Region Of Rapid Change Research Articles

An Epidemilogical Model Incorporating Vector Population Dynamics Applied to African Cassava Mosaic Virus Disease

An epidemiological model of African cassava mosaic virus disease (ACMD) was developed in which the dynamics within a locality, of healthy and infected cassava, and of infective and non-infective whitefly vectors, were specified. Cassava is propagated routinely from stem cuttings. If the exclusive use of virus-free cuttings was assumed then either a high rate of virus transmission by the vector or a large population of vectors could lead to persistent cycles in disease incidence. Such cycling could seriously impair cassava production. When frequency-dependent use of infected cuttings was admitted, however, persistent cycles of infection were no longer possible. Instead, three states could occur: disease eliminated (E), both healthy and diseased cassava persisted (P), or infection became total (T). The outcome depended on the particular parameter values, but some parameter combinations gave state P or T, depending on the initial conditions. In some instances, low initial incidence of ACMD posed a greater risk of progressing to state T than did higher levels of infection. In general, disease incidence exhibited a discontinuous, threshold response to changes in the proportion of infected cuttings used. An increase in their use could appear to have little impact on incidence, although the system was moving closer to a point of sudden increase in infection. Simulation of the removal (roguing) of diseased plants had little influence on disease incidence, but had an important and largely cryptic effect by helping to move the system away from the threshold of collapse of the healthy cassava population. Intensification of cassava production (represented in the model as an increase in the maximum possible abundance of cassava) had the opposite effect, moving the system closer to this threshold. Assuming a degree of disease 'reversion', whereby a proportion of the cuttings derived from infected plants are uninfected, had a stabilizing influence on the model, making even damped cycles of disease very unlikely. Total infection did not now occur except at very low reversion rates. The response of disease incidence to the increased use of infected cuttings was no longer discontinuous but remained nonlinear and there was a region of rapid change corresponding to the threshold seen in the absence of reversion.

Satellite studies of magnetospheric substorms on August 15, 1968: 1. State of the magnetosphere

The sequence of events occurring throughout the magnetosphere during a substorm has not been precisely determined. This paper introduces a collection of papers that attempts to establish this sequence for two substorms on August 15, 1968. Data from a wide variety of sources are used, the major emphasis being changes in the magnetic field. In this paper we use ground magnetograms to determine the onset times of two substorms that occurred while the Ogo 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetic tail (the region of rapid change from taillike to dipolar field). We conclude that at least two worldwide substorm expansions were preceded by growth phases. Probable beginnings of these phases were at 0330 and 0640 UT. However, the onset of the former growth phase was partially obscured by the effects of a preceding expansion phase around 0220 and a possible localized event in the auroral zone near 0320 UT. The onsets of the corresponding expansion phases were 0430 and 0714 UT. Further support for these determinations is provided by data discussed in the subsequent notes.

A map of Rayleigh-wave dispersion in the Pacific

The group velocity vs. frequency curves for 103 different Pacific paths have been analyzed to determine the group velocity dispersion as a function of latitude and longitude in the neighborhood of the group velocity maximum. The main features of the local group velocity dispersion curves can be specified by the maximum group velocity u0, the period t0 at which the maximum occurs, and a factor b related to the curvature of the dispersion curve at its maximum. The highest group velocities are found in the northwest Pacific where u0 exceeds 4.2 km/s. The Andesite line appears as a region of rapid change in u0, t0, and b. The East Pacific Rise is not delimited by the dispersion map, but it occurs in a region of low u0, t0, and b. The principal value of the maps appears to be that they present qualitative information on the crust and upper mantle structure.

The Role Of Electrical Conductivity Of Water In Shocking Lampreys (Petromyzon marinus)

A plot of the critical voltage gradient required to immobilize a sea lamprey in water of 15 °C, against the logarithm of the conductivity of the water, was described accurately by a rectangular hyperbola normal to the axes. The region of rapid change in critical gradient lies below a conductivity of 100 micromhos per centimetre cube. The average conductivity of a live lamprey at 15° was 2700 micromhos per centimetre cube.