect on subsequent movements or survivorship during the first year following release. Survivorship was not affected by distance of release from natal enclosures, which ranged from 546 m to 1.4 km. Survivorship through one year was similar for juveniles released in spring or autumn. After two years, most small juveniles had been killed by predators, but survivorship increased with body size and age. Juveniles over approximately 100 mm MCL (midline carapace length) and nine years of age when released exhibited high survivorship. However, following a long drought the previous two years, predation by Coyotes (Canis latrans) was heavy on these larger juveniles in the third year after release. Thus, survivorship after three years was relatively low (34%) with the youngest, smallest cohort (two years old when released) exhibiting the lowest survivorship (4%). We recommend releasing headstarted tortoises after they attain a body size of larger than 100 mm MCL and selecting release sites at least 546 m from enclosures
Recommended from our members
Head-started desert tortoises (Gopherus agassizii): Movements, survivorship and mortality causes following their release
We released and monitored 53 juvenile Agassiz’s Desert Tortoises (Gopherus agassizii), aged two to 15 y, that were hatched and head-started inside predator-resistant field enclosures. We set free these tortoises under a variety of conditions to evaluate effects of release distance, season of release, and age/body size on homing behavior and survivorship. Some juveniles moved large distances following release, but homing itself was undetectable. The lack of homing behaviors was likely due to release distance and selection of release sites out of the line-of-sight of natal enclosures. The use of small halfway-house enclosures to accustom some relocated juveniles to release sites for four months before release (i.e. “soft release”) had no effect on subsequent movements or survivorship during the first year following release. Survivorship was not affected by distance of release from natal enclosures, which ranged from 546 m to 1.4 km. Survivorship through one year was similar for juveniles released in spring or autumn. After two years, most small juveniles had been killed by predators, but survivorship increased with body size and age. Juveniles over approximately 100 mm MCL (midline carapace length) and nine years of age when released exhibited high survivorship. However, following a long drought the previous two years, predation by Coyotes (Canis latrans) was heavy on these larger juveniles in the third year after release. Thus, survivorship after three years was relatively low (34%) with the youngest, smallest cohort (two years old when released) exhibiting the lowest survivorship (4%). We recommend releasing headstarted tortoises after they attain a body size of larger than 100 mm MCL and selecting release sites at least 546 m from enclosures
Colonización en las tierras áridas de Norteamérica: El viaje de Agarito (Berberis trifoliolata) revelado por datos moleculares multilocus y restos fósiles de Packrat Midden
Here we conduct research to understand the evolutionary history of a shrubby species known as Agarito (Berberis trifoliolata), an endemic species to the Chihuahuan Desert. We identify genetic signatures based on plastid DNA and AFLP markers and perform niche modelling and spatial connectivity analyses as well as niche modelling based on records in packrats to elucidate whether orogenic events such as mountain range uplift in the Miocene or the contraction/expansion dynamics of vegetation in response to climate oscillations in the Pliocene/Pleistocene had an effect on evolutionary processes in Agarito. Our results of current niche modelling and palaeomodelling showed that the area currently occupied by Berberis trifoliolata is substantially larger than it was during the Last Interglacial period and the Last Glacial Maximum. Agarito was probably confined to small areas in the Northeastern and gradually expanded its distribution just after the Last Glacial Maximum when the weather in the Chihuahuan Desert and adjacent regions became progressively warmer and drier. The most contracted range was predicted for the Interglacial period. Populations remained in stable areas during the Last Glacial Maximum and expanded at the beginning of the Holocene. Most genetic variation occured in populations from the Sierra Madre Oriental. Two groups of haplotypes were identified: the Mexican Plateau populations and certain Northeastern populations. Haplogroups were spatially connected during the Last Glacial Maximum and separated during interglacial periods. The most important prediction of packrat middens palaeomodelling lies in the Mexican Plateau, a finding congruent with current and past niche modelling predictions for agarito and genetic results. Our results corroborate that these climate changes in the Pliocene/Pleistocene affected the evolutionary history of agarito. The journey of agarito in the Chihuahuan Desert has been dynamic, expanding and contracting its distribution range and currently occupying the largest area in its history.Aquí realizamos una investigación para comprender la historia evolutiva de una especie arbustiva conocida como Agarito (Berberis trifoliolata), una especie endémica del desierto de Chihuahua. Identificamos firmas genéticas basadas en marcadores de ADN plastidial y AFLP y realizamos análisis de modelado de nicho y de conectividad espacial, así como de modelado de nicho basado en registros en packrats para dilucidar si los eventos orogénicos como el levantamiento de la cordillera en el Mioceno o la dinámica de contracción/expansión de la vegetación en respuesta a las oscilaciones climáticas en el Plioceno/Pleistoceno tuvieron un efecto en los procesos evolutivos de Agarito. Nuestros resultados de la modelización del nicho actual y de la paleomodelización mostraron que el área ocupada actualmente por Berberis trifoliolata es sustancialmente mayor de lo que era durante el último período interglacial y el último máximo glacial. El agarito estaba probablemente confinado en pequeñas zonas del noreste y amplió gradualmente su distribución justo después del Último Máximo Glacial, cuando el clima en el desierto de Chihuahua y las regiones adyacentes se volvió progresivamente más cálido y seco. El área de distribución más contraída se predijo para el periodo interglaciar. Las poblaciones permanecieron en zonas estables durante el Último Máximo Glacial y se expandieron a principios del Holoceno. La mayor variación genética se produjo en las poblaciones de la Sierra Madre Oriental. Se identificaron dos grupos de haplotipos: las poblaciones de la Meseta Mexicana y ciertas poblaciones del Noreste. Los haplogrupos estuvieron conectados espacialmente durante el Último Máximo Glacial y se separaron durante los periodos interglaciares. La predicción más importante de la paleomodelación de los muladares de las ratas de carga se sitúa en la Meseta Mexicana, un hallazgo congruente con las predicciones actuales y pasadas de la modelización del nicho del agarito y los resultados genéticos. Nuestros resultados corroboran que estos cambios climáticos en el Plioceno/Pleistoceno afectaron a la historia evolutiva del agarito. El recorrido del agarito en el Desierto Chihuahuense ha sido dinámico, expandiendo y contrayendo su rango de distribución y ocupando actualmente la mayor área de su historia.Fil: Angulo, Diego F. Universidad Autónoma de Yucatán; México.Fil: Amarilla, Leonardo D. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina.Fil: Amarilla, Leonardo D. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Fil: Anton, Ana M. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina.Fil: Anton, Ana M. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Fil: Sosa, Victoria. Instituto de Ecología. Departamento de Biología Evolutiva; México