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Summary 1. Geographic information systems (GIS) have recently proved useful for estimating the environmental niche of species across broad geographic regions. However, the application of these niche-based GIS techniques has yet to be extensively applied to local systems. The assumptions of the methods are transferable across scales: species exist across a range of habitats, and these habitats represent a gradient of suitability that can be characterized using multivariate environmental data in association with known species occurrences. 2. Habitat availability and species’ niche characteristics are often considered to be important predictors of population density. However, seasonal habitat variability and stochastic events have been hypothesized to limit interactions among species and confound the relationship between available habitat, species niche characteristics and community structure. This research examines the relationship between environmental niche breadth and niche position and population density among species of stream fishes in a seasonally variable environment using a novel application of fine-scale GIS data. 3. Niche breadth and niche position were estimated for 11 species in a local assemblage using five fine-scale (0·5 m) GIS-based environmental data sets collected during four different times of the year (July, October, January, April). We compared niche measures to variation in population density among species to determine whether environmental niche characteristics, in the context of available habitat, explain variation in numbers of individuals among species. 4. Variation in population density among species in the October sample was predicted by niche breadth measures (R2 = 0·752), while variation in population density among species in the January sample was predicted by a model incorporating measures of both niche breadth and niche position (R2 = 0·953). Measures of niche breadth and niche position were not correlated with variation in population density in the July and April samples. 5. Species presence and local abundance are often assumed to be predictable based on the availability of suitable habitat. However, little effort has been directed at understanding the influence of both niche breadth and niche position on local abundance. Our results suggest that the amount and distribution of available habitat can be a strong predictor of interspecific variation in population density, even in seasonally variable environments.

This paper describes the analysis of the ν 3 and ν 6 fundamental bands of methyl chloride d 3 using a high-resolution spectrum (resolving power of the apparatus: 0.0015 cm −1 ). These bands are coupled by a weak Coriolis interaction and are then treated simultaneously. Spectroscopic constants are derived for the two isotopic species CD 3 35 Cl and CD 3 37 Cl.

The effect of secretin on forearm metabolism (muscle compartment) was examined in the presence of low and high concentrations of insulin. Continuous infusions (30 min. duration) of secretin 12 ng/min. (SEC), insulin 160 ng/min. (INS) or these doses of insulin and secretin together (INS + SEC) were delivered into the brachial arteries of normal volunteers. SEC alone was without significant effect on forearm metabolism. INS as previously reported, stimulated glucose and potassium uptake and inhibited the output of some amino acids from the deep venous bed. The two hormones together (INS + SEC) increased maximum lactate (p less than 0.025) and alanine output (p less than 0.005) beyond that observed with INS alone. A marked calcium output occurred (p less than 0.001) with the combined infusion which was not observed with either hormone alone. Potassium metabolism with INS + SEC was similar to that observed with INS alone. These results suggest that these two hormones, acting in concert, stimulate glycolysis and net calcium efflux. This joint action of insulin and secretin may have a physiological role in the immediate postprandial period.