Using fine-scale GIS data to assess the relationship between intra-annual environmental niche variability and population density in a local stream fish assemblage

P>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 center dot 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 center dot 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 center dot 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.