Heterogeneous responses of temperate-zone amphibian populations to climate change complicates conservation planning.

The pervasive and unabated nature of global amphibian declines suggests common demographic responses to a given driver, and quantification of major drivers and responses could inform broad-scale conservation actions. We explored the influence of climate on demographic parameters (i.e., changes in the probabilities of survival and recruitment) using 31 datasets from temperate zone amphibian populations (North America and Europe) with more than a decade of observations each. There was evidence for an influence of climate on population demographic rates, but the direction and magnitude of responses to climate drivers was highly variable among taxa and among populations within taxa. These results reveal that climate drivers interact with variation in life-history traits and population-specific attributes resulting in a diversity of responses. This heterogeneity complicates the identification of conservation 'rules of thumb' for these taxa, and supports the notion of local focus as the most effective approach to overcome global-scale conservation challenges.

Electrical conductivity of tissue at frequencies below 1 MHz.

A two-pronged approach, review and measurement, has been adopted to characterize the conductivity of tissues at frequencies below 1 MHz. The review covers data published in the last decade and earlier data not included in recent reviews. The measurements were carried out on pig tissue, in vivo, and pig body fluids in vitro. Conductivity data have been obtained for skeletal and myocardial muscle, liver, skull, fat, lung and body fluids (blood, bile, CSF and urine). A critical analysis of the data highlights their usefulness and limitations and enables suggestions to be made for measuring the electrical properties of tissues.

Variation of the dielectric properties of tissues with age: the effect on the values of SAR in children when exposed to walkie-talkie devices.

In vitro dielectric properties of ageing porcine tissues were measured in the frequency range of 50 MHz-20 GHz, and the total combined uncertainties of the measurements were assessed. The results show statistically significant reduction with age in both permittivity and conductivity of 10 out of 15 measured tissues. At microwave frequencies, the observed variations are mainly due to the reduction in the water content of tissues as an animal ages. The results obtained were then used to calculate the SAR values in children of age 3 and 7 years when they are exposed to RF induced by walkie-talkie devices. No significant differences between the SAR values for the children of either age or for adults were observed.

Complex permittivity of sodium chloride solutions at microwave frequencies.

The complex permittivity of aqueous solutions at 20 degrees C has been measured at concentrations between 0.001 and 5 mol/L and over a frequency range 0.13-20 GHz. The results were combined with literature values to derive empirical equations to predict the dielectric behavior of sodium chloride solutions between 0 and 5 mol/L and 5 degrees C-35 degrees C.

Admittance models for open ended coaxial probes and their place in dielectric spectroscopy.

Starting from a rigorous formulation for the admittance of an open ended coaxial probe, this paper identifies the simplifying assumptions that are introduced to obtain more practical solutions. The predictions of the models are compared to each other and to experimental data obtained on an independently calibrated experimental set-up. The advantages and limitations of a model are determined by the theoretical derivation and the numerical solution. The model identified as most suitable for the measurement of the dielectric properties of biological materials is used to determine the relative permittivity and conductivity of muscle and skull bone in the frequency range 1 MHz to 20 GHz.

Dielectric properties of the system bovine albumin: urea: betaine in aqueous solution.

Urea (about 5 mol l-1) causes an approximately five-fold increase of the dielectric increment of bovine serum albumin at 20 degrees C. The increase is reversed by betaine (about 2.5 mol l-1), or prevented if urea and betaine are added together. This result can be seen as an electric counterpart to the protective role of osmolytes which are secreted by cells and tissues subjected to various types of internal or environmental stress.

Dielectric behavior of aqueous solutions of plasmid DNA at microwave frequencies.

The relative permittivity and dielectric loss of aqueous solutions of plasmid (pUC8.c1 and pUC8.c2) DNA have been measured at 20 degrees C over the frequency range 100 MHz-10 GHz. The solutions had a concentration of 0.1% DNA, and were studied both in the relaxed and the supercoiled form. The dielectric measurements were made using a variety of techniques including frequency domain and time domain methods of operation. No evidence of any resonance absorption, nor of any other kind of enhanced absorption, was observed.

Microwave absorption in aqueous solutions of DNA.

In a recent Nature leading article the possibility of the occurrence of resonance absorption at microwave frequencies in DNA was discussed in terms of the latest available theoretical and experimental evidence, and its implications were assessed. Because of the far-reaching nature of these implications it is important that the existence or otherwise of such absorption be firmly established. Here we report on a concerted effort in two independent laboratories which has involved the measurement of the dielectric properties of aqueous solutions of circular DNA molecules in the frequency range 1-10 GHz. No resonance absorption or any form of enhanced absorption was demonstrated.

Comparison of the dielectric properties of normal and wounded human skin material.

Measurements have been made of the permittivity and conductivity of normal and wounded human skin material over the frequency range 10 MHz-10 GHz. The permittivity of the wounded tissue was found to be about 12% higher than that of the normal tissue. A similar percentage increase was observed for the conductivity. These differences are attributed to the presence of a higher proportion of bulk water in the wounded material.

Dielectric behavior of water in biological solutions: studies on myoglobin, human low-density lipoprotein, and polyvinylpyrrolidone.

The dielectric behavior of the aqueous solutions of three widely differing macromolecules has been investigated: myoglobin, polyvinylpyrrolidone (PVP), and human serum low-density lipoprotein (LDL). It was not possible to interpret unambiguously the dielectric properties of the PVP solution in terms of water structure. The best interpretation of the dielectric data on the myoglobin and LDL solutions was that, in both cases, the macromolecule attracts a layer of water of hydration one or two water molecules in width. For LDL, this corresponds to a hydration factor of only 0.05 g/g, whereas for myoglobin the figure is nearer 0.6 g/g. With myoglobin, part of the water of hydration exhibits its dispersion at frequencies of a few GHz, and the rest disperses at lower frequencies, perhaps as low as 10-12 MHz. The approximate constancy of the width of the hydration shell for two molecules as dissimilar in size as LDL and myoglobin confirms that the proportion of water existing as water of hydration in a biological solution depends critically on the size of the macromolecules as well as on their concentration.

Dielectric properties of ocular tissues in the supercooled and frozen states.

Values of the relative permittivity and conductivity of the rabbit retina and lens have been measured between 10 MHz and 10 GHz over the temperature range 20 to -20 degrees C. As the temperature was lowered from 20 degrees C the dielectric parameters changed smoothly until -9 degrees C where freezing occurred. At this temperature a sharp transition was observed in both conductivity and relative permittivity, the latter falling to a value of below 10 over most of the frequency range. Analysis shows that this is due to the unfreezable water, from which a value of lens hydration is calculated. This value agrees with that obtained previously at 37 degrees C using conventional dielectric mixture theory, but the associated margin of error is much smaller.

Dielectric properties of developing rabbit brain at 37 degrees C.

The dielectric properties of developing rabbit brain were measured at 37 degrees C between 10 MHz and 18 GHz using time domain and frequency domain systems. The results show a variation with age of the dielectric properties of brain. An analysis of the data suggests that the water dispersion in the brain of newly born animals can be represented by a Debye equation. This dispersion increases in complexity with age, and there is evidence of a smaller additional relaxation process centered around 1 GHz. It is concluded that the principal contribution to this subsidiary dispersion region arises from water of hydration.

Dielectric behavior of DNA solution at radio and microwave frequencies (at 20 degrees C).

The dielectric constant and conductivity of calf thymus DNA were investigated at frequencies between 0.1 MHz and 70 GHz. This work is to investigate the dielectric properties of DNA in low gigahertz region and also to study whether the dielectric behavior of the water is affected by the presence of highly charged DNA. The results of these measurements indicate the presence of two anomalous dispersions, the one between 1 MHz and 1 GHz and the second one above 1 GHZ. The dispersion at low frequencies is likely to arise from polar groups in the DNA molecule. The relaxation behavior of unbound water in DNA solution is only slightly affected by the presence of DNA at concentrations below 1%.

Dielectric properties of insulin in solution.

Values of the relative permittivity of a 5% solution of despentapeptide insulin (DPI) are given at six temperatures over a frequency range 0.2-50 MHz. The solvent was a mixture of water and ethylene glycol at pH = 3. From the dispersion curves a dipole moment of 72 +/- 1 was obtained for the insulin molecule at 25 degrees C. By combining the dielectric measurements with values of solvent viscosity the volume of the insulin molecule in solution was found to be only slightly larger than that in the crystal. This indicates that insulin possesses a markedly lower quantity of water of hydration than has been observed for globular proteins containing a higher proportion of hydrophilic groups on the surface of the molecule.

The dielectric properties of normal and tumour mouse tissue between 50 MHz and 10 GHz.

Microwaves are currently being investigated as a method of inducing hyperthermia. Values of the relative permittivity and conductivity of mouse muscle and tumour tissues over the frequency range 50 MHz to 10 GHz at a temperature of 37 degrees C are reported. The data were obtained by using an on-line computer-based time-domain spectrometer. The tumour tissues were the RIF/1 and KHT fibrosarcomas sited in the hind leg muscle of C3H mice. A significant difference in the relative permittivity between the normal and tumour tissue was shown to exist at frequencies up to 200 MHz.

Dielectric properties of ocular tissues at 37 degrees C.

Values of the relative permittivity, dielectric loss and conductivity are given for various rabbit ocular tissues at frequencies in the range 10 MHz-10 GHz. The tissues measured were cornea, retina, choroid, iris, and the cortical and nuclear zones of the lens. The dielectric parameters were determined using the technique of multiple response time domain spectroscopy. For all tissues the water relaxation could be characterised by a Debye dispersion with a relaxation time longer than that of pure water, indicating that its dielectric behaviour was affected by the presence of the biological macromolecules.

The dielectric method of investigating bound water in biological material: an appraisal of the technique.

Three independent dielectric methods for the measurement of water of hydration (bound water) in a biological material are described and discussed comparatively. For well-defined aqueous solutions of biological molecules, hydration can be obtained from direct observations made on the delta dispersion or from measurement of the dielectric values of the beta dispersion. For whole tissue, however, neither of these two methods is applicable, and to deduce the hydration, it is necessary to use the third technique in which the volume of the hydrated biological particle is obtained by measuring the effect of it on the known dielectric properties of pure water. The hydration can then be calculated by deducting the volume of the anhydrous particle from the experimentally determined volume of the hydrated particle. Owing to possible systemmatic errors the uncertainty in the absolute hydration value associated with this technique is rather larger than that obtained with the other two dielectric methods. For studying the differences between hydration in similar tissues, however, this objection disappears.