Induced Population Changes Research Articles

Landscape, cropping and field boundary influences on bird abundance

Given likely dramatic land use changes in the UK, there is a need to predict their impact upon biodiversity. Such predictions require an understanding of the habitat factors that determine species’ abundances. We relate British farmland bird abundance at the 1 km square scale to three variable sets: landscape composition (for example, areas of woodland, urban and arable), field boundary characteristics and cropping. We then use multi‐model inference to summarize model fits for each variable set and compare their explanatory power to identify the relative importance of the three forms of description of the farm landscape. The important variables across species within each set show the habitat features that are most significant for the farmland bird community.Models comprising landscape variables provided the best fits for 21 of 31 bird species, while field boundary variables were most important for seven species and cropping variables for one. After controlling for landscape influences, we then found that field boundary variation was slightly more influential than cropping. These patterns remained when farmland‐specialist species were considered alone. This shows that landscapes are the most important determinant of bird abundance and that cropping patterns and field boundary characteristics are influential only within the constraints of landscape structure. However, boundaries and, especially, crops remain important from a management perspective because they are more volatile, and can still drive population change. Important single variables that affected multiple species included woodland (predominantly negative even within farmland‐dominated landscapes), hedges with trees (providing nesting habitat and song‐posts) and heterogeneity both of landscapes and of cropping (predominantly positive effects). The results indicate which habitat features offer the most scope for induced population changes via management action and potentially provide relationships for input into predictive models with respect to land‐use change.

Deep Level Saturation Spectroscopy

We review the “Deep Level Saturation Spectroscopy” (DLSS) as the nonlinear method to study the deep local defects in semiconductors. The essence of a method is determined by the processes of sufficiently strong laser modulation (up to saturation) of quasistationar two-step absorption of the probe light via deep levels (DLs). DLSS is based on nonequilibrium processes of the optically induced population changes for deep levels which lead to the changes in an impurity absorption. This method allows us the separation of the spectral contributions from different deep centers (even in the case of their full spectral overlap), on the basis of the difference of their optical activity (photon capture cross-sections) and of their electroactivity difference (carriers capture coefficients). As shown, DLSS is allowed to determine directly the main set of phenomenological parameters (cross-sections, concentration, bound energy, etc.) for deep local defects, their content and energy position in the band gap. Some important aspects of DLSS were shown also: the possibility to connect directly the measured data to the local centers which are participating in radiative recombination, and also the possibility to study directly the phonon relaxation processes in the localized states of deep defects.

Generalized model for all-optical light modulation in bacteriorhodopsin

We present a generalized model for the photochemical cycle of bacteriorhodopsin (bR) protein molecule. Rate equations have been solved for the detailed light-induced processes in bR for its nine states: B→K↔L↔MI→MII↔N↔O↔P→Q→B. The complete steady-state intensity-induced population densities in various states of the molecule have been computed to obtain a general, exact, and analytical expression for the nonlinear absorption coefficient for multiple modulation pump laser beams. All-optical light modulation of different probe laser beam transmissions by intensity induced population changes due to one and two modulation laser beams has been analyzed. The proposed model has been shown to accurately model experimental results.

Analysis of spatial light modulation characteristics of C60

Spatial light modulation based on nonlinear absorption of light by excited-state molecules in C60–toluene solution is demonstrated. Amplitude modulation of probe laser beam transmissions at 747 and 885 nm, corresponding to the peak absorption of T1 and S1 states, by the modulation laser beam intensity induced population changes at 532 nm has been analyzed using the rate equation approach, for various values of the small-signal absorption coefficient. It is shown that for T1 and S1 states, modulation up to 100% and 94.5% can be achieved for a laser beam intensity of 800 W/cm2 and 1 kW/cm2 at 532 nm, respectively.

Fluorescence monitoring of laser induced population changes of 6P and 6D levels in cesium vapor

Observations of enhancement and reduction of the fluorescence intensity in the continuous-wave excitation of Cs atomic vapor is reported. The reduction is due to a depopulation of the 6 2P o 3/2 state upon illumination of a Cs vapor cell with light corresponding to the 6 2P o 3/2→6 2D 5/2 transition (917.23 nm). Collisional relaxation of the 6 2D 5/2 atoms to the 6 2D 3/2 state, followed by radiative decay to the 6 2P o 1/2 state (876.14 nm), creates an increase in the population of the 6 2P o 1/2 state. The increase in the excited state population results in an increase in the fluorescence intensity of the 6 2P o 1/2→6 2S 1/2 transition (894.35 nm). These laser induced population changes, monitored by the decrease or increase of the corresponding fluorescence signals, were modeled using a simplified density matrix formalism, which allowed an order of magnitude estimate of the rates of level mixing between 6 2D states and 6 2P states.

Numerical study of induced polarization dynamics in ultrafast spectral hole-burning experiments

We develop a theoretical description of time and frequency resolved transient spectroscopy in non-collinear geometry in a more complete way in respect to the interaction between the applied electric fields and the induced population changes in the sample. For this purpose we deduce a set of differential equations considering a vibrational mode exhibiting finite phase relaxation and an inhomogeneous broadening with cross-relaxation and rotation of the molecules with an average rotation time. In order to describe the electric fields according to experimental conditions a different chirp of the two fields is included in the model as well as calculation of probing components parallel and perpendicular in respect to the excitation. The coherent interaction is solely seen in the parallel probing signal. A time dependent evolution of the transient spectral hole is inferred from the calculations, strongly influenced by phase relaxation. Experimental data on diluted methanol with excitation of the OH-stretching mode demonstrates a temporal evolution of the transient hole in accordance to the calculations.

Chemisorption of sulfur on nickel: A study of cluster convergence in the linear combination of Gaussian-type orbitals local density functional approach

Chemisorption of sulfur at the (100), (110), and (111) surface of nickel has been studied, using the linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. Employing various cluster models consisting of 11 to 29 substrate atoms, adsorption at the experimentally known sites has been considered. Besides the equilibrium distance and the force constant of the vertical adsorbate motion, the dynamical dipole moment was evaluated and it turned out to provide a sensitive probe of cluster convergence. The influence of atoms from the third substrate layer on the various observables is found to be considerable in some cases. With increasing cluster size, bond lengths are stabilized to 0.02 Å, frequencies to 20 cm−1, and dipole moments to 0.1 D. The converged results agree very well with experiment. Adsorption induced population changes are restricted to only four to six neighboring substrate atoms of the modifier atom. However, energy resolved charge density differences reveal a possible mechanism for the transmission of the long-range electronic effects caused by the adatom.

Unusual Weather and Insect Population Dynamics: Lygaeus equestris during an Extinction and Recovery Period

A metapopulation of the seed feeding bug Lygaeus equestris has been censused from 1977 to 1990 in an area in SE Sweden with several small and scattered host-plant patches. Here I describe weather induced population changes during the last four years. During this period the population underwent a large scale extinction followed by recolonization and recovery, a pattern which differs radically from the previous ten years when fluctuations had been moderate and patch occupancy high. In the summer of 1987, with unusually cool weather and an exceptional deficit in sunshine hours, L. equestris populations declined to an unprecedented low level. Total extinction ensued in the following winter. Because weather effects are correlated over large areas the extinction coincided with low population density and patch occupancy in adjoining areas. Hence, following the extinction colonization rates were evidently at a minimum. The study area remained unpopulated until the summer of 1989. when some colonists arrived from outside areas. Because colonists and their offspring were so few establishment would most likely have failed had this been an ordinary year. However, unusually sunny weather resulted in a second generation, something that had not happened during the earlier years of study. This extra reproduction in combination with new immigration from growing populations outside the study area gave rise to a large autumn population, which survived the winter. However, at the end of 1990 population density was still lower than in an adjoining area which had not faced total extinction.

Relaxation of a non-equilibrium rotational distribution in HCl

A mixture of low pressure SF 6 and HCl gas is subjected to P(16)-CO 2 laser radiation. The ensuring fluorescent signals from the HCl vibration and ground-state rotations are spectroscopically resolved and their transient behaviour is measured. The fluorescent signals are used to monitor the collisionally induced population changes in the high rotational levels of HCl ( J = 10–22). The probabilities of collision-induced energy transfer between the rotational levels are calculated on the basis of the exponential gap law model used by Polanyi et al. Experiment and theory show the existence of a non-equilibrium state with a doubly peaked rotational distribution.

Population responses of Lake Michigan phytoplankton to nitrogen and phosphorus enrichment

A natural phytoplankton assemblage from Grand Traverse Bay, Lake Michigan, was treated with factorial enrichments of nitrate and phosphorus, with maintained nutrient concentrations ranging from 5 to 60 µg total soluble phosphorus liter−1 and 0.225 to 1.12 mg nitrate-nitrogen liter−1.One container was spiked with added vitamins, a chelator, and trace metals. The assemblage response was monitored at the species level. Significant differences in growth rates as a function of nutrient enrichment were detected at both the division and the species levels. Growth rates associated with the various levels of enrichment are reported for several diatom taxa. Many of the diatom taxa exhibited highly significant (P < 0.01) increases in growth rate after phosphorus enrichment, with the largest effects occurring between 5 and 15 µg total soluble phosphorus liter−1. Significant (P < 0.05) N effects were also observed, and the nature of these effects was found to be taxon-specific. Taxa also showed significant changes in percent composition, due both to time and to nutrient enrichment, indicating a substantial heterogeneity in response at the species level. Experimentally induced population changes were qualitatively similar to those observed in regions of the Great Lakes which have undergone anthropogenic eutrophication.

Energy transfer in A 2Σ+ OH. I. Rotational

We have used a frequency doubled, tunable dye laser to excite individual v′, N′, J′ levels of the A 2Σ+ state of the OH molecule. The fluorescence emitted in the presence of known pressures of various fill gases has permitted the determination of collisionally induced population changes, and hence state-to-state energy transfer rates. Here are reported results on rotational energy transfer within v′=0, using six different initially pumped levels for N2 as a collision partner, and two each for H2 and Ar. It is found that the rates are fast (5×10−10 cm3 sec−1 for a typical total transfer rate with N2), that a process having ΔN =ΔJ occurs faster than one where ΔN≠ΔJ, and that multiquantum transfer rates are nearly as large as single quantum transfer rates.

Effects of continuous competition in two species of cellular slime mold: Dictyostelium discoideum and Polysphondylium pallidum.

Competitively induced population changes were studied in the laboratory. The experimental organisms chosen for the work were two cellular slime mold species: Dictyostelium discoideum and Polysphondylium pallidum. Culturing these organisms on a temperature gradient of 15 °C to 30 °C induces long-term continuous competition for food and space. The organisms respond by changing their rates of resource exploitation and their susceptibility to interference.Before competition, D. discoideum interfered with P. pallidum fruiting, body formation. This effect was not observed after competition. Specifically, when grown alone before competition, P. pallidum fruited from 18° to 37 °C and D. discoideum from 9° to 27 °C. In mixed cultures, before competition. P. pallidum fruited from about 24° to 37 °C and D. discoideum from about 9° to 27 °C. In mixed cultures, after continued competition, P. pallidum fruited from about 20° to 37 °C and D. discoideum from about 9° to 24 °C. There is evidence to suggest that the change is genetic and related to parasexuality in P. pallidum.Rates of resource exploitation (which depend upon spore germination times and amoeba colony expansion rates) also changed. Germination times were unaltered but amoeba colony expansion rates increased.Apparently continued competition resulted in convergence and divergence with respect to resource use. Because P. pallidum gained the ability to fruit in the presence of D. discoideum it converged with respect to resource use. At the same time the two species diverged by increasing rates of resource use over different segments of the temperature range.