On the solution of mathematical models of herd immunity in human helminth infections

The general solution of the mathematical model of herd immunity to human helminth infections recently proposed by Anderson and May [3] is obtained. The numerical solution of a more accurate biological model is indistinguishable from the corresponding exact solution of a more tractable mathematical model. Computer simulations of some particular cases of this model support the notion that both ecological and immunological factors determine the observed convex patterns of age-prevalence and age-intensity curves of human helminth infections.This work was made thanks to the advise and support of Dr. Robert M. May while the author was Postdoctoral Fellow at Princeton University     

On choice of substrate and habitat in brachionid rotifers

Information on the distribution of 28 rotifers of the family Brachionidae from diverse waters in south and central Sweden was analyzed to reveal their relationships to substrate and habitat. Some brachionids are preferably planktic, others periphytic and/or benthic. Some non-planktic habitats are utilized more than others, but there is no evidence of a chemical attraction from any substrate. Instead, some substrates seem to be avoided, possibly depending on a poorer flora of periphytic algae. Besides substrate type, the following factors are found to be important for creating separate ecological niches in the brachionid family: temperature, oxygen content, trophic degree, chemical environment, food choice and sensitivity to predation. It is possible to delineate separate ecological niches for all brachionid rotifers, implying that Hutchinson's ideas about the plankton paradox are contradicted. Some species are specialists, other are generalists, the latter being characterized by a great morphological variation. The species are adapted in different ways to their preferential habitats, as regards foot, egg-carrying, protrusions and other lorical structures etc. Longer spines, for instance, are generally found in more transparent water, being a supposed protection against visual predators.     

Relative importance of temporal and spatial heterogeneity in the zooplankton community of an artificial reservoir

Zooplankton samples were collected monthly at three stations in Ogochi Reservoir during 1980, and the temporal and horizontal variations in the density of dominant taxa were evaluated by a standard 2-way ANOVA with random effects. The analysis revealed that horizontal is greater than temporal variation in the warm water season (> 18 °C), whereas temporal variation is greater in the cold water season (< 18 °C). Horizontal heterogeneity of the zooplankton community found in the warm water season seems to be associated with differences in site-specific conditions.     

Microgeographic distribution of immature Ixodes dammini ticks correlated with that of deer

In order to determine whether the small-scale distribution of immature Ixodes dammini Spielman et al. corresponds closely to the activity patterns of white-tailed deer, Odocoileus virginianus (Zimmerman), these relationships were examined in a site on Long Island, New York, U.S.A. We first determined the extent and temporal pattern of adult ticks feeding on deer by examining twenty-three resident deer tranquilized during September-December 1985. I. dammini adults infested deer throughout this fall period, most abundantly during October and November. With radio-telemetry collars attached to deer we determined the relative frequency that they occupied 0.25 ha quadrats of the study site. During the following summer, we examined white-footed mice, Peromyscus leucopus (Rafinesque), that inhabited these quadrats and removed immature ticks from each. 8975 larval and 163 nymphal I. dammini were removed from 208 mice trapped in forty-three such quadrats. The frequency of deer using these quadrats was positively correlated with both the number of larval and of nymphal ticks per mouse. These results suggest that risk of I. damminiborne zoonotic disease may be decreased by locally reducing deer density in sites that experience intense human activity.     

Pollination Ecology of Pedicularis punctata Decne. (Scrophulariaceae) in the Kashmir Himalaya

Abstract The pollination ecology of Pedicularis punctata was studied in the Pir Panjal Range of the Kashmir Himalaya in the summer of 1989. Its nectarless, rostrate, long-tubed flower was found to be pollinated exclusively by Bombus foragers vibrating pollen while the stigma contacted pollen in the pollinator's cervical crevice. Workers of Bombus tunicatus and B. flavothoracicus comprised 95% of its pollinators. Pollen-foraging fidelity of its pollinators was greatest where diversity of Bombus -pollinated plant species in three plant communities was least. Foragers on other plants carried virtually no Pedicularis pollen. P. punctata is a mid-season blooming species similar in its pollination syndrome to comparable species in other geographic regions. The enigmatic function of its long, nectarless corolla tube, even more exaggerated in other Asiatic species, requires further investigation.     

The allometric approach to species differences in brain size

Allometric methods can be used to test quantitative theories of the relationship between brain size and body size across species, and to search for ecological, behavioural, life history, and ontogenetic correlates of brain size. Brain size scales with an allometric exponent of around 0.75 against body size across mammals, but is closer to 0.56 for birds and for reptiles. The slope of the allometric line often varies depending upon the taxonomic level of analysis. However, this phenomenon, at least in mammals, may be a statistical artifact. Brain size for a given body size (relative brain size) varies among orders in birds and mammals, and some dietary associations with relative brain size have been found in particular taxa. Developmental status at birth is the most consistent correlate of relative brain size: precocial neonates have larger brains for a given maternal size than altricial neonates in both birds and mammals. Altricial neonates, however, have more brain growth following birth, and in birds also have larger relative adult brain sizes. Energetic explanations for differences in neonatal brain growth, although attractive on theoretical grounds, have largely failed to stand up to empirical tests.     

Social organization and ecology of proboscis monkeys (Nasalis larvatus) in mixed coastal forest in Sarawak

Data are presented from a 16-month study of proboscis monkeys in an area of mixed coastal forest in Sarawak. The population density, social organization, and feeding and ranging behavior are described in detail. Results are compared with those from other primates in an attempt to understand why females of certain species (including proboscis monkeys) transfer between social groups. The scarcity of available food and reasons for the limited habitat preferences of proboscis monkeys are also discussed.     

Forest-level analysis of stability under exploitation: depensation responses and catastrophe theory

Stability analysis of whole forests is proposed as a qualitative tool for the study of forest responses to partial or patchy harvests or mortality. Instead of modeling every tree or stand, aggregate tree biomass is modeled. In order to aggregate stands, spatial effects must be incorporated. It is shown that depensation growth responses (reduced growth at low biomass) are common because forests often modify harch environments to be more suitable for their growth. Depensation can result from thinning, partial mortality, patchy cutting, or clearcutting, depending on forest type and abiotic factors. Examples of these types of behaviors are given. Stability analysis of different growth regimes under exploitation are related to catastrophe theory and to optimal harvesting policies. Such qualitative analysis is shown to be applicable to data-poor regions such as the tropics where there is great concern over responses of forests to exploitation.     

SIMS microscopy in the biomedical field.

We attempted to indicate the requirements for biomedical applications of SIMS microscopy. Sample preparation methodology should preserve both the structural and the chemical integrity of the tissue. Furthermore, it is often necessary to correlate ionic and light microscope images. This implies a common methodological approach to sample preparation for both microscopes. The use of low or high mass resolution depends on the elements studied and their concentrations. To improve the acquisition and processing of images, digital imaging systems have to be designed and require both ionic and optical image superimposition. However, the images do not accurately reflect element concentration; a relative quantitative approach is possible by measuring secondary ion beam intensity. Using an internal reference element (carbon) and standard curves the results are expressed in micrograms/mg of tissue. Despite their limited lateral resolution (0.5 microns) the actual SIMS microscopes are very suitable for the resolution of biomedical problems posed by action modes and drug localization in human pathology. SIMS microscopy should provide a new tool for metabolic radiotherapy by facilitating dose evaluation. The advent of high lateral resolution SIMS imaging (less than 0.1 microns) should open up new fields in biomedical investigation.     

Scales and costs of habitat selection in heterogeneous landscapes

Summary Two scales of habitat selection are likely to influence patterns of animal density in heterogeneous landscapes. At one scale, habitat selection is determined by the differential use of foraging locations within a home range. At a larger scale, habitat selection is determined by dispersal and the ability to relocate the home range. The limits of both scales must be known for accurate assessments of habitat selection and its role in effecting spatial patterns in abundance. Isodars, which specify the relationships between population density in two habitats such that the expected reproductive success of an individual is the same in both, allow us to distinguish the two scales of habitat selection because each scale has different costs. In a two-habitat environment, the cost of rejecting one of the habitats within a home range can be expressed as a devaluation of the other, because, for example, fine-grained foragers must travel through both. At the dispersal scale, the cost of accepting a new home range in a different habitat has the opposite effect of inflating the value of the original habitat to compensate for lost evolutionary potential associated with relocating the home range. These costs produce isodars at the foraging scale with a lower intercept and slope than those at the dispersal scale.Empirical data on deer mice occupying prairie and badland habitats in southern Alberta confirm the ability of isodar analysis to differentiate between foraging and dispersal scales. The data suggest a foraging range of approximately 60 m, and an effective dispersal distance near 140 m. The relatively short dispersal distance implies that recent theories may have over-emphasized the role of habitat selection on local population dynamics. But the exchange of individuals between habitats sharing irregular borders may be substantial. Dispersal distance may thus give a false impression of the inability of habitat selection to help regulate population density.