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Single Period (single + period)
Selected AbstractsOptimal conservation planning for migratory animals: integrating demographic information across seasonsCONSERVATION LETTERS, Issue 3 2010Justin Sheehy Abstract Conservation strategies for migratory animals are typically based on ad-hoc or simple ranking methods and focus on a single period of the annual cycle. We use a density-dependent population model to examine one-time land purchase strategies for a migratory population with a breeding and wintering grounds. Under equal rates of habitat loss, we show that it is optimal to invest more, but never solely, in the habitat with the higher density dependence to habitat cost ratio. When there are two habitats that vary in quality within a season, the best strategy is to invest only in one habitat. Whether to purchase high- or low-quality habitat depends on the general life history of the species and the ratio of habitat quality to habitat cost. When carry-over effects are incorporated, it is almost always optimal to invest in high-quality habitat during the season that produces the carry-over effect. We apply this model to a threatened warbler population and show the optimal strategy is to purchase more breeding than wintering habitat despite the fact that breeding habitat is over ten times more expensive. Our model provides a framework for developing year-round conservation strategies for migratory animals and has important implications for long-term planning and management. [source] Pliocene forest dynamics as a primary driver of African bird speciationGLOBAL ECOLOGY, Issue 1 2010Gary Voelker ABSTRACT Aim, Montane tropics are areas of high endemism, and mechanisms driving this endemism have been receiving increasing attention at a global scale. A general trend is that climatic factors do not explain the species richness of species with small to medium-sized geographic ranges, suggesting that geological and evolutionary processes must be considered. On the African continent, several hypotheses including both refugial and geographic uplift models have been advanced to explain avian speciation and diversity in the lowland forest and montane regions of central and eastern Africa; montane regions in particular are recognized as hotspots of vertebrate endemism. Here, we examine the possible role of these models in driving speciation in a clade of African forest robins. Location, Africa. Methods, We constructed the first robustly supported molecular phylogenetic hypothesis of forest robins. On this phylogeny, we reconstructed habitat-based distributions and geographic distributions relative to the Albertine Rift. We also estimated the timing of lineage divergences via a molecular clock. Results, Robust estimates of phylogenetic relationships and clock-based divergences reject Miocene tectonic uplift and Pleistocene forest refugia as primary drivers of speciation in forest robins. Instead, our data suggest that most forest robin speciation took place in the Late Pliocene, from 3.2 to 2.2 Ma. Distributional patterns are complex, with the Albertine Rift region serving as a general east,west break across the group. Montane distributions are inferred to have evolved four times. Main conclusions, Phylogenetic divergence dates coincide with a single period of lowland forest retraction in the late Pliocene, suggesting that most montane speciation resulted from the rapid isolation of populations in montane areas, rather than montane areas themselves being drivers of speciation. This conclusion provides additional evidence that Pliocene climate change was a major driver of speciation in broadly distributed African animal lineages. We further show that lowland forest robins are no older than their montane relatives, suggesting that lowland areas are not museums which house ,ancient' taxa; rather, for forest robins, montane areas should be viewed as living museums of a late Pliocene diversification event. A forest refugial pattern is operating in Africa, but it is not constrained to the Pleistocene. [source] Mechanical Strain Stimulates Osteoblast Proliferation Through the Estrogen Receptor in Males as Well as FemalesJOURNAL OF BONE AND MINERAL RESEARCH, Issue 11 2000E. Damien Abstract Mechanical strain, testosterone, and estrogen all stimulate proliferation of primary cultures of male rat long bone (LOB)-derived osteoblast-like cells as determined by [3H]thymidine incorporation. The maximum proliferative effect of a single period of mechanical strain (3400 ,,, 1 Hz, and 600 cycles) is additional to that of testosterone (10,8 M) or estrogen (10,8 M). The cells' proliferative response to strain is abolished both by concentrations of tamoxifen that cause proliferation (10,8 M) and by those that have no effect (10,6 M). Strain-related proliferation also is reduced by the estrogen antagonist ICI 182,780 (10,8 M) but is unaffected by the androgen receptor antagonist hydroxyflutamide (10,7 M). Tamoxifen, ICI 182,780, and the aromatase inhibitor 4-dihydroandrostenedione, at concentrations that have no effect on basal proliferation, significantly reduce the proliferative effect of the aromatizable androgen testosterone but not that of the nonaromatizable androgen 5,-dihydrotestosterone. Hydroxyflutamide, at a concentration that has no effect on basal proliferation (10,7 M), eliminates the proliferative effect of 5,-dihydro-testosterone but had no significant effect on that caused by testosterone. Proliferation associated with strain is blocked by neutralizing antibody to insulin-like growth factor II (IGF-II) but not by antibody to IGF-I. Proliferation associated with testosterone is blocked by neutralizing antibody to IGF-I but is unaffected by antibody to IGF-II. These data suggest that in rat osteoblast-like cells from males, as from females, strain-related proliferation is mediated through the estrogen receptor (ER) in a manner that does not compete with estrogen but that can be blocked by ER modulators. Proliferation associated with testosterone appears to follow its aromatization to estrogen and is mediated through the ER, whereas proliferation associated with 5,-dihydrotestosterone is mediated by the androgen receptor. Strain-related proliferation in males, as in females, is mediated by IGF-II, whereas proliferation associated with estrogen and testosterone is mediated by IGF-I. [source] Habituation and Cross-Sensitization of Stress-Induced Hypothalamic-Pituitary-Adrenal Activity: Effect of Lesions in the Paraventricular Nucleus of the Thalamus or Bed Nuclei of the Stria TerminalisJOURNAL OF NEUROENDOCRINOLOGY, Issue 7 2002G. A. Fernandes Abstract Habituation of the hypothalamic-pituitary-adrenal (HPA) response to chronic intermittent restraint stress (30 min/day for 15 days) and the cross-sensitization to a heterotypic stress [i.p. lipopolysaccharide (LPS)] were investigated in intact male Sprague Dawley rats, and in rats bearing quinolinic acid lesions to the medial anterior bed nuclei of the stria terminalis (BST) or anterior region of the paraventricular nucleus of the thalamus (PVT). In intact animals, a single period of restraint increased plasma corticosterone levels at 30 min and led to an increase in corticotropin-releasing hormone (CRH) mRNA levels in the PVN at 3 h. LPS had a smaller effect on corticosterone and more variable effect on CRH mRNA. Chronic intermittent restraint stress caused a decrease in body weight and increase in adrenal weights, with concomitant increase in basal corticosterone levels. These animals also displayed marked habituation of the corticosterone and CRH mRNA responses to the homotypic stress of restraint, but no loss of the corticosterone response to the heterotypic stress of LPS and a cross-sensitization of the CRH mRNA response. This pattern of stress responses in control and chronically stressed animals was not significantly affected by lesions to the PVT or BST, two areas which have been implicated in the coping response to stress. Thus, these data provide evidence for independent adaptive mechanisms regulating HPA responses to psychological and immune stressors, but suggest that neither the medial anterior BST nor the anterior PVT participate in the mechanisms of habituation or cross-sensitization. [source] Optimal Dynamic Portfolio Selection: Multiperiod Mean-Variance FormulationMATHEMATICAL FINANCE, Issue 3 2000Duan Li The mean-variance formulation by Markowitz in the 1950s paved a foundation for modern portfolio selection analysis in a single period. This paper considers an analytical optimal solution to the mean-variance formulation in multiperiod portfolio selection. Specifically, analytical optimal portfolio policy and analytical expression of the mean-variance efficient frontier are derived in this paper for the multiperiod mean-variance formulation. An efficient algorithm is also proposed for finding an optimal portfolio policy to maximize a utility function of the expected value and the variance of the terminal wealth. [source] | |||||||||||||