A Geochemical Record of Precession-Induced Cyclic Eastern Mediterranean Sedimentation: Implications for Northern Sahara Humidity During the Pliocene

The chemical composition of lithogenic components in Pliocene sediments from the eastern Mediterranean displays periodic variations that are related to earth's orbital parameters owing to changes in insolation of the northern hemisphere. This can be explained by two different paleoclimatic/palaeoceanographic scenarios. During oligotrophic periods, similar to those persisting in the modern Mediterranean, sedimentation is rather uniform, and sediments receive high proportions of eolian material from the surrounding deserts. During more humid intervals sedimentation is dominated by fluviatile material that has been introduced by the Nile and northern borderland rivers. Higher nutrient loads associated with enhanced riverine input and a change in water circulation led to eutrophication, water column anoxia, and sapropel deposition in the eastern Mediterranean. Our investigations confirm earlier works claiming that in large parts of the northern Sahara palaeoclimatic conditions changed frequently from arid to more humid. Our results suggest that these changes in the Pliocene match with the approximately 22-ka insolation cyclicity.

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Zusammenfassung   Der Chemismus lithogener Komponenten plioz?ner Sedimente des ?stlichen Mittelmeers weist periodische ?nderungen auf, die mit den Orbitalparametern der Erde und dadurch hervorgerufenen Variationen der Sonneneinstrahlung im Bereich der Nordhemisph?re einhergehen. Dadurch entstehen zwei gegens?tzliche pal?oklimatische bzw. pal?ozeanographische Szenarien. In Zeiten mit geringen Niederschl?gen und oligotrophen Bedingungen im Mittelmeer, wie sie auch heute vorherrschen, ist die Sedimentation sehr stark von ?olischen Staubeintr?gen angrenzender Wüstengebiete gekennzeichnet. Demgegenüber dominiert in feuchten Klimaperioden die Sedimentation von fluviatilem Material, das durch den Nil und Flüsse der n?rdlich an das Mittelmeer grenzenden Gebiete eingetragen wird. Die durch Zirkulations?nderungen sowie den N?hrstoffeintrag der Flüsse induzierten eutrophen Bedingungen führten schlie?lich zur Ausbildung von Wassers?ulen-Anoxia und der Ablagerung von Sapropelen im ?stlichen Mittelmeer. Unsere Untersuchungen unterstützen frühere Forschungsergebnisse, wonach in weiten Teilen der n?rdlichen Sahara ein h?ufigerer Wechsel zwischen ariden und humideren Klimabedingungen stattfand. Nach unseren Ergebnissen folgte dieser rhythmische Wechsel im Plioz?n dem Insolationszyklus von ca. 22 ka.

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Received: 16 September 1998 / Accepted in revised form: 28 December 1998     

Translating effects of inbreeding depression on component vital rates to overall population growth in endangered bighorn sheep

Evidence of inbreeding depression is commonly detected from the fitness traits of animals, yet its effects on population growth rates of endangered species are rarely assessed. We examined whether inbreeding depression was affecting Sierra Nevada bighorn sheep (Ovis canadensis sierrae), a subspecies listed as endangered under the U.S. Endangered Species Act. Our objectives were to characterize genetic variation in this subspecies; test whether inbreeding depression affects bighorn sheep vital rates (adult survival and female fecundity); evaluate whether inbreeding depression may limit subspecies recovery; and examine the potential for genetic management to increase population growth rates. Genetic variation in 4 populations of Sierra Nevada bighorn sheep was among the lowest reported for any wild bighorn sheep population, and our results suggest that inbreeding depression has reduced adult female fecundity. Despite this population sizes and growth rates predicted from matrix-based projection models demonstrated that inbreeding depression would not substantially inhibit the recovery of Sierra Nevada bighorn sheep populations in the next approximately 8 bighorn sheep generations (48 years). Furthermore, simulations of genetic rescue within the subspecies did not suggest that such activities would appreciably increase population sizes or growth rates during the period we modeled (10 bighorn sheep generations, 60 years). Only simulations that augmented the Mono Basin population with genetic variation from other subspecies, which is not currently a management option, predicted significant increases in population size. Although we recommend that recovery activities should minimize future losses of genetic variation, genetic effects within these endangered populations-either negative (inbreeding depression) or positive (within subspecies genetic rescue)-appear unlikely to dramatically compromise or stimulate short-term conservation efforts. The distinction between detecting the effects of inbreeding depression on a component vital rate (e.g., fecundity) and the effects of inbreeding depression on population growth underscores the importance of quantifying inbreeding costs relative to population dynamics to effectively manage endangered populations.     

Rapid Extinction of Mountain Sheep Populations Revisited

Abstract: Predicting extinction probabilities for populations of various sizes has been a primary focus of conservation biology. Berger (1990) presented an empirically based extinction model for mountain sheep ( Ovis canadensis ) populations in five southwestern states that predicted disappearance within 50 years of all populations estimated to number 50 sheep or fewer, but essentially no loss in that time period of populations estimated at over 100. The majority of the 122 populations he used in his analysis were from California, but his analysis did not use many of the historical size estimates for these populations. I tested Berger's (1990) model using the complete data set from California and found—contrary to his results—that, for all size classes of population estimates, at least 61% of the populations persisted for 50 years. Also, two predictions from Berger's model were not consistent with the data from California: (1) 10 populations have increased from estimates of 50 or fewer animals to over 100, whereas the Berger model predicted that these populations would only decline to extinction; and (2) of 27 extant populations with long enough records, 85% were estimated at least 50 years ago to be 50 individuals or fewer and should therefore be extinct by now. Berger's model has now failed tests in three states and therefore does not support the strong population size effect on extinction probability that it first appeared to provide, and it may serve conservation poorly through misdirected effort if it is used as the basis for setting policies or taking actions.