76 research outputs found
Sequential Predation: A Multi-model Study
In many ecosystems food resources are available sequentially. The paper analyses a situation with
two competing prey species both of which are consumed by a common predator species. Within a
season the two prey species are available sequentially, although there may be an overlap. Three modelling
methodologies are applied to this system] discrete dynamical systems (difference equations),
individual-oriented event-driven simulations and cellular automata. The presence of the predator is shown
to have a strong impact on the outcome of the prey species competition. The system of coexisting prey
species changes to a system of founder-controlled competition. It appears that sequential predation can
even have counterintuitive evolutionary consequences for the prey species. The species which appears later
in the season will be more successful in its competition with the early species if it favours the predator;
for example, by a high leaf palatability. Spatial structuring and topological issues are found to play a
crucial role in both the ecological and evolutionary dynamics. The advantages of a multi!model approach
are discussed
Forest Development over a Twenty-Year Chronosequence of Reforested Urban Sites
Urbanization causes a variety of negative ecological impacts, impairing forests, streams, and other ecosystems. While urban forests are the subject of increasing research attention, planted urban forests are less well-understood than remnant forests; however, these systems may be distinct in terms of ecosystem structure and function. The current study investigates a chronosequence of reforested urban sites in Lexington, KY, USA, with a focus on overstory and understory woody plant community characteristics. Monitoring plots were established in each of the 20 sites; tree height, dbh, and species were surveyed for the overstory, and species and height were surveyed for the understory. Canopy height increased non-linearly with time since planting, rapidly increasing in years 6â10, but remaining relatively steady after year 15. While the overstory was dominated by planted native species, the understory was predominately non-native species, some of which are considered invasive. Overall, the nonlinear logistic relationship of canopy height to time since planting may be driven by species-specific effectsâwith trees such as ash (Fraxinus spp. L.) and black locust (Robinia pseudoacacia L.) important during the early years after planting, but declining over time due to pests and other pressures. Alternatively, this complex relationship of canopy height with time may be driven by uncontrolled factors such as site quality, landscape position, planted species, etc. The significance of invasive species in the understory suggests that the long-term health of these sites will be compromised without intentional ongoing maintenance to ensure continued forest development toward desired native community characteristics
The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids
Recently the first Earth Trojan has been observed (Mainzer et al., ApJ 731)
and found to be on an interesting orbit close to the Lagrange point L4 (Connors
et al., Nature 475). In the present study we therefore perform a detailed
investigation on the stability of its orbit and moreover extend the study to
give an idea of the probability to find additional Earth-Trojans. Our results
are derived using different approaches: a) we derive an analytical mapping in
the spatial elliptic restricted three-body problem to find the phase space
structure of the dynamical problem. We explore the stability of the asteroid in
the context of the phase space geometry, including the indirect influence of
the additional planets of our Solar system. b) We use precise numerical methods
to integrate the orbit forward and backward in time in different dynamical
models. Based on a set of 400 clone orbits we derive the probability of capture
and escape of the Earth Trojan asteroids 2010 TK7. c) To this end we perform an
extensive numerical investigation of the stability region of the Earth's
Lagrangian points. We present a detailed parameter study in the regime of
possible stable tadpole and horseshoe orbits of additional Earth-Trojans, i.e.
with respect to the semi-major axes and inclinations of thousands of fictitious
Trojans. All three approaches underline that the Earth Trojan asteroid 2010 TK7
finds himself in an unstable region on the edge of a stable zone; additional
Earth-Trojan asteroids may be found in this regime of stability.Comment: 11 pages, 16 figure
Improving Woody Biomass Estimation Efficiency Using Double Sampling
Although double sampling has been shown to be an effective method to estimate timber volume in forest inventories, only a limited body of research has tested the effectiveness of double sampling on forest biomass estimation. From forest biomass inventories collected over 9,683 ha using systematic point sampling, we examined how a double sampling scheme would have affected precision and efficiency in these biomass inventories. Our results indicated that double sample methods would have yielded biomass estimations with similar precision as systematic point sampling when the small sample was â„ 20% of the large sample. When the small to large sample time ratio was 3:1, relative efficiency (a combined measure of time and precision) was highest when the small sample was a 30% subsample of the large sample. At a 30% double sample intensity, there was a \u3c 3% deviation from the original percent margin of error and almost half the required time. Results suggest that double sampling can be an efficient tool for natural resource managers to estimate forest biomass
Development of Mine Soils in a Chronosequence of Forestry-Reclaimed Sites in Eastern Kentucky
Surface mining for coal has contributed to widespread deforestation and soil loss in coal mining regions around the world, and particularly in Appalachia, USA. Mined land reforestation is of interest in this and other regions where forests are the dominant pre-mining land use. This study evaluated mine soil development on surface-mined sites reforested according to the Forestry Reclamation Approach, representing a chronosequence of time ranging from 0 to 19 years after reclamation. Soils were sampled in depth increments to 50 cm and analyzed for a suite of soil physical and chemical characteristics. Overall, soil fines (silt + clay) tended to increase over time since reclamation (17% silt at year 0 increasing to 35% at year 11; 3.2% clay at year 0 increasing to 5.7% at year 14) while concentrations of metals (e.g., Al, Mg, Mn, Na) demonstrated varied relationships with time since reclamation. Concentrations of organic carbon (OC) tended to increase with time (0.9% OC at year 0 increasing to 2.3% at year 14), and were most enriched in near-surface soils. Some soil characteristics (e.g., Na, OC, Ca) demonstrated patterns of increasing similarity to the forest control, while others were distinct from the forest control throughout the chronosequence (e.g., Al, clay, Mn, gravel). Future surveys of these soils over time will elucidate longer-term patterns in soil development, and better characterize the time scales over which these soils might be expected to approximate forest soil conditions
Where are the Uranus Trojans?
The area of stable motion for fictitious Trojan asteroids around Uranus'
equilateral equilibrium points is investigated with respect to the inclination
of the asteroid's orbit to determine the size of the regions and their shape.
For this task we used the results of extensive numerical integrations of orbits
for a grid of initial conditions around the points L4 and L5, and analyzed the
stability of the individual orbits. Our basic dynamical model was the Outer
Solar System (Jupiter, Saturn, Uranus and Neptune). We integrated the equations
of motion of fictitious Trojans in the vicinity of the stable equilibrium
points for selected orbits up to the age of the Solar system of 5 billion
years. One experiment has been undertaken for cuts through the Lagrange points
for fixed values of the inclinations, while the semimajor axes were varied. The
extension of the stable region with respect to the initial semimajor axis lies
between 19.05 < a < 19.3 AU but depends on the initial inclination. In another
run the inclination of the asteroids' orbit was varied in the range 0 < i < 60
and the semimajor axes were fixed. It turned out that only four 'windows' of
stable orbits survive: these are the orbits for the initial inclinations 0 < i
< 7, 9 < i < 13, 31 < i < 36 and 38 < i < 50. We postulate the existence of at
least some Trojans around the Uranus Lagrange points for the stability window
at small and also high inclinations.Comment: 15 pages, 12 figures, submitted to CMD
Secular dynamics of a planar model of the Sun-Jupiter-Saturn-Uranus system; effective stability into the light of Kolmogorov and Nekhoroshev theories
We investigate the long-time stability of the Sun-Jupiter-Saturn-Uranus
system by considering a planar secular model, that can be regarded as a major
refinement of the approach first introduced by Lagrange. Indeed, concerning the
planetary orbital revolutions, we improve the classical circular approximation
by replacing it with a solution that is invariant up to order two in the
masses; therefore, we investigate the stability of the secular system for
rather small values of the eccentricities. First, we explicitly construct a
Kolmogorov normal form, so as to find an invariant KAM torus which approximates
very well the secular orbits. Finally, we adapt the approach that is at basis
of the analytic part of the Nekhoroshev's theorem, so as to show that there is
a neighborhood of that torus for which the estimated stability time is larger
than the lifetime of the Solar System. The size of such a neighborhood,
compared with the uncertainties of the astronomical observations, is about ten
times smaller.Comment: 31 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1010.260
Significance of Elevated Blood Metal Ion Levels in Patients with Metal-on-Metal Prostheses: An Evaluation of Oxidative Stress Markers
It is widely known that cobalt and chromium ions can enhance the production of reactive oxygen species, known to be damaging to cells by disturbing their redox status and then generating oxidative stress. The aim of the present study was to determine if increased metal ion levels induce a state of oxidative stress in patients with metal-on-metal (MM) hip arthroplasty. Results indicated that there was no significant difference in the concentration of oxidative stress markers (total antioxidants, peroxides, and nitrated proteins) in the patients with MM bearings compared to patients without prostheses. The activity antioxidant enzymes was stable (catalase and glutathione peroxidase) or slightly decreased (superoxide dismutase and heme oxygenase-1) over time. This work is the first to determine the biological effects of metal ions released from MM hip implants with regards to mid-term systemic oxidative stress and showed that the increased levels of Co and Cr ions are not associated with significant oxidative stress damage in the plasma of patients with these implants
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