Light, nutrients, and food-chain length constrain planktonic energy transfer efficiency across multiple trophic levels

The efficiency of energy transfer through food chains [food chain efficiency (FCE)] is an important ecosystem function. It has been hypothesized that FCE across multiple trophic levels is constrained by the efficiency at which herbivores use plant energy, which depends on plant nutritional quality. Furthermore, the number of trophic levels may also constrain FCE, because herbivores are less efficient in using plant production when they are constrained by carnivores. These hypotheses have not been tested experimentally in food chains with 3 or more trophic levels. In a field experiment manipulating light, nutrients, and food-chain length, we show that FCE is constrained by algal food quality and food-chain length. FCE across 3 trophic levels (phytoplankton to carnivorous fish) was highest under low light and high nutrients, where algal quality was best as indicated by taxonomic composition and nutrient stoichiometry. In 3-level systems, FCE was constrained by the efficiency at which both herbivores and carnivores converted food into production; a strong nutrient effect on carnivore efficiency suggests a carryover effect of algal quality across 3 trophic levels. Energy transfer efficiency from algae to herbivores was also higher in 2-level systems (without carnivores) than in 3-level systems. Our results support the hypothesis that FCE is strongly constrained by light, nutrients, and food-chain length and suggest that carryover effects across multiple trophic levels are important. Because many environmental perturbations affect light, nutrients, and food-chain length, and many ecological services are mediated by FCE, it will be important to apply these findings to various ecosystem types.     

Effect of preload on ischaemic and non-ischaemic left ventricular regional function

The response to preload of ischaemic and non-ischaemic regions of the left ventricle was studied in 14 dogs undergoing right heart bypass with mean aortic pressure and heart rate held constant. Regional function was measured by sonomicrometry before and after coronary artery occlusion. In the ischaemic region, as expected, there was paradoxical systolic lengthening (that is, systolic shortening was negative) but as stroke volume was progressively increased end diastolic length increased, whereas end systolic length changed little; thus systolic lengthening decreased (systolic shortening increased). Ischaemic regions that were dyskinetic at low stroke volumes were virtually akinetic at high stroke volumes. Additional studies showed that this response was not attributable to increased regional blood flow at high preloads and occurred over a wide range of heart rates and mean aortic pressures. Plots of systolic shortening against end diastolic length, expressing the regional Frank-Starling relation, were well described by linear regression in both ischaemic and non-ischaemic regions, although a few of these relations were better described by higher order polynomials. The slopes of these relations in the ischaemic region were 0.86(0.05) before and 0.83(0.06) after ligation, reflecting a small effect of preload on end systolic length. The data suggest that when contractility and afterload are constant preload determines the magnitude and in certain instances the sign of systolic shortening. In any ischaemic regions incapable of developing force the positive slope of the Frank-Starling relation is attributable to myocardial passive elastic properties. Paradoxical lengthening does not, however, necessarily indicate the absence of active force development; positive and negative values of systolic shortening describe a continuous spectrum of regional contractility. Thus the effects of preload and contractility on systolic shortening in ischaemic as well as non-ischaemic myocardium require differentiation.     

Pneumonia following closed head injury.

Pneumonia is common among patients with artificial airways in place. Most prior studies of such pneumonia involve a heterogeneous group of patients, usually with major medical or surgical illnesses. We studied the incidence of pneumonia in a group of patients with isolated closed head injury (CHI) in an effort to determine the pattern of the problem in the absence of other injuries and to determine whether the pattern of development of pneumonia in these patients was comparable to that in more heterogeneous groups of mechanically ventilated patients. We studied 109 initially comatose patients with isolated CHI who were ventilated 24 h or more. The mean age was 30.3 +/- 20.2 yr, 72% were male, and the admission Glasgow coma score was 4.9T +/- 1.4. Overall, 45 patients (41%) developed pneumonia, with the majority (29/45) occurring during the first 3 days of hospitalization. No patient developed pneumonia after the first week despite the fact that many were still ventilated, others remained intubated, and yet others were extubated but comatose. Patients who developed pneumonia experienced a longer ICU stay (10.5 +/- 5.4 days versus 7.2 +/- 4.3 days, p = 0.001) and hospital stay (34.8 +/- 27.6 versus 22.5 +/- 20.2 days, p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Why MSM in Rural South African Communities Should be an HIV Prevention Research Priority

Research into HIV and men who have sex with men’s (MSM) health in South Africa has been largely confined to the metropolitan centres. Only two studies were located making reference to MSM in rural contexts or same-sex behaviors among men in the same. There is growing recognition in South Africa that MSM are not only disproportionately affected by HIV and have been underserved by the country’s national response, but that they contribute significantly to sustaining the high number of new infections recorded each year. We argue that to meet the objectives of the country’s national strategic plan for HIV, STI and TB it is important we know how these behaviours may be contributing to the sustained rural HIV epidemic in the youngest age groups and determine what constitutes appropriate and feasible programmatic response that can be implemented in the country’s public sector health services.     

Research technique for epidemiologic investigation of nutrition and cancer

When investigating cancer etiology, epidemiologists often correlate estimates of current dietary practices to cancer incidence. There is evidence that diets may be basically stable over time due to deep‐rooted habits; however, changes in eating behavior are often associated with life‐style or health‐status changes. By using a diet‐related life‐event schedule, dietary changes resulting from life events may be more easily recalled and a more accurate characterization of diet patterns during neoplasm initiation may be made.