Animal dispersal of American ginseng (Panax quinquefolius L.)

American ginseng (Panax quinquefolius L.) is a long-lived forest herb found throughout Appalachia, well-known for its reputation as a medicinal plant species. Harvest of ginseng for the international medicinal plant trade has been a significant contributor to population decline of the species. As a declining species with life-history traits similar to many other herbaceous perennials and both cultural and economic value, American ginseng has become a focal species for many demographic and conservation based studies. Seed dispersal is a critical component of ginseng demography lacking empirical evidence for potential dispersal mechanisms. To date, gravity is considered to be the primary mechanism of dispersal, but the production of red, fleshy berries during the late fall suggest animal dispersal. Based on berry morphology, timing of ripening, and field-based observations, songbird and small mammal species were determined to be two likely candidates for ginseng dispersal. In Chapter 2, I investigated the frequency and type of songbird species interacting with ginseng berries using infrared, motion-activated wildlife cameras in the field and observed songbird digestive behavior in captivity. Thrushes were found to most frequently remove berries in the field and regurgitate viable seeds, on average, 16 minutes after ingestion. In Chapter 3, small mammal interactions were investigated using infrared, motion-activated wildlife cameras and cafeteria-style feeding boxes in the field. In addition, the impacts of high tree mast years, an environmental event linked to small mammal population dynamics, on seedling recruitment was investigated using historical mast indices and long-term ginseng population data. Small mammals more frequently interacted with ginseng berries in camera images compared to thrushes and mice were observed to predate ginseng seeds during feeding trials. The impacts of high tree mast years were found to differ between sites, but recruitment was found to decrease with increasing mast production at four sites. Overall, thrushes were found to be the primary animal disperser of American ginseng while small mammals are most often seed predators. As the primary animal disperser, thrushes provide the first documented opportunity for intermediate and long distance dispersal events. Dispersal events over such distances are becoming increasingly more important for ginseng to persist in the presence of intense harvest, deer browse, and climate change; three significant factors contributing to declining ginseng populations. Additionally, small mammal predation of ginseng seeds could further contribute to population declines

Cell-autonomous inhibition of alpha 7-containing nicotinic acetylcholine receptors prevents death of parasympathetic neurons during development

Neurotrophic molecules are key retrograde influences of cell survival in the developing nervous system, but other influences such as activity are also emerging as important factors. In the avian ciliary ganglion, half the neurons are eliminated between embryonic day 8 (E8) and E14, but it is not known how cell death is initiated. Because systemic application of alpha7-nicotinic acetylcholine receptor (nAChR) antagonists prevents this cell loss, we examined differences in receptor densities and responses of intracellular calcium to nicotine using the calcium-sensitive dye fura-2. In addition, we determined whether cell-autonomous inhibition of alpha7 activation in neurons prevented cell death. E8 neurons are heterogeneous with respect to alpha7-nAChR density, which leads to large increases in [Ca2+]i in some neurons; E8 neurons also exhibit a slower rate of Ca2+ decay after nicotinic stimulation than E13 neurons. Expressing alpha-bungarotoxin that is tethered to the membrane by a glycosylphosphatidylinositol linkage (GPIalpha btx) in ciliary ganglion neurons with the retroviral vector RCASBP(A) blocks increases in intracellular calcium induced by nicotine through alpha7-nAChRs and prevents neurons from dying. Expression of GPIalpha btx in surrounding non-neural tissues, but not in neurons, does not prevent cell loss. Furthermore, the GPIalpha btx is not efficiently expressed in the accessory oculomotor neurons, eliminating preganglionic inputs as another site for action of the antagonist. These results support the hypothesis that cholinergic inputs facilitate cell death in the developing autonomic nervous system by activating alpha7-nAChRs, possibly by leading to increases in intracellular calcium that exceed the threshold for cell survival

A Semiclassical Approach to Level Crossing in Supersymmetric Quantum Mechanics

Much use has been made of the techniques of supersymmetric quantum mechanics (SUSY QM) for studying bound-state problems characterized by a superpotential $\phi(x)$. Under the analytic continuation $\phi(x) \to i\phi(x)$, a pair of superpartner bound-state problems is transformed into a two-state level-crossing problem in the continuum. The description of matter-enhanced neutrino flavor oscillations involves a level-crossing problem. We treat this with the techniques of supersymmetric quantum mechanics. For the benefit of those not familiar with neutrino oscillations and their description, enough details are given to make the rest of the paper understandable. Many other level-crossing problems in physics are of exactly the same form. Particular attention is given to the fact that different semiclassical techniques yield different results. The best result is obtained with a uniform approximation that explicitly recognizes the supersymmetric nature of the system.Comment: 15 pages, Latex with lamuphys and psfig macros. Talk by first Author at the UIC "Supersymmetry and Integrable Models Workshop", Chicago, June 12-14, 1997; proceedings to be published in Springer Lecture Notes in Physics, H. Aratyn et al., eds. This paper also available at http://nucth.physics.wisc.edu/preprint

Conductivity of the classical two-dimensional electron gas

We discuss the applicability of the Boltzmann equation to the classical two-dimensional electron gas. We show that in the presence of both the electron-impurity and electron-electron scattering the Boltzmann equation can be inapplicable and the correct result for conductivity can be different from the one obtained from the kinetic equation by a logarithmically large factor.Comment: Revtex, 3 page

From laterally modulated two-dimensional electron gas towards artificial graphene

Cyclotron resonance has been measured in far-infrared transmission of GaAs/Al$_x$Ga$_{1-x}$As heterostructures with an etched hexagonal lateral superlattice. Non-linear dependence of the resonance position on magnetic field was observed as well as its splitting into several modes. Our explanation, based on a perturbative calculation, describes the observed phenomena as a weak effect of the lateral potential on the two-dimensional electron gas. Using this approach, we found a correlation between parameters of the lateral patterning and the created effective potential and obtain thus insights on how the electronic miniband structure has been tuned. The miniband dispersion was calculated using a simplified model and allowed us to formulate four basic criteria that have to be satisfied to reach graphene-like physics in such systems

CAT(0) spaces with polynomial divergence of geodesics

We construct a family of finite 2-complexes whose universal covers are CAT(0) and have polynomial divergence of desired degree. This answers a question of Gersten, namely whether such CAT(0) complexes exist