Single generation cycles and delayed feedback cycles are not separate phenomena.

We study a simple model for generation cycles, which are oscillations with a period of one or a few generation times of the species. The model is formulated in terms of a single delay-differential equation for the population density of an adult stage, with recruitment to the adult stage depending on the intensity of competition during the juvenile phase. This model is a simplified version of a group of models proposed by Gurney and Nisbet, who were the first to distinguish between single-generation cycles and delayed-feedback cycles. According to these authors, the two oscillation types are caused by different mechanisms and have periods in different intervals, which are one to two generation times for single-generation cycles and two to four generation times for delayed-feedback cycles. By abolishing the strict coupling between the maturation time and the time delay between competition and its effect on the population dynamics, we find that single-generation cycles and delayed-feedback cycles occur in the same model version, with a gradual transition between the two as the model parameters are varied over a sufficiently large range. Furthermore, cycle periods are not bounded to lie within single octaves. This implies that a clear distinction between different types of generation cycles is not possible. Cycles of all periods and even chaos can be generated by varying the parameters that determine the time during which individuals from different cohorts compete with each other. This suggests that life-cycle features in the juvenile stage and during the transition to the adult stage are important determinants of the dynamics of density limited populations.

Evolutionary food web model based on body masses gives realistic networks with permanent species turnover.

The networks of predator-prey interactions in ecological systems are remarkably complex, but nevertheless surprisingly stable in terms of long term persistence of the system as a whole. In order to understand the mechanism driving the complexity and stability of such food webs, we developed an eco-evolutionary model in which new species emerge as modifications of existing ones and dynamic ecological interactions determine which species are viable. The food-web structure thereby emerges from the dynamical interplay between speciation and trophic interactions. The proposed model is less abstract than earlier evolutionary food web models in the sense that all three evolving traits have a clear biological meaning, namely the average body mass of the individuals, the preferred prey body mass, and the width of their potential prey body mass spectrum. We observed networks with a wide range of sizes and structures and high similarity to natural food webs. The model networks exhibit a continuous species turnover, but massive extinction waves that affect more than 50% of the network are not observed.

The effects of Hurricane Mitch on a community in northern Honduras.

INTRODUCTION: Hurricane Mitch was an event described as one of the most damaging recent natural disasters in our hemisphere. This study examined its effects on a community of 5,000 residents in northern Honduras. METHODS: Survey responses of 110 attendants at an ambulatory clinic 4 months after the event were analyzed. Correlates were established between demographic and housing characteristics and morbidity and mortality. RESULTS: The availability of food, water, and medical care decreased significantly immediately after the hurricane, but by four months afterward returned to baseline values. Residents reported emotional distress correlated with the loss of a house or intrafamilial illness or mortality. Diarrheal illnesses more commonly were found in households with poor chronic access to medical care. The use of cement block housing correlated with availability of food or running water, with access to medical care and vaccinations, and with a reduced frequency of diarrhea or headaches in the immediate post-hurricane phase. CONCLUSIONS: Improvements in housing construction appear to be the most effective preventive measure for withstanding the effects of future hurricanes in tropical regions similar to northern Honduras.

Cutaneous malignant melanoma.

Melanoma is the deadliest form of skin cancer; 47,700 persons were expected to be diagnosed and 9,600 were expected to die from melanoma in 2000 (American Cancer Society, 2000). It is important for dermatology nurses to understand the epidemiology, risk factors, clinical presentation, diagnosis, treatment, prognosis, and prevention of cutaneous malignant melanoma.