Models of prey-predator systems incorporating the intrinsic population growth rates of each species were analyzed mathematically and were also simulated. The behavior of the t:ov~ b~sic models l.ed .to. a hypothesis: either a stable prey population possesses strong self-hm1tatwn, e.g., terntonahty, or the growth rate of the prey species is less than that of its !'redator. Intrinsic population growth rates calculated for eight species of prey and of their ~mportant pre~ators showed that the prey had higher rates than their predators in two cases, m bot~ of wh1ch the prey. species are known to be self-limited. The prey and predator have approximately ~qu~l rates 1~. the snow~hoe hare and lynx, known to be cyclic. Five species of ungulates, ~ackmg mtraspe~1f1c populatiOn controls, had lower growth rates than their predators. lncorporatw.n ?f two hab1tats, one with good and a second with poor cover for the prey, change~ a h~nt cycle to a stable focus; this fits some observations of snowshoe hares. The properties w1th the greatest effect on stability shown by these models are prey self-limitation and predator searching time.

Relative bioavailability of two iron fortificants, electrolytic Fe and ferric orthophosphate, was related to that of the reference ferrous sulfate with in vitro and rat model depletion-repletion methods in four laboratories to compare values directly with those obtained in a parallel human study. In vitro testing was performed on Fe compounds with both solubility and dialysis in a simulated in vitro gastrointestinal digestion system. Two depletion-repletion techniques, hemoglobin-regeneration efficiency (HRE) and an official method of the Association of Official Analytical Chemists (AOAC), were examined. AOAC relative biological values (RBV) of electrolytic Fe were 0.66 and 0.78 and of FePO4 were 0.25 and 0.34. HRE values were 0.78 and 0.58 for electrolytic Fe and FePO4, respectively. When compared with FeSO4 in a radiolabeled farina-based meal fed to humans, the RBV of FePO4 was 0.25 and electrolytic Fe 0.75. Results obtained with the AOAC method serve as the most reliable prediction of Fe bioavailability in the human although in vitro dialysis is a promising screening technique.

Many phonological processes can be affected by segmental context spanning word boundaries, which often lead to variable outcomes. This paper tests the idea that some of this variability can be explained by reference to production planning. We examine coronal stop deletion (CSD), a variable process conditioned by preceding and upcoming phonological context, in a corpus of spontaneous British English speech, as a means of investigating a number of variables associated with planning: Prosodic boundary strength, word frequency, conditional probability of the following word, and speech rate. From the perspective of production planning, (1) prosodic boundaries should affect deletion rate independently of following context; (2) given the locality of production planning, the effect of the following context should decrease at stronger prosodic boundaries; and (3) other factors affecting planning scope should modulate the effect of upcoming phonological material above and beyond the modulating effect of prosodic boundaries. We build a statistical model of CSD realization, using pause length as a quantitative proxy for boundary strength, and find support for these predictions. These findings are compatible with the hypothesis that the locality of production planning constrains variability in speech production, and have practical implications for work on CSD and other variable processes.

No abstract

The growth rate of a population at time t is defined as the change in numbers per unit population at that time period (rt = dn/Ntd). The rate usually changes with time as the population increases or decreases. The objective of this study was determine whether the growth rate of animal populations is or is not a function of the population density. In most mathematical models of populations whose changes resemble those of actual populations, the growth rate is a decreasing function of density, meaning that as the population increases the growth rate decreases and vice versa. Records of actual populations can be used to test the relation between growth rate and density by calculating correlation coefficients for these two characteristics. The methods of doing this are described and the limitations on the data are explained. The records of 111 different populations representing 71 species were analyzed by these methods. Of the 71 species, 7 were eliminated from further analysis because their census records were not significantly different from a series of random numbers, 47 had correlation coefficients that were negative and significantly different from zero, 16 had coefficients not significantly different from zero (all but one of these estimated coefficients were negative), and one, the world's human population, had a positive coefficient significantly different from zero. There were no differences between taxonomic groups (insects other invertebrates, fish, birds, mammals). The primary conclusion is that in most animal species a population's growth rate is a decreasing function of density. The explains the relative stability of animal populations, which never continue to increase at rates their fertility would allow, and rarely decrease to extinction. Tentative conclusions are presented regarding the processes regulating population numbers. Populations of herbivorous insects at low and moderate levels are regulated by disease, parasites, and predators causing mortality that is an increasing function of density. Favorable conditions may allow an increase in insect numbers so rapid that the population temporarily escapes regulation by its enemies. Nonterritorial species of vertebrates are normally controlled by predation and, when that fails, by competition; in both cases juvenile individuals are most affected. In territorial vertebrates competition for suitable territories determines the size of the breeding population. Populations of vertebrate species (expecting man) are in general regulated by the production of adult individuals being a decreasing function of population density.