Mediator structure and conformation change

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气候变化和人类活动对中国北方旱区植被变绿的定量贡献

气候变化和大规模的生态恢复使中国北方旱区植被发生了显著变化，量化气候变化和人类活动对植被动态的相对贡献，对于旱区生态系统管理和应对未来气候变化具有重要意义。目前，中国北方旱区植被变化影响因素的时间动态（2000年大规模生态恢复工程实施前后）和空间异质性（沿干旱梯度）仍需进一步的定量研究。基于多源数据，采用趋势分析、偏相关分析和随机森林模型等方法，分析了1981-2018年中国北方旱区气候和植被的时空变化规律，量化了2000年前后气候变化和人类活动对植被动态的相对贡献并分析其在干旱梯度上的空间差异性。结果表明：（1）1981-2018年期间，中国北方旱区的叶面积指数（LAI）平均增加速率为（0.0037±0.0443） a^-1，且增加速率沿干旱梯度增大。2000年前仅10.46%（P<0.05）的地区显著变绿，而2000年后达到36.84%，且植被变绿主要归因于非树木植被。（2）2000年后降水对植被变绿的正效应在不同干旱梯度均增加，而在半干旱区和亚湿润干旱区，温度对植被变绿由正向促进转为负向抑制，而辐射在干旱区由负效应转向正效应。（3）2000年前后，气候变化均主导着植被的动态，贡献率分别为96.07%和73.72%，人类活动的贡献在2000年后进一步增强（从3.93%增加到26.28%），且沿着干旱梯度而增加，其中人类活动对植被变绿的贡献在半干旱地区增加最显著（+0.0289 m² m^-2 a^-1，P<0.05）。研究结果可为未来气候变化下中国北方旱区的植被恢复和可持续发展提供科学依据。     

Temperature-Sensitive Behavior of Paramecium caudatum

SYNOPSIS. The effect of temperature on the behavior of swimming cells of Paramecium caudatum has been investigated by photographic analyses of their tracks in uniform temperature, in temperature gradient, or in temperature changing with time. When the cells were placed in the temperature gradient, the frequency of discontinuous directional changes of cells swimming toward the optimal temperature, the temperature of the culture, was much lower than that of the cells swimming in the opposite direction. This difference in the frequency of directional changes explained the observed accumulation of the cells at - the optimal temperature. When the temperature was suddenly changed toward the optimum, a transient decrease of the frequency of directional changes was observed and when the temperature was changed in the reverse direction, a transient increase of the frequency was noted. This transient response to the temperature change was the origin of the dependence of the frequency of directional changes on the swimming direction in the temperature gradient. Finally, the relation between the magnitude of the transient response and the rate of the temperature change was derived.     

EVOLUTION AND EXTINCTION IN A CHANGING ENVIRONMENT: A QUANTITATIVE-GENETIC ANALYSIS

Because of the ubiquity of genetic variation for quantitative traits, virtually all populations have some capacity to respond evolutionarily to selective challenges. However, natural selection imposes demographic costs on a population, and if these costs are sufficiently large, the likelihood of extinction will be high. We consider how the mean time to extinction depends on selective pressures (rate and stochasticity of environmental change, and strength of selection), population parameters (carrying capacity, and reproductive capacity), and genetics (rate of polygenic mutation). We assume that in a randomly mating, finite population subject to density-dependent population growth, individual fitness is determined by a single quantitative-genetic character under Gaussian stabilizing selection with the optimum phenotype exhibiting directional change, or random fluctuations, or both. The quantitative trait is determined by a finite number of freely recombining, mutationally equivalent, additive loci. The dynamics of evolution and extinction are investigated, assuming that the population is initially under mutation-selection-drift balance. Under this model, in a directionally changing environment, the mean phenotype lags behind the optimum, but on the average evolves parallel to it. The magnitude of the lag determines the vulnerability to extinction. In finite populations, stochastic variation in the genetic variance can be quite pronounced, and bottlenecks in the genetic variance temporarily can impair the population's adaptive capacity enough to cause extinction when it would otherwise be unlikely in an effectively infinite population. We find that maximum sustainable rates of evolution or, equivalently, critical rates of environmental change, may be considerably less than 10% of a phenotypic standard deviation per generation.     

Mass Coral Reef Bleaching: A Recent Outcome of Increased El Niño Activity?

Coral reefs are generally considered to be the most biologically productive of all marine ecosystems, but in recent times these vulnerable aquatic resources have been subject to unusual degradation. The general decline in reefs has been greatly accelerated by mass bleaching in which corals whiten en masse and often fail to recover. Empirical evidence indicates a coral reef bleaching cycle in which major bleaching episodes are synchronized with El Niño events that occur every 3–4 years on average. By heating vast areas of the Pacific Ocean, and affecting the Indian and Atlantic Oceans as well, El Niño causes widespread damage to reefs largely because corals are very sensitive to temperature changes. However, mass bleaching events were rarely observed before the 1970s and their abrupt appearance two decades ago remains an enigma. Here we propose a new explanation for the sudden occurrence of mass bleaching and show that it may be a response to the relative increase in El Niño experienced over the last two decades.     

An evaluation of the Lost World and Vertical Displacement hypotheses in the Chimantá Massif, Venezuelan Guayana

Aim To document the occurrence of vertical displacements of vegetation in the high plateaus of the Venezuelan Guayana (tepuis) over the last c. 6000 years, and to discuss their significance for the origin of their flora, especially the endemism patterns observed in their flat summits. Two hypotheses have been proposed for the origin of the summit flora. One (the Lost World hypothesis) proposes a long history of evolution in isolation from the surrounding plains, while the other (the Vertical Displacement hypothesis) suggests that vertical movements of vegetation during the Pleistocene glacial‐interglacial cycles would have resulted in floristic mixing within the lowlands, and genetic interchange among plateau summits. Location This work has been conducted on the flat summit of the Churí‐tepui, in the Chimantá massif, at 5°15′ Lat. N and 62°01′ Long. W, around 2250 m altitude. Methods Pollen analysis and radiocarbon dating of two peat outcrops, using modern analogue technique and numerical methods for palaeoecological interpretation were used. Results The replacement of a high‐altitude plant community (a paramoid Chimantaea shrubland) by a lower elevation (< 2300 m) Stegolepis meadow, occurred about 2500 years before present (yr bp ). This vegetation change is inferred to have resulted from a regional climatic shift to higher temperature and moisture. A subsequent decrease in temperature and moisture led to the establishment of present conditions after about 1450 yr bp . Main conclusions The highland vegetation of the tepuis responded to climate shifts with vertical displacements, supporting the hypothesis of vertical mixing. However, a physiographical analysis shows that around half of the tepuis would never have been connected by lowlands. Therefore, both hypotheses are needed to explain the origins of the summit flora in the tepuis.