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1.
In ecology textbooks prior to the 1970s, Aldo Leopold’s classic story of predator control, overpopulation of deer, and habitat
degradation on the Kaibab Plateau during the 1920s epitomized predator regulation of herbivore populations. However, the story
disappeared from texts in the late 20th century after several papers noted uncertainties in estimations of the deer population
and provided alternative explanations. We re-examined the case study by determining the age structure of aspen (Populus tremuloides Michx.) on the plateau. Aspen comprises the majority of deer browse in the summer, and the absence of a normal cohort of
aspen from the 1920s would indicate deer overpopulation. The number of aspen (at 1.4 m) dating to the 1920s was an order of
magnitude lower than the null expectation. Other periods of unusual numbers of aspen included high numbers of aspen dating
to the 1880s and 1890s (when regular surface fires ceased), few aspen dating from 1953 to 1962 (after a second irruption of
the deer population), and very high numbers from 1968 to 1992 (coincident with widespread logging). These convergent lines
of evidence support the idea of extreme deer herbivory in the 1920s, consistent with food limitation of deer at high populations
(bottom–up control) and predation limitation at low deer populations (top–down control). Some uncertainty remains within the
overall story, and this level of ambiguity is common in case studies that involve population ecology, land management, and
people at the scale of 1,000 km2 and 100 years. A complete version of the Kaibab deer story and its history would be a valuable, realistic case study for
ecology texts. 相似文献
2.
3.
David M. Seaborg 《Journal of molecular evolution》2010,71(2):87-99
The canonical genetic code is on a sub-optimal adaptive peak with respect to its ability to minimize errors, and is close
to, but not quite, optimal. This is demonstrated by the near-total adjacency of synonymous codons, the similarity of adjacent
codons, and comparisons of frequency of amino acid usage with number of codons in the code for each amino acid. As a rare
empirical example of an adaptive peak in nature, it shows adaptive peaks are real, not merely theoretical. The evolution of
deviant genetic codes illustrates how populations move from a lower to a higher adaptive peak. This is done by the use of
“adaptive bridges,” neutral pathways that cross over maladaptive valleys by virtue of masking of the phenotypic expression
of some maladaptive aspects in the genotype. This appears to be the general mechanism by which populations travel from one
adaptive peak to another. There are multiple routes a population can follow to cross from one adaptive peak to another. These
routes vary in the probability that they will be used, and this probability is determined by the number and nature of the
mutations that happen along each of the routes. A modification of the depiction of adaptive landscapes showing genetic distances
and probabilities of travel along their multiple possible routes would throw light on this important concept. 相似文献
4.
Janne Boone-Heinonen Ana V. Diez-Roux David C. Goff Catherine M. Loria Catarina I. Kiefe Barry M. Popkin Penny Gordon-Larsen 《PloS one》2013,8(12)
Background
Recent obesity prevention initiatives focus on healthy neighborhood design, but most research examines neighborhood food retail and physical activity (PA) environments in isolation. We estimated joint, interactive, and cumulative impacts of neighborhood food retail and PA environment characteristics on body mass index (BMI) throughout early adulthood.Methods and Findings
We used cohort data from the Coronary Artery Risk Development in Young Adults (CARDIA) Study [n=4,092; Year 7 (24-42 years, 1992-1993) followed over 5 exams through Year 25 (2010-2011); 12,921 person-exam observations], with linked time-varying geographic information system-derived neighborhood environment measures. Using regression with fixed effects for individuals, we modeled time-lagged BMI as a function of food and PA resource density (counts per population) and neighborhood development intensity (a composite density score). We controlled for neighborhood poverty, individual-level sociodemographics, and BMI in the prior exam; and included significant interactions between neighborhood measures and by sex. Using model coefficients, we simulated BMI reductions in response to single and combined neighborhood improvements. Simulated increase in supermarket density (from 25th to 75th percentile) predicted inter-exam reduction in BMI of 0.09 kg/m2 [estimate (95% CI): -0.09 (-0.16, -0.02)]. Increasing commercial PA facility density predicted BMI reductions up to 0.22 kg/m2 in men, with variation across other neighborhood features [estimate (95% CI) range: -0.14 (-0.29, 0.01) to -0.22 (-0.37, -0.08)]. Simultaneous increases in supermarket and commercial PA facility density predicted inter-exam BMI reductions up to 0.31 kg/m2 in men [estimate (95% CI) range: -0.23 (-0.39, -0.06) to -0.31 (-0.47, -0.15)] but not women. Reduced fast food restaurant and convenience store density and increased public PA facility density and neighborhood development intensity did not predict reductions in BMI.Conclusions
Findings suggest that improvements in neighborhood food retail or PA environments may accumulate to reduce BMI, but some neighborhood changes may be less beneficial to women. 相似文献5.
Joseph E. Donnelly Stephen D. Herrmann Kate Lambourne Amanda N. Szabo Jeffery J. Honas Richard A. Washburn 《PloS one》2014,9(1)
Background
The magnitude of the negative energy balance induced by exercise may be reduced due to compensatory increases in energy intake.Objective
To address the question: Does increased exercise or physical activity alter ad-libitum daily energy intake or macronutrient composition in healthy adults?Data Sources
PubMed and Embase were searched (January 1990–January 2013) for studies that presented data on energy and/or macronutrient intake by level of exercise, physical activity or change in response to exercise. Ninety-nine articles (103 studies) were included.Study Eligibility Criteria
Primary source articles published in English in peer-reviewed journals. Articles that presented data on energy and/or macronutrient intake by level of exercise or physical activity or changes in energy or macronutrient intake in response to acute exercise or exercise training in healthy (non-athlete) adults (mean age 18–64 years).Study Appraisal and Synthesis Methods
Articles were grouped by study design: cross-sectional, acute/short term, non-randomized, and randomized trials. Considerable heterogeneity existed within study groups for several important study parameters, therefore a meta-analysis was considered inappropriate. Results were synthesized and presented by study design.Results
No effect of physical activity, exercise or exercise training on energy intake was shown in 59% of cross-sectional studies (n = 17), 69% of acute (n = 40), 50% of short-term (n = 10), 92% of non-randomized (n = 12) and 75% of randomized trials (n = 24). Ninety-four percent of acute, 57% of short-term, 100% of non-randomized and 74% of randomized trials found no effect of exercise on macronutrient intake. Forty-six percent of cross-sectional trials found lower fat intake with increased physical activity.Limitations
The literature is limited by the lack of adequately powered trials of sufficient duration, which have prescribed and measured exercise energy expenditure, or employed adequate assessment methods for energy and macronutrient intake.Conclusions
We found no consistent evidence that increased physical activity or exercise effects energy or macronutrient intake. 相似文献6.
Dissipation of light energy was studied in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst., and in leaves of Spinacia oleracea L. and Arabidopsis thaliana (L.) Heynh., using chlorophyll fluorescence as an indicator reaction. Maximum chlorophyll fluorescence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea
(DCMU)-treated spinach leaves, as produced by saturating light and studied between +5 and −20 °C, revealed an activation energy
ΔE of 0.11 eV. As this suggested recombination fluorescence produced by charge recombination between the oxidized primary
donor of photosystem II and reduced pheophytin, a mathematical model explaining fluorescence, and based in part on known characteristics
of primary electron-transport reactions, was developed. The model permitted analysis of different modes of fluorescence quenching,
two localized in the reaction center of photosystem II and one in the light-harvesting system of the antenna complexes. It
predicted differences in the relationship between quenching of variable fluorescence F
v and quenching of basal, so-called F
0 fluorescence depending on whether quenching originated from antenna complexes or from reaction centers. Such differences
were found experimentally, suggesting antenna quenching as the predominant mechanism of dissipation of light energy in the
moss Rhytidiadelphus, whereas reaction-center quenching appeared to be important in spinach and Arabidopsis. Both reaction-center and antenna quenching required activation by thylakoid protonation but only antenna quenching depended
on or was strongly enhanced by zeaxanthin. De-protonation permitted relaxation of this quenching with half-times below 1 min.
More slowly reversible quenching, tentatively identified as so-called q
I or photoinhibitory quenching, required protonation but persisted for prolonged times after de-protonation. It appeared to
originate in reaction centers.
Received: 8 April 2000 / Accepted: 31 August 2000 相似文献
7.
Does Free-Air Carbon Dioxide Enrichment Affect Photochemical Energy Use by Evergreen Trees in Different Seasons? A Chlorophyll Fluorescence Study of Mature Loblolly Pine 总被引:8,自引:0,他引:8
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Graham J. Hymus David S. Ellsworth Neil R. Baker Stephen P. Long 《Plant physiology》1999,120(4):1183-1192
Previous studies of the effects of growth at elevated CO2 on energy partitioning in the photosynthetic apparatus have produced conflicting results. The hypothesis was developed and tested that elevated CO2 increases photochemical energy use when there is a high demand for assimilates and decreases usage when demand is low. Modulated chlorophyll a fluorescence and leaf gas exchange were measured on needles at the top of a mature, 12-m loblolly pine (Pinus taeda L.) forest. Trees were exposed to ambient CO2 or ambient plus 20 Pa CO2 using free-air CO2 enrichment. During April and August, periods of shoot growth, light-saturated photosynthesis and linear electron transport were increased by elevated CO2. In November, when growth had ceased but temperatures were still moderate, CO2 treatment had no significant effect on linear electron transport. In February, when low temperatures were likely to inhibit translocation, CO2 treatment caused a significant decrease in linear electron transport. This coincided with a slower recovery of the maximum photosystem II efficiency on transfer of needles to the shade, indicating that growth in elevated CO2 induced a more persistent photoinhibition. Both the summer increase and the winter decrease in linear electron transport in elevated CO2 resulted from a change in photochemical quenching, not in the efficiency of energy transfer within the photosystem II antenna. There was no evidence of any effect of CO2 on photochemical energy sinks other than carbon metabolism. Our results suggest that elevated CO2 may increase the effects of winter stress on evergreen foliage. 相似文献
8.
John Boakes John P. Peach Stuart M. McGill Author vitae 《Journal of electromyography and kinesiology》1998,8(6):423-427
Many low-back patients undergo electromyography (EMG)-based evaluations of muscle performance but present to the clinic after being prescribed muscle relaxants. The question that needed to be addressed was, do centrally acting muscle relaxants (methocarbamol; Robaxin®) affect the EMG spectral indices of muscle fatigue that are often used to assess muscle performance. Participants performed an isometric spine extension protocol involving a 30 s fatigue exertion trial, then 1 min rest, and finally a 10 s long repeat exertion trial, at a 60% maximum voluntary contraction (MVC) level of exertion. Seven men were tested on two separate days (approximately 3–7 days apart), one day while medicated (six doses) with Robaxin and on another while not medicated. Specifically, the following parameters were studied in the bilateral multifidus (L5), lower erector spinae (L3) and upper erector spinae (T9): the slope of median power frequencies (MPFs) over the duration of the trial and the initial y-intercept of the MPF. The results generally suggest that methocarbamol (Robaxin) does not have any significant affect on the EMG median power frequency of the extensors during a fatiguing contraction followed by a repeat exertion, at least in normal people (one exception was observed—one side of multifidus at L5). However, given that this appears to be the first study of its kind, and that a relatively small number of subjects were used in this study, further investigation is needed to make a definitive conclusion about the effects of this drug on the several features of the electromyogram, over a broad spectrum of the clinical population performing a wider variety of tasks. 相似文献