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121.
One of the main issues in community ecology is the detection of structure and the identification of its related causes. In
this study, co-occurrence null models were used to identify possible spatio-temporal patterns in the assemblage of aquatic
macrophytes in the Upper Paraná River floodplain. The samples were obtained through the Long Term Ecological Research (LTER)
Program at two different grains: (1) a coarser spatial grain in January and August 2001 (entire floodplain lagoons); (2) and
a finer spatial grain in November 2006 (1 m2 quadrats). The study was conducted in 36 lagoons, both connected and disconnected to the main river channel, located in the
sub-basins of the Baía, Ivinheima and Paraná rivers. Two null models of species co-occurrence, the C-Score and Checkerboard
indices, were used to test the null hypothesis of random structure of the aquatic macrophyte assemblages. The null models
showed that the aquatic macrophyte assemblages were spatially structured in the distinct spatial grains. However, despite
this general pattern, macrophyte assemblages are organized differently depending on the degree of connectivity, seasonal period
and, at a finer grain, depth. Species co-occurrences were random in the disconnected lagoons during flood periods, in deep
zones of the lagoons of the Baía River and in the shallow littoral zone in the lagoons of the Paraná River. Analysis of the
patterns of co-occurrence indicated that competition and/or habitat preferences are probably important influences on the nonrandom
structure of assemblages. However, we suppose that at least three important factors (disturbances by water level fluctuation,
dispersion and facilitation) counteract potential effects of competition in specific situations, leading macrophyte assemblages
to assume random structure. 相似文献
122.
V. E. Golimbet 《Molecular Biology》2008,42(5):738-746
Cognitive deficit is a key feature of schizophrenia. Genetic factors are thought to contribute to cognitive disturbances in schizophrenic patients. However, the role of specific genes in the development of cognitive deficit remains elusive. The review considers the current studies on the association between gene polymorphisms and cognitive dysfunction in schizophrenics. Main attention is drawn to the consistently reproducible association between the COMT polymorphism Val158Met and cognitive traits, which has a biological and neuropsychological support. The association studies with the genes for the dopamine and serotonin receptors, brain-derived neurotrophic factor, dysbindin, DISC1, D-amino acid oxidase, and D-amino acid oxidase activator are reviewed as well. 相似文献
123.
Disturbance of a Kenya Rift Valley stream by the daily activities of local people and their livestock 总被引:1,自引:1,他引:0
Human and domestic animal activities and visits to an impacted site in the Njoro River, Kenya, were recorded from 1994 to 1995. The activities of people in the humid and wet zones of the stream included linen washing, water abstraction, excretion, bathing and swimming. Human and animal visits and activities along the Njoro River were on a daily basis and were patterned according to the time of the day, weather and seasons. Women formed the first group to visit the stream at dawn, followed by men and lastly, children. The diurnal pattern of visits was bimodal, with major peaks between 0600 and 1100 h and from 1600 h until dusk. The intensity of all major activities peaked at around midday and donkeys and cattle formed the largest proportion of the domestic animals that visited the stream. Much of the small-scale water abstraction occurred between 0700 and 1100 h, coinciding with the time when most people visited the stream. Water abstraction was most intense during the dry season. A mean discharge of 1.7 litres per second was measured whilst 0.3 litre of water per second was abstracted at the impacted site, implying that about 20% of the flow volume of the Njoro River was abstracted at a single site in one day.The effects of the human and animal activities on the structure of macrozoobenthos in the wet zone of the Njoro River were studied on the impacted site in relation to an upstream reference site and a downstream reference site. Oligochaetes and chironomids dominated the fauna in all three sites. The impacted site had low patchiness and mean crowding, with the taxa distribution tending toward a randomly dispersed spatial pattern. The mean turnover (±SD) of the macroinvertebrates was 47±18, 48±26 and 36±22 in the impacted, downstream and upstream sites, respectively. The trampling of the streambed by humans and livestock could, therefore, alter the benthos structure through redistribution and reduction of faunal patchiness. The cumulative effect of small-scale, but widespread and frequent disturbances might have large impacts on whole river systems. This study demonstrated that, in the tropics, quantification of the daily activities of people and domestic animals is important for future management of the Njoro River and consequently, Lake Nakuru. 相似文献
124.
Aim of this study was to identify optimal conditions for suspension transfection in the absence of serum. Transfection parameters
for suspension culture can be very different to ones in adherent cells. Most transfection protocols have been developed and
optimizedfor adherent culture. Using green fluorescent protein (GFP) as reporter, FCS was eliminated from the transfection
process by altering critical parameters and by substituting serum with albumin. Using standard phosphate and calcium concentrations
for transfection in the absence of serum resulted in titers of only 1% of those observed in the presence of serum. A reduction
of the calcium concentration from 250 mM to 100 mM, yielded a 25-fold increase in the expression of the recombinant protein
compared to the serum-free standard conditions. Altering the phosphate concentration, 1.4 mM in the transfection buffer, did
not improve the protein expression. Interestingly, reduction of DNA quantity by half to a concentration of 0.5 μg per milliliter
of culture volume resulted in a two-fold increase of protein production. Addition of albumin to serum-free medium protected
the cells against the toxicity of the calcium phosphate transfection particles (CaPi) yielding higher protein expression.
All the experiments were executed in a shaken multi-well system, allowing high multiplicity parameter screening to speed up
optimizations. The culture system is inexpensive, simple and efficient, minimizing costs for labor and consumables.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
125.
In the southern boreal forest (Québec, Canada), tree harvesting is a major disturbance affecting the dominant black spruce (Picea mariana) stands already suffering from naturally recurrent insect and fire disturbances. Although recovery of the spruce forest after an insect infestation or a fire is possible under current site conditions, it is less likely when both types of disturbance occur during a short period of time. The addition of yet another disturbance, such as tree harvesting, can thus have catastrophic consequences. We analyzed the impact of three successive disturbances—tree harvesting, insect infestation, and fire—on the regeneration of boreal spruce–moss forests within a period of approximately 50 years. The spruce forests were harvested in the 1940s and the 1950s. Recovery from the logging consisted of advance regeneration (spruce layers less than 1 m high that were left intact during clear-cuts), which was burned in 1991. The vegetation cover (mostly heath and lichen species) and soil conditions (acidic, nutrient-poor podzolic soils developed from coarse materials) of the postfire sites that we studied were similar. Stand structure and tree regeneration were documented from large quadrats (0.25 ha) using age, size, and tree ring data from postlogged and postfire spruce. At an early stage of development, the growing advance regeneration was damaged by insect defoliators in the late 1970s and the mid-1980s, and several trees died a few years before the 1991 fire. The successive disturbances considerably reduced the number of seed-bearers, leading to the collapse of postfire regeneration and a shift to parkland. Through a successional trajectory far from the expected trend for boreal forests influenced by single disturbance, the shift resulted in the formation of divergent plant communities. The development of divergent communities at the landscape scale is generally overlooked due to their small size. They indicate, however, the weak resilience of boreal forests faced with cascading perturbations, which are likely to increase in intensively logged areas. 相似文献
126.
127.
Response of soil respiration to simulated N deposition in a disturbed and a rehabilitated tropical forest in southern China 总被引:3,自引:1,他引:2
Jiangming Mo Wei Zhang Weixing Zhu Yunting Fang Dejun Li Ping Zhao 《Plant and Soil》2007,296(1-2):125-135
Responses of soil respiration (CO2 emission) to simulated N deposition were studied in a disturbed (reforested forest with previous understory and litter harvesting)
and a rehabilitated (reforested forest with no understory and litter harvesting) tropical forest in southern China from October
2005 to September 2006. The objectives of the study were to test the following hypotheses: (1) soil respiration is higher
in rehabilitated forest than in disturbed forest; (2) soil respiration in both rehabilitated and disturbed tropical forests
is stimulated by N additions; and (3) soil respiration is more sensitive to N addition in disturbed forest than in rehabilitated
forest due to relatively low soil nutrient status in the former, resulting from different previous human disturbance. Static
chamber and gas chromatography techniques were employed to quantify the soil respiration, following different N treatments
(Control, no N addition; Low-N, 5 g N m−2 year−1; Medium-N, 10 g N m−2 year−1), which had been applied continuously for 26 months before the respiration measurement. Results showed that soil respiration
exhibited a strong seasonal pattern, with the highest rates observed in the hot and wet growing season (April–September) and
the lowest rates in winter (December–February) in both rehabilitated and disturbed forests. Soil respiration rates exhibited
significant positive exponential relationship with soil temperature and significant positive linear relationship with soil
moisture. Soil respiration was also significantly higher in the rehabilitated forest than in the disturbed forest. Annual
mean soil respiration rate in the rehabilitated forest was 20% lower in low-N plots (71 ± 4 mg CO2-C m−2 h−1) and 10% lower in medium-N plots (80 ± 4 mg CO2-C m−2 h−1) than in the control plots (89 ± 5 mg CO2-C m−2 h−1), and the differences between the control and low-N or medium-N treatments were statistically significant. In disturbed forest,
annual mean soil respiration rate was 5% lower in low-N plots (63 ± 3 mg CO2-C m−2 h−1) and 8% lower in medium-N plots (61 ± 3 mg CO2-C m−2 h−1) than in the control plots (66 ± 4 mg CO2-C m−2 h−1), but the differences among treatments were not significant. The depressed effects of experimental N deposition occurred
mostly in the hot and wet growing season. Our results suggest that response of soil respiration to elevated N deposition in
the reforested tropical forests may vary depending on the status of human disturbance.
Responsible Editor: Hans Lambers. 相似文献
128.
In various guises, feasible generalized least squares (FGLS) estimation has occupied an important place in regression analysis for more than 35 years. Past studies on the characteristics of the FGLS estimators are largely based on large sample evaluations, and the important issue of admissibility remains unexplored in the case of the FGLS estimator. In this paper, an exact sufficient condition for the dominance of a Stein‐type shrinkage estimator over the FGLS estimator in finite samples based on squared error loss is given. In deriving the condition, we assume that the model's disturbance covariance matrix is unknown except for a scalar multiple. Further, for models with AR(1) disturbances, it is observed that the dominance condition reduces to one that involves no unknown parameter. In other words, in the case of AR(1) disturbances and where the condition for risk dominance is met, the FGLS estimator is rendered inadmissible under squared error loss. 相似文献
129.
A Scaling Rule for Landscape Patches and How It Applies to Conserving Soil Resources in Savannas 总被引:12,自引:4,他引:8
Scaling issues are complex, yet understanding issues such as scale dependencies in ecological patterns and processes is
usually critical if we are to make sense of ecological data and if we want to predict how land management options, for example,
are constrained by scale. In this article, we develop the beginnings of a way to approach the complexity of scaling issues.
Our approach is rooted in scaling functions, which integrate the scale dependency of patterns and processes in landscapes
with the ways that organisms scale their responses to these patterns and processes. We propose that such functions may have
sufficient generality that we can develop scaling rules—statements that link scale with consequences for certain phenomena
in certain systems. As an example, we propose that in savanna ecosystems, there is a consistent relationship between the size
of vegetation patches in the landscape and the degree to which critical resources, such as soil nutrients or water, become
concentrated in these patches. In this case, the features of the scaling functions that underlie this rule have to do with
physical processes, such as surface water flow and material redistribution, and the ways that patches of plants physically
“capture” such runoff and convert it into plant biomass, thereby concentrating resources and increasing patch size. To be
operationally useful, such scaling rules must be expressed in ways that can generate predictions. We developed a scaling equation
that can be used to evaluate the potential impacts of different disturbances on vegetation patches and on how soils and their
nutrients are conserved within Australian savanna landscapes. We illustrate that for a 10-km2 paddock, given an equivalent area of impact, the thinning of large tree islands potentially can cause a far greater loss
of soil nitrogen (21 metric tons) than grazing out small grass clumps (2 metric tons). Although our example is hypothetical,
we believe that addressing scaling problems by first conceptualizing scaling functions, then proposing scaling rules, and
then deriving scaling equations is a useful approach. Scaling equations can be used in simulation models, or (as we have done)
in simple hypothetical scenarios, to collapse the complexity of scaling issues into a manageable framework.
Received 8 December 1998; accepted 17 August 1999. 相似文献
130.
Emmanuel Amoah Boakye Daniel Houle Yves Bergeron Martin P. Girardin Igor Drobyshev 《Ecology and evolution》2022,12(3)
Increasing air temperatures and changing precipitation patterns due to climate change can affect tree growth in boreal forests. Periodic insect outbreaks affect the growth trajectory of trees, making it difficult to quantify the climate signal in growth dynamics at scales longer than a year. We studied climate‐driven growth trends and the influence of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on these trends by analyzing the basal area increment (BAI) of 2058 trees of Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, Thuja occidentalis L., Populus tremuloides Michx., and Betula papyrifera Marsh, which co‐occurs in the boreal mixedwood forests of western Quebec. We used a generalized additive mixed model (GAMM) to analyze species‐specific trends in BAI dynamics from 1967 to 1991. The model relied on tree size, cambial age, degree of spruce budworm defoliation, and seasonal climatic variables. Overall, we observed a decreasing growth rate of the spruce budworm host species, A. balsamea and P. glauca between 1967 and 1991, and an increasing growth rate for the non‐host, P. tremuloides, B. papyrifera, and T. occidentalis. Our results suggest that insect outbreaks may offset growth increases resulting from a warmer climate. The observation warrants the inclusion of the spruce budworm defoliation into models predicting future forest productivity. 相似文献