Recent radar observations of Mercury have revealed the presence of anomalous radar reflectivity and polarization features near its north and south poles. Thermal model calculations show that, despite Mercury's proximity to the sun, the temperatures of flat, low-reflectivity surfaces at Mercury's poles are not expected to exceed 167 kelvin. The locations of the anomalous polar radar features appear to be correlated with the locations of large, high-latitude impact craters. Maximum surface temperatures in the permanently shadowed regions of these craters are expected to be significantly colder, as low as 60 kelvin in the largest craters. These results are consistent with the presence of water ice, because at temperatures lower than 112 kelvin, water ice should be stable to evaporation over time scales of billions of years. 相似文献
Individual plants and populations of scarlet gilia (Ipomopsis aggregata) shift from darker to lighter corolla colors during the flowering season. Shifts to lighter color coincide with emigration of hummingbirds from the system. In the absence of hummingbirds, lighter colors attract the remaining pollinator, a hawkmoth. Comparison of plants that shift to lighter colors with those that fail to shift shows that shifting is adaptive in that it enhances reproductive success because of the preference of hawkmoths for lighter colored flowers. Color shifting therefore provides a mechanism for plants to track changing pollinator abundances. 相似文献
Ferrihydrite, prepared in the presence of 0 to 20 mole % Cd in the solution, was used to study the transformation of ferrihydrite into crystalline products. The result showed that the presence of Cd strongly retards the transformation of ferrihydrite into crystalline products, suppressing the formation of goethite and leading to a product which eventually consists entirely of hematite at pH 8 and at 70 °C. The fraction of hematite in the transformation products increased with increasing level of Cd in the system. When 9 mole % Cd was present, the transformation product consisted entirely of hematite. The chemical analysis and XRD data showed that Cd was incorporated into the lattice of iron oxides, Cd-hematite and Cd-goethite being formed. The mole % Cd which replaced iron in the iron oxides increased with increasing level of Cd in the system below 9 mole % Cd. Above this value, but below 20 mole % the mole % of Cd incorporated in the lattice of iron oxides was constant at about 2.9 mole %. The volume of the unit cell of Cd-goethite increased with increasing level of Cd in the system until the goethite production was entirely suppressed. The volume of the unit cell of Cd-hematite also increased with increasing level of Cd, below 9 mole % of Cd in the system. Above this value, it was constant. Scanning electron microscopic examination showed that the presence of Cd affected the morphology of hematite more than that of goethite. The goethite grew from ferrihydrite as acicular crystals independent of the amount of Cd in the system. The shape of hematite particles varied from irregular platelets with lower Cd level, to ellipsoids, with higher Cd level in the system, and it also suggested that Cd prevented the formation of goethite by hindering the dissolution of ferrihydrite rather than by interfering with nucleation and growth of goethite from solution. The rate of transformation was studied at pH 8, 50 °C and 70 °C. The transformations were first order reactions at both temperatures. 相似文献
Ecological communities in urban ecosystems are assembled through ecological processes, such as species interactions, dispersal, and environmental filtering, but also through human factors that create and modify the landscape. These complex interactions make it difficult to untangle the relationships between social–ecological dynamics and urban biodiversity.
Objectives
As a result, there has been a call for research to address how human activities influence the processes by which ecological communities are structured in urban ecosystems. We address this research challenge using core concepts from landscape ecology to develop a framework that links social-ecological dynamics to ecological communities using the metacommunity perspective.
Methods
The metacommunity perspective is a useful framework to explore the assembly of novel communities because it distinguishes between the effects of local environmental heterogeneity and regional spatial processes in structuring ecological communities. Both are shaped by social–ecological dynamics in urban ecosystems.
Results
In this paper, we define social, environmental, and spatial processes that structure metacommunities, and ultimately biodiversity, in cities. We then address how our framework could be applied in urban ecosystem research to understand multi-scalar biodiversity patterns.
Conclusions
Our framework provides a theoretical and empirical foundation for transdisciplinary research to examine how social-ecological dynamics mediate the assembly of novel communities in urban ecosystems.
Acquired portosystemic shunts (PSS) are a clinical entity distinct from congenital PSS. Their apparent incidence in cats is low, which may reflect the rarity of predisposing hepatic parenchymal disease, such as cirrhosis, in this species. Two cats with acquired PSS associated with primary hepatobiliary disease are described. Relevant findings in acquired PSS are discussed, as are potential reasons for the apparently low incidence in the cat. 相似文献
Objectives were to evaluate risk factors affecting ovulatory responses and conception rate to the Ovsynch protocol. Holstein cows, 466, were submitted to the Ovsynch protocol [day 0, GnRH‐1; day 7, prostaglandin (PG) F2α; day 9, GnRH‐2] and 103 cows were inseminated 12 h after GnRH‐2. Information on parity, days in milk at GnRH‐1, body condition, milk yield, exposure to heat stress, pre‐synchronization with PGF2α and the use of progesterone insert from GnRH‐1 to PGF2α was collected. Ovaries were scanned to determine responses to treatments. Overall, 54.7%, 10.6%, 2.2%, 81.1%, 9.0%, 91.5% and 36.9% of the cows ovulated to GnRH‐1, multiple ovulated to GnRH‐1, ovulated before GnRH‐2, ovulated to GnRH‐2, multiple ovulated to GnRH‐2, experienced corpus luteum (CL) regression and conceived, respectively. Ovulation to GnRH‐1 was greater in cows without a CL at GnRH‐1, cows with follicles >19 mm and cows not pre‐synchronized with PGF2α 14 days before GnRH‐1. Multiple ovulations to GnRH‐1 increased in cows without CL at GnRH‐1 and cows with follicles ≤19 mm at GnRH‐1. Ovulation before GnRH‐2 was greater in cows without CL at PGF2α. Ovulation to GnRH‐2 increased in cows that received a progesterone insert, cows with a CL at GnRH‐1, cows with follicles not regressing from the PGF2α to GnRH‐2, cows with larger follicles at GnRH‐2, cows that ovulated to GnRH‐1 and cows not pre‐synchronized. Multiple ovulations after GnRH‐2 increased in cows with no CL at GnRH‐1, multiparous cows and cows that multiple ovulated to GnRH‐1. Conception rate at 42 days after AI increased in cows with body condition score > 2.75 and cows that ovulated to GnRH‐2. Strategies that optimize ovulation to GnRH‐2, such as increased ovulation to GnRH‐1, should improve response to the Ovsynch protocol. 相似文献