共查询到20条相似文献,搜索用时 15 毫秒
1.
Duncan N. L. Menge 《Ecosystems》2011,14(4):519-532
Human activity is drastically altering global nitrogen (N) availability. The extent to which ecosystems absorb additional
N—and with it, additional CO2—depends on whether net primary production (NPP) is N-limited, so it is important to understand conditions under which N can
limit NPP. Here I use a general dynamical model to show that N limitation at steady-state—such as in old-growth forests—depends
on the balance of biotically controllable versus uncontrollable N inputs and losses. Steady-state N limitation is only possible
when uncontrollable inputs (for example, atmospheric deposition) exceed controllable losses (for example, leaching of plant-available
soil N), which is the same as when uncontrollable losses (for example, leaching of plant-unavailable soil N) exceed controllable
inputs (biological N fixation). These basic results are robust to many model details, such as the number of plant-unavailable
soil N pools and the number and type of N fixers. Empirical data from old-growth tropical (Hawai’i) and temperate (Oregon,
Washington, Chile) forests support the model insights. Practically, this means that any N fixer—symbiotic or not—could overcome
ecosystem N limitation, so understanding N limitation requires understanding controls on all N fixers. Further, comparing
losses of plant-available N to abiotic inputs could offer a rapid diagnosis of whether ecosystems can be N-limited, although
the applicability of this result is constrained to ecosystems with a steady-state N cycle such as old-growth forests largely
devoid of disturbance. 相似文献
2.
3.
W. Greve 《Helgoland Marine Research》1995,49(1-4):811-820
Following the 19th century recognition and definition of basic ecological entities, functional analysis has been the highlight
of this century. The synthesis of these findings enables ecological prognosis. The population as the basic functional unit
has been repeatedly treated; in the marine field, fisheries management approaches developed into multi-species population
analysis. As in planktology, theoretical ecology, and classic biocoenotic research, the population interactions are of increasing
scientific interest. A mathematical model is suggested that combines these extrinsic and intrinsic functional relationships
in order to define the fit of the ecological niche to the environment, the decisive measure of the expected population success,
and thus of the probable population development needed for prognostic purposes. It is discussed in how far the orientation
towards the predictive power or — with respect to the “skill” of meteorological prognoses — the “ecological prognostic skill”
improvement may serve as a means to choose the best investigative strategy. 相似文献
4.
Dobzhansky studied mechanisms of balancing selection using systems of inversions in Drosophila and he soon found that changes
in inversion frequencies along generations in experimental populations conformed to the expectation for a simple model of
heterosis. However, other more complex modes of selection, like rare male advantage, were later found to affect the maintenance
of inversion polymorphisms. Here we show that a more realistic (and complex) model than heterosis—integrating all known fitness
component estimates obtained in independent experiments for the ST/CH system of inversions in Drosophila pseudoobscura—not only conforms to but actually also predicts the inversion frequencies. This concludes this line of work and points to
other selection mechanisms than heterosis that were also considered by Dobzhansky—frequency- and sex-dependent selection—as
potential mechanisms of balancing selection responsible for the maintenance of the inversion polymorphisms in Drosophila. 相似文献
5.
Puc M 《International journal of biometeorology》2012,56(2):395-401
Birch pollen is one of the main causes of allergy during spring and early summer in northern and central Europe. The aim of
this study was to create a forecast model that can accurately predict daily average concentrations of Betula sp. pollen grains in the atmosphere of Szczecin, Poland. In order to achieve this, a novel data analysis technique—artificial
neural networks (ANN)—was used. Sampling was carried out using a volumetric spore trap of the Hirst design in Szczecin during
2003–2009. Spearman’s rank correlation analysis revealed that humidity had a strong negative correlation with Betula pollen concentrations. Significant positive correlations were observed for maximum temperature, average temperature, minimum
temperature and precipitation. The ANN resulted in multilayer perceptrons 366 8: 2928-7-1:1, time series prediction was of
quite high accuracy (SD Ratio between 0.3 and 0.5, R > 0.85). Direct comparison of the observed and calculated values confirmed good performance of the model and its ability
to recreate most of the variation. 相似文献
6.
Joel D. Velasco 《Biology & philosophy》2008,23(4):455-473
Bayesian methods have become among the most popular methods in phylogenetics, but theoretical opposition to this methodology
remains. After providing an introduction to Bayesian theory in this context, I attempt to tackle the problem mentioned most
often in the literature: the “problem of the priors”—how to assign prior probabilities to tree hypotheses. I first argue that
a recent objection—that an appropriate assignment of priors is impossible—is based on a misunderstanding of what ignorance
and bias are. I then consider different methods of assigning prior probabilities to trees. I argue that priors need to be
derived from an understanding of how distinct taxa have evolved and that the appropriate evolutionary model is captured by
the Yule birth–death process. This process leads to a well-known statistical distribution over trees. Though further modifications
may be necessary to model more complex aspects of the branching process, they must be modifications to parameters in an underlying
Yule model. Ignoring these Yule priors commits a fallacy leading to mistaken inferences both about the trees themselves and
about macroevolutionary processes more generally. 相似文献
7.
Mythili P.K. Satyavathi V. Pavankumar G. Rao M.V.S. Manga V. 《Plant Cell, Tissue and Organ Culture》1997,50(3):171-178
Genetic analysis of five in vitro characters was made through a 5 × 5 diallel analysis using callus derived from immature
inflorescence segments of pearl millet (Pennisetum glaucum). The characters studied were:
— volume of total callus,
— frequency of embryogenic calli,
— embryogenic callus volume,
— growth rate in terms of increase in fresh weight, and
— frequency of regeneration.
High heritability values and heterosis were noticed for all these characters except for E callus frequency. Additive gene
action was predominant for callus growth rate and frequency of regeneration. Of the five inbreds, IP 1346 (= P5) was found
to be the best genetic background for embryogenic callus volume, embryogenic growth rate and frequency of regeneration.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
8.
Hoeks FW Boon LA Studer F Wolff MO van der Schot F Vrabél P van der Lans RG Bujalski W Manelius A Blomsten G Hjorth S Prada G Luyben KCh Nienow AW 《Journal of industrial microbiology & biotechnology》2003,30(2):118-128
Foam disruption by agitation—the stirring as foam disruption (SAFD) technique—was scaled up to pilot and production scale
using Rushton turbines and an up-pumping hydrofoil impeller, the Scaba 3SHP1. The dominating mechanism behind SAFD—foam entrainment—was
also demonstrated at production scale. The mechanistic model for SAFD defines a fictitious liquid velocity generated by the
(upper) impeller near the dispersion surface, which is correlated with complete foam disruption. This model proved to be scalable,
thus enabling the model to be used for the design of SAFD applications. Axial upward pumping impellers appeared to be more
effective with respect to SAFD than Rushton turbines, as demonstrated by retrofitting a 12,000 l bioreactor, i.e. the triple
Rushton configuration was compared with a mixed impeller configuration from Scaba with a 20% lower ungassed power draw. The
retrofitted impeller configuration allowed 10% more broth without risking excessive foaming. In this way a substantial increase
in the volumetric productivity of the bioreactor was achieved. Design recommendations for the application of SAFD are given
in this paper. Using these recommendations for the design of a 30,000 l scale bioreactor, almost foamless Escherichia coli fermentations were realised.
Electronic Publication 相似文献
9.
Geir Halnes Erik Ulfhielm Emma Eklöf Ljunggren Jeanette Hellgren Kotaleski Jean-Pierre Rospars 《Journal of computational neuroscience》2009,27(3):471-491
A biochemical model of the receptor, G-protein and effector (RGE) interactions during transduction in the cilia of vertebrate
olfactory receptor neurons (ORNs) was developed and calibrated to experimental recordings of cAMP levels and the receptor
current (RC). The model describes the steps from odorant binding to activation of the effector enzyme which catalyzes the
conversion of ATP to cAMP, and shows how odorant stimulation is amplified and delayed by the RGE transduction cascade. A time-dependent
sensitivity analysis was performed on the model. The model output—the cAMP production rate—is particularly sensitive to a
few, dominant parameters. During odorant stimulation it depends mainly on the initial density of G-proteins and the catalytic
constant for cAMP production. 相似文献
10.
Human beings are predisposed to think of evolution as teleological—i.e., having a purpose or directive principle—and the ways
scientists talk about natural selection can feed this predisposition. This work examines the suggestion that students’ teleological
thinking operates as an obstacle when the natural selection evolution model is taught. What we mean by obstacle is an established way of thinking that resists change due to its explanatory power. In light of this approach, the challenges
of teaching evolution in biology education have been revised, and improved methodological strategies aimed at a better comprehension
of the Darwinian evolution model are suggested. 相似文献
11.
New methods of determining the structural groups —COOH and —CH2— have been developed. The investigation of carboxyl groups is possible both after derivatization with p-fluorophenacylbromide and by quantitative interpretation of the Fourier transform infrared (FT IR) spectra. There exists
a linear relationship between the results of these two methods that is generally valid for the analysis of all brown coal
components. The maximum extinction coefficient of the symmetric stretching vibration band of the CH2 groups has been determined using model substances. This allows quantification of this structural group directly from the
FT IR spectrum. The results agree with the contents of methylene groups as determined by 13C-cross polarization–magic angle spinning–nuclear magnetic resonance (13C CPMAS NMR) spectroscopy. Using these methods, the COOH and CH2 groups contained in brown coals of the North Rhine region and in their bioconversion products have been quantified.
Received: 21 December 1999 / Received revision: 25 April 2000 / Accepted: 1 May 2000 相似文献
12.
Several tritrophic systems are characterized by local over-exploitation of the food source. Interactions between predatory
mites, spider mites and their host plants are an example of such systems: either the spider mites over-exploit local patches
of host plants or the spider mites are exterminated by predatory mites. It is often stated that modelling the overall population
dynamics of such systems in a realistic way would soon lead to an unmanageable edifice. We advocate, however, the use of physiologically
structured population models as a both general and formal mathematical framework. The advantage is that analytically tractable
models may be obtained from the complex ‘master’ model by time-scale arguments or special choices of model ingredients. In
this way a network of models can be derived, each concentrating on a particular aspect, all inadequate to cover the entire
spectrum, but together (we hope) providing a coherent set of insights the relative importance of which can be assessed by
computer experiments on the ‘master’ model.
In this paper a rather realistic model of predator/prey interactions in an ensemble of host-plant patches is presented and,
as an example of our approach, some special cases are derived from that model. Their analysis provided some first, useful
insights. It is shown that prolonged duration of the prey-dispersal phase and prey dispersal from predator (-invaded prey)
patches may result in a stable steady state, whereas a humped plant-production function may — under certain conditions — result
in two stable steady states. 相似文献
13.
GLOBIO3: A Framework to Investigate Options for Reducing Global Terrestrial Biodiversity Loss 总被引:3,自引:0,他引:3
Rob Alkemade Mark van Oorschot Lera Miles Christian Nellemann Michel Bakkenes Ben ten Brink 《Ecosystems》2009,12(3):374-390
The GLOBIO3 model has been developed to assess human-induced changes in biodiversity, in the past, present, and future at
regional and global scales. The model is built on simple cause–effect relationships between environmental drivers and biodiversity
impacts, based on state-of-the-art knowledge. The mean abundance of original species relative to their abundance in undisturbed
ecosystems (MSA) is used as the indicator for biodiversity. Changes in drivers are derived from the IMAGE 2.4 model. Drivers
considered are land-cover change, land-use intensity, fragmentation, climate change, atmospheric nitrogen deposition, and
infrastructure development. GLOBIO3 addresses (i) the impacts of environmental drivers on MSA and their relative importance;
(ii) expected trends under various future scenarios; and (iii) the likely effects of various policy response options. GLOBIO3
has been used successfully in several integrated regional and global assessments. Three different global-scale policy options
have been evaluated on their potential to reduce MSA loss. These options are: climate-change mitigation through expanded use
of bio-energy, an increase in plantation forestry, and an increase in protected areas. We conclude that MSA loss is likely
to continue during the coming decades. Plantation forestry may help to reduce the rate of loss, whereas climate-change mitigation
through the extensive use of bioenergy crops will, in fact, increase this rate of loss. The protection of 20% of all large
ecosystems leads to a small reduction in the rate of loss, provided that protection is effective and that currently degraded
protected areas are restored.
Author Contributions RA—Writing, study design, data analyses; MvO—Writing, research; LM— Writing, data analyses; CN—Contribution to method; MB—Research,
data analyses; BtB—Contribution to method. 相似文献
14.
This paper deals with information transfer from the environment and “self”-organization in open, nonlinear systems far from
thermodynamic equilibrium — in the presence of either non-stationary phase jitter noise, or amplitude stationary noise. By
“self”-organization we mean here the progressive formation within the system of sequential, ordered (coherent) relationships
between appropriate dynamical variables-like for example, the phase differences between the oscillating components of the
system. We take up (in Section II) the classical Laser as a specific example and examine in detail the influence of phase
jitter noise in the mode (phase) locking process. We find—as expected—that phase fluctuations in the cavity cause degradation
of the coherent behaviour (i.e. increase the entropy) of the system — which, however, levels off, or saturates with time.
Further (in Section III) we examine systems where the number of self-sustained oscillating components may vary with time in
such a way that the maximum entropy of the system increases faster than the overall instantaneous entropy. We put forth the
hypothesis that in such cases — because of the increase of the redundancy — the system gets organized not just in spite of,
but merely because of the presence of Noise. Possible applications in biological systems (especially concerning a model of
cerebral organization) are briefly discussed. It is understood here, that the system has to display some preliminary dynamical
structure before the organizing procedure takes over. What happens afterwards is the subject of this paper. 相似文献
15.
Yamada MO Okayama M Chikamori K Yamada G Moriwake Y Minami T Tohno S Takeuchi R Tohno Y 《Biological trace element research》2000,77(2):149-158
Marker elements were estimated from the red cosmetics collected from different ancient burials and mine ruins in three separate
districts of Japan. Element levels were displayed in reference to the relative amount to sulfur (RA/S), by which the cosmetics
were divided into five types: I—a low Hg/S with a low Fe/S; II—both moderate Hg/S and Fe/S; III—a moderate Hg/S with a high
Fe/S; III 2—a high Hg/S with a moderate Fe/S; IV—a high Hg/S with a high Fe/S. The cosmetics can be further characterized
by referring to other contaminants such as Zn, Cu, and Mn. These combined analyses with contaminant metals were capable of
characterizing the origins of the cosmetics; it is useful to compare them to each other. The cosmetics were identified as
being due to several groups of contaminants from ancient mines in Japan, and also with this system analysis of the markers
it is possible to identify them from neighboring countries. 相似文献
16.
Tomáš Pavlíček Csaba Csuzdi Mete Mısırlıoğlu Boris Vilenkin 《Biodiversity and Conservation》2010,19(7):1989-2001
The analysis presents earthworm distribution in eight designated tectonic units (C—Cyprus, EP—East Pondites, ET—East Taurus
block, Sc—Sakarya continent, Kb—Kırşehir block, L—the Levant, TM—Taurus–Menderes block, WP—West Pondites) in the East Mediterranean
region (EM). It represents a full list of earthworm species (N = 81) of the region and reveals significant faunal similarities between Sc + WP, and between Kb + TM. The new Sc + WP unit
is characterized by the presence of archaic (Spermophorodrilus) as well as modern earthworm fauna. In contrast, the second newly established Kb + TM unit is characterized by poorly presented
earthworm autochthonous fauna and by a lack of endemic species. The analysis done on the species list (N = 73) representing the autochthonous family Lumbricidae showed statistically supported separation of L and EP between themselves
and from other designated units. The Levant, part of the Arabian tectonic plate, is characterized by a high level of endemism
in ancient (Healyella) and modern (Dendrobaena, Perelia) genera, and by the lack of archaic genera. The EP harbors unique species representing archaic (Eophila), ancient (Healyella), and modern (Allolobophora, Aporrectodea,
Dendrobaena and Eisenia) earthworm genera in the EM. The analysis also indicates origin of earthworm fauna in TM and faunistic relationships among
the designated units. Clearly, the present-day division of the EM earthworm fauna shows recognizable influences of tectonic
history. 相似文献
17.
Matthias Wolf Christian Selig Tobias Müller Nicole Philippi Thomas Dandekar Jörg Schultz 《Biologia》2007,62(6):641-645
It was shown that compensatory base changes (CBCs) in internal transcribed spacer 2 (ITS2) sequence-structure alignments can
be used for distinguishing species. Using the ITS2 Database in combination with 4SALE — a tool for synchronous RNA sequence
and secondary structure alignment and editing — in this study we present an in-depth CBC analysis for placozoan ITS2 sequences
and their respective secondary structures. This analysis indicates at least two distinct species in Trichoplax (Placozoa) supporting a recently suggested hypothesis, that Placozoa is “no longer a phylum of one”.
The first two authors contributed equally to this work. 相似文献
18.
Quantifying the Routes of Transmission for Pandemic Influenza 总被引:1,自引:0,他引:1
Motivated by the desire to assess nonpharmaceutical interventions for pandemic influenza, we seek in this study to quantify
the routes of transmission for this disease. We construct a mathematical model of aerosol (i.e., droplet-nuclei) and contact
transmission of influenza within a household containing one infected. An analysis of this model in conjunction with influenza
and rhinovirus data suggests that aerosol transmission is far more dominant than contact transmission for influenza. We also
consider a separate model of a close expiratory event, and find that a close cough is unlikely (≈1% probability) to generate
traditional droplet transmission (i.e., direct deposition on the mucous membranes), although a close, unprotected and horizontally-directed
sneeze is potent enough to cause droplet transmission. There are insufficient data on the frequency of close expiratory events
to assess the relative importance of aerosol transmission and droplet transmission, and it is prudent to leave open the possibility
that droplet transmission is important until proven otherwise. However, the rarity of close, unprotected and horizontally-directed
sneezes—coupled with the evidence of significant aerosol and contact transmission for rhinovirus and our comparison of hazard
rates for rhinovirus and influenza—leads us to suspect that aerosol transmission is the dominant mode of transmission for
influenza. 相似文献
19.
Stephen Philip Pain 《Biosemiotics》2009,2(2):181-191
In this essay I shall examine the representation of aggression and its issues in the model animal, the Fruit Fly, Drosophila melanogaster. The Fruit Fly is the model animal for genetics and more recently neuroscience. On the basis of its behaviour conclusions
are being drawn that will help in the development of new treatments for clinical entities like aggression and anxiety disorders—the
author questions those findings and asks whether more should be done to focus on the actual biology and behaviour—the Umwelt, instead of trying to bridge the gulf between invertebrate and human behaviours. 相似文献
20.
A tissue model for the study of cell proliferation in vitro 总被引:2,自引:0,他引:2
Summary A procedure for the cultivation of mesentery is described, in which the culture is fully representative of the tissue of origin.
The intact mesenteric membrane—exposed to a minimum of trauma—was spread out over a hole in a filter paper strip in fluid
medium and was cultivated free-hanging. Specimens from rats and guinea pigs were used. The organ culture model appears especially
apt for cytochemical and proliferation studies. Proliferation variables based on Feulgen DNA analysis in individual, morphologically
defined cells and on mitotic counting and radiochemical analysis were estimated. The tissue was fully viable in chemically
defined growth medium and showed an almost unaltered light microscopical appearance after up to 52 hr in culture.
Supported by the Swedish Cancer Society. 相似文献