902 research outputs found
Selective Utilization of Microhabitats by Web-building Spiders
Natural enemies are members of complex ecological communities, and their ability to contribute to the biological control of pest organisms is strongly influenced by a convoluted network of ecological interactions with many other organisms within these communities. Researchers must develop an understanding of the mechanisms that shape trophic webs to predict and promote top-down effects of predators. The behavior of predators can have a strong influence on their potential as biological control agents.
Web-building spiders are a useful example organism for the study of natural enemy behavior because of the experimentally tractable nature of their foraging behavior. Specifically, patterns in microhabitat utilization and web construction by spiders provide insights into foraging behavior and pest-suppression potential.
In field collections, spiders were found to utilize microhabitats in a species-specific manner. Molecular gut-content analysis and a mathematical model showed that two spiders belonging to different web-building guilds differed in their dependence on microhabitat-specific prey activity-densities. In particular, the sheet-weaving guild constructed webs in microhabitats with the highest densities of springtails (Collembola). High dependence on this non-pest prey also correlated with evidence of increased intraspecific competition, and implies a potential negative effect of springtails on the consumption of pest insects, such as aphids.
In laboratory two-choice assays, sheet-weaving spiders selected microhabitats and constructed webs in a flexible, stepwise manner, which allowed spiders to regulate their investment of silk resources to match the profitability of the microhabitat. Spiders also exhibited prey-specific shifts in foraging behavior, constructing webs in the presence of mobile, non-pest springtails, but utilizing active foraging tactics in the presence of sedentary, pest aphids. However, in factorial no-choice assays, pest-consumption rates were not significantly affected by the presence of non-pest springtails, indicating that prey-specific foraging-mode shifts are compatible with biological control.
From these results, it is clear that the flexible foraging behavior of web-building spiders has a strong influence on their roles in ecological communities and their position within food webs. This dissertation highlights the importance of understanding the nuances of natural-enemy behavior for properly assessing and promoting biological control services
Local structure of liquid carbon controls diamond nucleation
Diamonds melt at temperatures above 4000 K. There are no measurements of the
steady-state rate of the reverse process: diamond nucleation from the melt,
because experiments are difficult at these extreme temperatures and pressures.
Using numerical simulations, we estimate the diamond nucleation rate and find
that it increases by many orders of magnitude when the pressure is increased at
constant supersaturation. The reason is that an increase in pressure changes
the local coordination of carbon atoms from three-fold to four-fold. It turns
out to be much easier to nucleate diamond in a four-fold coordinated liquid
than in a liquid with three-fold coordination, because in the latter case the
free-energy cost to create a diamond-liquid interface is higher. We speculate
that this mechanism for nucleation control is relevant for crystallization in
many network-forming liquids. On the basis of our calculations, we conclude
that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely
in gaseous planets
Scaling properties of critical bubble of homogeneous nucleation in stretched fluid of square-gradient density-functional model with triple-parabolic free energy
The square-gradient density-functional model with triple-parabolic free
energy is used to study homogeneous bubble nucleation in a stretched liquid to
check the scaling rule for the work of formation of the critical bubble as a
function of scaled undersaturation , the
difference in chemical potential between the bulk undersaturated
and saturated liquid divided by between the liquid
spinodal and saturated liquid. In contrast to our study, a similar
density-functional study for a Lennard-Jones liquid by Shen and Debenedetti [J.
Chem. Phys. {\bf 114}, 4149 (2001)] found that not only the work of formation
but other various quantities related to the critical bubble show the scaling
rule, however, we found virtually no scaling relationships in our model near
the coexistence. Although some quantities show almost perfect scaling relations
near the spinodal, the work of formation divided by the value deduced from the
classical nucleation theory shows no scaling in this model even though it
correctly vanishes at the spinodal. Furthermore, the critical bubble does not
show any anomaly near the spinodal as predicted many years ago. In particular,
our model does not show diverging interfacial width at the spinodal, which is
due to the fact that compressibility remains finite until the spinodal is
reached in our parabolic models.Comment: 10 pages, 10 figures, Journal of Chemical Physics accepted for
publicatio
Thoracic Spine Staples in a Collegiate Volleyball Player
CASE HISTORY: The participant in this research study was diagnosed with idiopathic scoliosis at the age of 8. PHYSICAL EXAM: Upon physical examination it was determined that she had a lateral curvature of the spine. DIFFERENTIAL DIAGNOSES: Herniated disk, erector spinae muscle strain, thoracic back pain, quadratus lumborum strain, or scoliosis. TESTS & RESULTS: She underwent many different x-rays and MRIs to get the best possible imaging of her spine. These images determined she had idiopathic scoliosis. She underwent seven total surgeries to repair the scoliosis present. The first of these surgeries was to place one Harrington Rod to correct the scoliosis present. Another surgery was performed to place 5 spinal staples into her thoracic spine. Three years after the first surgery a final surgery was performed to remove the Harrington Rod. FINAL DIAGNOSIS: The final diagnosis was idiopathic scoliosis. DISCUSSION: During the years between the surgery processes she was confined to a back brace which prevented her from participating in athletics. Approximately 6-9 months after her final surgery she began participating in volleyball. Research shows there is no measurable effect on upper extremity functionality for subjects who have undergone surgery to repair idiopathic scoliosis. Spinal fusion patients often learn how to perform sport movements effectively even though they are less mobile in the thoracic spine area. This is because patients have less range of motion in the spine but learn how to move their trunk and extremities to be effective in sports activities. If the thoracic spine is no longer able to move adequately, the cervical and lumbar spine will compensate, or the movement does not occur. Therefore, they are able to play sports that most people would believe they would not be able to do. Many of these individuals may also have a reduced equilibrium compared to others who have not undergone spinal fusion surgery to repair idiopathic scoliosis. Therefore, the person may not have a good understanding of their body positioning because of the lack of mobility in the thoracic spine. This could possibly be a negative effect on sport performance because of not being fully aware of the position of the body at all times. Lack of equilibrium could possibly be dangerous to the athlete if they are not able to detect their body angles when jumping or diving. Patients who have undergone corrective surgery may also show a decrease in physical activity compared to preoperative levels due to decreased flexibility and pain that could be present. OUTCOME OF THE CASE: The participant returned to activities of daily living and athletic competition post-surgery. RETURN TO ACTIVITY AND FURTHER FOLLOW-UP: The participant returned to athletic competition and currently competes in NCAA Division I athletics as a volleyball player. She has had no further complication with scoliosis since the procedures were performed
Crystal nucleation and cluster-growth kinetics in a model glass under shear
Crystal nucleation and growth processes induced by an externally applied
shear strain in a model metallic glass are studied by means of nonequilibrium
molecular dynamics simulations, in a range of temperatures. We observe that the
nucleation-growth process takes place after a transient, induction regime. The
critical cluster size and the lag-time associated with this induction period
are determined from a mean first-passage time analysis. The laws that describe
the cluster growth process are studied as a function of temperature and strain
rate. A theoretical model for crystallization kinetics that includes the time
dependence for nucleation and cluster growth is developed within the framework
of the Kolmogorov-Johnson-Mehl-Avrami scenario and is compared with the
molecular dynamics data. Scalings for the cluster growth laws and for the
crystallization kinetics are also proposed and tested. The observed nucleation
rates are found to display a nonmonotonic strain rate dependency
Crystal nucleation as the ordering of multiple order parameters
Nucleation is an activated process in which the system has to overcome a free
energy barrier in order for a first-order phase transition between the
metastable and the stable phases to take place. In the liquid-to-solid
transition the process occurs between phases of different symmetry, and it is
thus inherently a multi-dimensional process, in which all symmetries are broken
at the transition. In this Focus Article, we consider some recent studies which
highlight the multi-dimensional nature of the nucleation process. Even for a
single-component system, the formation of solid crystals from the metastable
melt involves fluctuations of two (or more) order parameters, often associated
with the decoupling of positional and orientational symmetry breaking. In other
words, we need at least two order parameters to describe the free-energy of a
system including its liquid and crystalline states. This decoupling occurs
naturally for asymmetric particles or directional interactions, focusing here
on the case of water, but we will show that it also affects spherically
symmetric interacting particles, such as the hard-sphere system. We will show
how the treatment of nucleation as a multi-dimensional process has shed new
light on the process of polymorph selection, on the effect of external fields
on the nucleation process, and on glass-forming ability.Comment: 20 pages, 10 figure
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