Hydrological studies of schistosomiasis transport in Sichuan Province, China.

Schistosomiasis is a water-bourne parasitic disease endemic to Sichuan Province of China. Long-term studies of infection and disease ecology in catchments in Sichuan have been supplemented by detailed hydrometric measurements to produce a model of water velocity and flow in an irrigation system. The model provides a means of estimating travel times of two infectious stages of the parasite from source sites to water contact exposure sites for individuals of both the human population and the intermediate vector snail populations. The hydrological transport model will be part of an overall model of schistosomiasis transmission in the catchments. A GIS system is used to manage spatial data of the drainage network, land use, infection sources and population centres. The development of the Three Gorges Dam in China will increase marshlands and irrigation in areas currently free of schistosomiasis. The potential for the spread of schistosomiasis into these new areas is a major concern. Hydrological models can be of particular importance in assessing future environmental risk.

Population toxicokinetics of tetrachloroethylene.

In assessing the distribution and metabolism of toxic compounds in the body, measurements are not always feasible for ethical or technical reasons. Computer modeling offers a reasonable alternative, but the variability and complexity of biological systems pose unique challenges in model building and adjustment. Recent tools from population pharmacokinetics, Bayesian statistical inference, and physiological modeling can be brought together to solve these problems. As an example, we modeled the distribution and metabolism of tetrachloroethylene (PERC) in humans. We derive statistical distributions for the parameters of a physiological model of PERC, on the basis of data from Monster et al. (1979). The model adequately fits both prior physiological information and experimental data. An estimate of the relationship between PERC exposure and fraction metabolized is obtained. Our median population estimate for the fraction of inhaled tetrachloroethylene that is metabolized, at exposure levels exceeding current occupational standards, is 1.5% [95% confidence interval (0.52%, 4.1%)]. At levels approaching ambient inhalation exposure (0.001 ppm), the median estimate of the fraction metabolized is much higher, at 36% [95% confidence interval (15%, 58%)]. This disproportionality should be taken into account when deriving safe exposure limits for tetrachloroethylene and deserves to be verified by further experiments.

The structural stability of a three-species food chain model.

A three-species food-chain model which was previously shown to exhibit chaotic dynamics was revisited. By exploring the sensitivity of that result this study found that complex behavior depended on the functional form chosen to model the interaction between the two highest species in the food chain. Two separate scenarios were explored: the gradual addition of refugia modeling the escape from predation at low prey densities; and the gradual addition of predator interference modeling territorial behavior. The addition of even a small amount of refugia provided a stabilizing influence as the chaotic dynamics collapsed to stable limit cycles. The results of adding interference to the model were more complex. Although the numerical simulations indicated that a low level of interference provided a stabilizing influence, the analytical results suggest that complex dynamics are possible for a range of parameter values that are biologically relevant. The sensitivity of the stability profile to functional changes in the model suggests two important ecological motivations for structural stability analysis. First, in ecological systems, environmental fluctuations cause continuous changes in the functional relationships between and within species, resulting in potential changes in the complexity of the dynamics over time. Second, slight changes in ecological structure may cause significant bifurcations; however, most ecological data are inadequate to distinguish such phenomena.

System issues for Controlled Ecological Life Support Systems.

There are several characteristics of a Controlled Ecological Life Support System that are distinct from commonly engineered systems. These are: 1) the uncertainty, due to limited data availability, and variability due to the heterogeneity of biological subsystems; 2) the closed, ecological nature of the system; and 3) the primary criterion of maximizing the probability of survival. Consequences of these features include: complex dynamics characterized by time scales ranging from milliseconds to months, posing difficult problems with respect to mathematical modeling and predictability; and the necessity for a unique controller design that can translate the high level requirement of survivability to low-level actuator tasks. Future research in the systems and control area should include an ecological perspective focusing on the unique dynamical characteristics of a Controlled Ecological Life Support System.