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1.
Alvarez LH 《Journal of mathematical biology》2000,40(5):383-405
We consider the determination of the harvesting strategy maximizing the present expected value of the cumulative yield from
the present up to extinction. By relying on a combination of stochastic calculus, ordinary nonlinear programming, and the
classical theory of diffusions, we show that if the underlying population evolves according to a logistic diffusion subject
to a general diffusion coefficient, then there is a single threshold density at which harvesting should be initiated in a
singular fashion. We derive the condition which uniquely determines the threshold and show that harvesting should be initiated
only when the option value of further preserving another individual falls below its opportunity cost. In this way, we present
a real option interpretation of rational harvesting planning. We also consider the comparative static properties of the value
of the harvesting opportunity and state a set of usually satisfied conditions under which increased stochastic fluctuations
(demographic or environmental) decrease the expected cumulative yield from harvesting and increase the optimal harvesting
threshold, thus postponing the rational exercise of the irreversible harvesting decision.
Received: 19 January 1999 / Revised version: 2 July 1999 / Published online: 16 February 2000 相似文献
2.
Lewis MA 《Journal of mathematical biology》2000,41(5):430-454
Despite the recognized importance of stochastic factors, models for ecological invasions are almost exclusively formulated
using deterministic equations [29]. Stochastic factors relevant to invasions can be either extrinsic (quantities such as temperature
or habitat quality which vary randomly in time and space and are external to the population itself) or intrinsic (arising
from a finite population of individuals each reproducing, dying, and interacting with other individuals in a probabilistic
manner). It has been long conjectured [27] that intrinsic stochastic factors associated with interacting individuals can slow
the spread of a population or disease, even in a uniform environment. While this conjecture has been borne out by numerical
simulations, we are not aware of a thorough analytical investigation.
In this paper we analyze the effect of intrinsic stochastic factors when individuals interact locally over small neighborhoods.
We formulate a set of equations describing the dynamics of spatial moments of the population. Although the full equations
cannot be expressed in closed form, a mixture of a moment closure and comparison methods can be used to derive upper and lower
bounds for the expected density of individuals. Analysis of the upper solution gives a bound on the rate of spread of the
stochastic invasion process which lies strictly below the rate of spread for the deterministic model. The slow spread is most
evident when invaders occur in widely spaced high density foci. In this case spatial correlations between individuals mean
that density dependent effects are significant even when expected population densities are low. Finally, we propose a heuristic
formula for estimating the true rate of spread for the full nonlinear stochastic process based on a scaling argument for moments.
Received: 19 October 1998 / Revised version: 1 September 1999 / Published online: 4 October 2000 相似文献
3.
Alvarez LH 《Journal of mathematical biology》2001,43(6):534-544
We consider the impact of increased stochastic fluctuations on the extinction date of an unstructured population subject
to either environmental or demographical stochasticity (or both). By modelling the population density as a general linear
diffusion, we state a set of typically satisfied conditions under which the decreasing minimal r-excessive mapping (and, therefore, the moment generating function) of the considered diffusion process is convex and, consequently,
under which the impact of increased stochastic fluctuations on the expected date at which the density becomes arbitrarily
small is unambiguously negative. In other words, we establish a set of sufficient conditions under which increased stochasticity
speeds up the extinction process independently of whether stochasticity is environmental or demographic. In this way, we are
able to confirm that increased stochasticity is detrimental for population growth.
Received: 25 April 2000 / Revised version: 18 April 2001 / Published online: 12 October 2001 相似文献
4.
Gourley SA 《Journal of mathematical biology》2000,41(3):272-284
We consider a scalar reaction-diffusion equation containing a nonlocal term (an integral convolution in space) of which Fisher‘s
equation is a particular case. We consider travelling wavefront solutions connecting the two uniform states of the equation.
We show that if the nonlocality is sufficiently weak in a certain sense then such travelling fronts exist. We also construct
expressions for the front and its evolution from initial data, showing that the main difference between our front and that
of Fisher‘s equation is that for sufficiently strong nonlocality our front is non-monotone and has a very prominent hump.
Received: 8 August 1999 / Revised: 3 March 2000 / Published online: 14 September 2000 相似文献
5.
We propose a new measure of synchronization of multichannel ictal and interictal EEG signals. The measure is based on the
residual covariance matrix of a multichannel autoregressive model. A major advantage of this measure is its ability to be
interpreted both in the framework of stochastic and deterministic models. A preliminary analysis of EEG data from three patients
using this measure documents the expected increased synchronization during ictal periods but also reveals that increased synchrony
persists for prolonged periods (up to 2 h or more) in the postictal period.
Received: 20 July 1997 / Accepted in revised form: 26 January 1999 相似文献
6.
Watkins JC 《Journal of mathematical biology》2000,41(3):253-271
In this paper we prove a consistency theorem (law of large numbers) and a fluctuation theorem (central limit theorem) for
structured population processes. The basic assumptions for these theorems are that the individuals have no statistically distinguishing
features beyond their class and that the interaction between any two individuals is not too high. We apply these results to
density dependent models of Leslie type and to a model for flour beetle dynamics.
Received: 24 February 1999 / Revised version: 23 July 1999 / Published online: 14 September 2000 相似文献
7.
In this paper we derive spatially explicit equations to describe a stochastic invasion process. Parents are assumed to produce
a random number of offspring which then disperse according to a spatial redistribution kernel. Equations for population moments,
such as expected density and covariance averaged over an ensemble of identical stochastic processes, take the form of deterministic
integro-difference equations. These equations describe the spatial spread of population moments as the invasion progresses.
We use the second order moments to analyse two basic properties of the invasion. The first property is permanence of form
in the correlation structure of the wave. Analysis of the asymptotic form of the invasion wave shows that either (i) the covariance
in the leading edge of the wave of invasion asymptotically achieves a permanence of form with a characteristic structure described
by an unchanging spatial correlation function, or (ii) the leading edge of the wave has no asymptotic permanence of form with
the length scales of spatial correlations continually increasing over time. Which of these two outcomes pertains is governed
by a single statistic, φ which depends upon the shape of the dispersal kernel and the net reproductive number. The second
property of the invasion is its patchy structure. Patchiness, defined in terms of spatial correlations on separate short (within
patch) and long (between patch) spatial scales, is linked to the dispersal kernel. Analysis shows how a leptokurtic dispersal
kernel gives rise to patchiness in spread of a population.
Received: 11 August 1997 / Revised version: 22 September 1998 / Published online: 4 October 2000 相似文献
8.
We explore a set of simple, nonlinear, two-stage models that allow us to compare the effects of density dependence on population
dynamics among different kinds of life cycles. We characterize the behavior of these models in terms of their equilibria,
bifurcations, and nonlinear dynamics, for a wide range of parameters. Our analyses lead to several generalizations about the
effects of life history and density dependence on population dynamics. Among these are: (1) iteroparous life histories are
more likely to be stable than semelparous life histories; (2) an increase in juvenile survivorship tends to be stabilizing;
(3) density-dependent adult survival cannot control population growth when reproductive output is high; (4) density-dependent
reproduction is more likely to cause chaotic dynamics than density dependence in other vital rates; and (5) changes in development
rate have only small effects on bifurcation patterns.
Received: 12 April 1999 / Published online: 3 August 2000 相似文献
9.
The population history of a 9-year-old roadside population of the invasive plant Bunias orientalis was reconstructed by demographic analysis including size, position, age (determined by herbchronology) and RAPD-PCR patterns
of individual plants. We evaluated emerging patterns of population growth and genetic structure during a full period of population
development under typical site conditions (anthropogenic disturbance) and their possible consequences for the invasion potential
of the species. The population has grown rapidly and continuously (though with slowing geometric population increase) during
the 9 years since its foundation, filling the space available in the study area. Genetic variation (RAPD markers) was already
high in the founder cohorts and remained at the same level throughout population development (variance fluctuations <15%).
Both results may be related to the mowing management at the site which seems to promote population growth of B. orientalis relative to other co-occuring species and to prevent the genetic drift and the development of spatial genetic structure that
would be expected under isolation-by-distance models. Large founder plants had comparatively low genetic variance and were
more closely related to younger cohorts than were small founder plants, indicating that selection acted during population
development. Overall, the current anthropogenic disturbance regimes may contribute to high genetic variability by artificially
increasing gene flow and thereby promoting the adaptability of invasive species to the often unpredictable conditions at disturbed
sites. Our approach using retrospective demographic investigation allows the detection of spatio-temporal microscale patterns
in genetic and phenotypic variation. Thus it allows a thorough understanding of local invasions of perennial herbaceous plants.
Received: 23 November 1998 / Accepted: 14 April 1999 相似文献
10.
The entomopathogenic nematode–bacterium complex Heterorhabditis megidis–Photorhabdus luminescens was cultured in 10-l internal loop bioreactors with marine impellers at aeration rates of 0.3 vvm and 0.7 vvm. Process parameters
like impeller velocity and oxygen saturation were controlled at equal set points. The bacterial density was assessed at 24 h.
Nematode dauer juveniles (DJ) were then inoculated and the development to adults after 8 days and final DJ yields after 16 days
were recorded. The bacterial population density and the nematode inoculum development was variable and was not influenced
by the aeration rate. A significant effect on the yield was recorded at the highest aeration rate. This result was confirmed
by a direct comparison in two 5-l internal loop glass bioreactors at 0.3 vvm and 1.0 vvm, which were inoculated with nematode
and bacterium pre-cultures from the same flask culture. Possible reasons for the positive correlation between aeration rate
and DJ yield are discussed.
Received: 27 September 1999 / Received revision: 21 January 2000 / Accepted: 23 January 2000 相似文献
11.
Elena Litchman 《Oecologia》1998,117(1-2):247-257
Light is a major resource in aquatic ecosystems and has a complex pattern of spatio-temporal variability, yet the effects
of dynamic light regimes on communities of phytoplankton are largely unexplored. I examined whether and how fluctuating light
supply affects the structure and dynamics of phytoplankton communities. The effect of light fluctuations was tested at two
average irradiances: low, 25 μmol quanta m−2 s−1 and high, 100 μmol quanta m−2 s−1 in 2- and 18-species communities of freshwater phytoplankton. Species diversity, and abundances of individual species and
higher taxa, depended significantly on both the absolute level and the degree of variability in light supply, while total
density, total biomass, and species richness responded only to light level. In the two-species assemblage, fluctuations increased
diversity at both low and high average irradiances and in the multispecies community fluctuations increased diversity at high
irradiance but decreased diversity at low average irradiance. Species richness was higher under low average irradiance and
was not affected by the presence or absence of fluctuations. Diatom abundance was increased by fluctuations, especially at
low average irradiance, where they became the dominant group, while cyanobacteria and green algae dominated low constant light
and all high light treatments. Within each taxonomic group, however, there was no uniform pattern in species responses to
light fluctuations: both the magnitude and direction of response were species-specific. The temporal regime of light supply
had a significant effect on the growth rates of individual species grown in monocultures. Species responses to the regime
of light supply in monocultures qualitatively agreed with their abundances in the community experiments. The results indicate
that the temporal regime of light supply may influence structure of phytoplankton communities by differentially affecting
growth rates and mediating species competition.
Received: 24 September 1997 / Accepted: 8 July 1998 相似文献
12.
Alexander P. Browning Jesse A. Sharp Tarunendu Mapder Christopher M. Baker Kevin Burrage Matthew J. Simpson 《Biophysical journal》2021,120(1):133-142
Bacteria invest in a slow-growing subpopulation, called persisters, to ensure survival in the face of uncertainty. This hedging strategy is remarkably similar to financial hedging, where diversifying an investment portfolio protects against economic uncertainty. We provide a new, to our knowledge, theoretical foundation for understanding cellular hedging by unifying the study of biological population dynamics and the mathematics of financial risk management through optimal control theory. Motivated by the widely accepted role of volatility in the emergence of persistence, we consider several models of environmental volatility described by continuous-time stochastic processes. This allows us to study an emergent cellular hedging strategy that maximizes the expected per capita growth rate of the population. Analytical and simulation results probe the optimal persister strategy, revealing results that are consistent with experimental observations and suggest new opportunities for experimental investigation and design. Overall, we provide a new, to our knowledge, way of conceptualizing and modeling cellular decision making in volatile environments by explicitly unifying theory from mathematical biology and finance. 相似文献
13.
Babu KR Swaminathan S Marten S Khanna N Rinas U 《Applied microbiology and biotechnology》2000,53(6):655-660
Escherichia coli TG1 transformed with a temperature-regulated interferon-α expression vector was grown to high cell density in defined medium
containing glucose as the sole carbon and energy source, utilizing a simple fed-batch process. Feeding was carried out to
achieve an exponential increase in biomass at growth rates which minimized acetate production. Thermal induction of such high
cell density cultures resulted in the production of ∼4 g interferon-α/l culture broth. Interferon-α was produced exclusively
in the form of insoluble inclusion bodies and was solubilized under denaturing conditions, refolded in the presence of arginine
and purified to near homogeneity, utilizing single-step ion-exchange chromatography on Q-Sepharose. The yield of purified
interferon-α was ∼300 mg/l with respect to the original high cell density culture broth (overall yield of ∼7.5% active interferon-α).
The purified recombinant interferon-α was found by different criteria to be predominantly monomeric and possessed a specific
bioactivity of ∼2.5 × 108 IU/mg based on viral cytopathic assay.
Received: 8 October 1999 / Received revision: 8 December 1999 / Accepted: 12 December 1999 相似文献
14.
Empirical evidence shows that childhood diseases persist in large communities whereas in smaller communities the epidemic
goes extinct (and is later reintroduced by immigration). The present paper treats a stochastic model describing the spread
of an infectious disease giving life-long immunity, in a community where individuals die and new individuals are born. The
time to extinction of the disease starting in quasi-stationarity (conditional on non-extinction) is exponentially distributed.
As the population size grows the epidemic process converges to a diffusion process. Properties of the limiting diffusion are
used to obtain an approximate expression for τ, the mean-parameter in the exponential distribution of the time to extinction
for the finite population. The expression is used to study how τ depends on the community size but also on certain properties
of the disease/community: the basic reproduction number and the means and variances of the latency period, infectious period
and life-length. Effects of introducing a vaccination program are also discussed as is the notion of the critical community
size, defined as the size which distinguishes between the two qualitatively different behaviours.
Received: 14 February 2000 / Revised version: 5 June 2000 / Published online: 24 November 2000 相似文献
15.
We analyse a simplified form of the frontal lobe architecture of cortico-basal ganglia-thalamo-cortical loops to determine
the manner in which they can learn temporal sequences as part of working memory activity. In particular, we consider how the
temporal duration of activity can arise in this setting. We start from a hard-wired version in which temporally extended activity
is created by the `long' loop of cortex → basal ganglia → thalamus → cortex, and show it arises from a near saddle-node bifurcation.
The manner in which the transition between patterns occurs is also considered. This is then extended to analyse the temporal
sequence storage and regeneration abilities of trained networks with a similar architecture. The temporal dynamics of this
activity is also analysed. Implications of this for other working memory activities and for understanding the architecture
of the frontal lobes are discussed in conclusion.
Received: 12 April 1999 / Accepted in revised form: 5 November 1999 相似文献
16.
Oscillators in networks may display a variety of activity patterns. This paper presents a geometric singular perturbation
analysis of clustering, or alternate firing of synchronized subgroups, among synaptically coupled oscillators. We consider
oscillators in two types of networks: mutually coupled, with all-to-all inhibitory connections, and globally inhibitory, with
one excitatory and one inhibitory population of oscillators, each of arbitrary size. Our analysis yields existence and stability
conditions for clustered states, along with formulas for the periods of such firing patterns. By using two different approaches,
we derive complementary conditions, the first set stated in terms of time lengths determined by intrinsic and synaptic properties
of the oscillators and their coupling and the second set stated in terms of model parameters and phase space structures directly
linked to parameters. These results suggest how biological components may interact to produce the spindle sleep rhythm in
thalamocortical networks.
Received: 9 September 1999 / Revised version: 7 July 2000 / Published online: 24 November 2000 相似文献
17.
Yu Zhou Tang Wei Zu Chen Cun Xin Wang Yun Yu Shi 《European biophysics journal : EBJ》1999,28(6):478-488
A method for constructing the suitable initial configuration of the membrane-protein system for molecular dynamics (MD) simulations
is presented. This method could provide some hydrated initial configurations and help us to determine the best surface area
of the system by contracting the surface area and comparing the optimized lowest energy of the system by energy minimization.
The gramicidin A (GA) channel in;the fully hydrated dimyristoylphosphatidylcholine (DMPC) bilayer was used as our model. Three
configurations with different surface areas were selected and applied for one 400 ps and two 300 ps MD simulations at constant
pressure and temperature. All simulations were fairly stable without any constraints. Through analysis of the MD trajectories
we found that the system with the best surface area was more stable than the other two systems, whose sizes were changed in
the simulations. Further analysis of the bilayer normal length and the order parameters of the lipid alkyl tails indicates
that the system with the best surface area shows some characteristics of the Lα phase, while both the smaller and the larger size systems have distinct deviations from the Lα phase that we expect. This illustrates that the correct surface area and the suitable initial configuration have an important
influence on the phase of the membrane in the MD simulation. In addition, by comparing the root mean square differences of
GA relative to the initial structure and interaction energy between different components of the system for all three systems,
we find that the state of the DMPC bilayer has exerted a significant influence on the structure of GA. All these results demonstrate
the validity of our method for constructing the initial configuration of the membrane-protein system for MD simulations.
Received: 10 September 1998 / Revised version: 19 March 1999 / Accepted: 19 March 1999 相似文献
18.
C. Escobar R. Hudson M. Martínez-Gómez R. Aguilar-Roblero 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2000,186(1):33-38
Young rabbits (Oryctolagus cuniculus) are only nursed for 3–5 min every 24 h. They show a circadian increase in activity in anticipation of this, which is entrained
by suckling. Our aim was to determine whether serum and liver metabolites show diurnal fluctuations which could act to regulate
this circadian pattern. Stomach weight, liver glycogen and serum metabolites were measured every 3 h in 7- to 8-day-old pups
when normally nursed (up to 24 h after suckling) and fasted (up to 48 h after suckling). The results suggest:
Accepted: 9 October 1999 相似文献
19.
M. C. de O. Souza I. C. Roberto A. M. F. Milagres 《Applied microbiology and biotechnology》1999,52(6):768-772
We investigated xylanase production by Thermoascus aurantiacus using semisolid fermentation. Multivariant statistical approaches were employed to evaluate the effects of several variables
(initial moisture in the medium, cultivation time, inoculum level, and bagasse mass) on xylanase production. The initial moisture
content and bagasse mass were the most important factors affecting xylanase activity. The xylanase activity produced by the
fungus under the optimized conditions (81% moisture content and 17 g bagasse) was found to be 2700 U per gram of initial dry
matter, whereas its value predicted by a polynomial model was 2400 U per gram of initial dry matter.
Received: 4 December 1998 / Received revision: 15 March 1999 / Accepted: 16 May 1999 相似文献
20.
Shigeru M. Shirai Ryoji Kuranaga Hideki Sugiyama Masahito Higuchi 《Ichthyological Research》2006,53(4):357-368
We analyzed patterns of genetic diversity in the sailfin sandfish (Arctoscopus japonicus), focusing on population subdivisions within the Sea of Japan. We observed 270 specimens from nine sampling sites in 1999–2000,
i.e., seven sites in the Sea of Japan and two sites from the Pacific coast of Hokkaido. An additional site (30 specimens)
was sampled from eastern Korea in the spawning season of 2004 for comparison. Forty haplotypes, compiled into three haplogroups
(A–C), were detected based on the comparison of a 400-bp sequence of the anterior part of the mitochondrial control region.
In accordance with previous hypotheses from morphological and molecular analyses, genetic discontinuity between the Sea of
Japan and the Pacific coast of Hokkaido was conspicuous. Within the Sea of Japan, eight sampling sites were not genetically
uniform, and most of the variations among sites were detected between eastern Korea [the “eastern Korea” (EK) population:
distributed from the Korean Peninsula to Mishima, Yamaguchi Prefecture] and the other sites along the coast of Japan [the
“western Japan” (WJ) population: from Oki Islands to western Hokkaido] (Φ
CT
= 0.096, P = 0.0183). The WJ population, having lower genetic variability, showed significant departure from neutrality, indicating
influences through a recent population expansion. The period of the expansion can be estimated to have begun on the order
of 104 years ago. We consider that the present Japan Sea populations have been formed through the invasion of a small ancestral
stock to the Sea of Japan and its population expansion during the last glacial period or later. On the other hand, we failed
to detect distinct evidence of a population expansion in the EK population. Haplogroup C, detected in a high frequency in
this population, was estimated to have mixed with haplogroup A after rapid differentiations of the latter. Therefore, the
EK population, strongly influenced by such a mixture, might possess haplogroup C in a higher frequency and a different haplotype
composition from the WJ population. 相似文献